†Department of Earth and Environmental Sciences, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada.
‡Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada.
Environ Sci Technol. 2015 May 5;49(9):5467-75. doi: 10.1021/es506223a. Epub 2015 Apr 15.
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 (ε=-0.2‰), while the slower homogeneous pathway exhibited a greater degree of fractionation (ε=-0.9‰ for the batch experiment, and ε=-1.5‰ for the column experiment).
铬同位素分析正迅速成为追踪地下水六价铬处理的一种有价值的辅助工具。对各种处理材料的评估表明，同位素分馏程度是反应机制的函数，其中六价铬向三价铬的还原会引起最大的分馏。然而，人们也观察到，均匀流条件会给同位素测量带来复杂性。在这里，进行了实验室批处理和柱实验，以评估在静态和饱和流动条件下零价铁还原六价铬过程中的铬同位素分馏。同位素测量伴随着传统的水地球化学测量（pH、Eh、浓度）和通过扫描电子显微镜和 X 射线吸收光谱进行的固相分析。δ(53)Cr 值的增加与 Cr(VI)浓度的降低相关，这表明发生了还原；固相分析表明铁上积累了 Cr(III)。反应传输模型采用双机制方法模拟实验中观察到的分馏。更快的非均相反应途径与最小的分馏（ε=-0.2‰）相关，而较慢的均相途径表现出更大程度的分馏（批处理实验中为 ε=-0.9‰，柱实验中为 ε=-1.5‰）。
