Department of Earth and Environmental Sciences, University of Waterloo, 200 University Ave W, Waterloo, ON N2L 3G1, Canada.
Environ Sci Technol. 2012 Jun 19;46(12):6783-9. doi: 10.1021/es2042383. Epub 2012 Jun 7.
Chromium isotopes are potentially useful indicators of Cr(VI) reduction reactions in groundwater flow systems; however, the influence of transport on Cr isotope fractionation has not been fully examined. Laboratory batch and column experiments were conducted to evaluate isotopic fractionation of Cr during Cr(VI) reduction under both static and controlled flow conditions. Organic carbon was used to reduce Cr(VI) in simulated groundwater containing 20 mg L(-1) Cr(VI) in both batch and column experiments. Isotope measurements were performed on dissolved Cr on samples from the batch experiments, and on effluent and profile samples from the column experiment. Analysis of the residual solid-phase materials by scanning electron microscopy (SEM) and by X-ray absorption near edge structure (XANES) spectroscopy confirmed association of Cr(III) with organic carbon in the column solids. Decreases in dissolved Cr(VI) concentrations were coupled with increases in δ(53)Cr, indicating that Cr isotope enrichment occurred during reduction of Cr(VI). The δ(53)Cr data from the column experiment was fit by linear regression yielding a fractionation factor (α) of 0.9979, whereas the batch experiments exhibited Rayleigh-type isotope fractionation (α = 0.9965). The linear characteristic of the column δ(53)Cr data may reflect the contribution of transport on Cr isotope fractionation.
铬同位素在地下水流动系统中 Cr(VI)还原反应的示踪中具有潜在的应用价值；然而，传输过程对 Cr 同位素分馏的影响尚未得到充分研究。本研究通过实验室批式和柱实验，评估了静态和控制流动条件下 Cr(VI)还原过程中 Cr 的同位素分馏。在批式和柱实验中，利用有机碳还原模拟地下水中 20mg/L 的 Cr(VI)。在批式实验中，对溶解态 Cr 样品进行同位素测量，对柱实验的流出物和剖面样品进行同位素测量。通过扫描电子显微镜（SEM）和 X 射线吸收近边结构（XANES）光谱分析残余固相材料，证实了 Cr(III)与柱固相中的有机碳有关。溶解态 Cr(VI)浓度的降低与 δ(53)Cr 的增加有关，表明 Cr(VI)还原过程中 Cr 同位素发生了富集。通过线性回归拟合柱实验的 δ(53)Cr 数据，得出分馏因子（α）为 0.9979，而批式实验则表现出瑞利型同位素分馏（α=0.9965）。柱中 δ(53)Cr 数据的线性特征可能反映了传输过程对 Cr 同位素分馏的影响。
