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非常规油气开采的快速增长，再次引发了人们对铬等重金属污染地表水体和浅层含水层的担忧。由于土壤和其他地质介质固有的复杂性，污染场地的修复往往因缺乏对土壤和沉积物中控制铬迁移和归宿的反应的机械理解而受到阻碍。 Cr(VI) 的修复通常是通过将其还原为 Cr(III) 来实现的，Cr(III) 的溶解度比 Cr(VI) 低得多，并且可以作为混合的氢氧化铁铬固体从地下水中沉淀出来。然而，最近的证据表明，非晶态 Cr(III) 物质和 Cr(III) 有机金属物质的同时形成，可能是补救措施后 Cr 的迁移载体。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) 物质形成的一系列事件 - 包括扩散、表面配位和电子转移。为了回答第一点，将使用含铁矿物磁铁矿和麦金矿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，铁矿物质对于生物修复场地中 Cr(VI) 的还原和固定至关重要。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）类型的 Cr 吸附和还原行为。生物矿物质是在生物修复区域观察到的，因此与现场条件相关。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，根据我们之前对铀进行的研究，我们假设钙和磷酸盐可能会增加形成的非结晶 Cr(III) 物质（相对于铁铬沉淀物）的比例。为了测试 Cr(III) 产品的稳定性，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。这些反应器的流出物将使用申请人实验室的新仪器来分析微量可溶性 Cr(III)-有机金属物质，这些物质可能是环境中的毒性媒介。此外，光谱技术，包括加拿大和斯坦福光源的同步加速器 X 射线分析以及红外 (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) 物质形成的事件顺序。该计划将为其奠定基础的长期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。这将涉及揭示电子转移前后矿物表面 Cr(VI) 的表面配位。通过重要的系统表征和规划，这些研究将对不稳定的 Cr(III) 物种如何在环境中形成提供机械理解。

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.