Redox Processes in Sulfide Minerals.

Abstract

Despite decades of research, the key controls of mechanisms and, hence, the rates of some of the most environmentally important redox mechanisms in sulfide minerals are poorly understood. Two of the main questions to be addressed are: 1) How do the semiconducting properties of the mineral mediate interactions between adsorbates on the surface or impurities within the bulk? 2) How do these interactions (i.e., structural, electronic, and magnetic) influence the adsorption mechanism, thermodynamics, and kinetics of geochemical processes? Experimental microbeam and spectroscopic techniques combined with theoretical approaches were employed to understand redox chemistry on iron and arsenic monosulfide nanoparticles, and coupled substitution mechanisms in galena. It has previously been shown that dissolved As(III) uptake from solution by mackinawite (FeS) nanoparticles is dependent on pH and redox conditions. The formation of amorphous arsenic sulfide precipitates accounts for As(III) uptake at pH 5 as shown by high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). The oxidation of the arsenic sulfide nanoparticles and the subsequent release of dissolved arsenic were investigated by quantum-mechanical calculations. At low pH, it has been shown that the paramagnetic (3O2) to diamagnetic (1O2) spin transition is the rate-limiting step in the oxidation of As4S4. At high pH, the rate-limiting step is oxygen dissociation on the surface. The theoretically calculated oxidation rate (~10-10 mol·m-2·s-1) is of the same order as empirically-derived rates from experiments at T = 298 K, pH = 8, and similar dissolved oxygen concentrations. The attenuation of the activation energy by co-adsorbed anions suggests the possibility of pH- or p(co-adsorbate)-dependent activation energies that can be used to redesign oxidation rate laws, which are based on actual reaction mechanisms. Apart from reaction mechanisms on the surface, redox processes have been shown to affect the ordering and phase relationships in solid solutions involving galena (i.e., AgSbS2-Pb2S2, AgBiS2-Pb2S2, and AgAsS2-Pb2S2). Quantum-mechanical with subsequent Monte-Carlo simulations were developed specifically to model coupled substitution. These allow for the derivation of phase diagrams and the determination of solubility limits for the coupled substitution of Ag in galena.