A FIRST PRINCIPLES STUDY OF THE OXIDATION ENERGETICS AND KINETICS OF REALGAR

Quantum-mechanical calculations allow resolving and quantifying important aspects of reaction mechanisms such as spin transitions and oxygen dissociation that can be the major rate-limiting steps in redox processes on sulfide and oxide surfaces. The interaction of As₄S₄ clusters with oxygen and co-adsorbed ions provides a model system for understanding the molecular-scale processes that underpin empirically-derived rate expressions, and provides clues to the oxidation mechanisms of other sulfides and oxides. Two activated processes are shown to dominate the kinetics of oxidation by molecular oxygen: i) a paramagnetic (³O₂) to diamagnetic (¹O₂) spin transition and ii) oxygen dissociation on the surface, in that order. The activation energies for the spin transition and O₂ dissociation step were determined to be 106 kJ/mol and 87 kJ/mol, respectively, for dry oxidation. ³O₂ transfers its spin to As₄S₄ and forms a low-spin, peroxo intermediate on the surface before dissociating. The adsorption of a hydroxide ion on the surface proximate to the ³O₂ adsorption site changes the adsorption mechanism by lowering the activation energy barriers for both the spin transition (29 kJ/mol) and the O₂ dissociation step (69 kJ/mol). Thus, while spin transition is rate limiting for oxidation with O₂ alone, dissociation becomes the rate-limiting step for oxidation with co-adsorption of OH^-. Thus, assuming that an As₄S₄ cluster with an adsorbed hydroxyl group is a reasonable approximation of the surface of As₄S₄ at high pH, the theoretically calculated oxidation rate (~10^-10 mol·m^-2·s^-1) is of the same order as empirically-derived rates from experiments at pH = 8. In addition, the co-adsorption of other anions found in alkaline waters (carbonate, bicarbonate, sulfate, and sulfite) are shown to energetically promote the oxidation of As₄S₄.

Activation-energy barriers due to spin transitions are rarely discussed in the literature as key factors for controlling oxidation rates of mineral surfaces, even though the magnitude of these barriers is enough to alter the kinetics significantly. Lowering the activation energy by co-adsorbed anions suggests the possibility of pH- or p(co-adsorbate)-dependent activation energies that can be used to refine oxidation rate laws for semiconducting minerals, such as sulfides and oxides.