How Are Scientists Tackling Difficult Drinking Water Contaminants?
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There exists a huge variety of different potential sources for water contamination — ranging from the physical silt and sediment that collects naturally in riverbeds, to harmful water-borne microbes, to manmade chemicals.
Normally these contaminants can be dealt with using relatively simple water treatment techniques, such as filtration, boiling, and/or chemical treatment to purify the water and remove any potentially harmful substances. But for some contaminants, these ordinary treatments are not enough.
To deal with these more resilient contaminants properly, scientists are continuously developing new, more effective methods for treating water contamination. By applying the best in advances from materials science, chemistry, physics, and environmental science, researchers are now able to tackle some of the most complex challenges in managing drinking water contamination.
Dealing with resilient “forever chemicals”
Per- and polyfluoroalkyl substances (PFAS) are one such family of contaminants that are particularly problematic to treat.
These manmade chemicals were first used commercially in the 1940s and 1950s, where their water and oil-repelling properties made them an ideal coating for non-stick pans, cleaning products, and food packaging.1 Since then, the use of PFAS has slowly declined as manufacturers became aware of their associated health risks; PFAS have been linked to reproductive, developmental, liver and kidney, and immunological problems in various animal studies.2
Yet despite this decline in use, PFAS chemicals still remain a major drinking water contamination risk today, with recent studies indicating that their prevalence in drinking water is even higher than previously believed.3
“PFAS are technically not difficult to “remove” from water because we could use adsorbent material, like activated carbon or ion exchange resins, to remove them from water via adsorption,” explains Christopher Sales, PhD, an associate professor of environmental engineering at Drexel University. “However, you create a contaminated adsorbent material [which] contains the PFAS that were removed from water.”
“PFAS are difficult to “destructively degrade”” Sales continues. “The fluoro-carbon bonds within PFAS are hard to break — they take a significant amount of energy to break and no living organism has demonstrated the ability to break them.”
This extreme stability has earned the PFAS class of compounds the nickname “forever chemicals”. Since the chemicals do not easily biodegrade, PFAS can easily leach out into ground and surface water from products that have been sitting in landfill for years — and the same is true for PFAS contaminated water filters. If these filters are not disposed of correctly, they can become an additional contamination source risk.
Sales and his team at Drexel University have been investigating alternative methods for dealing with these PFAS. Their latest work, published in January in the journal Environmental Science: Water Research & Technology4, describes the use of cold plasma technology to split these strong fluoro-carbon bonds apart, breaking PFAS down into much safer compounds.
Cold, or non-equilibrium, plasma technology uses a device called a ”gliding arc plasmatron” to create a rotating electromagnetic field, which excites electrons present in gas bubbles in a water sample. These high-energy electrons then begin to breakdown some chemical species in the water, and with enough energy can begin to emit ultraviolet radiation. The combination of this radiation and the high-energy electrons reaches a point where it is able to effectively split the strong fluoro-carbon bonds in the PFAS compounds.
In their study, the researchers at Drexel University were able to remove more than 90% of the long chain PFAS compounds included in their experimental water samples, and defluorinate approximately a quarter of the compounds using cold plasma technology. The process was also remarkably energy efficient, using between three and twenty times less energy than it would take to boil the same volume of water.
“The benefit of cold plasma technology is that there is evidence that it is able to destructively degrade PFAS into safe byproducts — carbon dioxide and fluoride,” Sales explains. “However, more research is needed to design cold plasma technologies to efficiently and effectively mineralize PFAS into carbon dioxide and fluoride.”
The researchers also want to investigate the effectiveness of the method at a much larger scale, and experiment with whether the method could also be adjusted to treat PFAS-contaminated soil.
Can nanotechnology treat pesticide-polluted waters?
Another of the more novel approaches that researchers are investigating for drinking water treatment is the use of engineered nanomaterials.
Professor My Ali El Khakani, an expert in nanostructured materials at the Institut National de la Recherche Scientifique (INRS), University of Québec, and his INRS colleague Professor Patrick Drogui, a specialist in electrotechnology and water treatment, have together been investigating the use of new materials in the treatment of pesticide-contaminated waters.
In a recent paper, published in Catalysis Today5, the two professors and their research teams describe using nanostructured tungsten-nitrogen codoped titanium oxide photoelectrodes to degrade atrazine in water.
Atrazine is a relatively common herbicide used widely on maize, sugar-cane, and sorghum grain crops to control the growth of weeds, although its use has now been restricted in several countries.6 A 2018 assessment from the United States Environmental Protection Agency found that the cumulative exposure to the herbicide from food, drinking water, and other residential sources can increase the risk of reproductive and developmental problems in humans.7
Like PFAS, atrazine can also be difficult to degrade with traditional water treatment chemicals, and so the researchers at the INRS sought to find a more effective alternative.
Their recent study utilized a UV/visible light driven photoelectro-catalysis method, or PEC, that had been optimized for atrazine. In essence, the method uses two light-sensitive electrodes (photoelectrodes) of opposite charges. When exposed to light and an electrical potential, free radicals are generated on the surface of the photoelectrodes. These radicals are then able to interact with the atrazine molecules that are present in water and degrade them easily.
By using specially engineered nanostructured materials, the scientists are able to maximize the available active surface area for the photoelectrodes, and in turn allow for large amounts of water to be treated using only very small photoelectrodes.
“One of the major benefits of using PEC processes is that we do not need to add chemical reactants, of which the residues can be harmful for the environment. It is like solving one problem, removing atrazine,by creating another one — releasing other chemicals in water and ultimately in the environment,” explains Professor El Khakani. “Another advantage here offered by the codoped TiO2:W,N anodes we developed is their capacity to absorb sunlight, which is available and renewable.”
Using their specially adapted PEC process, the researchers were able to eliminate around 60% of the atrazine present in atrazine-spiked samples of demineralized water. When using samples of real water, initially only 8% of the atrazine was successfully degraded, but after further coagulation and filtration treatment this rose to between 38 and 40%.
"The treatment was basically less efficient in real water compared to synthetic water samples because of the presence of many other substances in suspension — particulates, other bio-chemical components, etc.,” El Khakani explains. “The presence of such substances has two synergetic disadvantageous effects: first, the suspended matter limits the sunlight absorption of the anode and second, the presence of other hydrocarbon or chemical species in raw samples can poison the surface of the TiO2:W,N electrode limiting thereby its full photocatalytic potential.”
The authors suggest that their optimized PEC process could be effective as a tertiary water treatment — to be applied after standard filtration and coagulation steps — to catch atrazine and perhaps also other similar emerging contaminants.
References
- State of Rhode Island Department of Health. PFAS Contamination of Water. Retrieved from: https://health.ri.gov/water/about/pfas/ [Last Accessed: March 19, 2020].
- United States Environmental Protection Agency. (2018, December 6). Basic Information on PFAS. Retrieved from: https://www.epa.gov/pfas/basic-information-pfas [Last Accessed: March 19, 2020].
- Evans et al. (2020, January 22). PFAS Contamination of Drinking Water Far More Prevalent Than Previously Reported. Retrieved from: https://www.ewg.org/research/national-pfas-testing/ [Last Accessed: March 19, 2020].
- Lewis et al. (2020). Rapid degradation of PFAS in aqueous solutions by reverse vortex flow gliding arc plasma. Environ. Sci.: Water Res. Technol. DOI: https://doi.org/10.1039/C9EW01050E
- Komtchou et al. (2020). Photo-electrocatalytic oxidation of atrazine using sputtured deposited TiO2: WN photoanodes under UV/visible light. Catal. Today. DOI: https://doi.org/10.1016/j.cattod.2019.04.067
- World Health Organization. (2003). Atrazine in drinking water. Retrieved from: https://apps.who.int/iris/bitstream/handle/10665/75363/WHO_SDE_WSH_03.04_32_eng.pdf [Last Accessed: March 19, 2020].
- United States Environmental Protection Agency. (2018, July 26). Chlorotriazines: Cumulative Risk Assessment - Atrazine, Propazine, and Simazine. Retrieved from: https://www.regulations.gov/document?D=EPA-HQ-OPP-2013-0266-1160 [Last Accessed: March 19, 2020].
