Fundamental mechanisms in the formation of labile non-crystalline species during reductive transformation of heavy metals

六价铬（Cr(VI)）污染的土壤，沉积物和地下水是一个普遍存在的问题，由于采矿，钻井和工业活动。在加拿大，通过油砂开采和水力压裂等技术进行非常规油气开采的快速增长，重新引发了人们对地表水体和浅层含水层受到铬等重金属污染的担忧。由于土壤和其他地质介质固有的复杂性，由于缺乏对土壤和沉积物中控制铬迁移和命运的反应的机制理解，污染场地的修复常常受到阻碍。铬（VI）的修复通常是通过将其还原为铬（III）来实现的，而铬（III）的可溶性远不如铬（VI），并且可以作为混合铁铬氢氧化物固体从地下水中沉淀出来。然而，最近的证据表明，伴随形成的非结晶Cr（III）物种和Cr(III)-有机金属物种可能是Cr在补救措施后的迁移载体。本研究计划有短期和长期目标：（短期）确定控制Cr（VI）还原过程中形成的产物类型的环境因素；（长期）确定导致不稳定Cr（III）形成的扩散、表面配位和电子转移等事件链。为了回答第一点，我们将利用含铁矿物磁铁矿和麦金石以及土壤微生物进行实验室Cr（VI）吸附和还原实验。众所周知，铁矿物在经过生物修复的位点上对Cr（VI）的还原和固定化至关重要。特别是，我们将比较两种矿物的化学沉淀和微生物产生（生物源）类型的Cr吸附和还原行为。生物成因矿物是在生物修复带观察到的，因此与野外条件有关。水的化学性质也可能极大地影响最终形成的Cr（III）产物，因此我们将系统地改变铁矿物和微生物还原实验中的溶质组成。特别是，根据我们先前对铀进行的研究，我们假设钙和磷酸盐可能会增加形成的非结晶Cr（III）物种的比例（相对于铁铬沉淀）。为了测试Cr（III）产品的稳定性，将进行流动反应器实验。通过改变输入溶液的化学性质，我们将确定导致Cr再活化的条件。将使用申请人实验室的新仪器分析这些反应器流出的痕量可溶性Cr(III)-有机金属物种，这些物种可能是环境中的毒性载体。此外，光谱技术，包括加拿大和斯坦福光源的同步加速器x射线分析和红外（IR）光谱学，将用于表征Cr产品。本应用程序概述的5年计划将限制Cr（VI）还原过程中的速率，吸附行为和产物形成控制。我们的最终目标是确定导致非晶体Cr（III）形成的事件顺序。长期计划将为该计划奠定基础，该计划将使用时间分辨x射线吸收光谱测量来确定Cr（VI）在还原后如何成为非晶体物质。这将涉及在电子转移之前和之后揭示矿物表面Cr（VI）的表面配位。通过重要的系统表征和规划，这些研究将提供对环境中不稳定Cr（III）物种如何形成的机制理解。

Hexavalent chromium (Cr(VI)) contamination of soils, sediments, and ground water is a pervasive problem worldwide due to mining, drilling, and industrial activities. In Canada, the rapid growth of unconventional oil and gas recovery by techniques such as oil sands extraction and hydraulic fracturing has renewed concerns about the contamination of surface water bodies and shallow aquifers by heavy metals such as chromium. Because of the inherent complexity of soils and other geologic media, the remediation of contaminated sites is often hampered by the lack of a mechanistic understanding of the reactions in soils and sediments that control chromium mobility and fate. Remediation of Cr(VI) is typically achieved by reducing it to Cr(III), which is much less soluble than Cr(VI) and can precipitate out of groundwater as mixed iron-chromium hydroxide solids. However recent evidence points to the concomitant formation of non-crystalline Cr(III) species and Cr(III)-organometallic species that could be mobility vectors for Cr following remedial action. This research program has short-term and long-term aims: (Short) to determine the environmental factors that control the type of products formed during the reduction of Cr(VI), and (Long) to determine the chain of events – including diffusion, surface coordination, and electron transfer – that lead to the formation of labile Cr(III) species formation.To answer the first point, laboratory Cr(VI) adsorption and reduction experiments will be conducted using the iron-containing minerals magnetite and mackinawite, and soil microbes. Iron minerals are known to be critical in the reduction and immobilization of Cr(VI) at sites that have undergone bioremediation. In particular we will compare the Cr adsorption and reduction behaviours of chemically precipitated and microbially-produced (biogenic) types of both minerals. The biogenic minerals are observed in zones of bioremediation and so are relevant to field conditions. Water chemistry may also greatly impact the final Cr(III) product formed, so we will systematically vary the solute composition in iron mineral and microbial reduction experiments. In particular, we hypothesize that calcium and phosphate may increase the fraction of non-crystalline Cr(III) species (versus iron-chromium precipitates) that form, based on prior studies we conducted with uranium. To test the lability of the Cr(III) products, flow-through reactor experiments will be conducted. By varying the input solution chemistry, we will determine conditions that lead to Cr remobilization. The effluent from these reactors will be analyzed for trace soluble Cr(III)-organometallic species that are likely toxicity vectors in the environment, using new instrumentation in the applicant’s laboratory. Additionally spectroscopic techniques, including synchrotron X-ray analyses at the Canadian and Stanford Light Sources and infrared (IR) spectroscopy, will be used to characterize the Cr products.The 5-year program outlined in this application will constrain the rates, adsorption behaviour, and product formation controls during Cr(VI) reduction. Our ultimate goal is to determine the sequence of events that leads to the formation of non-crystalline Cr(III) species. The long-term program, for which this program will lay the groundwork, will use time-resolved X-ray absorption spectroscopy measurements to determine how Cr(VI) becomes a non-crystalline species following its reduction. This will involve uncovering the surface coordination of Cr(VI) at the mineral surface prior to and after electron transfer. Through significant system characterization and planning, these studies will provide a mechanistic understanding of how labile Cr(III) species form in the environment.