Lactate injection by electric currents for bioremediation of tetrachloroethylene in clay

Biological transformation of tetrachloroethylene (PCE) in silty clay samples by ionic injection of lactate under electric fields is evaluated. To prepare contaminated samples, a silty clay slurry was mixed with PCE, inoculated with KB-1® dechlorinators and was consolidated in a 40 cm long cell. A current density between 5.3 and 13.3 A m−2 was applied across treated soil samples while circulating electrolytes containing 10 mg L−1 lactate concentration between the anode and cathode compartments to maintain neutral pH and chemically reducing boundary conditions. The total adsorbed and aqueous PCE was degraded in the soil to trichloroethylene (TCE), cis-1,2-dichloroethene (cis-DCE), vinyl chloride (VC) and ethene in 120 d, which is about double the time expected for transformation. Lactate was delivered into the soil by a reactive transport rate of 3.7 cm2 d−1 V−1. PCE degradation in the clay samples followed zero order transformation rates ranging from 1.5 to 5 mg L−1 d−1 without any significant formation of TCE. cis-DCE transformation followed first order transformation rates of 0.06–0.10 per day. A control experiment conducted with KB-1 and lactate, but without electricity did not show any significant lactate buildup or cis-DCE transformation because the soil was practically impermeable (hydraulic conductivity of 2 × 10−7 cm s−1). It is concluded that ionic migration will deliver organic additives and induce biological activity and complete PCE transformation in clay, even though the transformation occurs under slower rates compared to ideal conditions.