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

由于采矿、钻探和工业活动，土壤、沉积物和地下水的六价铬（Cr（VI））污染是世界范围内普遍存在的问题。在加拿大，通过油砂开采和水力压裂等技术进行的非常规石油和天然气开采迅速增长，重新引起了人们对铬等重金属污染地表水体和浅水层的关切。由于土壤和其他地质介质的固有复杂性，污染场地的修复往往受到土壤和沉积物中控制铬的流动性和归宿的反应缺乏机械理解的阻碍。Cr（VI）的修复通常通过将其还原为Cr（III）来实现，Cr（III）的溶解度比Cr（VI）低得多，并且可以作为混合的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，伴随形成的非结晶铬（III）物种和铬（III）-有机金属物种，可能是流动性载体铬以下补救措施。该研究计划有短期和长期目标：（短）确定控制Cr（VI）还原过程中形成的产物类型的环境因素，（长）确定导致不稳定Cr（III）物种形成的事件链--包括扩散、表面配位和电子转移。要回答第一点，将使用含铁矿物磁铁矿和麦基诺石以及土壤微生物进行实验室Cr（VI）吸附和还原实验。铁矿物是已知的关键，在减少和固定的铬（VI）在网站上进行了生物修复。特别是，我们将比较化学沉淀和微生物产生的（生物）类型的两种矿物的铬吸附和还原行为。在生物修复区观察到生物矿物，因此与现场条件有关。水化学也可能极大地影响最终形成的Cr（III）产品，因此我们将在铁矿物和微生物还原实验中系统地改变溶质成分。特别是，我们假设，钙和磷酸盐可能会增加非结晶铬（III）物种（相对于铁铬沉淀物），形成的分数，根据先前的研究，我们进行了铀。为了测试Cr（III）产物的不稳定性，将进行流通式反应器实验。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堆的流出物中痕量可溶性Cr（III）-有机金属物质，这些物质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大学光源的同步加速器X射线分析和红外（IR）光谱，将用于表征Cr产品。本申请中概述的5年计划将限制Cr（VI）还原过程中的速率、吸附行为和产品形成控制。我们的最终目标是确定导致非晶态Cr（III）物种形成的事件顺序。该计划将为长期计划奠定基础，将使用时间分辨X射线吸收光谱测量来确定Cr（VI）如何在还原后成为非晶体物质。这将涉及在电子转移之前和之后在矿物表面处揭示Cr（VI）的表面配位。通过显着的系统表征和规划，这些研究将提供一个机械的理解如何不稳定的铬（III）物种在环境中形成。

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.