Relative Contributions of <i>Dehalobacter</i> and Zerovalent Iron in the Degradation of Chlorinated Methanes

The role of bacteria and zerovalent iron (Fe<sup>0</sup>) in the degradation of chlorinated solvents in subsurface environments is of interest to researchers and remediation practitioners alike. Fe<sup>0</sup> used in reactive iron barriers for groundwater remediation positively interacted with enrichment cultures containing <i>Dehalobacter</i> 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 <i>Dehalobacter</i>-containing enrichment culture was combined with Fe<sup>0</sup> compared with Fe<sup>0</sup> alone. The dichloromethane-fermenting culture transformed dichloromethane up to three times faster with Fe<sup>0</sup> 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/Fe<sup>0</sup> reaction was large at −29.4 ± 2.1‰, while that for chloroform respiration by <i>Dehalobacter</i> 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 Fe<sup>0</sup> 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.