On the water frontNew way to purify water

Published 16 July 2008

Water-attracting materials seem to repel impurities, thus leaving a layer of pure water near their surface; making tubes from these particle-excluding materials would allow for a new way to purify water — if, for now, in relatively small quantities

An entirely new way of purifying water has been discovered by U.S. scientists. The technique is based on the discovery that water-attracting materials seem to repel impurities, leaving a layer of pure water near their surface. In 2003 Gerald Pollack and his colleagues at the University of Washington in Seattle, discovered a process known as the “particle-exclusion phenomenon.” They found that particles dissolved in water naturally move away from a hydrophilic, or water-loving, surface, leaving pure water behind. What was really surprising was quite how far the particles would move — up to several tenths of a millimetre. This is much further than predicted by conventional theories. Pollack believes the phenomenon is caused by water molecules forming into a liquid-crystal-like array that sits on the hydrophilic surface. “This liquid crystalline zone excludes particles in much the same manner as ice excludes particles,” he told New Scientist’s Jon Evans. Pollack believes the phenomenon has many potential applications, with water treatment being one of the most obvious. So as a proof-of-principle he has designed a system based around a needle-sized tube made of the hydrophilic polymer Nafion. He then passed water containing tiny latex spheres, soil, clay, bacteria such as Escherichia coli, and viruses through the tube and measured what came out. He found that the particles would move to the middle of the tube, leaving a stream of pure water next to the tube walls. To collect these two different streams, Pollack inserted two concentric steel tubes into one end of the Nafion tube. The particle-rich stream flowed into the smaller, central steel tube, while the pure-water stream was siphoned off into the outer steel tube.

While promising, the technique has its limitations. Almost 97 percent of the regularly sized latex spheres were removed, but only 70 to 90 percent of the more irregular soil and clay particles. Another challenge is the flow rate, which is a mere fifteen millilitres per hour. Pollack says this could be increased by packing thousands of these tubes together. He believes the device has great potential, especially for the developing world. “The need for simple, inexpensive purification devices is huge, as the technical expertise required to maintain sophisticated systems is not always available in third-world villages,” he says. “Our filtration device contains no physical filters and is therefore simple; and flow through the device can probably be achieved by gravity.” Eric Hoek from the University of California, Los Angeles, who develops novel membranes for water treatment, calls this work “scientifically elegant,” but says the system may be better suited for small-scale biological separations than large-scale water purification.

Journal Reference: Environmental Science & Technology (DOI: 10.1021/es703159q)