Approximating the impact of sorption on biodegradation kinetics in soil-water systems

Naphthalene sorption and biodegradation were quantified in three soils of varying organic C content using miscible displacement and batch incubation techniques. As anticipated, sorption increased with organic C content; the sorption rate coefficient and equilibrium sorption constant (K-d) were inversely related, Organic matter in solution (NOM) from the high-organic-matter soil decreased apparent sorption, which was also quantified using methanol as a cosolvent, A cosolvency power of 3.8 or 4.1 was determined, The initial biodegradation rate decreased with increasing sorption and was a function of naphthalene concentration in solution, The extent of biodegradation was greatest in the high-organic-matter soil and decreased when NOM was removed from the system, The observed dependence of biodegradation kinetics on sorption (this study and in the literature) prompted us to develop a simple approach for approximating a biodegradation rate constant for soil-water systems, This approach combines independently determined sorption parameters with the biodegradation rate constant determined for aqueous (soil-free) solution, The approach was applied to our data and to published data where sorption was shown to control naphthalene biodegradation, The approximated biodegradation rate constant was within 10% of the measured values for three cases, 20% for a fourth case, and was underestimated by five- to sevenfold for the high-organic-matter soil, The approach will be useful when applying management models for predicting contaminant fate and transport that require a degradation rate constant, or for estimating biodegradation rates in situations where cultured organisms are introduced for biodegradation.