Modeling the dynamics of coupled sorption and anaerobic biodegradation kinetics of trichloroethene on colloidal activated carbon.

Colloidal Activated Carbon (CAC) coupled with bioaugmentation has been increasingly used in Permeable Reactive Barriers (PRBs) to remediate chlorinated solvent plumes. A linear isotherm utilizing partition coefficient Kd is often used in contaminant fate and transport modeling to simplify the coupled sorption and biodegradation. However, strong sorption of trichloroethene (TCE) on CAC is highly non-linear, and the dynamic interaction between nonlinear sorption, desorption, and anaerobic biodegradation remains insufficiently understood. Moreover, some studies have suggested that biodegradation alone is rate-limiting and sorption can be considered at equilibrium. This proposition has not been verified with mathematical models that couple biodegradation with non-linear sorption of TCE on CAC. This study evaluated whether biodegradation, rather than TCE desorption from CAC, controls the overall removal rate under well-mixed batch conditions. Specifically, this study combined the model parameters obtained from independent batch experiments of abiotic TCE sorption and desorption on CAC with anaerobic biodegradation by a dechlorinating consortium in absence of CAC and applied them to a coupled Freundlich-Michaelis-Menten model to assess the goodness of fit to the experimental data for biodegradation in presence of CAC. TCE sorption was non-linear (Freundlich n = 0.39), and more than 80% of the desorbed mass was released within 10 h, substantially faster than the multi-day biodegradation response. The apparent Michaelis-Menten parameters were KM = 0.27 μM and μmax = 23.84 μM*day-1, however KM was not identifiable within the tested concentration range. The coupled equilibrium-sorption model explained the biodegradation data in CAC microcosms with R2 = 0.91. These results support biodegradation, rather than desorption, as the dominant rate-limiting process under the well-mixed, nine-day batch conditions. Modeling the interaction between non-linear sorption and biodegradation will allow practitioners to more realistically and accurately predict PRB performance. However, careful considerations are required to translate batch system results to field-scale PRBs. NOVELTY STATEMENT: Colloidal Activated Carbon demonstrated an equilibrium non-linear sorption in a coupled sorption and biodegradation rate experiment. This is the first study to verify the rate-limiting hypothesis using a coupled non-linear sorption-biodegradation model for CAC-TCE systems.