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The impact of bioremediation on PCB leachability
Polychlorinated biphenyls (PCBs) are common contaminants of concern due to their hazardous properties. These properties include: possible carcinogenicity; persistence in the environment; low water solubilities and slow biodegradation rates. It is commonly assumed that remediation of a PCB contaminated media will result in reductions of PCB leachability based on linear partitioning theory. However, it has been observed that a multitude of events, which take place during bioremediation, may alter the association of PCBs with the solid media leading to changes in the partitioning relationship. Because PCBs typically were discharged in oils and tend to remain partitioned in these oils due to their hydrophobicity, the relationship between non-aqueous phase liquids (NAPL) and PCBs is of particular interest. At the initiation of this study it was hypothesized that biodegradation of PCB contaminated media may result in higher PCB leachability from that media by two possible mechanisms. These mechanisms are: 1) degradation of the non-aqueous phase liquid (NAPL) at a higher rate than PCBs may result in increased PCB mole fraction in the NAPL matrix, which in turn causes increased PCB partitioning into the surrounding aqueous phase, based on Raoult's La w; and/or 2) during bioremediation, biological populations may produce biosurfactants to enhance the solubility of the non-aqueous phase liquid (NAPL), in their attempt to biodegrade it, which in turn, enhances PCB solubilization from the non-aqueous matrix to water. If this hypothesis is true, efforts to employ bioremediation may be counterproductive to managing the health risks associated with PCB contaminated sites. To evaluate this hypothesis, laboratory studies were conducted. The experiments included: time series of four month bioremediation studies and subsequent measurements of PCBs in both aqueous and solid phases, biological population growth (for PCB and oil degraders separately), tracking of biodegradation of the organic soil matrix (NAPL), biosurfactant accumulation, as well as other supplementary experiments. From the experimental studies conducted, it was found that aqueous phase PCB concentrations in the biodegradation reactors showed an increasing trend with bioremediation. During this same time, NAPL content of the soil was found to decrease and aqueous phase organics as measured by chemical oxygen demand and dissolved organic carbon were found to increase. Surface tension, used as a surrogate for surfactant concentration, showed a slight reduction during the experiments but was considerably higher than typically associated with surfactant-facilitated solubilization. Unexpectedly, soil phase total PCB concentrations were found to increase during the study. It was suspected that observed increases in soil phase PCB concentration were associated to mixing related dissagglomeration of the soil, which increased extraction efficiency. From the data it was not possible to conclude with certainty that biodegradation rates for NAPL or PCBs were significantly different as increases in PCBs were observed and the reduction in NAPL concentration corresponded closely with increased aqueous phase organic concentrations. However, it was concluded that increased aqueous phase PCB concentrations resulted from changes in PCB mole fractions in the NAPL, and that the oil matrix remained in the soil controlled the aqueous phase concentrations.