Kinetics of aerobic cometabolism of chlorinated solvents

The objectives of this paper are to review the wide range of kinetic models that have been introduced to describe the cometabolic oxidation of chlorinated solvents, to compare modeling approaches and associated experimental data, and to discuss knowledge gaps in the general topic of cometabolism kinetics. To begin, a brief description of the mechanism of oxygenase enzyme metabolism and its qualitative effects on cometabolic degradation kinetics is given. Next, a variety of kinetic expressions that have been used to describe cometabolism, ranging from adaptations of simple metabolic relationships to the development of complex equations that account for intracellular concentrations of key reaction species, are presented. A large number of kinetic coefficients published for a variety of oxygenase populations degrading a broad range of chlorinated solvents are categorized and compared. The discussion section of the paper contains an exploration of knowledge gaps that exist in our understanding of the kinetics of aerobic chlorinated solvent cometabolism. Specific topics covered include: the use of half saturation constants (Ksc and Ksg) as estimates for inhibition constants (Kisc and Kisg) in saturation modeling expressions, the specific nature of chlorinated solvent induced product toxicity and the capability for cells to recover from toxic effects, and methods for incorporating reducing energy limitations into cometabolism models. Finally, the applicability of the broad range of kinetic modeling approaches to scale-up and field applications for in situ bioremediation of chlorinated solvents is discussed.