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optofluidics xuming zhang

Xuming Zhang / Hong Kong Polytechnic University

Researchers at Hong Kong Polytechnic University say reactors such as this one use sunlight to purify water trapped in tiny tubes. 

Researchers purify water with trapped sunlight

by Chris Bentley
Jan 28, 2011

As the adage goes, sunshine may indeed be the best disinfectant. Researchers at Hong Kong Polytechnic University may have figured out how to clean water by forcing it through tiny channels exposed to sunlight. 

The resulting design could use free, abundant solar energy to purify wastewater if it can be scaled up. But as the water channels shrink in size, the potential costs and physical challenges grow, researchers said. 

“We may not be able to say microfluidics makes purification possible, but we are confident that microfluidics makes water purification closer to practical uses,” says Xuming Zhang, assistant professor of applied physics at Hong Kong Polytechnic University and co-author of the study. 

The study, entitled “Optofluidic planar reactors for photocatalytic water treatment using solar energy” was published Jan. 11 in Biomicrofluidics — a journal published by the American Institute of Physics. 

 Photocatalysis — the process that uses light to break down impurities — has been studied before, but only on water carried in larger tubes. Smaller tubes, often thinner than a human hair, improve the efficiency of the process by increasing surface area and facilitating a new reactor design. 

Instead of mixing the light-reactive compound TiO2 into the water and filtering it out later, Zhang says microfluidics make it possible to use stationary TiO2 films and circumvent the energy-intensive filtration process. When exposed to sunlight, TiO2 emits free radicals that can degrade organic compounds, namely pesticides, agricultural chemicals and other contaminants that may give rise to strange colors or tastes. Researchers have dubbed the intersection of  microfluidics and photocatalysis “optofluidics.” 

“Using solar light is almost the only hope for photocatalysis to go to large-scale uses,” Zhang says. Photocatalysis works best under ultraviolet light, but sunlight has only about 3 to 5 percent of its energy in the UV range. While artificial UV lights are readily available, Zhang notes that they offset most of the energy-savings of the original design. 

George Whitesides, a Harvard University professor of chemistry and chemical biology, is skeptical about the technology’s ability to scale up. “My instinct is that this will not be the answer to large-scale purification, but it might be useful for certain kinds of local purification problems,” he said.  

Whitesides’ research group develops new applications for microfluidic systems, among other things. 

As for the scale, researchers were able to purify about 54 milliliters of water per hour — the equivalent of a Coca-Cola can every six hours. The Metropolitan Water Reclamation District of Greater Chicago, by contrast, processes an average of 1.4 billion gallons of wastewater each day. 

Zhang said the research group hopes to scale up its solar reactors to 1,000 liters per hour — just fractions of a percent of Chicago’s needs.  

Granted, Chicago’s water reclamation district is one of the world’s largest facilities of its kind. Still issues of scale loom. Zhang noted that photocatalysis isn’t meant to replace existing methods of water treatment, just to complement them. 

Whitesides said the technology could find a place alongside solar-thermal water heaters, which already provide hot water to households in sunny climates at very low-cost. “And potable water is more valuable than water that is simply warm,” he said, “so the economics might work.”  

Zhang agreed, noting that while it wasn’t needed in Hong Kong, it could be a good idea to combine heating and purification in colder regions.