Cheap Nanotech Filter Clears Hazardous Microbes and Chemicals from Drinking Water

About 780 million people—a tenth of the world’s population—do not have access to clean drinking water. Water laced with contaminants such as bacteria, viruses, lead and arsenic claims millions of lives each year. But an inexpensive device that effectively clears such contaminants from water may help solve this problem.

Thalappil Pradeep and his colleagues at the Indian Institute of Technology Madras developed a $16 nanoparticle water filtration system that promises potable water for even the poorest communities in India and, in the future, for those in other countries sharing the same plight. Although cheap filtration systems have been developed previously, this is the first one to combine microbe-killing capacity with the ability to remove chemical contaminants such as lead and arsenic. Because the filters for microbes and chemicals are separate components, the system can be customized to rid water of microbial contaminants, chemical contaminants or both, depending on the user’s needs.

A prototype of the filtration system. Credit: Thalappil Pradeep, Indian Institute of Technology Madras

In a report published yesterday in Proceedings of the National Academy of Sciences, Pradeep and his collaborators explain that the microbe filter relies on silver nanoparticles embedded in a cage made of aluminum and chitosan, a carbohydrate derived from the chitin in crustacean shells. The cage blocks macroscale water contaminants as well as protects the nanoparticles from sediments that would otherwise accumulate on their surfaces, thereby preventing them from releasing microbe-zapping ions.

The team used nanoparticles that release iron- and arsenic-trapping ions to make its chemical filter. But Pradeep notes that the “cage” technique can be used with other nanoparticles to target contaminants such as mercury.

The membrane filter at the top kills bacteria and viruses, and the axial block at the bottom can be custom fitted with a second filter for lead or arsenic. Credit: Thalappil Pradeep, Indian Institute of Technology Madras

The materials are added one by one into water and self-assemble into small sheets that resemble clay (see image below). These sheets are the “cages” that then hold on to the silver nanoparticles. Production requires no electricity because the claylike filters are made at room temperature. Every liter of water used to make the material goes to filtering 500 liters of water. “This is a room-temperature green synthesis, which means it can be deployed in any part of the world,” Pradeep says.

Chitosan fibers combine with aluminum hydroxide nanoparticles (AlOOH Np) to form a claylike “cage” that can protect embedded silver nanoparticles from deposits that would reduce their microbe-killing power. Credit: Thalappil Pradeep, Indian Institute of Technology Madras

“This is probably the strongest aspect of the study,” says John Georgiadis, professor of bioengineering at the University of Illinois at Urbana–Champaign. “Other systems are very expensive and have very low green profiles.”

James Smith, professor of environmental and civil engineering at the University of Virginia called the new work “promising and exciting” but foresees problems with the filter’s production in countries like India and Africa. “The method involves both strong acids and bases and likely would not allow for manufacturing in a developing-world setting,” Smith commented via e-mail. Depending on how high the region’s contamination levels are, the filter must be boiled in water for about four hours every six months to remove deposits that reduce nanoparticle potency. According to Smith, this cleaning may be “difficult for developing-world families to accomplish on a regular basis.” Smith is the co-developer of PureMadi, a clay filtering pot coated with nanoparticles that gets rid of harmful bacteria—but not chemicals—in water that is currently being used in South Africa.

Pradeep has already run small-scale field trials and found the filters to be effective. Working with a Madras-based start-up that will make and assemble the filters, his team plans to distribute 2,000 supersize versions of the filtration system, each meant to serve about 300 people. These community units will provide water for about 600,000 people in the state of West Bengal and allow Pradeep’s team to test how effective their technology is at clearing contaminants—especially arsenic, which occurs naturally in groundwater there—on a larger scale.

“Water means health, education and overall well-being of a society,” Pradeep observes. “This is how these technologies are going to make an impact in places such as India.”

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