24 Mar 2015
Microbe-free drinking water is hard to come by in many areas of India.
Chemists at the Indian Institute of Technology Madras have developed a portable, inexpensive water filtration system that is twice as efficient as existing filters. The filter doubles the well-known and oft-exploited antimicrobial effects of silver by employing nanotechnology. The team, led by Professor Thalappil Pradeep, plans to use it to bring clean water to underserved populations in India and beyond.
Left alone, most water is teeming with scary things. A recent study showed that your average glass of West Bengali drinking water might contain E. coli, rotavirus, cryptosporidium, and arsenic. According to the World Health Organization, nearly a billion people worldwide lack access to clean water, and about 80% of illnesses in the developing world are water-related. India in particular has 16% of the world’s population and less than 3% of its fresh water supply. Ten percent of India’s population lacks water access, and every day about 1,600 people die of diarrhea, which is caused by waterborne microbes.
Pradeep has spent over a decade using nanomaterials to chemically sift these pollutants out. He started by tackling endosulfan, a pesticide that was hugely popular until scientists determined that it destroyed ozone and brain cells in addition to its intended insect targets. Endosulfan is now banned in most places, but leftovers persist in dangerous amounts. After a bout of endosulfan poisoning in the southwest region of Kerala, Pradeep and his colleagues developed a drinking water filter that breaks the toxin down into harmless components. They licensed the design to a filtration company, who took it to market in 2007. It was “the first nano-chemistry based water product in the world,” he says.
But Pradeep wanted to go bigger. “If pesticides can be removed by nanomaterials,” he remembers thinking, “can you also remove microbes without causing additional toxicity?” For this, Pradeep’s team put a new twist on a tried-and-true element: silver.
Silver’s microbe-killing properties aren’t news—in fact, people have known about them for centuries, says Dr. David Barillo, a trauma surgeon and the editor of a recent silver-themed supplement of the journal Burns.
“Alexander the Great stored and drank water in silver vessels when going on campaigns” in 335 BC, he says, and 19th century frontier-storming Americans dropped silver coins into their water barrels to suppress algae growth. During the space race, America and the Soviet Union both developed silver-based water purification techniques (NASA’s was “basically a silver wire sticking in the middle of a pipe that they were passing electricity through,” Barillo says). And new applications keep popping up: Barillo himself pioneered the use of silver-infused dressings to treat wounded soldiers in Afghanistan. “We’ve really run the gamut—we’ve gone from 300 BC to present day, and we’re still using it for the same stuff,” he says.
No one knows exactly how small amounts of silver are able to kill huge swaths of microbes. According to Barillo, it’s probably a combination of attacks on the microbe’s enzymes, cell wall, and DNA, along with the buildup of silver free radicals, which are studded with unpaired electrons that gum up cellular systems. These microbe-mutilating strategies are so effective that they obscure our ability to study them, because we have nothing to compare them to. “It’s difficult to make something silver-resistant, even in the lab where you’re doing it intentionally,” Barillo says.
But unlike equal-opportunity killers like endosulfan, silver knocks out the monsters and leaves the good guys alone. In low concentrations, it’s virtually harmless to humans. “It’s not a carcinogen, it’s not a mutagen, it’s not an allergen,” Barillo says. “It seems to have no purpose in human physiology—it’s not a metal that we need to have in our bodies like copper or magnesium. But it doesn’t seem to do anything bad either.”
Though silver’s mysterious germ-killing properties are old news, Pradeep is taking advantage of them in new ways. The particles his team works with are less than 50 nanometers long on any one side—about four times smaller than the smallest bacteria. Working at this level allows him greater control over desired chemical reactions, and the ability to fine-tune his filters to improve efficiency or add specific effects. Two years ago, his team developed their biggest hit yet—a combination filter that kills microbes with silver and breaks down chemical toxins with other nanoparticles. It’s portable, works at room temperature, and doesn’t require electricity. Pradeep is working with the government to make these filters available to underserved communities. Currently 100,000 households have them; “by next year’s end,” he hopes, “it will reach 600,000 people.”
The latest filter goes one better: it “tunes” the silver with carbonate, a negatively-charged ion that strips protective proteins from microbe cell membranes. This leaves the microbes even more vulnerable to silver’s attack. “In the presence of carbonate, silver is even more effective,” he explains, so he can use less of it: “Fifty parts per billion can be brought down to .” Unlike the earlier filter, this one kills viruses, too—good news, since according to the National Institute of Virology, most do not.
Going from 50 parts per billion of silver to 25 may not seem like a huge leap. But for Pradeep—who aims to help a lot of people for a long time—every little bit counts. Filters that contain less silver are less expensive to produce. This is vital if you want to keep costs low enough for those who need them most to buy them, or to entice the government into giving them away. He estimates that one of his new filter units will cost about $2 per year, proportionately less than what the average American pays for water.
Using less silver also improves sustainability. “Globally, silver is the most heavily used nanomaterial,” Pradeep says, and it’s not renewable: anything we use “is lost for the world.” If all filters used his carbonate trick, he points out, we could make twice as many of them before we run out of raw materials—and even more if, as he hopes, his future tunings bring the necessary amount down further. This will become especially important if his filters catch on in other places with no infrastructure and needy populations. “Ultimately, I want to use the very minimum quantity of silver,” he says.
“Pradeep’s work shows enormous potential,” says Dr. Theresa Dankovich, a water filtration expert at the University of Virginia’s Center for Global Health. But, she points out, “carbonate anions are naturally occurring in groundwater and surface waters,” so “it warrants further study to determine how they are already enhancing the effect of silver ions and silver nanoparticles,” even without purposeful manipulation by chemists. Others see potential shortcomings. James Smith, a professor of environmental engineering at the University of Virginia and the inventor of a nanoparticle-coated clay filtering pot, worries that the nanotech-heavy production process “would not allow for manufacturing in a developing world setting,” especially if Pradeep’s continuous tweaking of the model deters large-scale companies from actually producing it.
Nevertheless, Pradeep plans to continue scaling up. “If you can provide clean water, you have provided a solution for almost everything,” he says. When you have the lessons of history and the technology of the future, why settle for anything less?
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