Diffusion Kinetics of Adsorbed Species on Pyrite Surfaces

Surface diffusion can bring redox pairs closer together, which is necessary for electron transfer to happen, thereby strongly influencing the overall kinetics. However, little is known about the diffusion kinetics of redox-active species on pyrite and other sulfides, which aid in catalyzing many redox reactions. Here, we calculate the diffusion of oxidant (UO22+) and reductant Fe2+, HS-) species on pyrite {100} surfaces using quantum-mechanical calculations. Energy curves along different diffusion paths are derived for both inner- and outer-sphere complexes by moving the species in small increments (0.05-0.25 angstrom). The diffusion path along the molecular ridges formed by disulfide groups on the uppermost pyrite surface has the lowest energy barrier for the diffusion of all species tested. Single-particle diffusion coefficients along their optimal diffusion pathways are derived from diffusion energy barriers and attempt frequencies. Calculations are performed on flat defect-free pyrite surfaces, while on actual surfaces, diffusion is affected by defects, steps, and impurities. Calculated mobilities of the outer-sphere complexed uranyl and ferrous iron are about 4-5 times faster than their inner-sphere ones. Although the results here focus on single-particle diffusion, UO22+-HS- was used as an example for interdependent multiple-particle diffusion on the pyrite surface. Interactions between diffusing species (uranyl vs HS-), and to a limited degree jump correlations, were derived quantum-mechanically. Interactions are a combination of electronic interactions underneath the mineral surface and through the aqueous near-surface region; their interdependent diffusion can be approximated by apparent Coulomb interactions (with a dielectric constant of similar to 7.7) for processing in subsequent Monte Carlo simulations.