Modeling cis-Dichloroethene Partitioning in Tetrachloroethene-NAPL Using Intra-aggregate Diffusion Sorption Models

Microbial facilitated dechlorination is being employed for treatment and polishing of source zones containing chlorinated ethenes. Metabolic reductive dechlorination converts tetrachloroethene (PCE) and trichloroethene (TCE) to lesser chlorinated ethenes and ultimately ethene. Where metabolic reductive dechlorination occurs in proximity to nonaqueous phase liquid (NAPL), the process may enhance the rate of NAPL dissolution by increasing the concentration driving force. Accumulation of cis-dichloroethene (cis-DCE) due to incomplete dechlorination, however, is commonly noted in reports of both laboratory- and field-scale studies of microbially enhanced dissolution. There is growing evidence that cis-DCE may partition into NAPL present within the source zone, thereby, enriching the organic phase with cis-DCE. Partitioning of cis-DCE into the NAPL may create a persistent source of cis-DCE contamination. A series of batch and column laboratory experiments undertaken to investigate this phenomenon provide a data set to investigate the fundamental processes controlling cis-DCE partitioning in PCE-NAPL. Analytical solutions previously developed to model sorption due to intra-aggregate diffusion were employed here to simulate column data. These models include aqueous-phase and non- aqueous phase resistance to mass transfer. Simulation results suggest that cis-DCE partitioning is dominated by the rate of diffusion within the entrapped NAPL ganglia, indicating that intra-NAPL diffusion may be a controlling mechanism for dechlorination rates observed in DNAPL source zones. Further insights were gained through a sensitivity analysis employing these analytical models. Preliminary conclusions from this study suggest that these rate limitations should be incorporated into multiphase compositional simulators used to describe metabolic reductive dechlorination.