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Formation of Aluminum Hydroxide-Doped Surface Passivating Layers on Pyrite for Acid Rock Drainage Control

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journal contribution
posted on 19.09.2018, 00:00 by Yan Zhou, Rong Fan, Michael D. Short, Jun Li, Russell C. Schumann, Haolan Xu, Roger St. C. Smart, Andrea R. Gerson, Gujie Qian
The aim of this study was to test the performance of a novel method for acid rock drainage (ARD) control through the formation of Al­(OH)3-doped passivating surface layers on pyrite. At pH 2.0 and 4.0, there was no obvious inhibition of the pyrite oxidation rate on addition of 20 mg L–1 Al3+ (added as AlCl3·6H2O). In comparison, the pyrite oxidation rate at circumneutral pH (7.4 ± 0.4) decreased with increasing added Al3+ with ≈98% reduction in long-term (282 days) dissolution rates in the presence of 20 mg L–1 Al3+. Al3+ was added to the solution and allowed to equilibrate prior to pyrite addition (2 g L–1). Consequently almost all Al3+ (>99.9%) was initially present as aluminum hydroxide precipitates at pH 7.4. X-ray photoelectron spectroscopy analysis showed a significant concentration of Al3+ (20.3 at. %) on the pyrite surface reacted at pH 7.4 with 20 mg L–1 added Al3+, but no Al3+ on pyrite surfaces reacted at pH 2.0 and 4.0 with added Al3+. Transmission electron microscopy and synchrotron X-ray absorption near edge spectroscopy analyses indicated that compact surface layers containing both goethite and amorphous or nanocrystalline Al­(OH)3 formed in the presence of 20 mg L–1 Al3+ at circumneutral pH, in contrast to the porous goethite surface layers formed on pyrite dissolved in the absence of Al3+ under otherwise identical conditions. This work demonstrates the potential for novel Al-based pyrite passivation of relevance to the mining industry where suitable Al-rich waste materials are available for ARD control interventions.