Soils and Mineral Boundaries

Phototransformations in the presence of soils and clays have been reported by several authors. The penetration of light into soils and sediments is greatly inhibited below the first 0.2 mm or so (Hebert and Miller, 1989), but often surface photoreactions can proceed at significant rates. For example, sediment-sorbed DDE (6) photo-lyzes faster than when it is dissolved, and the product mixture differs in a way that suggests an environment rich in H-donors, probably organic matter (Miller and Zepp, 1979 Zafiriou et al., 1984). Parathion (7) has been shown to be rapidly photolyzed on the surfaces of dust and soil particles to paraoxon (8), a more toxic substance (Spencer et al., 1980). The pyrethroid insecticide fenpropathrin (9) was rapidly photolyzed (ti/j about 1 day) on soils high in organic carbon (Takahashi et al., 1985). 

The photophysics and photochemistry of organic molecules adsorbed on some mineral surfaces have been reviewed by Oelkrug et al. (1986). The mobility of adsorbed substrates is usually restricted, and therefore photochemical transformations that require bond shifts or group rotations may be impeded on the other hand, if concentrations are high enough the closer proximity of adsorbed molecules may 

Solid squares show concentrations in untreated controls. From Draper and Crosby (1987). Reprinted by permission. Copyright 1987 American Chemical Society. 

and even mineralize many organic compounds all the way to CO2 in fair-to-excellent yields (Matthews, 1986 Ollis et al., 1991). 

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