Liquid-liquid mass transfer of partitioning electron donors in chlorinated solvent source zones

A combination of batch and column experiments evaluated the mass transfer of two candidate partitioning electron donors (PEDs), n-hexanol (nHex) and n-butyl acetate (nBA), for enhanced bioremediation of trichloroethene (TCE)-dense nonaqueous phase liquid (DNAPL). Completely mixed batch reactor experiments yielded equilibrium TCE-DNAPL and water partition coefficients (KNW) for nHex and nBA of 21.7 ± 0.27 and 330.43 ± 6.7, respectively, over a range of initial PED concentrations up to the aqueous solubility limit of ca. 5000 mg/L. First-order liquid-liquid mass transfer rates determined in batch reactors with nBA or nHex concentrations near the aqueous solubility were 0.22 min(-1) and 0.11 min(-1), respectively. Liquid-liquid mass transfer under dynamic flow conditions was assessed in one-dimensional (1-D) abiotic columns packed with Federal Fine Ottawa sand containing a uniform distribution of residual TCE-DNAPL. Following pulse injection of PED solutions at pore-water velocities (vp) ranging from 1.2 to 6.0 m/day, effluent concentration measurements demonstrated that both nHex and nBA partitioned strongly into residual TCE-DNAPL with maximum effluent levels not exceeding 35% and 7%, respectively, of the applied concentrations of 4000 to 5000 mg/L. PEDs persisted at effluent concentrations above 5 mg/L for up to 16 and 80 pore volumes for nHex and nBA, respectively. Mathematical simulations yielded KNW values ranging from 44.7 to 48.2 and 247 to 291 and liquid-liquid mass transfer rates of 0.01 to 0.03 min(-1) and 0.001 to 0.006 min(-1) for nHex and nBA, respectively. The observed TCE-DNAPL and water mass transfer behavior suggests that a single PED injection can persist in a treated source zone for prolonged time periods, thereby reducing the need for, or frequency of, repeated electron donor injections to support bacteria that derive reducing equivalents for TCE reductive dechlorination from PED fermentation.