A Half-Century Perspective on the Microbial Bioremediation of PCBs

Microbial biotransformation of PCBs was first observed in the field and in laboratory studies nearly five decades ago. Following years of investigation (on both aerobic biodegradation or mineralization and anaerobic dechlorination), bioremediation technologies are emerging for PCB-contaminated sediments. This perspective examines the arc of scientific and engineering developments that eventually brought us to today's approach of combining anaerobic and aerobic metabolism in an in situ bioaugmentation application. Although an understanding of aerobic biodegradation of lesser chlorinated biphenyls was developed early on, more extensively chlorinated congeners were found to be resistant to oxidative transformation. Major discoveries in the microbiology of anaerobic PCB dechlorination remained necessary, including understanding of pathways, PCB-dechlorinating bacteria, kinetics, and the interaction of biological processes with the physical environment of sediment-associated PCBs. Recent developments linking microbial kinetics to dissolved-phase PCBs and identification of factors that limited bacterial growth on PCBs are eliminating bottlenecks and have facilitated field translation. A focus in the last decade on scaling the growth of a PCB-dechlorinating bacterial species to a commercially viable level, and developments in field application methods for in situ bioaugmentation have facilitated multiple field demonstrations of PCB bioremediation. These field trials, in synergy with in situ remediation of sediments with activated carbon amendment, provide a path to achieve rapid reduction of PCB exposure to the ecosystem while significantly enhancing the rate of biodegradation.