Nanofilter’s electropotential eliminates contaminants The fiber feels like soft paper, but it’s made of nanosized strings of minerals. When water flows through this material, its nanosized, aluminum-oxide-based strands grab and hold microscopic pathogens and other tiny particles. The science behind it seems simple: the flowing water sets up a positive charge on the nanofibers, which can adsorb passing particles. Those particles include pathogens such as E. coli and Salmonella according to tests conducted by the fiber’s maker, Ahlstrom Filtration LLC, and other companies that use the material in their products. The filter is made of nanofibers of a type of aluminum oxide called boehmite (also known in the water industry as alumina). The nanoalumina filaments, about 2 × 250 nanometers, are grafted onto glass fibers. These are made into a nonwoven, paper-like material that can be folded into filters in various configurations. Already on the market for 2 years for some applications, these filters could eventually serve as an ultraefficient removal method for pathogens in drinking water. Meanwhile, a Florida community of more than 38,000 people is about to experiment with using the filters to treat wastewater. Prototypes and demonstrations of the nanoalumina filters already show some success in keeping reverse-osmosis (RO) filters clear of biofouling in large utilities, its makers say. The filters even remove particulate matter and metals from water for car manufacturers. One Toyota plant uses the nanoalumina cartridges to protect its RO system and has cut its costs 5-fold, according to inventor Fred Tepper, founder of the nanofiltration-focused company
Argonide, and Rick Lancaster of Toyota’s water management group. The plant needs to clean its RO filter only every 9 monthssinstead of replacing it within 3 months. Tepper created the material in the 1990s with funding in part from NASA, the U.S. Air Force, and the U.S. EPA. He and co-workers from the University of Florida put the filter through its paces and found that the nanoalumina also removes organic material, disinfection byproducts, chlorine, iodine, and endocrine disrupters with some success. Rodney Komlenic of Ahlstrom adds lead, mercury, and arsenic to that list. The filter performs well at pH 5-9 and in varying saline conditions. Its electrostatic charge disappears once the filter is removed from water, but trapped particles remain adsorbed, according to un published data from Ahlstrom and Argonide. But although these studies have shown that pathogens remain trapped and are unlikely to escape, the fate of the nanoalumina itself has yet to be studied. Throwing away the filters presents potential drawbacks, comments Qilin Li of Rice University. Landfill disposal, for example, could allow contaminant-laden nanoparticles to escape into the environment. Li notes that this is the first time she has seen a water treatment product that uses such fine fibers; they could have unknown impacts on human health and ecosystems once released into the environment. Other potential concerns include worker safety. James Economy of the University of Illinois Urbana-Champaign notes that the brittle nanofibers could end up in the lung alveoli of work-
7736 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / November 1, 2008
ers processing the filaments and making the final products. Komlenic says that Ahlstrom reviewed the material carefully, with an eye toward human and environmental safety. Tepper adds that natural nanoalumina has been used in vaccines and analgesics for decades without harm to people. The nanoalumina filters also have been certified as safe for drinking water in accordance with NSF/ANSI Standard 53, Tepper says, referring to standards developed for EPA purposes and tested by the Water Quality Association, a nonprofit trade organization. Other highly efficient technologies already allow makers to tailor the pore sizes and surface chemistry of filters to remove viruses and bacteria by using materials that include iron and zinc oxide and silver- or titanium-coated glass fibers, Economy says. Still, he allows that nanoalumina’s catalytic surface also could be useful. And at about $30 a pound, Economy says, cost is a selling point. Even cheaper materials will be important for developing countries such as Bangladesh. By the end of the year, Ahlstrom’s product could enter developed countries’ markets for smallscale use, says Komlenic. Products in development include filters for “point of use” and “point of entry” (where, for example, water enters a building’s distribution system). Tepper also envisions the nanomaterial’s future: cleaning wastewater from gas drilling sites, complementing seawater desalination, and supplementing large-scale municipal wastewater treatment. “It’s coming,” he says. “We have to find out more about the application[s].” —NAOMI LUBICK
10.1021/es8021914
2008 American Chemical Society
Published on Web 09/03/2008
