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

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.