The attenuation of Ni, Zn and Cu, by secondary Fe phases of different crystallinity from surface and ground water of two sulfide mine tailings in Manitoba, Canada

Abstract Secondary Fe phases, which could attenuate Cu, Zn and Ni, formed in samples of unacidified filtered surface and ground water from two deposits of sulfide mine tailings at Thompson and Leaf Rapids, Manitoba. The precipitates were separated from the solutions by filtration, and analyzed by X-ray diffraction, scanning electron microscopy and electron microprobe. Selective dissolution with 0.2 M ammonium oxalate solution at pH 3 was used to separate phases of different crystallinity. An assemblage of goethite and schwertmannite precipitated from initially anoxic ground water with a pH of 5.7–6.2, whereas jarosite and/or schwertmannite were identified in acidic oxygenated (pH 2.7–3.2) surface water. This difference in mineralogy is attributed to the redox and acidity conditions. In the surface water, activities of SO 4 2 - and H + control the proportions of minerals in the assemblage and the composition of schwertmannite. More Cu, Zn and Ni were precipitated from the ground water than from surface waters due to the higher initial content of Fe and higher pH of ground water. The concentrations of the metals were not significantly changed in the surface water because of limited sorption of cations at low pH. The affinity of the metals to the precipitate rather than the solution is in the order Cu > Zn > Ni. Sequential extraction indicated that Ni and Zn accumulate in poorly crystalline schwertmannite, while Cu concentrates in crystalline phases (goethite and jarosite). However, schwertmannite, as the principal phase in most of the precipitates, contains a significant proportion of Cu. During the recrystallization of metastable schwertmannite to goethite, the metals could be redistributed and released back into the mine water.