Structure-activity relationships for the degradation of a mixture of organic chemicals in soil

The degradation rates of a mixture of volatile and semi-volatile organic compounds (N - 16) were determined by monitoring the removal of parent chemicals from two different soil types, a Captina silt loam (Typic Fragiudult) and a McLaurin sandy loam (Typic Paleudults), over a 7-day period. The compounds were applied to the soil in a mixture such that the concentration of each individual chemical was 100 μ/g soil (dry weight). Soil samples, along with sterile (autoclaved) controls, were incubated in the dark at 20° C in 8 x 5-cm glass jars equipped with teflon-lined stoppers and charcoal traps. Samples were disassembled, extracted with methanol, and analyzed on days 0, 2, 3, 6, and 7 of the experiment. Linear regression analysis was used to model the disappearance of the compounds from the soils. First-order degradation rate constants and half-lives were calculated. Half-lives for the 16 organic compounds were relatively short (<23 days). Correlations between physicochemical parameters and half-lives were poor (r ≤ 0.63) for the entire data set (N - 16), however, a group of structurally related benzene derivatives (N - 7) gave good correlations in both soil types with log₁₀ octanol-water partition coefficient (r ≥ 0.89), log₁₀ molecular weight (r ≥ 0.84), log water solubility (r ≥ 0.77), and molecular connectivity (r ≥ 0.83). The present study illustrates the possibility of using structure-activity relationships for predicting the degradation of organic compounds in complex systems such as soil and provides data on the degradation of a mixture of volatile and semi-volatile organic compounds under defined experimental conditions.