Relative Contributions of Dehalobacter and Zerovalent Iron in the Degradation of Chlorinated Methanes

The role of bacteria and zerovalent iron (Fe0) in the degradation of chlorinated solvents in subsurface environments is of interest to researchers and remediation practitioners alike. Fe0 used in reactive iron barriers for groundwater remediation positively interacted with enrichment cultures containing Dehalobacter strains in the transformation of halogenated methanes. Chloroform transformation and dichloromethane formation was up to 8-fold faster and 14 times higher, respectively, when a Dehalobacter-containing enrichment culture was combined with Fe0 compared with Fe0 alone. The dichloromethane-fermenting culture transformed dichloromethane up to three times faster with Fe0 compared to without. Compound-specific isotope analysis was employed to compare abiotic and biotic chloroform and dichloromethane degradation. The isotope enrichment factor for the abiotic chloroform/Fe0 reaction was large at −29.4 ± 2.1‰, while that for chloroform respiration by Dehalobacter was minimal at −4.3 ± 0.45‰. The combined abiotic/biotic dechlorination was −8.3 ± 0.7‰, confirming the predominance of biotic dechlorination. The enrichment factor for dichloromethane fermentation was −15.5 ± 1.5‰; however, in the presence of Fe0 the factor increased to −23.5 ± 2.1‰, suggesting multiple mechanisms were contributing to dichloromethane degradation. Together the results show that chlorinated methane-metabolizing organisms introduced into reactive iron barriers can have a significant impact on trichloromethane and dichloromethane degradation and that compound-specific isotope analysis can be employed to distinguish between the biotic and abiotic reactions involved.