Solar detoxification

Solar energy is being investigated as a potential means to destroy environmental contaminants. In solar detoxification , photons in sunlight are used to break down contaminants into harmless or more easily treatable products. Solar detoxification is a "destructive" technology—it destroys contaminants as opposed to "transfer-of-phase" technologies such as activated carbon or air stripping, which are more commonly used to remove contaminants from the environment .

In a typical photocatalytic process, water or soil containing organic contaminants is exposed to sunlight in the presence of a catalyst such as titanium dioxide or humic and fulvic acids. The catalyst absorbs the high energy photons, and oxygen collects on the catalyst surface, resulting in the formation of reactive chemicals referred to as hydroxyl free-radicals and atomic oxygen (singlet oxygen). These reactive chemicals transform the organic contaminants into degradation products, such as carbon dioxide and water. Solar detoxification can be accomplished using natural sunlight or by using inside solar simulators or outside solar concentrators, both of which can concentrate light 20 times or more. The effectiveness of solar detoxification in both water and soil is affected by sorption of the toxic compounds on sediments or soil and the depth of light penetration. Solar detoxification of volatilized toxic compounds may also occur naturally in the atmosphere .

For solar detoxification to be successfully accomplished, toxic chemicals should be converted to thermodynamically stable, non-toxic end products. For example, chlorinated compounds should be transformed to carbon dioxide, water, and hydrochloric acid through the general sequence:

organic pollutants → aldehydes → carboxylic acids → carbon dioxide

Photocatalytic degradation such as this usually results in complete mineralization only after prolonged irradiation. If intermediates formed in the degradation pathway are nontoxic, however, the reaction does not have to be driven completely to carbon dioxide and water for acceptable detoxification to have occurred. In the solar detoxification of pentachlorophenol (PCP) and 2,4-dichlorophenol, for example, toxic intermediates were detected; however, with extended exposure to sunlight, the compounds were rendered completely nontoxic, as measured by respiration rate measurements in activated sludge .

Many applications of solar detoxification implemented at ambient temperatures have only 90–99% efficiency and do not completely mineralize the compounds. The rate of some photolytic reactions can be increased by raising the temperature of the reaction system, but the production of stable reaction intermediates (some of which may be toxic) is reduced. The changes in reaction rate have been attributed to a combination of a thermally induced increase in the photon-absorption rate, an increase in the quantum yield of the primary photoreaction, and the initiation of photoinduced radical-chain reactions.

Research conducted at the Solar Energy Research Institute (SERI), a laboratory funded by the U.S. Department of Energy , has demonstrated that chlorinated hydrocarbons , such as trichloroethylene (TCE), trichloroethane (TCA), and vinyl chloride , are vulnerable to photocatalytic treatment. Other toxic chemicals shown to be degraded by solar detoxification include a textile dye (Direct Red No. 79), pinkwater, a munitions production waste, and many types of pesticides, including chlorinated cyclodiene insecticides, triazines, ureas, and dinitroaniline herbicides. In conjunction with ozone and hydrogen peroxide, both of which are strong oxidants, ultraviolet light has been shown to be effective in oxidizing some refractory chemicals such as methyl ethyl ketone, a degreaser, and polychlorinated biphenyls (PCBs).

[Judith Sims ]

RESOURCES

BOOKS

Mill, T., and W. Mabey. "Photodegradation in Water." In Environmental Exposure from Chemicals, edited by W. B. Neely and G. E. Blau. CRC Press, Boca Raton, FL: 1985.

PERIODICALS

Al-Ekabi, H., et al. "Advanced Technology for Water Purification by Heterogenous Photocatalysis." International Journal of Environment and Pollution 1, nos. 1–2 (1991): 125–136.

Manilai, V. B., et al. "Photocatalytic Treatment of Toxic Organics in Wastewater: Toxicity of Photodegradation Products." Water Research 26, no. 8 (1992): 1035–1038.

Stephenson, F. A. "Chemical Oxidizers Treat Wastewater." Environmental Protection 3, no. 10 (1992): 23–27.

OTHER

Solar Energy Research Institute, Development and Communications Office. Solar Treatment of Contaminated Water. SERI/SP-220-3517. Goldon, CO: Solar Heat Research Division, Solar Energy Research Institute, 1989.