Carbon isotope fractionation in the reductive dehalogenation of carbon tetrachloride at iron (hydr)oxide and iron sulfide minerals

Compound-specific isotope analysis (CSIA) is used increasingly in contaminant hydrology in the attempt to assess the nature as well as the extent of in situ transformation reactions. Potentially, variations of stable isotope ratios along a contaminant plume may be used to quantify in situ degradation. In the present study, the abiotic dehalogenation of CCl₄ by Fe(II) present at the surface of different iron minerals has been characterized in terms of the reaction rates and carbon isotopic fractionation (δ¹³C) of carbon tetrachloride (CCl₄) as well as the yields and isotopic signatures of chloroform (CHCl₃), one of the main transformation products. The abiotic reductive dehalogenation of CCl ₄ was associated with substantial carbon isotopic enrichment effects. The observed enrichment factors, ε, correlated neither with the surface-normalized reaction rate constants nor with the type of products formed but fell into two distinctly different ranges for the two principal groups of minerals studied. With iron (hydr)oxide minerals (goethite, hematite, lepidocrocite, and magnetite) and with siderite, the ε-values for CCl ₄ dehalogenation were remarkably similar (-29 ± 3‰). Because this value matches well with the theoretical estimates for the cleavage of an aliphatic C-Cl bond, we suggest that dissociative electron transfer to CCI4 controls the reaction rates for this group of iron minerals. Conversely, CCl₄ transformation by different preparations of the iron sulfide mackinawite was accompanied by a significantly lower carbon istotopic fractionation (ε = -15.9 ± 0.3‰), possibly due to the presence of nonfractionating rate-determining steps or a significantly different transition state structure of the reaction. Isotopically sensitive branching of the reaction pathways (i.e., the effect of different product distributions on isotope fractionation of CCl₄) did not play a significant role in our systems. The extensive data set presented in this study opens new perspectives toward an improved understanding of the factors that determine reaction mechanisms and isotopic fractionation of dehalogenation reactions by Fe(II) at iron containing minerals.