| Abstract
| - Compound-specific isotope analysis (CSIA) is usedincreasingly in contaminant hydrology in the attempt toassess the nature as well as the extent of in situ transformationreactions. Potentially, variations of stable isotope ratiosalong a contaminant plume may be used to quantify in situdegradation. In the present study, the abiotic dehalogenationof CCl4 by Fe(II) present at the surface of different ironminerals has been characterized in terms of the reactionrates and carbon isotopic fractionation (δ13C) of carbontetrachloride (CCl4) as well as the yields and isotopic signaturesof chloroform (CHCl3), one of the main transformationproducts. The abiotic reductive dehalogenation of CCl4was associated with substantial carbon isotopic enrichmenteffects. The observed enrichment factors, ε, correlatedneither with the surface-normalized reaction rate constantsnor with the type of products formed but fell into twodistinctly different ranges for the two principal groups ofminerals studied. With iron (hydr)oxide minerals (goethite,hematite, lepidocrocite, and magnetite) and with siderite,the ε-values for CCl4 dehalogenation were remarkably similar(−29 ± 3‰). Because this value matches well with thetheoretical estimates for the cleavage of an aliphatic C−Clbond, we suggest that dissociative electron transfer toCCl4 controls the reaction rates for this group of iron minerals.Conversely, CCl4 transformation by different preparationsof the iron sulfide mackinawite was accompanied by asignificantly lower carbon istotopic fractionation (ε = −15.9± 0.3‰), possibly due to the presence of nonfractionatingrate-determining steps or a significantly different transitionstate structure of the reaction. Isotopically sensitive branchingof the reaction pathways (i.e., the effect of differentproduct distributions on isotope fractionation of CCl4) didnot play a significant role in our systems. The extensive dataset presented in this study opens new perspectivestoward an improved understanding of the factors thatdetermine reaction mechanisms and isotopic fractionationof dehalogenation reactions by Fe(II) at iron containingminerals.
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