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 contami nated with PCBs and hydraulic oils, Untreated and biotreated sludge-so il samples were obtained from a two-month, pilot-scale field test of P CB biodegradation in land treatment units. Column leachate comprised m ainly di-and trichlorobiphenyls, In comparison to untreated samples, c olumn leachate from biotreated samples showed 34 to 63% reduction in P CBs; average values of total PCB concentrations in the column effluent ranged from 0.36 to 0.82 mu gl(-1) for untreated samples and 0.25 to 0.30 mu gl(-1) for biotreated samples. Only a small fraction, less tha n 6%, of the PCB mass in the untreated sludge-soil mixtures was remove d by prolonged flushing with up to 2400-3000 pore volumes of water, su ggesting long-term, slow release of PCBs at low aqueous concentrations . It should be noted that flushing rates for the column experiments we re much higher than typical ground-water flow rates and the two-month period for column experiments can be qualitatively compared to a great ly extended duration for ground-water flow. The leaching of PCBs from a nonaqueous phase liquid (NAPL) in the column experiments was modelle d as a predominantly dissolution-governed process based on first-order kinetics. The estimated dissolution rate coefficients ranged from 0.0 2 to 0.04 min(-1) for various PCB homolog groups. This investigation h elps support the concept of biostabilization, in which contaminated ma terials may be actively biotreated to remove the potentially mobile or ganic contaminants leaving a residual material that is more stable aga inst leaching. Natural attenuation processes like sorption and intrins ic biodegradation further reduce the PCB leachability potential. Addit ional long-term monitoring of the land treatment units under passive c onditions is under progress to evaluate such reductions. (C) 1997 Else vier Science B.V.