Sphalerite oxidation simulating acidic, circumneutral and alkaline conditions to account for weathering behavior and Zn release

There is little evidence of sphalerite oxidation accounting for interfacial behavior, evolution of its oxidation capacity and kinetics processes under simulated weathering conditions. Accordingly, the present study combines surface analyses of pristine and leached low Fe-bearing and Pb-bearing sphalerite samples (PbS-ZnS) including X-ray Photoelectron Spectroscopy (XPS), Glow Discharge Optical Emission Spectroscopy (GDOES), Raman spectroscopy, Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM), along with the chemical evolution of leachates obtained after 24 h of mineral contact with 0.1 M NaOH, 0.1 M NaNO₃, 0.1 M H₂SO₄ or 0.1 M HClO₄ solution. An electrochemical study using Cyclic voltammetry (CV), Chronopotentiometry (CP), Chronoamperometry (CA), Linear sweep voltammetry (LSV) and Tafel plots (TP) of PbS-ZnS and marmatite-like sphalerite ((Fe, Zn)S) samples were also conducted to evaluate the oxidation capacity of the mineral interfaces in the solutions. Results reveal slow sphalerite oxidation linked to formation and heterogeneous polysulfides distribution, thus defining bare modifications of surface as indicated by SEM, AFM, GDOES, XPS and Raman studies. The highest oxidation was obtained in NaOH solution (NaOH > NaNO₃ > HClO₄ > H₂SO₄). While the corresponding K_f·C_R^b parameters were 1.07 × 10⁻⁷ and 3.14 × 10⁻⁸ mol·s⁻¹ for PbS-ZnS and (Fe, Zn)S samples, respectively. A clear oxidation trend involving the progressive passivation of (Fe, Zn)S sample was revealed during evolution of sphalerite oxidation, whereas an alternative passive to transpassive oxidation was observed for the PbS-ZnS sample. We suggest weathering mechanisms for sphalerite, and their environmental implications are discussed.