EFFECT OF DIFFUSION AND SORPTION ON THE KINETICS OF BIODEGRADATION - THEORETICAL CONSIDERATIONS

Organic chemicals subject to biodegradation by microorganisms in soil are often limited in their availability by sorption and diffusion to unavailable sites. To quantitatively describe biodegradation under conditions of substrate limitation, a process-based deterministic model called the diffusion-sorption-biodegradation (DSB) model was developed to describe the biodegradation of an organic chemical in the presence of spherical aggregates. Diffusion is described by Fick's second law, sorption by a linear isotherm, and biodegradation by one of various rate equations including first-order kinetics. Sensitivity analyses were performed to determine the dependence of a chemical's half-life on soil physical and chemical properties. Half-lives in the presence of aggregates were substantially greater than equivalent half-lives in the absence of aggregates when the sorption partition coefficient was greater-than-or-equal-to 3.2 dm3kg-1 and the aggregate radius was greater-than-or-equal-to 0.32 cm. The two-compartment model provided a better fit than did a simple first-order model to simulations of the biodegradation of a sorbing chemical in the presence of large (0.25-cm radii) aggregates, whereas there was no difference between the two models for aggregates of much smaller radii. When simple first-order kinetics was assumed without accounting for diffusion and sorption, long-term extrapolation of chemical persistence in the presence of aggregates overestimated the degradation of a chemical with a sorption partition coefficient of 10 dm3kg-1. Considering that many organic pollutants sorb strongly and numerous soils have some degree of aggregate structure and organic matter, taking into account the physical and chemical processes affecting a chemical's concentration may greatly improve kinetics models of biodegradation in soil.