Quantification of Hexavalent Cr Reduction in Groundwater Using Cr Stable Isotopes

该项目将开发铬（Cr）稳定同位素比率，作为了解这种有毒元素如何在受污染的地下水中移动的新工具。铬污染很普遍，因为电镀、皮革制革和木材处理行业使用大量高浓度的六价铬溶液。铬（VI）作为一种高度可溶的氧阴离子很容易随地下水移动，但在许多地下水系统中，它经过化学还原成铬（III），后者的可溶性较低，毒性较低。该小组的初步工作表明，这种还原反应涉及稳定同位素53Cr和52Cr的分馏。还原反应的产物相对于反应物富集较轻的同位素52Cr，随着还原的进行，剩余未还原的Cr（VI）相对于其起始成分逐渐富集较重的同位素。本项目的工作假设是，地下水中Cr（VI）的53Cr/52Cr比值提供了Cr（VI）减少量的定量指标。这个项目解决了在以前的工作中提出的几个基本问题，在这种新方法能够在实际应用中定量和有信心地使用之前，必须回答这些问题。主要目标是1)评估Cr（VI）还原过程中同位素分馏大小的可变性；2)确定由Cr（VI）吸附和Cr（III）氧化诱导的同位素分馏（如果有的话）；3)评估污染源中53Cr/52Cr的可变性。这项工作将包括在实验室进行几项实验，分析来自铬（VI）污染羽流的地下水样本，以及分析工业和天然铬。这项以实验室为基础的研究将得到其他来源资助的实地研究的补充。它将作为三个独立的项目进行，研究生将接受环境地球化学、水文地质学和同位素地球化学方面的培训。重点是地下水污染，但这里学到的大部分知识可以应用于地球化学和海洋学的其他领域。

0229079JohnsonThis project will develop chromium (Cr) stable isotope ratios as a new tool for understanding how this toxic element moves in contaminated groundwater. Cr contamination is widespread, because the electroplating, leather tanning, and wood treatment industries use large volumes of highly concentrated Cr(VI) (hexavalent Cr) solutions. Cr(VI) moves readily with groundwater as a highly soluble oxyanion, but in many groundwater systems, it undergoes chemical reduction to Cr(III), which is less soluble and less toxic. Preliminary work by this group has demonstrated that this reduction reaction involves a fractionation of the stable isotopes 53Cr and 52Cr. The products of the reduction reaction are enriched in 52Cr, the lighter isotope, relative to the reactants, and as reduction proceeds, the remaining unreduced Cr(VI) becomes progressively more enriched in the heavier isotope relative to its starting composition. The working hypothesis of this project is that the 53Cr/52Cr ratio of the Cr(VI) in groundwater provides a quantitative indication of the amount of Cr(VI) reduction that has occurred. This project addresses several basic questions raised in previous work that must be answered before this new approach can be used quantitatively and with confidence in practical applications. The primary objectives are 1) to assess variability in the size of the isotopic fractionation during Cr(VI) reduction, 2) determine the isotope fractionations, if any, that are induced by Cr(VI) sorption and Cr(III) oxidation, and 3) assess 53Cr/52Cr variability among contaminant sources. The work will involve several experiments in the laboratory, analysis of groundwater samples from Cr(VI) contamination plumes, and analysis of industrial and natural Cr. This laboratory-based study will be complemented by a field study to be funded from other sources. It will be carried out as three separate projects for graduate students, who will be trained in environmental geochemistry, hydrogeology, and isotope geochemistry. The focus is on groundwater contamination, but most of what is learned here may be applied to other areas of geochemistry and oceanography.