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Research Article

Research on drinking water purification technologies for household use by reducing total dissolved solids (TDS)

Roles Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing

* E-mail: [email protected]

Affiliation Redlands East Valley High School, Redlands, California, United States of America

• Bill B. Wang

• Published: September 28, 2021
• https://doi.org/10.1371/journal.pone.0257865
• Reader Comments

This study, based in San Bernardino County, Southern California, collected and examined tap water samples within the area to explore the feasibility of adopting non-industrial equipment and methods to reduce water hardness and total dissolved solids(TDS). We investigated how water quality could be improved by utilizing water boiling, activated carbon and sodium bicarbonate additives, as well as electrolysis methods. The results show that heating is effective at lower temperatures rather than long boils, as none of the boiling tests were lower than the original value. Activated carbon is unable to lower TDS, because it is unable to bind to any impurities present in the water. This resulted in an overall TDS increase of 3.5%. However, adding small amounts of sodium bicarbonate(NaHCO 3 ) will further eliminate water hardness by reacting with magnesium ions and improve taste, while increasing the pH. When added to room temperature tap water, there is a continuous increase in TDS of 24.8% at the 30 mg/L mark. The new findings presented in this study showed that electrolysis was the most successful method in eliminating TDS, showing an inverse proportion where an increasing electrical current and duration of electrical lowers more amounts of solids. This method created a maximum decrease in TDS by a maximum of 22.7%, with 3 tests resulting in 15.3–16.6% decreases. Furthermore, when water is heated to a temperature around 50°C (122°F), a decrease in TDS of around 16% was also shown. The reduction of these solids will help lower water hardness and improve the taste of tap water. These results will help direct residents to drink more tap water rather than bottled water with similar taste and health benefits for a cheaper price as well as a reduction on plastic usage.

Citation: Wang BB (2021) Research on drinking water purification technologies for household use by reducing total dissolved solids (TDS). PLoS ONE 16(9): e0257865. https://doi.org/10.1371/journal.pone.0257865

Editor: Mahendra Singh Dhaka, Mohanlal Sukhadia University, INDIA

Received: June 22, 2021; Accepted: September 14, 2021; Published: September 28, 2021

Copyright: © 2021 Bill B. Wang. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the manuscript and its Supporting Information files.

Funding: The author received no specific funding for this work.

Competing interests: The authors have declared that no competing interests exist.

Introduction

The concentration of total dissolved solids(TDS) present in water is one of the most significant factors in giving water taste and also provides important ions such as calcium, magnesium, potassium, and sodium [ 1 – 3 ]. However, water with high TDS measurements usually indicates contamination by human activities, such as soil and agricultural runoff caused by irrigation, unregulated animal grazing and wildlife impacts, environmentally damaging farming methods such as slash and burn agriculture, and the overuse of nitrate-based fertilizer [ 4 , 5 ], etc. Around tourist areas as well as state parks, these factors will slowly add up over time and influence the water sources nearby [ 5 ]. Water that flows through natural springs and waterways with high concentrations of organic salts within minerals and rocks, or groundwater that originates from wells with high salt concentration will also result in higher particle measurements [ 6 ].

Water sources can be contaminated by substances and ions such as nitrate, lead, arsenic, and copper [ 7 , 8 ] and may cause many health problems related to heavy metal consumption and poisoning. Water reservoirs and treatments plants that do not consider water contamination by motor vehicles, as well as locations that struggle to provide the necessary components required for water treatment will be more prone to indirect contamination [ 9 – 11 ]. Many plants are effective in ensuring the quality and reduction of these contaminants, but often leave out the secondary considerations, The United States Environmental Protection Agency(US EPA)’s secondary regulations recommend that TDS should be below 500 mg/L [ 2 ], which is also supported by the World Health Organization(WHO) recommendation of below 600 mg/L and an absolute maximum of less than 1,000 mg/L [ 3 ]. These substances also form calcium or magnesium scales within water boilers, heaters, and pipes, causing excess buildup and drain problems, and nitrate ions may pose a risk to human health by risking the formation of N -nitroso compounds(NOC) and less public knowledge about such substances [ 12 – 15 ]. Nitrates can pose a non-carcinogenic threat to different communities, but continue to slip past water treatment standards [ 15 ]. Furthermore, most people do not tolerate or prefer water with high hardness or chlorine additives [ 16 ], as the taste changes tremendously and becomes unpreferable. Even so, TDS levels are not accounted for in mandatory water regulations, because the essential removal of harmful toxins and heavy metals is what matters the most in water safety. Some companies indicate risks in certain ions and alkali metals, showing how water hardness is mostly disregarded and is not as well treated as commercial water bottling companies [ 17 , 18 ].

In Southern California, water quality is not as well maintained than the northern counties as most treatment plants in violation of a regulation or standard are located in Central-Southern California [ 19 ], with southern counties having the largest number of people affected [ 20 ]. This study is focused on the Redlands area, which has had no state code violations within the last decade [ 21 ]. A previous study has analyzed TDS concentrations throughout the Santa Ana Basin, and found concentrations ranging from 190–600 ppm as treated wastewater and samples obtained from mountain sites, taking into account the urban runoff and untreated groundwater as reasons for elevated levels of TDS but providing no solution in helping reduce TDS [ 22 ]. Also, samples have not been taken directly through home water supplies, where the consumer is most affected. Other water quality studies in this region have been focused on the elimination of perchlorates in soil and groundwater and distribution of nitrates, but such research on chemicals have ceased for the last decade, demonstrated by safe levels of perchlorates and nitrates in water reports [ 23 , 24 ]. In addition to these studies, despite the improving quality of the local water treatment process, people prefer bottled water instead of tap water because of the taste and hardness of tap water [ 25 ]. Although water quality tests are taken and documented regularly, the taste of the water is not a factor to be accounted for in city water supplies, and neither is the residue left behind after boiling water. The residue can build up over time and cause appliance damage or clogs in drainage pipes.

This study will build upon previous analyses of TDS studies and attempt to raise new solutions to help develop a more efficient method in reducing local TDS levels, as well as compare current measurements to previous analyses to determine the magnitude to which local treatment plants have improved and regulated its treatment processes.

Several methods that lower TDS are reviewed: boiling and heating tap water with and without NaHCO₃, absorption by food-grade activated carbon [ 26 , 27 ], and battery-powered electrolysis [ 28 – 30 ]. By obtaining water samples and determining the difference in TDS before and after the listed experiments, we can determine the effectiveness of lowering TDS. The results of this study will provide options for residents and water treatment plants to find ways to maintain the general taste of the tap water, but also preserve the lifespan of accessories and pipelines. By determining a better way to lower TDS and treat water hardness, water standards can be updated to include TDS levels as a mandatory measurement.

Materials and methods

All experiments utilized tap water sourced from Redlands homes. This water is partially supplied from the Mill Creek (Henry Tate) and Santa Ana (Hinckley) Water Sheds/Treatment Plants, as well as local groundwater pumps. Water sampling and sourcing were done at relatively stable temperatures of 26.9°C (80.42°F) through tap water supplies. The average TDS was measured at 159 ppm, which is slightly lower than the reported 175 ppm by the City of Redlands. Permission is obtained by the author from the San Bernardino Municipal Water Department website to permit the testing procedures and the usage of private water treatment devices for the purpose of lowering water hardness and improving taste and odor. The turbidity was reported as 0.03 Nephelometric Turbidity Units (NTU) post-treatment. Residual nitrate measured at 2.3mg/L in groundwater before treatment and 0.2 mg/L after treatment and perchlorate measured at 0.9 μg/L before treatment, barely staying below the standard of 1 μg/L; it was not detected within post-treatment water. Lead content was not detected at all, while copper was detected at 0.15 mg/L.

For each test, all procedures were done indoors under controlled temperatures, and 20 L of fresh water was retrieved before each test. Water samples were taken before each experimental set and measured for TDS and temperature, and all equipment were cleaned thoroughly with purified water before and after each measurement. TDS consists of inorganic salts and organic material present in solution, and consists mostly of calcium, magnesium, sodium, potassium, carbonate, chloride, nitrate, and sulfate ions. These ions can be drawn out by leaving the water to settle, or binding to added ions and purified by directly separating the water and ions. Equipment include a 50 L container, 1 L beakers for water, a graduated cylinder, a stir rod, a measuring spoon, tweezers, a scale, purified water, and a TDS meter. A standard TDS meter is used, operated by measuring the conductivity of the total amount of ionized solids in the water, and is also cleaned in the same manner as aforementioned equipment. The instrument is also calibrated by 3 pH solutions prior to testing.All results were recorded for and then compiled for graphing and analysis.

Heating/Boiling water for various lengths of time

The heating method was selected because heat is able to break down calcium bicarbonate into calcium carbonate ions that are able to settle to the bottom of the sample. Four flasks of 1 L of tap water were each heated to 40°C, 50°C, 60°C, and 80°C (104–176°F) and observed using a laser thermometer. The heated water was then left to cool and measurements were made using a TDS meter at the 5, 10, 20, 30, and 60-minute marks.

For the boiling experiments, five flasks of 1 L of tap water were heated to boil at 100°C (212°F). Each flask, which was labeled corresponding to its boiling duration, was marked with 2, 4, 6, 10, and 20 minutes. Each flask was boiled for its designated time, left to cool under open air, and measurements were made using a TDS meter at the 5, 10, 20, 30, 60, and 120-minute marks. The reason that the boiling experiment was extended to 120 minutes was to allow the water to cool down to room temperature.

Activated carbon as a water purification additive

This test was performed to see if food-grade, powdered activated carbon had any possibility of binding with and settling out residual particles. Activated carbon was measured using a milligram scale and separated into batches of 1, 2, 4, 5, 10, 30, and 50 mg. Each batch of the activated carbon were added to a separate flask of water and stirred for five minutes, and finally left to settle for another five minutes. TDS measurements were recorded after the water settled.

Baking soda as a water purification additive

To lower scale error and increase experimental accuracy, a concentration of 200 mg/L NaHCO₃ solution was made with purified water and pure NaHCO₃. For each part, an initial TDS measurement was taken before each experiment.

In separate flasks of 1 L tap water, each labeled 1, 2, 4, 5, 10, and 30 mg of NaHCO 3 , a batch was added to each flask appropriately and stirred for 5 minutes to ensure that everything dissolved. Measurements were taken after the water was left to settle for another 5 minutes for any TDS to settle.

Next, 6 flasks of 1 L tap water were labeled, with 5 mg (25 mL solution) of NaHCO₃ added to three flasks and 10 mg (50 mL solution) of NaHCO₃ added to the remaining three. One flask from each concentration of NaHCO₃ was boiled for 2 mins., 4 mins., or 6 mins., and then left to cool. A TDS measurement was taken at the 5, 10, 20, 30, 60, and 120-minute marks after removal from heat.

Electrolysis under low voltages

This test was performed because the ionization of the TDS could be manipulated with electricity to isolate an area of water with lower TDS. For this test, two 10cm long graphite pieces were connected via copper wiring to a group of batteries, with each end of the graphite pieces submerged in a beaker of tap water, ~3 cm apart.

Using groups of 1.5 V double-A batteries, 4 beakers with 40mL of tap water were each treated with either 7.5, 9.0, 10.5, and 12.0 V of current. Electrolysis was observed to be present by the bubbling of the water each test, and measurements were taken at the 3, 5, 7, and 10 minute marks.

Results/Discussion

Heating water to various temperatures until the boiling point.

The goal for this test was to use heat to reduce the amount of dissolved oxygen and carbon dioxide within the water, as shown by this chemical equation: Heat: Ca(HCO 3 ) 2 → CaCO 3 ↓ + H 2 O + CO 2 ↑.

This would decompose ions of calcium bicarbonate down into calcium carbonate and water and carbon dioxide byproducts.

Patterns and trends in decreasing temperatures.

The following trend lines are based on a dataset of changes in temperature obtained from the test results and graphed as Fig 1 .

• PPT PowerPoint slide
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• TIFF original image

https://doi.org/10.1371/journal.pone.0257865.g001

To predict the precise temperature measurements of the tap water at 26.9°C, calculations were made based on Fig 1 . The fitting equations are in the format, y = a.e bx . The values for the fitting coefficients a and b, and correlation coefficient R 2 are listed in Table 1 as column a, b and R 2 . The calculated values and the target temperature are listed in Table 1 .

https://doi.org/10.1371/journal.pone.0257865.t001

Fig 2 was obtained by compiling TDS results with different temperatures and times.

https://doi.org/10.1371/journal.pone.0257865.g002

The fitting equations for Fig 2 are also in the format, y = a.e bx . The fitting coefficients a and b, and correlation coefficient R 2 values are listed in Table 2 . Based on the fitting curves in Fig 2 and the duration to the target temperature in Table 1 , We calculated the TDS at 26.9°C as listed in column calculated TDS in Table 2 based on the values we reported on Fig 2 .

https://doi.org/10.1371/journal.pone.0257865.t002

Based on the heating temperature and the calculated TDS with the same target water temperature, we obtained the following heating temperature vs TDS removal trend line and its corresponding fitting curve in Table 2 .

In Fig 1 , a trend in the rate of cooling is seen, where a higher heating temperature creates a steeper curve. During the first five minutes of cooling, the water cools quicker as the absorbed heat is quickly released into the surrounding environment. By the 10-minute mark, the water begins to cool in a linear rate of change. One detail to note is that the 100°C water cools quicker than the 80°C and eventually cools even faster than the 60°C graph. Table 1 supports this observation as the duration to target temperature begins to decrease from a maximum point of 94.8 mins to 80.95 mins after the 80°C mark.

As shown in Fig 2 , all TDS values decrease as the temperature starts to cool to room temperature, demonstrating a proportional relationship where a lower temperature shows lower TDS. This can partially be explained by the ions settling in the flasks. Visible particles can also be observed during experimentation as small white masses on the bottom, as well as a thin ring that forms where the edge of the water contacts the flask. When the water is heated to 40°C and cooled, a 3.8% decrease in TDS is observed. When 50°C is reached, the TDS drops at its fastest rate from an initial value of 202 ppm to 160 ppm after 60 minutes of settling and cooling. The TDS measurements in these experiements reach a maximum of 204 ppm at the 60°C mark. However, an interesting phenomenon to point out is that the water does not hit a new maximum at 100°C. meaning that TDS reaches a plateau at 60°C. Also, the rate of decrease begins to slow down after 20 minutes, showing that an unknown factor is affecting the rate of decrease. It is also hypothesized that the slight increase in TDS between the 5–20 minute range is caused by a disturbance in the settling of the water, where the temperature starts to decrease at a more gradual and constant rate. The unstable and easy formation of CaCO 3 scaling has also been the subject of a study of antiscaling methods, which also supports the result that temperature is a significant influence for scale formation [ 12 ].

In Table 2 , calculations for TDS and the time it takes for each test to cool were made. Using the data, it is determined that the test with 50°C water decreased the most by 16% from the initial measurement of 159ppm. This means that it is most effective when water is heated between temperatures of 40–60°C when it comes to lowering TDS, with a difference of ~7–16%. When water is heated to temperatures greater than 80°C, the water begins to evaporate, increasing the concentration of the ions, causing the TDS to increase substantially when cooled to room temperature.

Finally, in Fig 3 , a line of best fit of function f(x) = -0.0007x 3 + 0.1641x 2 –10.962x + 369.36 is used with R 2 = 0.9341. Using this function, the local minimum of the graph would be reached at 48.4°C.

https://doi.org/10.1371/journal.pone.0257865.g003

This data shows that heating water at low temperatures (i.e. 40–50°C) may be more beneficial than heating water to higher temperatures. This study segment has not been presented in any section within the United States EPA Report on water management for different residual particles/substances. However, warmer water temperatures are more prone to microorganism growth and algal blooms, requiring more intensive treatment in other areas such as chlorine, ozone, and ultraviolet disinfection.

Using the specific heat capacity equation, we can also determine the amount of energy and voltage needed to heat 1 L of water up to 50°C: Q = mcΔT, where c, the specific heat capacity of water, is 4.186 J/g°C, ΔT, the change in temperature from the experimental maximum to room temperature, is 30°C, and m, the mass of the water, is 1000 g. This means that the amount of energy required will be 125580 J, which is 0.035 kWh or 2.1 kW.

After taking all of the different measurements obtained during TDS testing, and compiling the data onto this plot, Fig 4 is created with a corresponding line of best fit:

https://doi.org/10.1371/journal.pone.0257865.g004

In Fig 4 , it can be observed that the relationship between the temperature of the water and its relative TDS value is a downwards facing parabolic graph. As the temperature increases, the TDS begins to decrease after the steep incline at 50–60°C. The line of best fit is represented by the function f(x) = -0.0142x 2 + 2.258x + 105.84. R 2 = 0.6781. Because the R 2 value is less than expected, factors such as the time spent settling and the reaction rate of the ions should be considered. To determine the specifics within this experiment, deeper research and prolonged studies with more highly accurate analyses must be utilized to solve this problem.

Boiling water for various amounts of time

Trend of boiling duration and rate of cooling..

Using the same methods to create the figures and tables for the previous section, Fig 5 depicts how the duration of time spent boiling water affects how fast the water cools.

https://doi.org/10.1371/journal.pone.0257865.g005

As seen in Fig 5 , within the first 10 minutes of the cooling time, the five different graphs are entwined with each other, with all lines following a similar pattern. However, the graph showing 20 minutes of boiling is much steeper than the other graphs, showing a faster rate of cooling. This data continues to support a previous claim in Fig 2 , as this is most likely represented by a relationship a longer the boil creates a faster cooling curve. This also shows that the first 5 minutes of cooling have the largest deviance compared to any other time frame.

The cooling pattern is hypothesized by possible changes in the orderly structure of the hydrogen bonds in the water molecules, or the decreased heat capacity of water due to the increasing concentration of TDS.

Effect on TDS as boiling duration increases.

In Fig 6 , all lines except for the 20-minute line are clustered in the bottom area of the graph. By excluding the last measurement temporarily due to it being an outlier, we have observed that the difference between the initial and final TDS value of each test decreases.

https://doi.org/10.1371/journal.pone.0257865.g006

Despite following a similar trend of an increase in TDS at the start of the tests and a slow decrease overtime, this experiment had an interesting result, with the final test measuring nearly twice the amount of particles compared to any previous tests at 310 ppm, as shown in Fig 6 . It is confirmed that the long boiling time caused a significant amount of water to evaporate, causing the minerals to be more concentrated, thus resulting in a 300 ppm reading. Fig 6 follows the same trend as Fig 2 , except the TDS reading veers away when the boiling duration reaches 20 minutes. Also, with the long duration of heating, the water has developed an unfavorable taste from intense concentrations of CaCO₃. This also causes a buildup of a thin crust of CaCO₃ and other impurities around the container that is difficult to remove entirely. This finding is in accordance with the introductory statement of hot boiling water causing mineral buildups within pipes and appliances [ 9 ]. A TDS reading of 300ppm is still well below federal secondary standards of TDS, and can still even be compared to bottled water, in which companies may fluctuate and contain 335ppm within their water [ 1 , 2 ].

This experiment continues to stupport that the cooling rate of the water increases as the time spent boiling increases. Based on this test, a prediction can be made in which an increased concentration of dissolved solids lowers the total specific heat capacity of the sample, as the total volume of water decreases. This means that a method can be derived to measure TDS using the heat capacity of a tap water mixture and volume, in addition to current methods of using the electrical conductivity of aqueous ions.

Adding food-grade activated carbon to untreated tap water

Fig 7 presents a line graph with little to no change in TDS, with an initial spike from 157 to 163 ppm. The insoluble carbon remains in the water and shows no benefit.

https://doi.org/10.1371/journal.pone.0257865.g007

The food-grade activated carbon proved no benefit to removing TDS from tap water, and instead added around 5–7 ppm extra, which settled down to around +4 ppm at 120 minutes. The carbon, which is not 100% pure from inorganic compounds and materials present in the carbon, can dissolve into the water, adding to the existing concentration of TDS. Furthermore, household tap water has already been treated in processing facilities using a variety of filters, including carbon, so household charcoal filters are not effective in further reducing dissolved solids [ 18 ].

Adding sodium bicarbonate solution to boiled tap water

As seen in Fig 8 , after adding 1 mg of NaHCO 3 in, the TDS rises to 161 ppm, showing a minuscule increase. When 4 mg was added, the TDS drops down to 158 ppm. Then, when 5 mg was added, a sudden spike to 172 ppm was observed. This means that NaHCO 3 is able to ionize some Ca 2+ and Mg 2+ ions, but also adds Na + back into the water. This also means that adding NaHCO 3 has little to no effect on TDS, with 4mg being the upper limit of effectiveness.

https://doi.org/10.1371/journal.pone.0257865.g008

To examine whether or not the temperature plays a role in the effectiveness in adding NaHCO 3 , a boiling experiment was performed, and the data is graphed in Fig 9 .

https://doi.org/10.1371/journal.pone.0257865.g009

Fig 9 presents the relationship between the amount of common baking soda(NaHCO₃) added, the boiling time involved, and the resulting TDS measurements. After boiling each flask for designated amounts of time, the results showed a downward trend line from a spike but does not reach a TDS value significantly lower than the initial sample. It is apparent that the NaHCO₃ has not lowered the TDS of the boiling water, but instead adds smaller quantities of ions, raising the final value. This additive does not contribute to the lowering of the hardness of the tap water. However, tests boiled with 5 mg/L of baking soda maintained a downward pattern as the water was boiled for an increasing amount of time, compared to the seemingly random graphs of boiling with 10 mg/L.

In some households, however, people often add NaHCO₃ to increase the pH for taste and health benefits. However, as shown in the test results, it is not an effective way of reducing TDS levels in the water [ 10 , 16 ], but instead raises the pH, determined by the concentration added. Even under boiling conditions, the water continues to follow the trend of high growth in TDS, of +25–43 ppm right after boiling and the slow drop in TDS (but maintaining a high concentration) as the particles settle to the bottom.

Utilizing the experimental results, we can summarize that after adding small batches of NaHCO3 and waiting up to 5 minutes will reduce water hardness making it less prone to crystallizing within household appliances such as water brewers. Also, this process raises the pH, which is used more within commercial water companies. However, the cost comes at increasing TDS.

Using electrolysis to treat TDS in tap water

Different voltages were passed through the water to observe the change in TDS overtime, with the data being compiled as Fig 10 .

https://doi.org/10.1371/journal.pone.0257865.g010

The process of electrolysis in this experiment was not to and directly remove the existing TDS, but to separate the water sample into three different areas: the anode, cathode, and an area of clean water between the two nodes [ 19 ]. The anions in the water such as OH - , SO 4 2- , HCO 3 - move to the anode, while the cations such as H + , Ca 2+ 、Mg 2+ 、Na + move to the cathode. The middle area would then be left as an area that is more deprived of such ions, with Fig 10 proving this.

As shown in Fig 10 , electrolysis is effective in lower the TDS within tap water. Despite the lines being extremely tangled and unpredictable, the general trend was a larger decrease with a longer duration of time. At 10 minutes, all lines except 10.5 V are approaching the same value, meaning that the deviation was most likely caused by disturbances to the water during measurement from the low volume of water. With each different voltage test, a decrease of 12.7% for 6.0 V, 14.9% for 9.0 V, 22.7% for 10.5 V. and 19.5% for 12.0 V respectfully were observed. In the treatment of wastewate leachate, a study has shown that with 90 minutes of electrical treatment, 34.58% of TDS content were removed, supporting the effectiveness of electricity and its usage in wastewater treatment [ 29 ].

This experiment concludes that electrolysis is effective in lowering TDS, with the possibility to improve this process by further experimentation, development of a water cleaning system utilizing this cathode-anode setup to process water. This system would be a more specific and limited version of a reverse osmosis system by taking away ions through attraction, rather than a filter.

The Southern Californian tap water supply maintains TDS values below the federal regulations. However, crystalline scale buildup in household appliances is a major issue as it is hard to clean and eliminate. To easily improve the taste and quality of tap water at home as well as eliminating the formation of scales, the following methods were demonstrated as viable:

• By heating water to around 50°C (122°F), TDS and water hardness will decrease the most. Also, the boiling process is effective in killing microorganisms and removing contaminants. This process cannot surpass 10 minutes, as the concentration of the ions in the water is too high, which poses human health risks if consumed. These, along with activated carbon and NaHCO₃ additives, are inefficient methods that have minimal effects for lowering TDS.
• Electrolysis is one of the most effective methods of eliminating TDS. Experiments have proven that increased current and duration of time helps lower TDS. However, this method has yet to be implemented into conventional commercial water filtration systems.

Also, some observations made in these experiments could not be explained, and require further research and experimentation to resolve these problems. The first observation is that TDS and increasing water temperature maintain a parabolic relationship, with a maximum being reached at 80°C, followed by a gradual decrease. The second observation is that when water is boiled for an increased duration of time, the rate of cooling also increases.

This experiment utilized non-professional scientific equipment which are prone to mistakes and less precise. These results may deviate from professionally derived data, and will require further study using more advanced equipment to support these findings.

Acknowledgments

The author thanks Tsinghua University Professor and PLOS ONE editor Dr. Huan Li for assisting in experimental setups as well as data processing and treatment. The author also thanks Redlands East Valley High School’s Dr. Melissa Cartagena for her experimental guidance, and Tsinghua University Professor Dr. Cheng Yang for proofreading the manuscript.

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Recent advancements in water treatment

For immediate release, acs news service weekly presspac: january 19, 2022.

Generating clean, safe water is becoming increasingly difficult. Water sources themselves can be contaminated, but in addition, some purification methods can cause unintended harmful byproducts to form. And not all treatment processes are created equal with regard to their ability to remove impurities or pollutants. Below are some recent papers published in ACS journals that report insights into how well water treatment methods work and the quality of the resulting water. Reporters can request free access to these papers by emailing  newsroom@acs.org .

“Drivers of Disinfection Byproduct Cytotoxicity in U.S. Drinking Water: Should Other DBPs Be Considered for Regulation?” Environmental Science & Technology Dec.15, 2021

In this paper, researchers surveyed both conventional and advanced disinfection processes in the U.S., testing the quality of their drinking waters. Treatment plants with advanced removal technologies, such as activated carbon, formed fewer types and lower levels of harmful disinfection byproducts (known as DBPs) in their water. Based on the prevalence and cytotoxicity of haloacetonitriles and iodoacetic acids within some of the treated waters, the researchers recommend that these two groups be considered when forming future water quality regulations.

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Sometimes drinking water smells foul or “off,” even after treatment. In this first-of-its-kind study, researchers identified the major odorants in raw water. They also report that treatment plants using a combination of ozonation and activated carbon remove more of the odor compounds responsible for the stink compared to a conventional process. However, both methods generated some odorants not originally present in the water.

“Self-Powered Water Flow-Triggered Piezocatalytic Generation of Reactive Oxygen Species for Water Purification in Simulated Water Drainage” ACS ES&T Engineering Nov. 23, 2021

Here, researchers harvested energy from the movement of water to break down chemical contaminants. As microscopic sheets of molybdenum disulfide (MoS2) swirled inside a spiral tube filled with dirty water, the MoS2 particles generated electric charges. The charges reacted with water and created reactive oxygen species, which decomposed pollutant compounds, including benzotriazole and antibiotics. The researchers say these self-powered catalysts are a “green” energy resource for water purification.

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Filtration process and alternative filter media material in water treatment.

1. Introduction

2. filtration for water treatment, 2.1. filtration process.

• Straining . It is not desirable as collected particles clog the upper part of the bed (blinding), preventing an efficient use of the filter [ 12 ].
• Sedimentation . This is favoured when the density of the suspended material is greater than that of water. The particle will deviate from the streamline because of gravity and it will impact the medium surface [ 12 , 20 ]. This depends upon particle density and temperature [ 16 ], the diameter of the particle and more generally on the ratio between the settling velocity of the particle and the velocity of the fluid approaching the media [ 12 ]. Larger particles and lower filtration velocities will lead to higher collection efficiency for this mechanism [ 9 ].
• Interception . This occurs when a particle is transiting within a distance equal to its radius from the surface of the grain. The contact between the particle and the grain can result in attachment (12). The mechanism is very similar to straining, but smaller particles are involved [ 6 , 12 , 21 ]; it depends on the ratio of the particle diameter to the media diameter [ 12 ]. Its efficiency increases with increasing particle size and decreasing collector size [ 9 ].
• Diffusion . This is due to the thermal energy of the fluid, which is transferred to the particles. This causes them to drift from the streamlines to impact the surface of the grain or on other particles [ 9 ]. As mentioned previously, diffusion is efficient for sizes below 1 µm because viscous drag is not restricting the particles; the lower the particle size, the more significant the mechanism [ 12 ].
• Ives [ 12 ] adds inertia . The streamlines tend to diverge from the grains when approaching them, but particles with sufficient inertia might proceed unchanged and impact on the grains. It is, however, negligible for water filtration because of small mass and density differences [ 12 , 22 ].
• Furthermore, every particle is subjected to hydrodynamic action , caused by the velocity gradients within pore openings. As it experiences higher velocities on one side, the particle tends to rotate and create an additional spherical field, which causes the particle to move across the flow field. Because of deformable non-spherical shapes and non-ideal flow conditions, the results are non-predictable random paths, leading to movement across the streamlines and collision with the grains [ 12 , 23 ]. This is usually negligible; however, it appears to be more effective for lower particle–grain size ratios [ 11 ].

2.2. Filtration Operating Setup

2.3. process performance monitoring and filter backwash, 3. development and testing of new filtration media, 3.1. expanded aluminosilicate–filtralite, 3.2. glass-based media, 3.3. polypropylene fibre, 3.4. sand with granular activated carbon, 4. conclusions and future work recommendation, supplementary materials, author contributions, acknowledgments, conflicts of interest.

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Cescon, A.; Jiang, J.-Q. Filtration Process and Alternative Filter Media Material in Water Treatment. Water 2020 , 12 , 3377. https://doi.org/10.3390/w12123377

Cescon A, Jiang J-Q. Filtration Process and Alternative Filter Media Material in Water Treatment. Water . 2020; 12(12):3377. https://doi.org/10.3390/w12123377

Cescon, Anna, and Jia-Qian Jiang. 2020. "Filtration Process and Alternative Filter Media Material in Water Treatment" Water 12, no. 12: 3377. https://doi.org/10.3390/w12123377

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A Review on Reverse Osmosis and Nanofiltration Membranes for Water Purification

1 Department of Materials Science and Engineering, The Ohio State University, 2041 N. College Road, Columbus, OH 43210, USA

Zhiyuan Feng

2 State Key Laboratory of Precision Measurement Technology and Instrument, Tianjin University, Tianjin 300072, China

3 Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China

Zhien Zhang

4 William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, OH 43210, USA

Sustainable and affordable supply of clean, safe, and adequate water is one of the most challenging issues facing the world. Membrane separation technology is one of the most cost-effective and widely applied technologies for water purification. Polymeric membranes such as cellulose-based (CA) membranes and thin-film composite (TFC) membranes have dominated the industry since 1980. Although further development of polymeric membranes for better performance is laborious, the research findings and sustained progress in inorganic membrane development have grown fast and solve some remaining problems. In addition to conventional ceramic metal oxide membranes, membranes prepared by graphene oxide (GO), carbon nanotubes (CNTs), and mixed matrix materials (MMMs) have attracted enormous attention due to their desirable properties such as tunable pore structure, excellent chemical, mechanical, and thermal tolerance, good salt rejection and/or high water permeability. This review provides insight into synthesis approaches and structural properties of recent reverse osmosis (RO) and nanofiltration (NF) membranes which are used to retain dissolved species such as heavy metals, electrolytes, and inorganic salts in various aqueous solutions. A specific focus has been placed on introducing and comparing water purification performance of different classes of polymeric and ceramic membranes in related water treatment industries. Furthermore, the development challenges and research opportunities of organic and inorganic membranes are discussed and the further perspectives are analyzed.

1. Introduction

Human welfare has been promoted by continued economic growth, which is accounted for by mechanization and industrialization. However, increasing income and wealth would cause ecological problems, since natural resources are used as inputs of several products, and the pollution of the environment is directly linked to the production scale [ 1 , 2 ]. Water shortage is one of the problems caused by global industrialization. In developing countries, untreated wastewater entered rivers and seas, leading to ground water contamination and limited clean water supply. In order to protect the environment and save mankind, various actions have been taken to tackle industrial pollutants [ 3 , 4 , 5 , 6 , 7 ]. On the other hand, continued population expansion and urbanization also lead to increasing residential water demand. The United Nations predicts that with the current population growth rate, in ten years half of the geographic regions of the world will be impacted by water scarcity [ 8 ]. Water purification and desalination have been used more and more around the world to provide people with fresh and clean water, especially in water-stressed countries such as Qatar, the United Arab Emirates, and Israel. These regions need inventive and viable approaches for safe water supply to support population growth. Since 1980, filtration systems equipped with nanoporous membranes have been commercialized and membrane separation has become a rapidly emerging technology in many industrial applications such as food industry, petroleum industry, chemical processing industry, pulp and paper industry, pharmaceuticals and electronic industry [ 9 , 10 , 11 , 12 , 13 , 14 ]. In these industries, wastewater purification is an essential process that involves membrane separation technique. According to particle size of retained species, water purification systems such as reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF) have been introduced globally [ 15 , 16 , 17 , 18 ]. A description of membrane types with corresponding pore diameter and retained species is shown in Figure 1 . Meanwhile, significant progress has been made in research on RO membranes made from different materials for desalination applications [ 19 ].

Classification of membranes for water purification in terms of pore size and retained species.

It is well known that polymeric membranes are currently used the most in seawater desalination and wastewater treatment industries due to their well-developed and outstanding performance [ 20 , 21 , 22 ]. Research is still being conducted to solve problems related to performance limitations and post-treatment process. Fouling is one of the main drawbacks of polymeric membranes. Surface structure and materials have been modified to suppress fouling effect. Introduction of materials that contain inorganic fillers in organic matrix such as mixed matrix membranes (MMMs) is a significant achievement for underlying issues. In addition to slow improvement achieved in polymeric membranes, inorganic membranes have gained growing interest due to their long-term chemical and thermal stabilities and high mechanical strength [ 23 ]. In general, inorganic membranes include metal oxide membranes and carbon-based membranes ( Figure 2 ). Alumina, zirconia, titania and their mixtures are the most commercialized metal oxide membranes in the market. Almost all inorganic membranes share a common structure, containing a macro-porous support and a meso- or micro-porous barrier layer. In the industry, ceramic membranes are usually used in systems whose operating conditions are challenging to polymeric membranes (high temperature, corrosive effluent, etc.). However, recent studies on cost-effective preparation method using cheap materials indicate a commercialization potential for ceramic membranes [ 24 , 25 ]. In addition, ceramic membranes synthesized from advanced porous materials such as carbon nanotubes (CNTs) and graphene oxide (GO) have been identified as the most promising inorganic membranes in thin film technology [ 26 , 27 ]. These membranes have excellent permeability and selectivity, and their structures offer high productivity and practically efficient performance in desalination and water purification processes.

Representative reverse osmosis (RO) and nanofiltration (NF) membranes for water treatment.

This paper critically reviews the growth and achievement in organic and inorganic membrane studies for RO and NF procedures. The review will start by introducing the synthesis method and structural properties of recent RO and NF membranes, followed by discussing and comparing water purification performance of representative RO and NF membranes made from organic and inorganic materials. The wide scope of this review highlights the potential of RO and NF membranes made from new materials for further research and improvement. Finally, challenges and remaining issues that need to be addressed for further work are summarized.

2. Reverse Osmosis and Nanofiltration Membranes

2.1. polymeric membranes.

Polymeric/organic RO and NF membranes have dominated the global market since 1980 due to their excellent performance and low cost. Some state-of-the-art polymeric RO and NF membranes are listed in Table 1 together with manufacturer, selective layer composition, operation condition, and purification performance. It can be seen that current market is dominated by thin-film composite (TFC) membranes due to their outstanding performance. Important polymers that are being used for making RO and NF membranes are polyamides, cellulose acetate, cellulose diacetate, cellulose triacetate, piperazine, etc. Polyamide is a macromolecule containing recurring amide (-CO-NH-) groups, and can be found both naturally and artificially. Examples of natural polyamide are wool, silk, and angora. Cellulose-based polymers are usually prepared by phase inversion method, as introduced in Section 2.1.1 . In this section, two classes of organic membranes made from different polymeric materials are reviewed.

Commercial polymeric RO and NF membranes for water purification.

2.1.1. Cellulose-Based Membranes

Cellulose-base (CA) membranes have been developed and commercialized for more than 60 years. In 1955, cellulose acetate membranes were prepared and introduced by Reid et al. using acetone as the solvent [ 28 ]. The general synthesis process of CA membrane is called phase inversion method: cellulose triacetate is first dissolved in an organic solvent or solvent mixture to form a casting solution. Then the solution is coated on a flat or tubular support. Finally, the support is immersed in a non-solvent bath, where polymer coagulation occurs and a CA membrane forms. Although CA membranes made by Reid at al. had good selectivity, the water permeability was extremely low and could not be used for practical applications. In 1963, Loeb et al. invented the first efficient RO membrane: cellulose diacetate (CDA) membrane. CDA membranes had much higher flux compared to CA membranes but were prone to biological attack [ 29 ]. The invention of CDA membranes accelerated the development of cellulose triacetate (CTA) membranes, which had slightly stronger thermal, chemical, and biological stabilities [ 30 ]. With asymmetric morphologies, cellulose-based membranes have anisotropic structures, consisting of an upper skin layer on a porous sublayer [ 31 ]. Both the skin layer and porous sublayer have identical chemical composition. The filtration performance of CA membranes depends on the degree of acetylation. For instance, CA membrane with 40 wt% acetate and a 2.7 degree of acetylation had a salt rejection between 98% and 99% [ 32 ]. Higher acetylation will result in higher selectivity but lower water permeability. CA membranes are stable in pH range 4–6. In acidic and basic feed solutions, hydrolysis reaction will happen and lower the selectivity.

Though membranes with better separation performances and comparable costs were fabricated, some studies were reported to improve CA membranes. Chou et at. found dispersing silver nanoparticles on CA membrane surface would increase its biological stability while maintain the permeability and salt rejection [ 33 ]. Coating phospholipid polymer on CA membrane during phase conversion resulted in a fouling-resistant membrane with high water flux [ 34 ]. A small percentage of mineral fillers such as aluminum oxide improved the compaction resistance of CA membranes remarkably [ 35 ]. During the past four decades, thin-film composite (TFC) membranes, whose permeability and rejections surpass those of CA membranes, have dominated the market. However, CA membrane still exists due to its overall exceptional chlorine resistance, which depends on several parameters such as polymer type, synthesis procedure, and pH of feed solution. Since feed water disinfection is a necessary step in RO and NF installations and chlorine is the most common choice of disinfectants, it is important to have chlorine-tolerant membranes for water treatment. Table 2 shows effects of various processing methods on chlorine resistance. Current research mainly focuses on modifications of TFC membranes for chlorine resistance improvement.

Effects of various processing methods on chlorine resistance.

2.1.2. Thin-Film Composite Membranes

TFC membranes were invented by Cadotte in the 1970s, but were not widely used until the second half of the 1980s [ 43 ]. Polyamide (PA) membranes were developed by Hoehn and Richter and had good water purification performance. The main drawback of PA membranes was susceptibility to free chlorine attack [ 44 ]. After development of TFC membranes, it was found the PA TFC membranes had outstanding separation performance as well as better chlorine resistance. As shown in Figure 3 , the structure of a PA TFC membrane consists of a thin selective barrier layer on a porous support [ 45 , 46 , 47 ]. The support has a microporous structure (UF membrane), providing mechanical strength and high water flux, and the barrier layer has a function of ion separation. Compared with CA membranes, which can only be made from linear, soluble polymers, TFC membranes have more desirable characteristics. Many materials (linear and crosslinked polymers) and approaches can be used to synthesize or modify the porous support and barrier layer individually to optimize the thermal and chemical stabilities, permeability, salt rejections, etc. Many papers focus on improving TFC membranes for RO applications have been published. On the other hand, the manufacturing cost of TFC membranes is higher than that of CA membranes since at least two membrane fabrication steps are needed: synthesis of microporous support followed by synthesis and deposition of barrier layer on microporous support.

Thin-film composite membrane structure.

The porous support plays an important role in providing mechanical strength to withstand high pressure during RO and NF processes. Meanwhile to form a defect-free barrier layer, the surface of the support needs to be uniform and smooth. Polysulfone is one of the most significant microporous supports for TFC membranes [ 48 ]. The surface pore size of polysulfone support ranges from 1.9 nm to 15 nm, with a surface porosity up to 16% [ 49 , 50 ]. The selectivity generally increases with decreasing pore size [ 51 ]. Since polysulfone shows good structural stability in a wide pH range, barrier layers made from highly acidic or alkaline precursors can be coated on polysulfone substrates. The disadvantages of polysulfone include poor weatherability, low chlorine resistance, and prone to stress cracks. Adding nanoparticles and applying new preparation methods are two main approaches to improve polysulfone supports. A chlorine-resistant TFC membrane can be made by metalation sulfochlorination of polysulfone [ 52 ]. Plasma treatment on polysulfone support results in the exhibition of hydrophobicity, which optimizes chlorine resistance and water permeability [ 53 , 54 ]. In addition to polysulfone, CA, polyimide, polypropylene, polyketone and polyethylene terephthalate (PET) have also been used as porous supports [ 55 , 56 , 57 , 58 ]. A hydrolyzed PA CA membrane has been fabricated and the covalent bond between porous CA support and selective PA barrier layer indicates a chemical stable structure. This membrane exhibits a NaCl rejection up to 97% [ 58 ]. In addition, TFC membranes synthesized by heat and plasma treatments using electrospun nanofibers as supports showed remarkable filtration performance [ 59 ]. Yoon et al. have prepared a PA TFC membrane using polyacrylonitrile (PAN) nanofibrous scaffold as porous support. The experimental result showed the PA PAN composite membrane has similar sulfate rejection rate (98%) but 38% higher water permeability compared to commercial NF membranes (NF270) [ 60 ]. Several recent studies focus on the effect of support pore size on barrier layer formation and water purification performance, but there have been no consistent conclusions so far [ 61 , 62 ].

Most selective barriers of TFC membranes are synthesized by interfacial polymerization, which occurs at an interface between two immiscible monomers/solvents [ 63 , 64 ]. Once a layer forms at the interface, solvents from both sides cannot pass through it and therefore the reaction stops, producing a membrane thinner than 200 nm ( Figure 4 ). Heat treatment is necessary since interfacial polymerization happens at elevated temperature. The purification performance of TFC membranes is primarily determined by barrier layer, which is affected by solvent type and concentration, curing condition and temperature. Table 3 summarizes precursors for preparing TFC membranes by interfacial polymerization method for water purification in recent studies. Due to their good mechanical property and outstanding rejection ratio, TFC membranes are used in a large number of purification tasks, especially in desalination. The main problem associated with TFC membranes is their flux and salt rejection decrease gradually as a result of fouling, particularly in treating with wastewater containing bacteria and nutrients. According to Mansourpanah et al., TFC membranes with antifouling property can be prepared by grafting functional groups or adding hydrophilic additives on membrane surface through radiation or plasma treatment [ 65 ]. The altered barrier layer becomes smooth, hydrophilic and has similar surface charge as foulants. Therefore the interaction between contaminants and membrane surface is reduced. It is also found that TFC membranes blended with polyacrylamide and polymethacrylic acid exhibit biofouling resistance [ 66 ]. Deposition of natural hydrophilic polymers such as sericin would increase surface hydrophilicity of TFC membranes, and improves selectivity and fouling resistance [ 67 ]. Another drawback of TFC membranes is poor chlorine resistance. During water purification process, chlorine (frequently used as disinfectant) changes the hydrogen bounding in TFC membranes, resulting in performance decay [ 68 ]. Thus, it is essential to increase chlorine resistance of TFC membranes. A chlorine-resistant TFC membrane has been invented by Yao et al. by secondary interfacial polymerization method to eliminate the interaction between unreacted amino groups and free chlorine [ 69 ]. Experimental results indicated TFC membranes blended with layered double hydroxides (LDHs) have high porosity and hydrophilicity, exhibiting superior chlorine resistance and anti-fouling capacity [ 70 ]. Similar studies focus on enhancing chlorine resistance of TFC membranes by incorporating additives are available in literature [ 71 , 72 , 73 ]. From a technique perspective, methods such as atomic layer deposition (ALD) controls membrane thickness precisely through sequential surface reactions [ 74 ]. Hydrophilic selective barriers synthesized using this technology have excellent fouling and chlorine resistance.

Mechanism of interfacial polymerization.

Monomers and performance evaluation for thin-film composite (TFC) membranes prepared by interfacial polymerization method.

2.2. Ceramic Membranes

Although ceramic/inorganic RO and NF membranes have only been studied for 30 years and are in early stage of commercialization, their encouraging performance, as exemplified in Table 4 , offers great potential for water purification. In this section, two classes of ceramic membranes made from different inorganic materials are discussed.

State-of-the-art inorganic RO and NF membranes for water purification.

2.2.1. Metal Oxide Membranes

Compared to polymeric membranes, inorganic membranes offer higher chemical stability and stronger mechanical properties. Metal oxides such as alumina, zirconia, and titania form an important class of ceramic membranes. Conventionally, a RO metal oxide membrane has an asymmetric structure consisting of a thick macroporous (>50 nm) support, an intermediate mesoporous (2–5 nm) layer, and a thin selective (<1 nm) top layer. A NF metal oxide membrane has similar structure as RO metal oxide membrane but contains no selective top layer [ 101 , 102 , 103 ]. The most widely used approach for preparing metal oxide ceramic membranes is sol-gel method, which converts precursor solutions into solid membranes in four steps: precipitation reaction first happens between hydrolyzed precursors, followed by a peptization reaction in which precipitation transforms into a colloid sol. The stable sol is then coated on a porous support and gelates during drying. Finally high temperature sintering is applied to the membrane to optimize mechanical properties and pore structure [ 8 , 89 ]. In order to make homogeneous membranes with less defects, colloidal particles are dispersed uniformly in the solvent by stabilizers such as nitric acid, ethanolamine (MEA) and triethylenetetramine (TETA) [ 104 , 105 , 106 ]. Since complex fabrication process of multi-layered membranes as well as expensive precursor materials indicating high manufacturing cost, simplified synthesis method and use of cheap materials will reduce the production cost and accelerate the development and commercialization of ceramic membranes.

One of the most widely studied inorganic membranes is alumina membrane, which has an average pore size of 2–5 nm (MWCO of 3000–1000 Da) and is commonly used in NF systems or as an intermediate layer in RO membranes [ 107 ]. Alumina membranes with pore size smaller than 1 nm has been made, but showed low permeability (5 LMH/bar) and cannot be used for industrial purposes [ 8 ]. Wang el al. have prepared a supported γ-Al 2 O 3 hollow fiber membrane with a mean pore size of 1.61 nm that demonstrates a high water permeability of 17.4 LMH/bar [ 85 ]. This membrane exhibits good selectivity for multivalent ions such as Ca 2+ (84.1%), Mg 2+ (85%), Al 3+ (90.9%) and Fe 3+ (97.1%), but very low retention of monovalent ions such as NH 4 + (27.3%) and Na + (30.7%). Recent studies focus on surface modification of alumina membrane to further improve its purification performance. For instance, a mixed matrix carbon molecular sieve (CMS) and α-Al 2 O 3 membrane fabricated by vacuum-assisted impregnation method has a water flux up to 25 kg m −2 h −1 and a salt rejection between 93% and 99% when tested using 3.5 wt% NaCl (seawater) at 75 °C [ 87 ]. Ren et al. changed the surface of a porous alumina membrane from hydrophilic to hydrophobic by fluoroalkylsilane (FAS) grafting, resulting in a water flux of 19.1 LMH and salt rejection over 99.5% [ 88 ]. Such outstanding salt retention and water permeability hold promise for practical desalination applications. In addition to surface modification, using cheap precursor materials provides both economic and environmental benefits. Researchers have used Al 2 O 3 hollow fiber supports and coal fly ash, a byproduct of coal burning, to synthesize Al 2 O 3 -NaA zeolite membranes successfully. The Al 2 O 3 -NaA zeolite membrane has been used to treat wastewater containing lead ions (Pb(Ⅱ), 50 mg L −1 ) and possesses a Pb(Ⅱ) removal rate of 99.9% [ 108 ].

Zirconia and titania are other popular materials for ceramic membranes. In sol-gel method, zirconium alkoxides are often used as precursors to prepare zirconia sols [ 109 , 110 ]. However, some zirconium alkoxides such as zirconium propoxide is water-reactive, which could produce agglomerates rather than stable nanoparticles. Therefore at the beginning few laboratories had successfully synthesized zirconia membranes. In 1998, Garem et al. discovered that adding 13 mol% magnesium would enhance the chemical and thermal stabilities of zirconia sols [ 111 ]. Since then many stabilizers have been investigated for preparing zirconia membranes. Glycerol has been introduced into the sol-gel process to make ZrO 2 NF membranes for treating high-salinity wastewater. More specifically, glycerol binds to the surface of ZrO 2 nanoparticles as a capping agent and prevents phase transformation during calcination. The crack-free ZrO 2 NF membrane exhibits a permeability of 13 LMH/bar and approximately 68% rejection rate when filtering NaCl solutions with mass fraction up to 24.92% [ 90 ]. Lu et al. have used zirconium salts and titanium alkoxides as sol-gel precursors to prepare a TiO 2 -doped ZiO 2 NF membrane [ 91 ]. The addition of Ti 4+ suppresses zirconia phase transformation, narrows the pore size distribution and increases the specific surface area. This membrane has high water permeability above 35 LMH/bar with a MWCO of 500 Da, and simulated retention rates of 99.6% for Co 2+ and 99.2% for Sr 2+ , indicating its attractive potential for radioactive wastewater treatment. Compared with alumina and zirconia membranes, the surface pore size and phase composition of titania membranes can be controlled by synthesis procedure. Anatase is the most preferable crystal form of titania due to its exceptional stability and narrow pore size distribution. A TiO 2 membrane with a pore diameter of 4 nm has been fabricated successfully by gentle heat treatment and remained stable in various solutions (brackish water, sea water and brine water) for over 350 h [ 89 ].

In addition to traditional metal oxide membranes, composite membranes made of two or more metal oxides is a current research focus. For example, a bilayer membrane containing a TiO 2 layer on top of a ZnAl 2 O 4 layer has been prepared and evaluated. It has been proved that compare to single layer membrane made from 50 mol% TiO 2 and 50 mol% ZnAl 2 O 4 with similar pore size, the bilayer membrane which has opposite surface charges could increase the electric interactions between membrane pores and filtered ions, and therefore produces a higher salt rejection, especially for divalent salts [ 112 ]. Another example of inorganic composite membranes is CoO-SiO 2 membrane synthesized by Elma et al. for desalination applications [ 94 ]. The effects of cobalt addition (up to 35 mol%), feed solution concentration (0.3–7.5 wt% NaCl), and operation temperature (22–60 °C) on purification performance were investigated systematically. Experimental results showed the volume fraction of silica mesopores increases with cobalt concentration, and with over 99.7% NaCl retention rate at all times, the highest water flux of 20 kg m −2 h −1 was achieved for 0.3 wt% feed solution at 60 °C. Furthermore, a series of studies confirm that silica membranes blended with cobalt oxide exhibit not only excellent desalination performance but also robust structures compared to single-element SiO 2 membranes [ 92 , 93 ].

In spite of prominent outcomes of metal oxide RO and NF membranes, certain shortcomings such as raw material cost and membrane thickness have hindered their commercialization for water purification. These issues can be overcome by further reducing the membrane thickness or exploring other cheap materials that have great chemical and thermal stabilities. Membranes that have strong surface charges in aqueous environment are also attractive.

2.2.2. Carbon-Based Membranes

In recent years, ordered mesoporous materials (OMMs) have attracted increasingly research interests in addressing water pollution and water shortage problems [ 113 , 114 ]. Among all kinds of OMMs, ordered mesoporous carbons (OMCs) such as carbon nanotubes (CNTs) and graphene possess important properties including large specific surface area, highly uniform structure with tunable pore size and strong atomic bonds, thus have been selected as promising candidates for wastewater treatment applications [ 115 , 116 , 117 ]. As one of fullerene derivatives, CNTs are cylindrical molecules composed of rolled-up graphite sheets with diameter ranges from 1 nm to several centimeters [ 118 ]. Based on the layers of graphite sheets, CNTs can be further classified into single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs) and multi-walled carbon nanotubes (MWCNTs). For water desalination and purification applications, CNTs can be fabricated into standalone membranes or incorporated with other materials in many formats. An investigation of a highly stable and electrochemically active membrane made solely of CNTs, which could find significant applications in chemical and biological wastewater treatment, was undertaken by Sadia et al. [ 119 ]. Such CNTs membrane maintained a phenol removal rate over 85% for 4 h with an average oxidation rate of ~0.059 mol h −1 m −2 when operated with H 2 O 2 . Since water molecules can transport through CNTs structure without much impedance, some CNTs membranes used in RO systems with outstanding salt rejections as well as high water permeabilities have been reported [ 120 , 121 , 122 ]. On the other hand, the incorporation of CNTs into polymeric or inorganic matrix makes it possible to modify membrane properties and further improve surface hydrophilicity, fouling resistance, structural stability and salt retention. Yang et al. have confirmed a polyvinyl alcohol (PVA) based carboxylic MWCNTs membrane synthesized by interfacial adhesion method has better thermal stability and separation performance than a PVA membrane without carboxylic MWCNTs [ 123 ]. This PVA/C-MWCNT membrane exhibits a water flux of 6.96 kg m −2 h −1 and a NaCl rejection of 99.91% at 22 °C. In the work conducted by Peydayesh et al., hyperbranched polyethyleneimine modified MWCNTs were incorporated into polyethersulfone matrix to form a positively charged NF membrane, which had a average pore size of 0.81 nm and an enhanced water flux of 75.7 LMH [ 124 ]. The hybrid membrane showed superior retention rates for heavy metals (i.e., 99.06% for Zn 2+ , 94.63 for Ni 2+ , and 93.93% for Pb 2+ ) and antifouling property due to effective membrane surface charge and hydrophilicity, respectively.

Despite advantages of CNTs, drawbacks such as high cost and low selectivity for certain ions (arsenate, arsenic, and sodium) have limited their commercialization [ 118 ]. Graphene, a cost-effective two-dimensional carbon allotrope that consists of a monolayer of carbon atoms arranged in hexagonal lattice, has been found to be a highly permeable and selective material for water purification processes [ 125 , 126 ]. Since water flux across a membrane is inversely proportional to the membrane thickness, single-atom-thick graphene offers an opportunity for exceptional permeability and efficient energy utilization [ 127 ]. Pure graphene has a closely packed structure which is impermeable to gas and liquid molecules. Therefore to improve permeability and ion selectivity defects or functional groups must be generated designedly. Nanoporous graphene can be fabricated either by electrochemical modification of pristine graphene or by growth on supports from different chemical reactions [ 128 ]. The most commonly applied techniques to generate nanosized pores on graphene structure include high-temperature oxidation, ultraviolet (UV) ozone treatment and plasma etching [ 129 , 130 , 131 ]. Sub-nanometer-sized pores on monolayer graphene have been created successfully for nanofiltration and desalination applications [ 132 ]. During synthesis process, small defects were first introduced by ion bombardment and further enlarged by oxidative etching. The experimental results revealed that the separation mechanisms of the porous graphene membrane at short and long oxidation periods are electrostatic repulsion and streric size exclusion, respectively. Graphene oxide (GO), chemically converted from graphene nanosheets, has oxygen functional groups such as hydroxyl and epoxy which enable it to have better water dispersibility than graphene [ 133 , 134 ]. Nair et al. invented a GO membrane consisting of closed-packed GO sheets that only allow water molecules to travel through and concurrently hinder the motion of other species [ 135 ]. Similarly, Zhao et al. designed a free-standing GO membrane in which the GO sheets are crosslinked by Ca 2+ from Congo red dye [ 136 ]. More specifically, this GO membrane with tunable interlayer spacing was prepared by facile and thermal reduction methods using hot pressing method. Accompanied by relative high water permeability (17.1 LMH/bar), the resulting membrane showed excellent removal rates for heavy metal ions (i.e., 98.6% for Cu 2+ , 97.2% for Pb 2+ , 99.1% for Cd 2+ and 97.2% for Ni 2+ ). Although there have been many breakthroughs and exciting achievements for porous graphene and GO membranes in water filtration, special synthesis techniques for large-area porous membranes and fabrication reproducibility remain challenges towards commercialization.

2.3. Mixed Matrix Membranes

Mixed matrix membranes (MMMs), a currently popular area of research, are made by incorporating inorganic fillers into organic matrices. Although TFC membranes have excellent salt removal performance, there is a trade-off between permeability and selectivity. The main advantage of MMMs is to combine the low manufacturing cost, outstanding selectivity and high packing density of polymeric materials with long-term stabilities, high mechanical strength and regeneration capability of ceramic materials. One type of MMMs is a polymeric membrane blended with inorganic nanoparticles, which can be prepared by dispersion crosslinking, interfacial polymerization, or dip coating. Inorganic fillers that have been investigated for this purpose include titania, zeolite, silica, alumina, etc., and experimental results indicate the addition of inorganic nanoparticles alter the polymeric structures and effect the transportation of molecules through membrane pores [ 137 , 138 , 139 , 140 , 141 ]. Therefore it is not surprising that small inorganic nanoparticles would improve the water purification performance of organic membranes. Titania is widely used in anti-fouling coating due to its photocatalytic property. Kim et al. studied the influence of TiO 2 fillers on the properties of carboxylate groups functionalized TFC membranes and found the carboxylate groups help the adsorption of titania on TFC membrane surface, which result in very good anti-biofouling properties, especially under UV excitation [ 142 ]. Such a hybrid RO membrane also has stable surface structure since no significant loss of titania particles was observed after being tested for 168 h [ 143 ]. Researchers also recognized the addition of zeolite and silica nanoparticles increases the surface roughness, contact angle, and water flux [ 144 , 145 ]. NaA zeolite nanoparticles are the first successfully synthesized zeolite particles with low contact angle (<5°) and RO ranged pores (~0.5 nm) [ 146 ]. MMMs prepared with NaA zeolite fillers by interfacial polymerization method have many outstanding properties, that is, more negatively charged and hydrophilic surface with increasing zeolite content, enhanced water permeability, and better water purification performance [ 147 ].

Composite membrane synthesized from carbon-based materials and organic materials is another type of MMMs. Majumder et al. reported a polystyrene membrane incorporated with MWCNTs which have an average diameter of 7 nm [ 148 ]. The MWCNTs were grown and aligned by catalytic chemical vapor deposition (cCVD) method, followed by spin coated on polystyrene matrix to seal gaps between CNTs. The tips of MWCNTs were opened by plasma etching approach, and the water flux of the synthesized composite membrane was 4–5 orders of magnitude higher than that calculated from Hagen-Poiseuille theory, indicating macroscale hydrologic mechanism. On the other hand, some researchers explained the ultra-high water flux was due to the formation of a layer of water molecules along MWCNTs walls, which reduce the friction significantly when bulk mater molecules come through [ 149 ]. Furthermore, to simplify the complex fabrication steps of MMMs, a patient has been published recently about dispersing 0.8 nm diameter CNTs into cross-linking solutions during the formation of polymeric membranes, so that the CNTs can be embedded into the organic barrier layer on top of microporous polyethersulfone support [ 150 ]. After being functionalized by octadecylamine, tests were performed on membranes made with and without CNTs to demonstrate the improved water flux generated by CNTs pathways. Experimental results showed the flux of membrane containing CNTs was approximately twice as much as that without CNTs (44 L m −2 day −1 bar −1 compared with 26 L m −2 day −1 bar −1 ), and MMMs with CNTs also had a slightly better salt rejection (97.7% compared with 96.2%). Even though MMMs combine the benefits of both polymeric and ceramic membranes, they are difficult to study since the interface between various materials may have unwanted structure and certain great materials become insoluble in each other. In addition, studies on MMMs with larger surface area are necessary before developing manufacturing apparatus for large-scale production.

3. Challenges and Future Perspectives

Although the water purification market has been occupied by polymeric membranes for more than 10 years, research and development activities in polymeric membranes are reaching the bottleneck and many industries still use traditional TFC membranes such as PA membrane which was introduced nearly 40 years ago. Despite expansions of TFC membranes and related techniques, it is time to upgrade RO technology to a new height or develop another cutting-edge technology for water purification. Addition of functional materials such as inorganic fillers, lyotropic crystals, CNTs, MWCNTs, and aquaporins can optimize the water flux and/or salt rejection, but the high cost issue associated with synthesis and blending these materials needs to be addressed before scale-up production and commercialization [ 151 , 152 ]. Meanwhile, new models are needed to predict the performance of composite membranes. Traditional polymeric RO and NF membranes are commonly modeled based on extended Nernst-Planck equation, which needs to be modified for carbon-based MMMs [ 153 ]. Recent models applied to calculate water flux and salt rejection of charged membranes for aqueous electrolyte solutions are listed in Table 5 . For organic membranes blended with CNTs, CNTs can be simplified as circular cylinders, the fluid transport of which can be modeled using Hagen-Poiseuille equation. The flow through pores outside the CNTs and within the polymeric matrix can still be studied by extended Nernst-Planck model concerning dielectric exclusion since the dielectric constants for feed water, CNTs and organic matrix are different and electrostatic interactions will happen between ions in feed solution and polarization charges formed along the boundary of various dielectric media [ 154 ]. Assuming that the CNTs are distributed uniformly in polymeric base, the predicted model for such MMMs is likely to be extended Nernst-Planck formula plus an additional Hagen-Poiseuille term. Both terms are re-written according to their corresponding concentration before addition. The modeling of MMMs with GO fillers is more complicated and depends on the insertion direction: if GO is blended vertically into organic membrane like CNTs, similar equation of CNTs MMMs can be used for GO MMMs; If GO is added horizontally, the tortuosity factor in the extended Nernst-Planck equation needs to be revised due to the fact that the ion transport path inside GO is different from that in polymeric matrix. Additionally, since the functional groups located on the surface of GO (types of functional groups are determined by synthesis method, precursor materials, etc.) can react with ions in fluid and form complexes, the flux and permeability may change with time, indicating possible process-model mismatch. On the other hand, advanced techniques including rapid thermal processing (RTP) and nanorods fabrication enable the generation of defect-free membranes for water treatment applications. In addition to the use of new materials and leading-edge technologies, membrane diameter also plays an important role in enhancing filtration performance. Membranes with large surface area could reduce capital cost and energy consumption by approximately 15% [ 68 ]. Furthermore, different water treatment plants have specific difficulties to overcome. For instance, low recovery rate of seawater, disposal of brine and high capital cost are the biggest challenges that nowadays desalination plants confront. Tarquim et al. have developed a method to minimize produced brines, which results in good recovery rate, but more research and equipment are needed to reduce brine disposal [ 155 ]. Moreover, integration of traditional synthesis process with renewable energy may make green fabrication of nanocomposite membranes possible.

Recent models for transport of aqueous electrolytes through charged membranes.

The excellent filtration performance of inorganic membranes, as stated in Table 4 , indicates the capacity of ceramic membranes for most water purification applications, and the low acceptance of inorganic membranes in the past is because of the sheer dominance of polymeric RO and NF membranes in large-scale water treatment systems. Recent research on preparation of advanced inorganic membranes such as free-standing CNTs membranes and interlayer free membranes enables efficient filtration process with better purification performance and lower facility cost [ 8 , 162 ]. According to Weschenfelder et al., the operation expense and total cost of a water treatment plant using ceramic membranes with a flow rate of 2 m/s and water recovery rate of 95% are US $0.23/m 3 and US $3.21/m 3 , respectively [ 163 ]. Similar to polymeric membranes, the development and manufacturing costs of ceramic membranes remains a significant problem for their industrialization. For example, although there have been rapid growth and development for CNTs and MWCNTs membranes in laboratory-scale, the commercial applications of carbon-based membranes are ongoing in a low pace due to the high cost of synthesizing CNTs and MWCNTs. Thanks to recent advancements in fabrication technology including cCVD, large-scale synthesis of high-quality CNTs economically is achievable. However, the reproducibility and feasibility of these methods for making membranes are in doubt. For traditional metal oxide membranes, high cost of supports is a challenging issue for commercialization. Current research focuses on studying alternative inorganic membranes made from cheaper or waste materials such as coal fly ash to reduce the manufacturing investment.

4. Conclusions

Tremendous amount of effort has been made to overcome the clean water scarcity and nanotechnology is a strong candidate with fast development. Study and commercialization of polymeric RO and NF membranes started in the early 1960s. So far the water desalination market is dominated by two kinds of membranes: cellulose-based (CA) membranes and thin-film composite (TFC) membranes. The most representative products such as TS40, TS80 and AD-90 were developed more than 30 years ago and due to their low manufacturing costs and high salt rejections, no major change has been made since then. New research directions for barrier layers in TFC membranes include improvement of fouling resistance as well as chemical and thermal stabilities. Meanwhile microporous supports can be optimized to increase the mechanical strength and permeability.

Inorganic RO and NF membranes have been studied in lab scale for water purification since the 1980s. The most representative ceramic membranes are metal oxide membranes and carbon-based membranes. The main synthesis method for metal oxide membranes is sol-gel technique, which needs further optimization to control the particle size and distribution. The performance of mixed matrix membranes (MMMs) made with both organic and inorganic nanomaterials is excellent, yet they are too expensive compared with other membranes. Hence it is important to realize the economic competitiveness of MMMs, as well as their potential applications. While nanotechnology is leading the way in developing RO and NF membranes for water purification, there are still technical and scientific problems that need to be solved before more benefits can be realized. Despite the challenges to be overcome, it is highly possible that ceramic membranes will be commercialized and industrialized in water purification and desalination fields in the near future.

This research received no external funding.

Conflicts of Interest

The author declares no conflict of interest.

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• Published: 07 April 2022

Solar water purification with photocatalytic nanocomposite filter based on TiO 2 nanowires and carbon nanotubes

• E. Horváth 1 ,
• J. Gabathuler   ORCID: orcid.org/0000-0003-4544-8952 1 ,
• G. Bourdiec 1 ,
• E. Vidal-Revel 1 ,
• M. Benthem Muñiz 1 ,
• M. Gaal 2 ,
• D. Grandjean 2 ,
• F. Breider   ORCID: orcid.org/0000-0002-5698-0314 2 ,
• L. Rossi 1 ,
• A. Sienkiewicz   ORCID: orcid.org/0000-0003-3527-7379 1 , 3 &
• L. Forró   ORCID: orcid.org/0000-0002-3513-328X 1   nAff4  

npj Clean Water volume  5 , Article number:  10 ( 2022 ) Cite this article

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• Environmental chemistry
• Nanoscale materials

Water contamination due to environmental conditions and poor waste management in certain areas of the world represents a serious problem in accessing clean and safe drinking water. This problem is especially critical in electricity-poor regions, where advanced water purification methods are absent. Here, we demonstrate that titanium dioxide nanowires (TiO 2 NWs)-based photocatalytic filters assisted only with sunlight can efficiently decontaminate water. Moreover, interweaving TiO 2 NWs with carbon nanotubes (CNTs) leads to the formation of a TiO 2 NWs/CNTs composite material and offers an additional water decontamination channel, that is of pasteurization with the visible part of the solar emission spectrum. Our results demonstrate that this nanoporous filter can successfully intercept various types of microbial pathogens, including bacteria and large viruses. In addition, photo-catalytically generated reactive oxygen species (ROS) on the surface of the TiO 2 NWs/CNTs-based filter material under exposure to sunlight contribute to an efficient removal of a broad range of organic compounds and infective microbes. A pilot study also yielded encouraging results in reducing traces of drugs and pesticides in drinking water.

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Introduction.

Universal access to clean and safe drinking water is essential for life. Since 1990 in emerging countries, the quality of water has remarkably improved due to economic growth and technological progress. Yet, even nowadays, numerous locations throughout the world are constantly exposed to risks of contamination and still suffer from the lack of sanitary infrastructure. According to the World Health Organization (WHO) [ 1 ], at least 1.8 billion people, mainly in rural areas, consume water that is contaminated with feces. According to the United Nations Children’s Fund (UNICEF), an estimated number of 1,800 children die every day (diarrhea) because of unsafe water supply [ 2 ]. WHO reports that by 2040, a large portion of the world will endure water stress in relation to the dramatically insufficient drinking water resources. For this reason, the same source predicts a global crisis regarding universal access to clean and safe drinking water.

This problem may prove to be especially acute in countries where sanitary infrastructure is not at place or disrupted (e.g., at remote, rural or war-affected areas), as well as in electrical energy-disadvantaged communities, where running advanced purification methods is not feasible. Under such circumstances, people are forced to drink water contaminated by feces, parasites, life-threatening harmful bacteria, viruses, etc. These pathogenic species could be of various nature, like, e.g., Escherichia Coli , Salmonella , Cryptosporidium , or Hepatitis A , which could cause not only stomach pain, fever, headache, painful diarrhea and fatigue, but even death, as in the case of Legionella Pneumophila infection. It is then important to find a simple and efficient way to eliminate them to provide clean, safe and affordable drinking water in a cost-effective manner.

One of the possible solutions would be to make use of the photocatalytic water treatment by combining sunlight with nanoscale forms of titanium dioxide (TiO 2 ). In fact, this approach for water decontamination has been extensively investigated since 1985 [ 3 , 4 , 5 , 6 , 7 , 8 ]. TiO 2 is a wide-band gap semiconductor, which efficiently absorbs the ultra-violet (UV) part of the sunlight spectrum. For typical photocatalytic applications, due to their large specific surface area, the nano-sized forms of TiO 2 , e.g., TiO 2 nanoparticles (TiO 2 NPs) or TiO 2 nanowires (TiO 2 NWs), are of particular importance. The electron-hole pairs induced by the absorbed UV photons diffuse to the surface of TiO 2 NPs or TiO 2 NWs, where, in contact with water and oxygen molecules, reactive oxygen species (ROS) are produced. In the aquatic environment this photocatalytic mechanism generates various kinds of ROS, the most important of which are: hydroxyl radical (OH . ), superoxide radical anion (O 2 .− ), hydrogen peroxide (H 2 O 2 ), and singlet oxygen ( 1 Δ g ) [ 9 ]. In turn, ROS formed on the surface of photo-excited TiO 2 NPs, before they decay, can readily attack and react with organic contaminants and pathogens, if there are any in the immediate vicinity.

Thus, due to the high specific surface area of nano-sized photocatalytic particles, their efficient photo-generation of ROS, as well as large contact surface and direct interactions, the photocatalytic systems based on nanoparticle forms of TiO 2 provide a powerful disinfecting strategy against waterborne microorganisms and pathogens [ 10 , 11 ]. However, despite the significant potential and many advantages of such an approach, no operational photocatalytic sunlight-driven titania-based devices have been elaborated up to date. The current situation can be explained in part by the technical complications, which are directly related to the nanoparticle form of titania. More specifically, in slurry-type reactors, after completed photocatalytic cycles, it is difficult to collect and recycle TiO 2 NPs (20–100 nm in size), which are dispersed in suspension containing the pollutants.

To overcome this problem, different techniques of immobilization of TiO 2 NPs on various types of substrates have been developed [ 12 , 13 ]. However, TiO 2 NPs immobilized in the form of compact thin films and porous coatings usually reveal mechanical and structural instabilities, because the powdery deposits tend to detach and fall apart exposed to the action of even weak friction and shear forces.

In this regard, here, we report the design and characterization of a highly efficient and durable photocatalytic water filter, which is based on a nanocomposite material consisting of titanium dioxide nanowires (TiO 2 NWs) and carbon nanotubes (CNTs).

The device has a very simple (planar) configuration, in which the contaminated water passes through a multi-layered filter material, the pathogens are trapped on the filter surface, whereas the UV component of the sunlight, being strongly absorbed by TiO 2 NWs, generates ROS. Therefore, in addition to the mechanical retention of pathogens, the photo-triggered ROS attack and kill diverse range of pathogenic species, including bacteria and germs, thus leading to even more efficient water decontamination. Furthermore, implementation of CNTs in the herein developed composite contributes to enhancing the photocatalytic action under exposure to sunlight [ 14 , 15 ]. Especially, electrically-conducting CNTs absorb a broad spectrum of the sunlight radiation, which heats up the filter material and thus provides an additional channel for water disinfection via pasteurization [ 16 , 17 ].

To further improve the photocatalytic properties of the composite filter material elaborated herein, we have also proposed to combine the TiO 2 NWs/CNTs composite with gold nanoparticles (AuNPs). The issue of incorporation of AuNPs into the structure of the filter material and its influence on the resulting photocatalytic properties is discussed in a more complete manner in the final part of this study (Summary).

In brief, this study reports a tailored synthetic method to prepare a solar water purifier, its basic active components, as well as its thorough characterization in terms of photo-generation of ROS––towards the removal of a wide spectrum of toxic chemicals and pathogens from drinking water.

Results and discussion

General concept of the tio 2 nws/cnts nanocomposite-based water purification filter.

As mentioned above, the essential building elements of the herein elaborated nanoporous photocatalytic filter material are TiO 2 NWs and CNTs. A simple representation of the working principle of this TiO 2 NWs/CNTs nanocomposite for water filtration is shown in Fig. 1 .

a A representative selection of microorganisms, which could contaminate the drinking water. They are trapped at the surface of the TiO 2 NWs/CNTs composite - based filter. The sketches of the pathogens are shown on the same scale as the filter material, except for the Hepatitis A virus, the diameter of which is of 24 nm. b The principle of ROS generation at the surface of TiO 2 NWs upon UV illumination. Since the lifetime of ROS (OH . , H 2 O 2 and O 2 .− ) is short, the volume of action is also indicated. Incorporation of CNTs into the TiO 2 NWs/CNTs nano-construct efficiently separates electron-holes pairs photo-generated within TiO 2 NWs, thus reducing their recombination rate and consequently increasing the efficiency of photocatalysis. (The scalebar in a) is 1 μm).

In particular, thanks to its nanoporous structure, the composite filter material efficiently retains waterborne microbial pathogens (a typical few of them are schematically depicted in Fig. 1a ). Moreover, the filter paper decontaminates the polluted water through the action of ROS, which are photo-catalytically generated under the influence of the UV component of sunlight on the TiO 2 NWs surface due to their direct contact with water and oxygen molecules [ 18 ] (Fig. 1b ).

When it comes to the function played by CNTs in the TiO 2 NWs/CNTs nanocomposite material developed by us, it is generally accepted that the presence of CNTs enhances the photocatalytic effect in two ways. Specifically, numerous reports have shown that CNTs in combination with either TiO 2 NPs or TiO 2 NWs can act as electron acceptors to more effectively separate charges photo-generated within TiO 2 nanostructures and thus significantly improve the photocatalytic properties of such constructs [ 14 , 15 ]. In addition, the broadband absorption of solar radiation leads to heating of the filter material, thereby allowing photo-thermal disinfection of water [ 16 , 17 ]. Indeed, we show that the herein elaborated combination of TiO 2 NWs with CNTs, gave a considerable improvement in the functioning of the filter material.

Synthesis and structural characterization of TiO 2 NWs/CNTs nanocomposite

High resolution TEM images of the individual nano-structural components, that is of a single TiO 2 NW and a single CNT, are shown in Fig. 2a, b , respectively. For the practical implementation of numerous variants of the filter material, a large-scale production of both TiO 2 NWs and CNTs was develop in our laboratory. The detailed description of the large-scale synthesis routes of TiO 2 NWs can be found in ref. [ 19 ]. and in the U.S. patent ‘Titanium oxide aerogel composites’ filed by us [ 20 ].

a , b HR TEM images of the essential individual nano-structural components of the composite filter material, i.e., a single TiO 2 NW and a single multiwalled CNT, respectively. The length of both nanoelements can reach several micrometers (The scalebars are 10 nm).; ( c ) Photos of selected samples of TiO 2 NWs-based filter paper with various wt% contents of CNTs, which alters their transparency (the diameter of the filter paper disc is of 20 mm); d Characteristic SEM image of the filter paper (The scalebar is 5 μm is 5b, ( e ) Photo of large area filter papers synthesized from pure TiO 2 NWs (front) and containing 1 wt% content of CNTs (grayish). (The scalebar is 10 cm).

In short, the first step of the synthesis yields titanate (H 2 Ti 3 O 7 ) nanowires, which recrystallize into anatase TiO 2 NWs during the subsequent heat treatment process. While using this technological approach, in our laboratory conditions we were able to produce an amount of ~0.3–1.0 kg of TiO 2 NWs per day in a single synthesis cycle. However, it is worth mentioning that prospective industrial production would not be subject to such quantitative restrictions.

The mass production of CNTs was accomplished by a process of catalytic chemical vapor deposition (CVD) performed in an inclined rotary tube furnace operating in continuous mode [ 21 ]. As explained in Methods, the overnight synthesis produced ~1.2 kg of CNTs.

The above-described high-efficiency synthesis processes of both nano-structural components, i.e., TiO 2 NWs and CNTs, allowed us to fabricate photocatalytic filtering papers with large surface areas. To this end, TiO 2 NWs and CNTs were mixed in different proportions in order to optimize the filter’s performance in terms of water sterilization. Subsequently, the corresponding mixtures were processed by doctor blading into films of various thicknesses, ranging from 2 μm up to 300 μm. The thin layers obtained this way could easily be detached from the hydrophobic support material. After air drying at 120 °C and calcination in vacuum at 600 °C, a phase transformation from titanate to anatase was achieved. It is worth mentioning here that that the calcination process fuses TiO 2 NWs together, thus leading to the formation a thin filtering membrane with a tightly compact structure, which, in normal use (i.e., under standard volume flow-rates), does not release nano-structural ingredients. In short, the technology developed herein made it possible to produce freestanding, flexible thin filtering papers with large surfaces, which, depending on their thickness, could even be transparent. Examples of such thin disc-shaped filtering membranes (50 μm thick, 20 mm in diameter), with various wt% contents of CNTs, are shown in Fig. 2c .

The surface appearance of the filter is shown in an SEM image in Fig. 2d , while the large surface area (~0.3 m 2 ), self-standing filter papers used for the fabrication of the solar-thermal water purification prototype are displayed in Fig. 2e , for pure TiO 2 NWs (white) and with 1 wt% content of CNTs (grayish).

Design of a prototype of a photo-catalytic filter and characterization of its flow-rate and photo-thermal properties

The large area filter paper made it possible to fabricate a prototype of a solar-thermal water purification system (Fig. 3a ). For this purpose, a freestanding 30 μm thick filter paper with dimensions of 37.5 cm × 28.0 cm was positioned and fixed between two 1.5 mm thick borosilicate glass windows. In our design, borosilicate glass was chosen because of its low absorbance in the UV range. The water inlet and outlet were placed on the opposite diagonal sides of the device (Fig. 3 b). The stacked assembly was sealed and mechanically secured by a rectangular aluminum frame. For receiving the highest photon flux, the sun-facing surface area of ~0.1 m 2 could easily be positioned at an optimal angle ( θ ) with respect to solar radiation. The total internal volume of the device was estimated to be of 200 ± 50 ml.

a Photograph of the device (with a visible random mirror image of the surroundings on the outer glass surface of the filter) and ( b ) the sketch of the cross-section of the prototype. The flow-rate through the device depends on the water pressure through the difference in elevation levels between the contaminated water tank and the clean water outlet (symbolically marked as height, h). The orientation of the device can easily be adjusted to an optimal angle (Θ) with respect to solar radiation; ( c ) Temperature of the solar-thermal device when exposed to the sun. The limit temperatures are of 52.2 and 52.9 °C for the fit curves for the infrared camera and for the thermocouple, respectively. The inset shows the infrared image of the prototype at one temperature point during its exposure to sunlight; ( d ) Purified water volume flowing through the device as a function of time for three values of h: 75, 120 and 160 cm. The rectilinear slopes of the plots confirm the dependence of the volume flow-rate on h according to the Pascal’s law.

The possibility of efficient heating of the influent water due to the photo-thermal effect is also an important attribute of the developed filtering device. In our opinion, this functional property is related to the presence of CNTs in the structure of the filtering layer. Absorbing a broad spectrum of sunlight, these carbon nanowires with a metallic character heat up the filter and water in its immediate vicinity, thus enabling sterilization by elevated temperatures. In particular, temperatures of the order of 50 °C could be attained inside the prototype after approximately 1 h. of exposure to solar radiation (during a sunny afternoon on June 11th in Lausanne, Switzerland). To illustrate the thermal characteristics of the prototype, temperature was measured with both a thermocouple (on the surface of the device) and an infrared camera. The corresponding time dependencies of the device temperature exposed to the sun radiation for about 60 min are shown in Fig. 3c . It is worth mentioning that no forced water flow was used in this experiment. Therefore, the outcome of this experiment reflects the maximum potential temperature that the device can attain under similar meteorological conditions.

An important parameter of the filtering system is its flow-rate ( Q ), defined as the purified water volume ( V ) passing through the device per unit time ( t ), i.e., Q  =  V / t . This volume flow-rate is usually described the Darcy’s law [ 22 ], which states that the fluid flow-rate through a filtration device is directly proportional to the corresponding pressure gradient, that is: Q  =  κA Δp / μL where Q [ m 3  ∙  s −1 ] is the volume flow-rate, κ [ m 2 ] is the permeability of the medium, A [ m 2 ] is the filter area, Δp [ Pa ] is the pressure drop across the filter membrane of thickness L [ m ], and μ [ Pa ∙ s ] is the viscosity of the fluid.

The drop of pressure across the filtering device can be varied by changing the vertical separation between the polluted water tank and the filter outlet, in accordance to the Pascal’s law: Δp  =  gρh , where g is the gravitational constant, ρ stands for the fluid density and h abbreviates the vertical separation distance (height). For the meaning of the symbols see Fig. 3b . The measured dependencies of the water volume flowing through the device as a function of time for three values of h : 75, 120 and 160 cm is shown in Fig. 3d . Qualitatively, the Darcy’s law is satisfied, although for higher h values, there might be some pressure loss, since V versus h is not strictly linear. As can be seen, even this small surface area filter can supply ca . 2.0 liters of decontaminated water per day.

Overview of experimental techniques characterizing the photo-catalytic performance of the filter

The photo-catalytic efficiency of the filtering composite material prepared in this work was checked for a broad spectrum of potential targets, ranging from small molecular compounds to bacteria. In particular, for characterization of the photocatalytic generation of ROS the following methods were implemented: electron spin resonance (ESR), optical spectroscopy and standard microbiological bacteria survival tests. In addition, for measuring the variation of minute concentrations of molecules, such as drugs and pesticides in water, mass spectrometry (MS) coupled with high-performance liquid chromatography (HPLC) was applied.

Photo-catalytic generation of ROS tracked by ESR test based on TEMPOL decay

To confirm the efficient photocatalytic generation of ROS in an aqueous environment by the TiO 2 NWs/CNTs-based filtering composite material, ESR in combination with spin-trapping was employed. Specifically, a stable nitroxide radical, 4-Hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL), was used as a target molecule for the photo-generated ROS, i.e., OH ∙ and O 2 .−∙ radicals [ 20 , 23 ]. The ESR-active TEMPOL is a water-soluble antioxidant, which reportedly acts as a superoxide dismutase (SOD) mimicking agent [ 24 , 25 ].

The reactive pathways of TEMPOL with ROS have been reported to introduce structural changes at the 1- and 4-positions of this nitroxide molecule, for O 2 − and OH ∙ radicals, respectively [ 26 ]. Acting in concert, these structural changes induce the decay of the ESR-active TEMPOL, as well as lead to the concurrent formation of another ESR-active radical, the 4-oxo-TEMPO (TEMPONE).

In aqueous solutions, both TEMPOL and TEMPONE reveal easily detectable ESR spectra, which are characteristic for NO-centered radicals bearing one unpaired electron ( S  = 1/2). Specifically, the corresponding ESR signals consist of three well-resolved features resulting from the 14 N atom-related hyperfine splitting ( I  = 1). Due to the differences in the hyperfine splitting constants, A 14 N , and the spectroscopic g -factors values, being of 17.1 G/2.0057 and 16.1 G/2.0056, for TEMPOL and TEMPONE, respectively, these two nitroxide radicals can be very easily distinguished [ 27 ].

The principle of the ESR experiment allowing for an indirect detection and evaluation of photo-catalyzed ROS, with using TEMPOL as a target molecule, is shown schematically in Fig. 4a . In particular, the diagram in this figure depicts the two most important reaction pathways related to the attack of O 2 − and OH ∙ radicals on TEMPOL, thus leading to the ESR-silent TEMPOL-hydroxylamine (TEMPOL-H) and the ESR-active TEMPONE, respectively.

a Principle of oxidative stress evaluation with using a stable nitroxide radical, TEMPOL. The reducing/oxidative action of the photo-generated ROS deprives the TEMPOL molecule of spin and makes it ESR-silent. Through a competing process, the OH ∙ radical attacks the TEMPOL molecule at the 4-position and converts it to another ESR-active nitroxide, TEMPONE. b The ROS-inducted progressive decay of the ESR signal of TEMPOL is accompanied by the occurrence of the ESR signal of TEMPONE, as observed over 120 min time under exposure to UV-A. c Time-evolutions of the ESR signal intensities of TEMPOL monitored at the entrance and outlet of the TiO 2 NWs/CNTs––based photocatalytic filter.

Considering the course of the ESR signals of TEMPOL and TEMPONE as a function of time, it seems clear that their time-evolutions confirm the presence of ROS, but with divergent dependencies. In particular, as a function of time, the ESR signal intensities of TEMPOL and TEMPONE, gradually decay and increase, respectively.

Typical results of ROS detection by means of ESR using TEMPOL as a molecular target are shown in Fig. 4b . Specifically, it can be seen that for the generation of ROS by TiO 2 -based photo-catalyst, after a prolonged illumination with UV-A light, the ESR signals of TEMPOL and TEMPONE clearly decrease and increase, respectively. Note that only the low-field hyperfine components of the ESR spectra of TEMPOL and TEMPONE are shown in Fig. 4b .

During this experiment, both sides of the filtering device were illuminated by a UV-A light source, which was operating at the wavelength of 367 nm with the power intensity of ~1–3 mW/cm 2 .

To check the efficiency of the prototype in photo-generation of ROS when exposed to sunlight, the samples for ESR measurements were taken regularly over an extended period of time from the inlet and outlet of the filter (see Fig. 3b ). Before staring the experiment, the upper tank of the device was filled with the volume of two liters of 400 μM aqueous solution of TEMPOL.

Changes in the intensities of the ESR spectra during the filter’s exposure to sunlight were assessed by comparing the double integrals of the corresponding ESR signals acquired sequentially as a function of time (for more details see METHODS).

The corresponding plots of the ESR signal intensities monitored at the entrance and outlet of the prototype filtering device over ~2.5-h. period of exposure to sunlight, during the sunny early afternoon of April 6 th in Lausanne, Switzerland, are displayed in Fig. 4c . As shown, the ESR signal intensity of TEMPOL decreases strongly with time after its passage through the filter, which is the consequence of the ROS-induced damage. It is worth mentioning that the simultaneous creation of the ESR-active TEMPONE compensates to a relatively small extent the herein observed overwhelming decay of the paramagnetic TEMPOL. In fact, as mentioned before, both these processes confirm the divergent mechanisms of ROS-mediated actions on the same target molecule (TEMPOL). Thus, this observation validates the final conclusion that TiO 2 NWs/CNTs-based composite is very efficient in generating ROS and the accompanying oxidative stress on its surface.

Photodegradation of Methyl orange–a UV–VIS assay

The colorful dye, methyl orange (C 14 H 14 N 3 NaO 3 S, abbreviated as MO) is often used as a model organic pollutant for evaluation of the ROS generation efficiency and the oxidative stress conditions in various environments [ 28 , 29 , 30 ]. The usefulness of MO for this type or research is due to the fact that the ROS-mediated damage to MO reduces its strong absorption band at 464 nm. In particular, under the influence of photo-oxidative stress, the MO molecules undergo the successive stages of hydroxylation, demethylation and oxidation, which result in photo-bleaching and, ultimately, complete loss of color. The corresponding changes in the molecular structure of the dye can be associated to its concentration and easily be monitored by the changes in the dye’s absorbance ( A ), according to the Beer–Lambert law expressed as A  =  ε l c, where ε is the molar attenuation coefficient, l is the optical path, and c is the MO concentration.

In this experiment, MO dissolved in distilled water (Millipore), with an initial concentration c 0  = 20 mg/mL was poured into Petri dishes (90 mm in diameter and 15 mm height) containing the test samples in the form of discs (50 mm in diameter) at their bottom. For the purpose of exposure to UVA light, a UV spot light source, LC-8 Lightingcure™ ( λ exc  = 365 nm, 16 mW/cm 2 , from Hamamatsu Photonics (Japan), was used. The photocatalytic activity of the herein developed composite material based on TiO 2 NWs/CNTs was tested, as well as two reference materials: a commercial standard photocatalyst, TiO 2 NPs (Degussa P25), having similar specific surface area (~40 m 2 /g), and a nano-structured aluminum oxide (Al 2 O 3 ).

The photodegradation of the dye and the corresponding changes in its concentration were monitored by measuring the characteristic MO absorbance peak at 464 nm with a UV–Vis spectrophotometer, Varian Cary 50, Agilent (USA). In order to obtain the time dependencies, small aliquots (2.5 mL) were taken every 10 min from Petri dishes and transferred to the cuvettes of the UV–VIS spectrophotometer.

All these test measurements were performed under static conditions, i.e ., without flow or mixing/stirring the MO solution in Petri dishes. As shown in Fig. 5a , the filter paper elaborated herein is twice as efficient as the P25 itself.

a The normalized concentration (c/c 0 ) of MO versus UVA illumination time in the presence of the TiO 2 NWs/CNTs filter (red dots). The herein monitored MO degradation efficiency was compared to that of P25 TiO 2 NPs (Degussa) deposited on an anodic aluminum oxide (Al 2 O 3 ) membrane (gray circles) and also compared to the efficiency of the same anodic Al 2 O 3 membrane without any coverage (black dots); For illumination with UVA, a UV spot light source (λ exc  = 365 nm, 16 mW/cm 2 ), model LC-8 Lightingcure™ (Hamamatsu Photonics, Japan) was implemented; ( b ) After the first passage through the TiO 2 NWs/CNTs photocatalytic filter, both micropollutants slightly desorb, which coincides with starting of UV-A illumination (due to thermal desorption). For subsequent passages, however, the photo-generated ROS take over and destroy these harmful molecules.

It has been shown that CNTs in combination with other semiconducting nanomaterials, such as, e.g., TiO 2 NPs or zinc oxide NPs (ZnONPs), substantially reinforce the photo-catalytic efficiency of composite photo-catalytic materials [ 31 ]. In particular, it has been demonstrated that the presence of CNTs in theses composite materials enhances their photocatalytic efficiencies through increased total active surface area and electrical conductivity [ 32 ]. Therefore, in the presence of CNTs, these contributions increase lifetimes of the photo-generated electron-hole pairs generated by semiconducting nanomaterials, thus improving their overall photo-catalytic performance [ 16 ].

Degradation of micropollutants

Micropollutants, such as drug residues, pesticides, maintenance products, hormones, or cosmetics, contaminate drinking water worldwide and may have a long-term toxic effect on living organisms and the ecosystem [ 33 , 34 , 35 ]. In this regard, we also examined whether our photocatalytic filter was able to reduce their concentrations. To this end, the Central Environmental Laboratory of EPFL prepared a “cocktail” of nine micropollutants (five drugs, three pesticides and one maintenance product: atrazin, carbamazepin, diclofenac, mecoprop, metolachlor, bezotriazol, lomeprol, gabapentin, metformin). They were dissolved in 5 L of distilled water, representing a concentration of hundreds nanograms per liter (ng/L). After each passage through the filter, the micropollutants were separated by high-performance liquid chromatography (HPLC), and various bands were analyzed with mass spectrometry (MS). Figure 5b shows the results of the successful removal of two selected micropollutants, i.e., gabapentin (painkiller) and metformin (drug for diabetes).

The first passage performed in the dark (sampling 1) showed a global decrease in concentrations of both gabapentin (blue trace) and metformin (red trace), that can be attributed to the adsorption of a certain part of the compounds on the filter surface. Specifically, as can be seen in Fig. Fig. 5 b, the concentrations of both compounds decreased by ~25% during the 1st sample run. This downward trend in the concentration of both chemicals was partially stopped during the 2nd sampling period. The herein observed inhibition of the filtration process and the accompanying slight increase in the concentration of both chemicals may be related to thermal desorption. Actually, it also coincides with the beginning of exposure of the filter to illumination with UVA light (365 nm, 1 ÷ 3 mW/cm 2 ). However, during the next sampling runs (samplings 3 and 4), a small drop in concentrations of both micropollutants was observed, what can be attributed to the ROS-mediated photo-decomposition (see Fig. 5b ). This small, though measurable change might be due to very low concentrations of gabapentin and metformin, and their relatively short residence time under flow conditions within the volume where ROS are photo-generated (see Fig. 1b ). An additional argument supporting this rather minor effect of micropollutant photodegradation can also be related the very low power density of the incident UVA radiation in this experiment. Although the observed decrease in concentration is slight, it nevertheless suggests the potential of the herein developed TiO 2 NWs/CNTs-based photocatalytic filter for the removal of micropollutants from water.

Microbiological tests of the mechanical removal and photo-oxidative sanitation of waterborne pathogens

The capability of the device to filter out germs from water was verified using the Colilert and Quanti-Tray method [ 36 , 37 ]. This is a standard method to indicate the sanitary quality of water, e.g., the evidence of fecal contamination with coliform bacteria, with special emphasis on E. coli . In practical application of this method, the enzymatic activity of coliform bacteria metabolizes two nutrient-indicating molecules, i.e., ortho-nitrophenyl-β-D-galactopyranoside (ONPG) and 4-methyl-umbelliferyl-β-D-glucuronide (MUG). More specifically, in the general case of the presence of coliform group bacteria, the metabolized ONPG turns the colorless water into yellow, whereas in the specific case of the presence of E. coli , MUG renders the aqueous milieu fluorescent upon UV illumination. In this approach, the enumeration of coliforms and E. coli in water samples is performed using so-called colony-forming units (CFU).

The tested water sample was taken from the river La Chamberonne (passing nearby the laboratory). Prior to performing the contamination test, an aliquot of 1 ml of this water was 100 times diluted with distilled water. Next, before and after passing through the filter, the water was mixed with the Colilert reagent, poured into Quanti-trays and incubated at 35 °C for 24 h. The results are shown in Fig. 6a . The Quanti-tray containing the untreated water turned completely yellow and became also fluorescent under UV light (not shown), while the Quanti-tray with the filtered solution did not change color. Moreover, as can be seen in the upper portion of Fig. 6a , all the cells of the Quanti-tray containing the untreated water are yellow, thus confirming the contamination with coliform bacteria at the level higher than 200 CFU/100 ml. In contrast, the assessment of the Quanti-tray containing the treated water (lower portion of Fig. 6a ), points to the contamination level well below 1 CFU/100 ml.

a The presence of coliform bacteria in the 100 times diluted river water is indicated by the yellow color of the Quanti-tray (upper part of the image), whereas the colorless Quanti-tray confirms that no bacteria passed through the TiO 2 NWs/CNTs filter (lower part of the image); ( b ) The bacterial inactivation performance of the TiO 2 NWs/CNTs filter upon UVA and visible light illumination as compared to the control surface of the humid Teflon filter. After 30 sec of illumination with UVA light and after more than 300 sec when exposed to visible light, the concentrations of bacteria are below the detection limit. Inset: schematic representation of the experiment showing the location of the bacteria-containing spots on the filter’s surface.

Thus, the Colilert and Quanti-Tray procedure confirmed the efficiency of the TiO 2 NWs/CNTs-composite-based filter for the mechanical removal of waterborne pathogens, including E-coli bacteria. This high filtration efficiency can be attributed to the small cut-off size of the filter pores (<1 μm).

Below we show that apart from the mechanical retention of pathogens, the photocatalytic properties of the herein developed filter material also allow for their complete deactivation. To this end, a second microbiological test was performed. Specifically, we employed the conventional plate count technique, in which, similarly to the previously discussed Colilert and Quanti-Tray test, CFU units are taken into account to estimate the number of viable microorganisms in the sample [ 37 ].

The E. coli bacteria survival was measured under three conditions: (i) deposited on TiO 2 NWs/CNTs filter and illuminated with UV light (at 365 nm with power density of 16 mW/cm 2 ), for time intervals of 20, 60, 100 and 180 sec; (ii) deposited on the same filter, but illuminated with daily sun light extended up to 600 sec; and iii) deposited on Teflon substrate, illuminated by the UV and visible lights for the corresponding time intervals - as control measurement. In all cases, the starting E. coli concentration was 2*10 8 CFU/mL.

From the suspensions prepared that way, 5-μL aliquots were taken, dropped on the filter surface and exposed to illumination with UVA light. After each illumination run, the filter fragments were placed in Eppendorf tubes containing physiological salt and then rinsed for 60 min. This treatment allowed to the dead E. coli to be removed and to enter the survived cells back into solution. Next, serial dilutions were prepared from the solution containing cells and 10 μL aliquots of each dilution were spread onto cultivation agar in Petri dishes. Subsequently, they were incubated at 37 °C for 24 h. The enumeration of bacteria per mL (CFU/mL) was extrapolated from the colonies counted at the appropriate spots for TiO 2 NWs filter, and the survival rate could be established. The results shown in Fig. 6b superbly demonstrate the antibacterial activity of the TiO 2 NW/CNTs filter. Already, after 60 sec of UVA light exposure the number of survived bacteria drops below the detection limit (red trace in Fig. 6b ). Even after exposure for 30 sec to UVA light, the damaged bacteria could be noticed in the SEM image (not shown) . A direct impact of high energy UV photons can damage the E. coli bacteria as shown in the control experiment on the wet Teflon filter surface, but the corresponding decrease in their number is non-comparable with that of the TiO 2 NWs-based photocatalytic filter.

It is also worth mentioning that in the experiment described above, the time scale in which E. coli inactivation occurred was much shorter than for any other tests of light-dependent oxidative stress conducted herein. The rapid photo-inactivation of E. coli observed in this experiment can be attributed to the fact that the bacteria were trapped on the filter surface, thus being in the immediate range of ROS photo-generated by immobilized photocatalyst particles. In addition, the entire experiment was carried out under static conditions (without water flow). Therefore, the photo-catalytically induced ROS, before their disappearance, had a better chance of reaching the bacteria. This is in contrast to other experiments, in which photodegradation of waterborne chemical compounds, such as TEMPOL or MO, was observed under flow conditions.

The apparent differences in photo-deactivation rates of adsorbed bacteria and water-soluble chemicals can then be associated to the limited volume of action of ROS generated mainly at the filter surface. Specifically, both the small volume of the direct ROS-mediated deactivation (symbolically represented by the bluish cylinder in Fig. 1b ) and dynamic conditions of water flow led to the observed reduction in photocatalytic deactivation efficiency of waterborne chemicals.

Furthermore, another interesting observation is the fact that that exposure only to sunlight exerts a profound sterilizing impact and significantly eliminates E. coli bacteria. This sterilizing effect is related, in part, to the UV component of the sunlight spectrum, which induces photocatalytic damage to bacteria, as well as to pasteurization due to the increased temperature of the filter material containing CNTs in its structure (see Fig. 3d ). Regardless of the contributions of the above-mentioned mechanisms, the photo-thermal water filter developed in this work seems to be an interesting proposition for large-scale applications, allowing for effective water purification in the absence of specific infrastructures. The device, using solely sunlight, exerts a sterilizing effect by destroying microbial pathogens, such as bacteria or even viruses, thus rendering the polluted water safe and drinkable.

We have reported a simple approach to filter and purify water, which is based on the herein developed thermo-photocatalytic composite material, TiO 2 NWs/CNTs, and is using solely sunlight as a source of energy. The filter can efficiently entrap, degrade and eliminate waterborne pathogens, including bacteria, viruses and worms. Furthermore, it has also proven efficiency in photocatalytic decomposition and neutralization of waterborne organic micropollutants, such as drugs, pesticides and detergents.

In the filtering device constructed in this work, the sunlight-mediated disinfection of waterborne pathogens occurs due to the combined action of photo-generated ROS and thermal pasteurization, i.e., the respective contributions of photocatalytic properties of TiO 2 NWs and photo-thermal input of CNTs, resulting from their strong absorption in broad optical spectral range.

A small prototype of the water-filtering device with surface of 0.3 m 2 can supply 2 L/day of decontaminated water, which could be easily scaled up by increasing the filter surface. The great advantage of this device is that it is cost-effective, thermally stable, chemically inert, and capable of promoting oxidation of organic compounds via photo-catalytic generation of ROS. We believe that this simple method could be a good solution for drinkable water-stressed areas.

As an outlook, to further improve the photocatalytic properties of the composite filtering material elaborated in this work, we also propose a few of possible improvements. The first could be the incorporation of gold nanoparticles (AuNPs) into the TiO 2 NWs/CNTs-based composite fabric, as shown in Fig. 7a, b . In this regard, earlier studies have shown that AuNPs exhibit enhanced photon absorption properties and can be used to efficiently generate local heating upon illumination through photo-induced resonant plasmons [ 38 ]. Therefore, composites consisting of the photoactive TiO 2 and AuNPs can be conveniently employed as plasmonic photocatalysts, enabling a number of important reactions to occur with high efficiency and under minimal light exposure [ 39 , 40 , 41 ]. Although the exact underlying mechanisms are still not well understood, this direction of improvement of the composite material seems to be promising. In particular, the expected improvement in photocatalytic composite material also incorporating AuNPs is related to the plasmonic effect, which works for the whole spectral range of sunlight.

a TEM images of a large assembly of the TiO 2 NWs/AuNPs composite (the scalebar is 200 nm), and ( b ) Enlarged image fragment selected from the image shown in ( a ) (the scalebar is 5 nm); ( c and e ) SEM images of the filter material without and with surface structuring, respectively (the scalebar is 2 μm). d Two distinctly different optical responses upon visible light illumination of non-structured (left) and structured (right) versions of the filter material (the scalebar is 10 mm).

A second route for improvement could be the surface structuring of the filter, in order to better trap and conduct the incident light into the bulk of the material, since, as mentioned before, both ROS generation and the resulting water pasteurization are proportional to the illuminated volume. In general, the question of more efficient light trapping is very vivid in photovoltaics in order to improve the light-matter interaction within the device’s photo-active layer and thus to enhance the solar cell efficiency. One way of achieving this is through elaboration of periodic light trapping structures. Recently, numerous such structures have been developed, including nanowires [ 42 ], nanoparticles [ 43 ], gratings [ 44 , 45 ], and many more. In particular, gratings could be designed to diffract the incident light at the TiO 2 –air interface at angles bigger than the critical angle (depending on the grating periodicity), so that most of the potentially scattered or reflected energy could be returned to the photo-active filter layer. It has previously been shown that such surface structuring results in an effective enhancement of the optical path length and better light absorption [ 45 ].

In this regard, in Fig. 7c–e , we show that it is possible to fabricate the TiO 2 NWs/CNTs filter to have a grating pattern structure on its surface. Specifically, the SEM images of the as-deposited filter on a flat and lithography-modified substrate surfaces are shown in Fig. 7c, e , respectively. The grating pattern structure can easily be recognized on the surface of the filter material deposited on the lithography-engineered substrate. Moreover, as shown in Fig. 7d , these two surfaces have distinctly different optical responses, due to the enhanced light interference on the surface with the grating pattern.

Further optimization of structuring of the filter surface as well as exploration of its advantageous properties are the subjects of forthcoming studies related to the TiO 2 NWs/CNTs filter material.

Synthesis of TiO 2 NWs

TiO 2 nano-powder with size distribution of 20–50 nm (Nanoshell LLC) was dissolved in a base solution of NaOH (Merck), and heated up to 70 °C and exposed to turbulent mixing. In a time-window of 1–24 h a high fraction of mesoporous titanates was formed with a jelly-like appearance. By centrifugation the (H 2 Ti 3 O 7 ) NWs were separated from the reaction residues and transformed into anatase TiO 2 NWs by heat treatment at 600 °C (ref. [ 20 ]). The specific surface of TiO 2 NWs is in the 150 m 2 /g range.

Synthesis of CNTs

The continuous production method of CNTs implemented in this work was based on chemical vapor deposition (CVD) performed in an inclined rotary tube furnace [ 21 ]. This approach overcomes the limited capacity and scalability of more frequently used fixed bed reactors [ 46 , 47 ]. The furnace used here was equipped with a quartz tube with a diameter of 80 mm. During the CVD deposition process, the catalyst (bimetallic Fe 2 Ni catalytic particles on CaCO 3 supporter) was continuously introduced into the reaction tube with an endless screw placed at the end of the catalyst container. Acetylene and argon were fluxed at 10 L/h and 80 L/h, respectively. The material was purified by dissolving the catalyst and the support in a 1.5 M hydrochloric acid. Subsequently, MWCNTs were filtered, washed with distilled water, and dried at 120 °C overnight. A production rate of about 1.2 kg per day was achieved. The specific surface of CNTs 700 m 2 /g rage.

Mechanical filtration

The mean pore size distribution of the filters was characterized by polystyrene dispersion of 1.97 μm , 870 nm , 530 nm and 110 nm particles of concentration of 13.7 mg/L, which were passed through the filter. Due to intrinsic absorbance and the light scattering of polymer colloid particles, the dispersion has a characteristic absorbance spectrum. Therefore, comparing the initial spectra with the filtered solution spectra yields information about the concentration and retention ability.

Light intensity measurements

The light intensities were measured with the optical intensity handheld meter ML9002A (Anritsu). The silicone-based detector measured the total power incident onto the 9 mm-diameter circular surface of the probe.

Thermal imaging

Thermal images were captured with the FLIR ONE infrared camera. It measured the intensity of infrared light (7.5 μm <  λ  < 14 μm) emitted by the objects in the field of view of the camera over an area of 160 by 120 pixels to compute their temperature. In the limit of sensibility of −20 °C to 120 ̊ the temperature resolution was of 0.1 °C.

Colilert-18 test

The water sample/nutrient-indicator mixture was introduced into a Quanti-Tray (array of plastic reservoirs) and sealed with IDEXX Quanti-Tray Sealer. The sealed tray was placed in an incubator at 44.5 ± 0.2 °C for 18 h. The numbers of positive wells (yellow color) were translated into the Most Probable Numbers by a table provided with the trays.

Throughput determination of viable bacteria by the colony-forming units (CFU) method

Evaluation of viable coliforms and E. coli in water samples was performed using the so-called colony-forming units (CFU) method. The CFU method is generally accepted for the evaluation and assessment of viable microorganisms, as well as for testing the impact of antimicrobial techniques and substances under various experimental conditions.

The initial concentration of E. coli bacteria used in this study was of 2–10 8 CFU/ml. All manipulations with the bacteria were performed using clean, sterile equipment, including pipette tips and Eppendorf tubes, solidified agar media, Petri dishes, and the bacterial cell spreader. To expose the E. coli bacteria in control experiments to UVA light, a UVA-light spot source, LC-8 Lightingcure™ (λ = 365 nm, 16 mW/cm 2 ) from Hamamatsu Photonics (Japan) was used. In experiments aiming at characterizing the disinfecting potential of the filter material based on the TiO 2 NWs /CNTs nanocomposite, E. coli bacteria deposited on this filter were exposed to sunlight.

ESR spectroscopy

The ESR experiments were carried out at room temperature using a cw-ESR X-band spectrometer, Bruker EleXsys E500 (Bruker BioSpin GmbH, Karlsruhe, Germany), equipped with a high-Q cylindrical cavity (Model ER 4122 SHQE). Water with 400 μM concentration of TEMPOL was passed through the filter and exposed to sunlight (as shown in Fig. 3 ) over a period of ~2.5 h. Small aliquots of about 20 μL were taken both before and after the filter at regular time intervals, transferred into 0.7 mm ID and 0.87 mm OD glass capillary tubes (VitroCom, NJ, USA), with a sample height of ca . 30 mm, and sealed on both ends with Cha-seal (Tube sealing compound, Chase Scientific Glass, Rockwood, TN, USA). The typical instrumental settings were: microwave frequency ~9.4 GHz; microwave power 0.633 mW; magnetic field sweep 120 G; magnetic field modulation frequency 100 kHz; magnetic field modulation amplitude 0.5 G; time constant 10.24 ms; spectral resolution 2048 points; conversion time 40.96 ms; resulting magnetic field sweeping time 84 s; two scans were accumulated per one trace. To evaluate the number of spins in the individual samples, the respective ESR spectra were double-integrated using a software tool (OriginPro2019) and the obtained signal intensities were compared with the intensity of the reference signal (400 μM TEMPOL).

Methyl orange solution (MO) with an initial concentration of 20 mg/L was poured into Petri dishes containing the test samples in the form of discs (50 mm in diameter) at their bottom. For the purpose of exposure to UVA light, a UV spot light source, LC-8 Lightingcure™ (λ exc  = 365 nm, 16 mW/cm 2 ), from Hamamatsu Photonics (Japan), was used. The photodegradation of the dye and the corresponding changes in its concentration were monitored by measuring the characteristic MO absorbance peak at 464 nm with a UV–Vis spectrophotometer, Varian Cary 50, Agilent (USA). In order to obtain the time dependencies, small aliquots (2.5 mL) were taken every 10 min from Petri dishes and transferred to the cuvettes of the UV–VIS spectrophotometer.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

E.H. gratefully acknowledges the Global Water Award of the United Arab Emirates, the Swiss-South African collaboration grant, and the support of the Karl Zeno Schindler Foundation. We would also like to thank Lenke Horváth and Rita Smajda, for their assistance in the early stage of the experiments.

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Present address: Stavropoulos Center for Complex Quantum Matter, University of Notre Dame, Indianapolis, IN, USA

Authors and Affiliations

Laboratory of Physics of Complex Matter, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland

E. Horváth, J. Gabathuler, G. Bourdiec, E. Vidal-Revel, M. Benthem Muñiz, L. Rossi, A. Sienkiewicz & L. Forró

Central Environmental Laboratory, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland

M. Gaal, D. Grandjean & F. Breider

ADSresonances Sàrl, CH-1028, Préverenges, Switzerland

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E.H. was the project leader and synthesized the materials. L.R. did the electron microscopy. J.G. worked with the prototype and characterized the filter paper with G.B., E.V.-R. and M.B.-M. The micropollutant test was performed by M.G., D.G. and F.B. The ESR measurements were done by A.S. who has participated in the paper writing. L.F. is the team leader and wrote the paper. All authors approved the manuscript.

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Correspondence to E. Horváth or L. Forró .

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Horváth, E., Gabathuler, J., Bourdiec, G. et al. Solar water purification with photocatalytic nanocomposite filter based on TiO 2 nanowires and carbon nanotubes. npj Clean Water 5 , 10 (2022). https://doi.org/10.1038/s41545-022-00157-2

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Received : 03 October 2021

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DOI : https://doi.org/10.1038/s41545-022-00157-2

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