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

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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.