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 Al ³⁺ (added as AlCl ₃ ·6H ₂ O). In comparison, the pyrite oxidation rate at circumneutral pH (7.4 ± 0.4) decreased with increasing added Al ³⁺ with ≈98% reduction in long-term (282 days) dissolution rates in the presence of 20 mg L ^-1 Al ³⁺ . Al ³⁺ was added to the solution and allowed to equilibrate prior to pyrite addition (2 g L ^-1 ). Consequently almost all Al ³⁺ (>99.9%) was initially present as aluminum hydroxide precipitates at pH 7.4. X-ray photoelectron spectroscopy analysis showed a significant concentration of Al ³⁺ (20.3 at. %) on the pyrite surface reacted at pH 7.4 with 20 mg L ^-1 added Al ³⁺ , but no Al ³⁺ on pyrite surfaces reacted at pH 2.0 and 4.0 with added Al ³⁺ . 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 Al ³⁺ at circumneutral pH, in contrast to the porous goethite surface layers formed on pyrite dissolved in the absence of Al ³⁺ 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.