Michigan Tech Researchers Use Solar Power
to Clean Contaminated Groundwater
by Kristine Bradof
This article originally appeared in the July 1996 issue of the Wellspring newsletter, published by the MTU Regional Groundwater Education in Michigan (GEM) Center, now the Center for Science and Environmental Outreach at Michigan Technological University.
A rooftop may seem an unlikely place for cleaning contaminated groundwater, but researchers from Michigan Technological University's Department of Civil and Environmental Engineering didn't think so. To them, a roof was a logical location to test a now-patented process that relies on the sun's energy to convert hazardous chemicals into little more than carbon dioxide and water. The process, called solar photocatalysis, offers several advantages over other technologies.
Most cleanup methods, such as air stripping and adsorption, merely transfer the toxic contaminants from the groundwater to the air or to another medium, which must then be handled as hazardous waste. With solar photocatalysis, gone is the need to transport and dispose of harmful byproducts of the treatment process itself. The contaminants are actually destroyed on-site, as are compounds that add unpleasant tastes, odors, or colors to water. Photocatalysis also degrades organic matter that occurs naturally in water. When organics are removed from drinking water, byproducts of the chlorine disinfection process can't react with them to form trihalomethanes like chloroform, which have been linked to cancer.
Many of the chemicals that photocatalysis can eliminate from groundwater are listed as "priority pollutants" by the U.S. Environmental Protection Agency. They include dioxins, polychlorinated biphenyls (PCBs), herbicides, pesticides, solvents, and benzene, toluene, ethylbenzene, and xylene (BTEX) from fuel spills or leaking storage tanks. In field testing of the solar photocatalysis technology, fuel-contaminated groundwater at Tyndall Air Force Base in Florida was successfully cleaned at a cost comparable to that of conventional technologies. A larger-scale test at K. I. Sawyer Air Force Base destroyed chlorinated compounds in contaminated groundwater.
The ingredients essential to solar photocatalysis are an adsorbent, a photocatalyst, and ultraviolet (UV) light from the sun. The adsorbent medium, usually granular activated carbon or a resin, attracts organic pollutants from the groundwater passing through it. The photocatalyst titanium dioxide, commonly used as a brightener in household products like toothpaste, is bonded to the adsorbent. When the adsorbent is saturated with organic molecules, it is exposed to sunlight. The UV energy excites electrons in the photocatalyst, setting up a reaction in which the pollutants are destroyed structurally and rendered harmless.
The process doesn't require full sunlight. It works even on rainy days, though less efficiently. In fact, the main limitation, clogging of the adsorbent bed by particles and dissolved substances in the groundwater, has nothing to do with light. Pretreatment is needed to prevent clogging problems and improve reaction time. During the testing phase, pretreatment consisted of filtration, oxygen addition, and ion exchange. The cost, about $5 per 1,000 gallons of groundwater, may be reduced in the future if less expensive water pretreatment methods prove successful. Current research seeks to improve process efficiency by developing a near-UV translucent support matrix for the photocatalyst to replace the adsorbent. Silica gel is one material under investigation, according to Michigan Tech researcher and associate professor David Hand.
Other applications of solar photocatalysis may include disinfection of drinking water without the use of chlorine, an increasingly controversial chemical. Titanium dioxide, the material used as a photocatalyst, has already been shown to inactivate Escherichia coli, a form of bacteria associated with fecal contamination.
The solar photocatalytic process is an exciting clean technology that uses only sunlight, water, and a reusable reaction medium to remove toxins from groundwater. Nothing ends up in a landfill or hazardous waste incinerator. As Dr. Hand observes, the basic process is very similar to the sun's ability to "bleach" stains from fabrics. So, the next time you hang your laundry out to dry on a sunny day, remember that somewhere, not far away, that same sunlight just might be cleaning your drinking water.
UV-light process renders poisons harmless. Biophotonics International, July/August 1995, p. 19.
Zhang, Y., J. C. Crittenden, and D. W. Hand. The solar photocatalytic decontamination of water. Chemistry & Industry, 19 September 1994, pp. 714-716.