NEWS TECHNOLOGY Titanium dioxide photocatalysis: Treatment hype or hope? Scientists disagree whether a novel process for cleaning polluted water and air is a breakthrough in environmental treatment or an academic curiosity. Research scientists are enthusiastic about the potential of titanium dioxide to clean up everything from organic contaminants and heavy metals in water to indoor air pollution. But some environmental engineers argue that these degradation processes are too inefficient to ever pass into widespread commercial Titanium dioxide works as a catalyst with ultraviolet light to generate highly reactive oxidizing agents that attack and destroy organic pollutants in water or air. This photocatalyst can also remove heavy metal ions from solution, converting them into less toxic, readily recoverable metallic forms, according to materials scientist Marc Anderson at the University of Wisconsin. "We can use photoactivity to degrade contaminants in water wastewater and air " he said. "Although these cesses have not yet moved from the laboratory to commercial application they have great promise Already in Japan titanium dioxide particles are being incorporated into bathroom tiles to degrade indoor air pollutants " Other such advanced oxidation processes using different catalysts have gained commercial acceptance. Hydrogen peroxide is used for water purification at about 200 major installations in North America, whose output ranges from a few gallons to 1500 gallons per minute, according to James Bolton at the photochemistry unit, University of Western Ontario. But titanium dioxide-catalyzed processes are unlikely to succeed commercially, according to Bolton. "This is an example of scientific hype. Titanium dioxide is interesting scientifically, but the fundamental questions are, how efficient is it and how much does it cost?" The quantum yield, a funda-
mental measure of process efficiency, is only 4% for titanium dioxide processes, Bolton said, whereas the quantum yield for hydrogen peroxide processes is 100%. "This low efficiency equals an expensive process that can't compete in the market," he said. Anderson acknowledged that the poor quantum yield of current titanium dioxide systems is a problem, but one that he believes can be solved by applied research. "What the field needs is a better catalyst and better reactor design. This work is already under way." Examples include pretreating titanium dioxide with carbon compounds to improve the catalyst's efficiency by increasing the affinity between titanium dioxide and contaminants in solution, said Robert Peters, research leader in charge of remediation studies at the Department of Energy's Argonne National Labora-
Robert Peters of Argonne National Laboratory uses photocatalysts based on ultraviolet light to remove, detoxify, and recover heavy metals from groundwater and soils while destroying organics.
tory. "By modifying the surface properties we get the catalyst to act like an ion exchange column and remove metals. We also get the photooxidation of organic contaminants," he said. Anderson added that reactor design improvements include using fiber optics to get light directly to the catalytic surfaces. REBECCA RENNER
"Greener" dry-cleaning alternative pushed by industry, environmental groups An alternative technology to dry cleaning and its use of the suspected carcinogen perchloroethylene is being actively promoted by a new coalition of cleaning industry groups, environmentalists, and EPA. Called "wet cleaning," the technology is currently undergoing detailed evaluations at cleaners in Chicago and Los Angeles. Wet-cleaning machines use water and special pH-neutral detergents in place of conventional dry-cleaning solvents. Unlike traditional dry cleaning, wet cleaning involves agitating clothes but in a slower, more controlled way than that of commercial washing machines. Operators program temperature and water extraction speed for specific types of garments and fabrics. Perchloroethylene, or "perc," has been linked to cancer and neurological and liver damage; it is listed as a hazardous air pollut-
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ant under the Clean Air Act. EPA ranks it as a "possible to probable human carcinogen," and the Occupational Safety and Health Administration limits perc exposure to 100 ppm per day. The Ontario government is considering dropping its perc exposure limit from 50 to 10 ppm. Perc use has declined by 40% in the past five years, according to William Seitz, executive director of the 3000-member Neighborhood Cleaners Association. Although Seitz acknowledged that this drop has resulted from the introduction of improved drycleaning equipment, he expects that wet cleaning will lead to additional reductions in perc use. A six-month Canadian study published in October 1995 found that the technology successfully cleaned 30-80% of the garments brought into the five wet cleaners evaluated. The study also re-
