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首页> 外文学位 >Using chromium stable isotopes to monitor chromium reactive transport: Oxidation experiments and field studies.
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Using chromium stable isotopes to monitor chromium reactive transport: Oxidation experiments and field studies.

机译：使用铬稳定同位素监测铬的反应性运输：氧化实验和现场研究。

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摘要

Chromium (Cr) is a common contaminant found at a number of sites globally. Cr sources can be either anthropogenic or natural. Cr occurs in nature in two valences, with Cr(VI) being carcinogenic, more toxic and mobile than Cr(III). Reduction of Cr(VI) to Cr(III) is a common remediation strategy, but monitoring Cr reduction using only concentration and speciation analyses is a difficult endeavor because concentrations can be affected by dilution and advection in addition to reduction. Cr isotopes are fractionated during reduction processes, so monitoring isotopic changes in Cr may be a faster and more effective way to monitor reduction. This research involved both measuring Cr isotopic fractionation caused by Cr(III) oxidation and a field study in an area of well documented Cr contamination problem.;Back reaction of Cr(III) to Cr(VI) is possible under some conditions and we need to be able to characterize that reaction when monitoring Cr(VI) contamination. Previous work measured isotopic fractionation factors for Cr(VI) reduction and through this study we now have fractionation factors for Cr(III) oxidation. Cr(III) oxidation experiments using pyrolusite (beta-MnO 2) at pH 3 and 4 were performed. Cr(VI) formed in the first stages of the experiments is enriched in delta53Cr by 1.1‰, and is independent of pH or matrix or electrolyte used. This enrichment in the Cr(VI) product relative to the reactant cannot be explained by a simple kinetic isotopic effect, because during kinetic interactions one would expect the product to become enriched in the lighter Cr isotopes relative to the reactant. Moreover, the results do not fit a Rayleigh distillation curve behavior. We also suspect that this is not a reflection of isotopic equilibrium, as that should cause stronger fractionation. Seemingly a combination of equilibrium and kinetic isotopic effects govern the multistep oxidation reaction.;The second part of the study was conducted in Leon valley, Guanajuato, Mexico. In 1975, high Cr concentrations were detected in groundwater. Previous work proposed an anthropogenic origin for a high concentration plume near the Quimica Central (QC) factory located in Buenavista (BV), while the larger but less contaminated Cr plume located near San Juan de Otates (SJO) is assumed to be caused by weathering of ultramafic rocks. In this study surface and groundwater samples collected in 2007 and 2008 in both areas showed enriched isotopic values ranging from +0.124‰ to +1.40‰ near QC and +0.749‰ to +2.853‰ in San Juan de Otates. The samples suggested a decrease in concentrations of Cr(VI). In BV concentrations ranged from 0.005 mg/l and 121 mg/l for surface and groundwater in 2007 to 0.002 mg/l and 95.1 mg/l for surface and groundwater samples in 2008. In 2007 Cr(VI) concentrations in San Juan de Otates ranged from 0.01 mg/l to 0.012 mg/l in surface and from 0.01 mg/l to 0.016 mg/l in groundwater, while in 2008 Cr(VI) concentrations were 0.001 mg/l in surface water.;Fractionation in Cr isotopes measured in leaching experiments from the waste material piles and delta53Cr values from groundwater, suggests that: (1) Cr in the waste piles was fractionated, during "weathering", (2) the decrease in delta53Cr values is caused by an unknown process, and/or (3) indicates the presence of new Cr sources with unique Cr isotopic compositions. Cr (VI) concentrations and Cr isotopes were also measured in a nearby landfill. Low Cr(VI) concentrations and enriched delta 53Cr values suggest that reduction is occurring in the landfill. Additional sampling suggests that weathering of ultramafic rocks in San Juan de Otates outcrop resulted in Cr(VI) concentrations in surface and groundwater below USEPA and Mexican Maximum Permissible Contaminant Levels in 2007 and 2008.
机译：铬（Cr）是全球许多站点发现的常见污染物。铬源可以是人为的或天然的。天然存在六价铬，六价铬比六价铬具有致癌性，毒性和移动性。 Cr（VI）还原为Cr（III）是一种常见的修复策略，但是仅使用浓度和形态分析来监测Cr的还原是一项艰巨的工作，因为除还原外，浓度还会受到稀释和对流的影响。 Cr同位素在还原过程中被分离，因此监测Cr的同位素变化可能是监测还原的更快，更有效的方法。这项研究涉及测量由Cr（III）氧化引起的Cr同位素分馏，以及在一个有据可查的Cr污染问题的领域中进行的现场研究。;在某些条件下Cr（III）可以反应成Cr（VI），我们需要在监测Cr（VI）污染时能够表征该反应。以前的工作测量了Cr（VI）还原的同位素分馏因子，通过这项研究，我们现在有了Cr（III）氧化的分馏因子。使用pH值为3和4的软锰矿（β-MnO2）进行Cr（III）氧化实验。在实验的第一阶段形成的Cr（VI）在delta53Cr中富集了1.1‰，并且与所用的pH值或基质或电解质无关。相对于反应物而言，Cr（VI）产物的这种富集无法通过简单的动力学同位素效应来解释，因为在动力学相互作用期间，人们期望产物相对于反应物而富集于较轻的Cr同位素中。此外，结果不符合瑞利蒸馏曲线行为。我们还怀疑这不是同位素平衡的反映，因为这会引起更强的分级分离。似乎平衡和动力学同位素效应的结合决定了多步氧化反应。研究的第二部分是在墨西哥瓜纳华托州利昂山谷进行的。 1975年，在地下水中发现了高浓度的Cr。先前的工作提出了人为起源，原因是位于布埃纳维斯塔（BV）的Quimica Central（QC）工厂附近的高浓度羽流，而假定位于San Juan de Otates（SJO）附近的较大但污染较少的Cr羽流是由风化引起的超镁铁质岩石。在这项研究中，2007年和2008年在两个地区采集的地表水和地下水样品在QC附近的富同位素值范围从+ 0.124‰至+ 1.40‰，在San Juan de Otates范围从+ 0.749‰至+ 2.853‰。样品表明Cr（VI）浓度降低。 BV的浓度范围从2007年的地表水和地下水0.005 mg / l和121 mg / l到2008年的地表水和地下水样品的0.002 mg / l和95.1 mg /l。2007年San Juan de Otates的Cr（VI）浓度地表水的范围从0.01 mg / l至0.012 mg / l，地表水的范围从0.01 mg / l至0.016 mg / l，而2008年地表水中的Cr（VI）浓度<0.001 mg / l。在废料堆浸试验中测得的水和地下水中的delta53Cr值表明：（1）在“风化”过程中，废料堆中的Cr被分馏了；（2）未知过程导致了delta53Cr值的降低，并且/或（3）表示存在具有独特Cr同位素组成的新Cr源。还测量了附近垃圾填埋场的Cr（VI）浓度和Cr同位素。低Cr（VI）浓度和富集的53Cr值表明垃圾填埋场正在减少。进一步的采样表明，圣胡安奥塔特山露头的超镁铁质岩石风化导致2007年和2008年地表和地下水中的Cr（VI）浓度低于USEPA和墨西哥的最大允许污染物水平。

著录项

  • 作者

    Villalobos Aragon, Alejandro.;

  • 作者单位

    The University of Texas at El Paso.;

  • 授予单位 The University of Texas at El Paso.;
  • 学科 Geochemistry.
  • 学位 Ph.D.
  • 年度 2009
  • 页码 74 p.
  • 总页数 74
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 语言学;
  • 关键词

  • 入库时间 2022-08-17 11:38:25

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