Authors: Gillow, J; Horst, J

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Gillow, J & Horst, J 2008, 'Harnessing Iron Mineral Transformations for the Geochemical Stabilization of Mining Impacts', in AB Fourie, M Tibbett, I Weiersbye & P Dye (eds), Mine Closure 2008: Proceedings of the Third International Seminar on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 539-545,

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Engineering a successful treatment strategy for inorganics associated with mining residuals requires that equal consideration be given to the solid and dissolved phases. Because of its chemistry and relative abundance, the role of iron in controlling the mobility of many other elements is pronounced. This applies to a variety of metals, radionuclides, and certain non-metals. This paper discusses the potential for harnessing biologically-mediated iron mineral transformations in order to stabilize metal impacts stemming from mining activities, and presents several examples. Iron minerals can be harnessed through biologically-mediated transformations within, and down-gradient of an in situ anaerobic treatment zone. This process involves a variety of possible products, but of primary importance are the formation of iron sulphide mineral phases within the anaerobic zone, and the formation of oxy-hydroxide mineral phases in the down-gradient redox recovery zone. These biologically-mediated mineral phases can be used to sequester sulphur derived from sulphate, create a long-term source of reductive poise which can help immobilize certain minerals, or create sorptive capacity to control the mobility of various inorganics. These reactions may utilize natural iron present in aquifer soil and available sulphate – such as acid rock drainage, but can also be engineered with a supplemental source of iron and/or sulphate. One example where this concept applies is uranium. Uranium is highly soluble across a wide pH range and under oxic conditions due to the propensity of the uranyl cation to form complexes with a variety of common ligands (carbonate, phosphate, and hydroxide) as well as a multitude of low molecular weight organics and natural organic matter. All of these uranium complexes persist in oxidizing environments and can be resistant to surface reactions. In contrast, the in situ precipitation of insoluble forms of uranium can be accomplished under anaerobic and reducing conditions, but a critical component to this approach is the creation of excess reductive poise (through the concurrent mineralization of iron sulphides) to buffer the environment over the long term and mitigate re-oxidation of the precipitated uranium phases. Also important is the additional sorptive capacity that can be created to control the mobility of any uranium that is re-oxidized. This same approach can be applied to other metals that are conducive to precipitation as sulphides themselves, where the excess iron sulphide phases can increase the long-term stability of other sulphide mineral phases present. The success of this type of approach requires an adequate understanding of site mineralogical and hydrological characteristics, speciation of the dissolved phases being targeted, and the mineralization dynamics and associated biogeochemistry. This will allow development of an integrated and effective treatment design for difficult mine-related soil and groundwater impacts from metals and radionuclides.

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