Case Study in Environmental Chemistry....

Authors: Sarunya Hengpraprom and Cindy Lee, Environmental Engineering and Science, Clemson University.

Abstract : This case study considers the rate at which exposure to sunlight can transform a pesticide known as terbufos. Transformation of a contaminant by photolysis will change its behavior in a given situation. In some cases, a product of photolysis is just as toxic or more toxic than the parent compound. In other cases, the products of photolysis are rendered nontoxic and the transformation can be considered beneficial. The experiment described in this case study produced some of the first basic kinetics data available to evaluate the importance of photolysis as a process to remove terbufos from the environment. For more detailed information about this research, see Lee, C. M.; Anderson, B.; and Elzerman, A. W. 1999. Photochemical oxidation of terbufos. Environmental Toxicology and Chemistry. 18(7):1349-1353.

Transport and Transformation of Terbufos

Terbufos is released to the environment through its use as an insecticide and less commonly by accidental spills. If released to soil, terbufos is expected to be only slightly mobile, and therefore very little leaching or transport in soil is expected. In soil environment, terbufos undergoes rapid biological degradation and produces the oxidative by-products (terbufos sulfoxide and sulfone) in a step-wise manner (9). Photolysis in surface waters and in soils would be expected to also produce terbufos sulfoxide and sulfone, however, little published information is available. In aqueous media, terbufos predominately undergoes chemical degradation (hydrolysis) rather than microbial degradation. Formaldehyde was reported as a major hydrolysis product (9). The combined biological and chemical degradation half-lives in various soils plus the hydrolysis half-lives in the aqueous phase are summarized in Table 3. Additionally, terbufos is expected to degrade in the ambient atmosphere quickly by reaction with photochemically produced hydroxyl radicals. Terbufos is reported to have a half-life of 0.004 day in the atmosphere (7). Bioconcentration in aquatic organisms and adsorption to sediments are expected to be other important fate processes for terbufos (11). Numerous studies have been reported on hydrolysis and biological reactions; on the other hand, photolysis reactions have been less examined. Therefore, photolysis was selected as a potentially significant transformation process that should be investigated.