Dual Mechanism Conceptual Model for Cr Isotope Fractionation during Reduction by Zerovalent Iron under Saturated Flow Conditions

摘要

Chromium isotope analysis is rapidly becoming a valuable complementary tool for tracking Cr(Ⅵ) treatment in groundwater. Evaluation of various treatment materials has demonstrated that the degree of isotope fractionation is a function of the reaction mechanism, where reduction of Cr(Ⅵ) to Cr(Ⅲ) induces the largest fractionation. However, it has also been observed that uniform flow conditions can contribute complexity to isotope measurements. Here, laboratory batch and column experiments were conducted to assess Cr isotope fractionation during Cr(Ⅵ) reduction by zerovalent iron under both static and saturated flow conditions. Isotope measurements were accompanied by traditional aqueous geochemical measurements (pH, Eh, concentrations) and solid-phase analysis by scanning electron microscopy and X-ray absorption spectroscopy. Increasing δ~(53)Cr values were associated with decreasing Cr(Ⅵ) concentrations, which indicates reduction; solid-phase analysis showed an accumulation of Cr(Ⅲ) on the iron. Reactive transport modeling implemented a dual mechanism approach to simulate the fractionation observed in the experiments. The faster heterogeneous reaction pathway was associated with minimal fractionation (e = -0.2‰), while the slower homogeneous pathway exhibited a greater degree of fractionation (ε = -0.9‰ for the batch experiment, and e = - 1.5‰ for the column experiment).