Mineral-Water Interactions in Acid Mine Drainage Environments: A TEM, AEM, and EFTEM Study

Abstract

R. J. Harrison A. Putnis C. Putnis Background. Mineral-water interfacial interactions associated with working and abandoned sulphidebearing mines and mining wastes are among the most complex, dynamic, and environmentally important of all near-surface rock-water systems (e.g. Jambor and Blowes, 1994). These mining sites, numbering approximately 200,000 in the United States alone, typically release large amounts of heavy metals into the environment ranging up to hundreds of kilometers down hydrologic gradients in relatively short times. Contamination occurs when high solute concentrations of iron form during the weathering of pyrite and associated metal sulphides. Acidic effluent flowing from mine adits and waste piles mix with air and oxygenated surface water and precipitate iron oxides, oxyhydroxides, and/or hydroxysulphates. These phases form mineral/rock coatings and ironrich sediments that may contain very high concentrations of associated metals (up to percent levels of, for example, Cu, Zn, and Pb). Field and laboratory studies suggest that both the growth/dissolution of discrete metal-bearing precipitates and adsorption/desorption reactions are the most important processes in the attenuation/release of toxic metals. However, despite thermodynamic predictions, as well as mineralogical (e.g. XRD) and geochemical (e.g. extraction) techniques, many of the phases and interfaces which contain metals other than iron have generally not been identified with reliable certainty (e.g. Jambor, 1994). An understanding of the minerals and mineral coatings present, as well as their composition, degree of crystallinity, and microfabric are all critical in understanding toxic metal partitioning (including sorption/desorption reactions) under different Eh/pH environments in these complex natural systems. Further, modelling the formation of AMD environments, and the transport of toxic metals through and away from these sites, would be greatly aided by a Department of Geological Sciences, Virginia Polytechnic Institute, Blacksburg, VA 24061, USA