Microbe-level investigation of sequential anaerobic/aerobic bioremediation of polychlorinated biphenyls in soil

Environmental polychlorinated biphenyl (PCB) contamination is often comprised of mixtures of congeners with a range of degree of chlorination. Complete biodegradation of PCB mixtures is rarely achieved, since highly chlorinated PCBs are not metabolized under aerobic conditions and less chlorinated congeners are not degraded by anaerobic microorganisms. Sequential anaerobic-aerobic treatment for PCB contamination is promising, yet little is known about specific parameters important for its successful outcome. Laboratory-scale, PCB-spiked, soil slurry microcosms were used to investigate the effects of engineering parameters on PCB biodegradation and the microbial community.; In microcosms incubated anaerobically, dechlorination of several hexachlorobiphenyls to penta- and tetrachlorobiphenyls by indigenous organisms was detected in soil microcosms within 12 weeks. Biphenyl-amended microcosms achieved the highest extent of dechlorination with 22.5% dechlorination achieved over 60-wk. Use of a denaturing gradient gel electrophoresis method combined with sequencing showed that most of the cloned sequence types were related to the 16S rRNA sequences of aerobic species, and several to acidiphilic bacteria. In addition, several sequence types were similar to known microorganisms involved in either iron reduction or iron oxidation.; Both 2,3',4'-PCB and biphenyl were completely degraded in aerobic microcosms in 7 wk. Microcosms not amended with biphenyl showed decreased degradation rates compared with those amended with biphenyl. Although total biomass remained constant in all treatments, the anaerobic microbial community decreased in size and there was a subsequent increase in aerobic lipid biomarkers in biphenyl-amended microcosms.; The sequential treatment process was more effective for overall PCB removal than either the anaerobic or aerobic processes alone. A total reduction in PCB mass of 57.5% in sequential microcosms was observed versus 27% and 25% for anaerobic and aerobic microcosms alone. Because the soil was aerobic at the onset of the experimentation, highly reducing conditions were never obtained. The less-reducing conditions of the anaerobic process allowed the aerobic microorganisms to survive the 60 wk and rebound to degrade the less chlorinated congeners produced.