Enrichment of anaerobic polychlorinated biphenyl dechlorinators from sediment with iron as a hydrogen source

Little is known about anaerobic polychlorinated biphenyl (PCB) dechlorination, although it is believed that some microorganisms are capable of respiring PCBs, gaining energy for growth from PCB dechlorination. If this is the case, the amendment of appropriate electron donors to contaminated sediment should stimulate dechlorination. The effect of elemental iron (Fe⁰) addition, an easily amended electron donor, on the microbial dechlorination of the PCB congeners 3,4,5-trichlorobiphenyl (3,4,5-CB) and 2,2′,3,4, 4′,5,5′-heptachlorobiphenyl (2,2′,3,4,4′,5,5′-CB) was investigated in microcosms containing estuarine sediment from Baltimore Harbor. Results showed that the addition of 0.1 g Fe⁰/g sediment reduced the lag time for removal of doubly flanked para chlorines by approximately 100 days. Because Fe⁰ is a source of cathodic hydrogen (H₂), the effect of direct H₂ addition to sediment microcosms was also tested. The addition of 0.001 atm H₂ in the headspace generated the same dechlorination activity and reduction in lag time as the addition of 0.1 g Fe⁰/g. Higher concentrations of Fe ⁰ or H₂ increased the lag prior to dechlorination. Additional results showed that an alkaline pH (≥7.5), high [Fe²⁺] (3.3 g/L), or HS^- (≥0.1 mg/L total sulfide) inhibited dechlorination. Elevated concentrations of Fe²⁺, OH^-, and HS^- are products of Fe⁰ oxidation or increased microbial activity (methanogenesis, homoacetogenesis, and sulfate reduction), both of which would result from the amendment of large quantities of Fe⁰ or H₂ to sediment. This research shows that not only can PCB dechlorination be stimulated through the addition of electron donor, but implies that the dechlorinators are enriched by the continuous addition of low concentrations of H₂, similar to other known dechlorinators, such as the dehalorespirer Dehalococcoides ethenogenes. These results suggest that the direct addition of controlled amounts of Fe⁰ to sediments may be an effective remediation tool to reduce the lag period prior to dechlorination at PCB-impacted sites. They also suggest that PCB dechlorinators may be enriched using techniques similar to those used with known dehalorespirers.