Paradis Charles J, Jagadamma Sindhu, Watson David B, McKay Larry D, Hazen Terry C, Park Melora, Istok Jonathan D
Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, United States.
Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, United States.
J Contam Hydrol. 2016 Apr;187:55-64. doi: 10.1016/j.jconhyd.2016.02.002. Epub 2016 Feb 11.
Reoxidation and mobilization of previously reduced and immobilized uranium by dissolved-phase oxidants poses a significant challenge for remediating uranium-contaminated groundwater. Preferential oxidation of reduced sulfur-bearing species, as opposed to reduced uranium-bearing species, has been demonstrated to limit the mobility of uranium at the laboratory scale yet field-scale investigations are lacking. In this study, the mobility of uranium in the presence of nitrate oxidant was investigated in a shallow groundwater system after establishing conditions conducive to uranium reduction and the formation of reduced sulfur-bearing species. A series of three injections of groundwater (200 L) containing U(VI) (5 μM) and amended with ethanol (40 mM) and sulfate (20 mM) were conducted in ten test wells in order to stimulate microbial-mediated reduction of uranium and the formation of reduced sulfur-bearing species. Simultaneous push-pull tests were then conducted in triplicate well clusters to investigate the mobility of U(VI) under three conditions: 1) high nitrate (120 mM), 2) high nitrate (120 mM) with ethanol (30 mM), and 3) low nitrate (2 mM) with ethanol (30 mM). Dilution-adjusted breakthrough curves of ethanol, nitrate, nitrite, sulfate, and U(VI) suggested that nitrate reduction was predominantly coupled to the oxidation of reduced-sulfur bearing species, as opposed to the reoxidation of U(IV), under all three conditions for the duration of the 36-day tests. The amount of sulfate, but not U(VI), recovered during the push-pull tests was substantially more than injected, relative to bromide tracer, under all three conditions and further suggested that reduced sulfur-bearing species were preferentially oxidized under nitrate-reducing conditions. However, some reoxidation of U(IV) was observed under nitrate-reducing conditions and in the absence of detectable nitrate and/or nitrite. This suggested that reduced sulfur-bearing species may not be fully effective at limiting the mobility of uranium in the presence of dissolved and/or solid-phase oxidants. The results of this field study confirmed those of previous laboratory studies which suggested that reoxidation of uranium under nitrate-reducing conditions can be substantially limited by preferential oxidation of reduced sulfur-bearing species.
溶解相氧化剂对先前还原并固定的铀进行再氧化和迁移，这对修复受铀污染的地下水构成了重大挑战。与还原态含铀物种相比，还原态含硫物种的优先氧化已被证明在实验室规模上限制了铀的迁移，但缺乏现场规模的调查。在本研究中，在建立有利于铀还原和还原态含硫物种形成的条件后，研究了浅层地下水系统中硝酸盐氧化剂存在下铀的迁移情况。在十个测试井中进行了一系列三次注入含U(VI)（5 μM）并添加乙醇（40 mM）和硫酸盐（20 mM）的地下水（200 L），以刺激微生物介导的铀还原和还原态含硫物种的形成。然后在三个重复的井组中进行同步推挽试验，以研究在三种条件下U(VI)的迁移情况：1）高硝酸盐（120 mM），2）高硝酸盐（120 mM）加乙醇（30 mM），3）低硝酸盐（2 mM）加乙醇（30 mM）。乙醇、硝酸盐、亚硝酸盐、硫酸盐和U(VI)的稀释调整后的突破曲线表明，在36天测试期间的所有三种条件下，硝酸盐还原主要与还原态含硫物种的氧化耦合，而不是U(IV)的再氧化。在所有三种条件下，相对于溴化物示踪剂，推挽试验期间回收的硫酸盐量（而非U(VI)量）大大超过注入量，这进一步表明在硝酸盐还原条件下还原态含硫物种被优先氧化。然而，在硝酸盐还原条件下且不存在可检测到的硝酸盐和/或亚硝酸盐时，观察到了一些U(IV)的再氧化。这表明在溶解态和/或固相氧化剂存在的情况下，还原态含硫物种可能无法完全有效地限制铀的迁移。该现场研究结果证实了先前实验室研究的结果，即硝酸盐还原条件下铀的再氧化可通过还原态含硫物种的优先氧化而得到显著限制。
