Leaching of PCB compounds from untreated and biotreated sludge-soil mixtures

Column experiments were conducted to evaluate the aqueous leachability of polychlorinated biphenyls (PCBs) from sludge-soil mixtures contaminated with PCBs and hydraulic oils. Untreated and biotreated sludge-soil samples were obtained from a two-month, pilot-scale field test of PCB biodegradation in land treatment units. Column leachate comprised mainly di- and trichlorobiphenyls. In comparison to untreated samples, column leachate from biotreated samples showed 34 to 63% reduction in PCBs; average values of total PCB concentrations in the column effluent ranged from 0.36 to 0.82 μg 1−1 for untreated samples and 0.25 to 0.30 μg 1−1 for biotreated samples. Only a small fraction, less than 6%, of the PCB mass in the untreated sludge-soil mixtures was removed by prolonged flushing with up to 2400–3000 pore volumes of water, suggesting long-term, slow release of PCBs at low aqueous concentrations. It should be noted that flushing rates for the column experiments were much higher than typical ground-water flow rates and the two-month period for column experiments can be qualitatively compared to a greatly extended duration for ground-water flow. The leaching of PCBs from a nonaqueous phase liquid (NAPL) in the column experiments was modelled as a predominantly dissolution-governed process based on first-order kinetics. The estimated dissolution rate coefficients ranged from 0.02 to 0.04 min−1 for various PCB homolog groups. This investigation helps support the concept of biostabilization, in which contaminated materials may be actively biotreated to remove the potentially mobile organic contaminants leaving a residual material that is more stable against leaching. Natural attenuation processes like sorption and intrinsic biodegradation further reduce the PCB leachability potential. Additional long-term monitoring of the land treatment units under passive conditions is under progress to evaluate such reductions.