Nanoscale Water Purification for Macroscale Results
By Montserrat Tang-Holmberg
~ 3 minutes ~
The future of drinking water may hinge on a solution so small that it belongs in an Ant-Man movie, yet the technology behind it is anything but fantastical. Nanotechnologies—engineered structures less than 100 nanometers in diameter—are smaller than individual wavelengths of visible light. Their potential, however, is monumental, with researchers believing that nanoscale solutions will be the next frontier in water reclamation and purification. By shrinking materials down to these scales, scientists increase surface area and exaggerate physical properties beyond what is possible at the macroscale, allowing for the production of potable water through filtration, adsorption, and catalysis processes.
In 1991, Japanese scientist Iijima Sumio observed carbon nanotubes (CNTs) using an electron microscope. These structures are long, cylindrical arrangements of carbon atoms arranged hexagonally. Since this discovery, CNTs have become one of the most discussed examples of water purification nanotechnology. They work through membrane filtration: CNTs are exceptionally permeable due to their aligned channels and hydrophobic walls, but they reject contaminants like salts and microbes. CNTs are produced through various thermal processes that remove carbon atoms from carbon-bearing materials. Manufacturers then use these atoms to construct hexagonal lattices, with the possibility of surface and structure customization for specialized filtration. Furthermore, the conductive nature of CNTs has been utilized to kill microorganisms and break down heavy metals through electricity. With this in mind, journals such as Nature and ScienceDirect, as well as government agencies NASA and the U.S. Army Public Health Command, have presented CNTs as potential solutions to the shortage of clean drinking water.
Taking a different approach, researchers at Yale University, Rice University, Arizona State University, and the University of Texas at El Paso recently identified gold nanoparticles (NPs) as effective photocatalysts. This means that by converting solar energy into chemical energy, the NPs could degrade pesticides, pharmaceuticals, and perfluorooctanoic acid, a carcinogen, in water. Gold NPs were discovered by Michael Faraday in 1857 and have been studied in various capacities, with these researchers now focusing on targeting micropollutants that evade conventional purification strategies. In another application, scientists have synthesized gold nanorods that absorb sunlight and convert it to localized heat. Through photocatalysis, the nanorods use solar radiation to disinfect water and break down contaminants that typically require expensive chemical treatments. Following these results, Naomi Halas, director of the Laboratory for Nanophotonics at Rice University reflected:
“This is really nanoengineering at its best, a novel nanoparticle designed to solve an important problem in what would otherwise be an impossible environment.” (Naomi Halas, director of the Laboratory for Nanophotonics, Rice University)
In yet another example of nanopurification technology, research on silver-based NPs has exploded in recent years. Silver nitrate is favored in the production of these NPs because it easily dissociates into silver ions in water. These positively charged particles are cytotoxic to microorganisms, serving as an efficient avenue for water sanitization. Further, silver nitrate NPs prevent biofouling, the accumulation of living organisms and organic matter, act as co-catalysts in degradation reactions, and are effective adsorbents. Recently, a more specialized function for these NPs has emerged: acting as a coating for other metals with desirable properties. Magnetite is one such metal, being supermagnetic, a type of magnetism unique to NPs. Multiple researchers have used silver nitrate as a coating for magnetite NPs, serving as a surface chemistry modifier. In one study, researchers at the College of Chemistry and Chemical Engineering at Yangzhou University found that the resulting nanocomposites effectively reduce 4-nitrophenol, a hazardous toxin, and degrade methylene blue dye in water. Similarly, researchers at the Czech Academy of Sciences found that these NPs remove 98.4% of arsenic and large quantities of hexavalent chromium, a carcinogen, from water. This “core-shell” configuration is one of the most efficient, combining silver’s purification properties with magnetite’s magnetism. This enables post-use magnetic retrieval, which is both sustainable and prevents secondary contamination.
Despite their merits, nanotechnologies have been attracting skepticism for their safety, scalability, and economic feasibility. Most importantly, some NPs inherently degrade natural environments. For example, the same property that makes silver nitrate an effective sanitizing agent also means it is toxic to vital microbial communities and aquatic organisms. Most health risks can be circumvented by stabilizing and immobilizing NPs, but the initial hope that nanotechnology could be used for direct environmental remediation is currently nonviable. Still, the scientific community remains hopeful for future improvements and applications.
The use of nanoscale devices for water purification is receiving significant interest and investment. Wastewater treatment plants are increasingly turning to nanomaterials for filtration, disinfection, and specialized decontamination processes. Commercial products are starting to incorporate these technologies for market use, and emerging companies are developing nano-enabled desalination plants. While the adoption of water-purifying nanotechnology remains limited, the field is moving from laboratory research into practical application. Scientists are offering a breakthrough for water reclamation efforts, a solution that has already proven effective enough to sanitize medical equipment and provide potable drinking water. With the UN recently declaring an “era of global water bankruptcy,” the world is thirsty for innovative approaches. Though perhaps only Ant-Man will actually see this technology, the water-hungry world is sure to feel the difference.
