The kinetics and electrochemical rate-determining step of aqueous pyrite oxidation

Abstract

Rate data available in the literature have been compiled for the reaction of pyrite with dissolved oxygen (DO) to produce a rate law that is applicable over four orders of magnitude in DO concentration over the pH range 2–10. The valid rate law is ▪ where r is the rate of pyrite destruction in units of mol m−2 s−1. A series of batch and mixed flow reactor experiments were performed to determine the effect of SO42−, Cl−, ionic strength, and dissolved oxygen on the rate of reaction of pyrite with ferric iron. Of these, only dissolved oxygen was found to have any appreciable effect. Experimental results of the present study were combined with kinetic data reported in the literature to formulate rate laws that are applicable over a six order of magnitude range in Fe3+ and Fe2+ concentration for the pH range ~0.5–3.0. In N2-purged solution, the rate law is ▪ and when dissolved oxygen is present, ▪ where r is the rate of pyrite destruction in mol m−2 s−1.Experiments were also performed in which a single pyrite sample was repeatedly reacted with ferric iron solutions of the same composition and identical surface area to mass of solution ratio (A/M). For each subsequent experiment, the rate of reaction slowed and the original behavior of the pyrite could not be reestablished by washing the pyrite with concentrated HNO3 or EDTA. This behavior was interpreted as representative of a change in the electrochemical properties of the solid pyrite. Pretreating pyrite samples with aqueous solutions of ferrous iron and EDTA did not change the reaction rate with ferric iron; however, pretreatment with hydroxylamine hydrochloride lowered the rate significantly.The data presented are best modeled by a nonsite-specific Freundlich multilayer isotherm. Good correlation was found between Eh and rate for the aqueous oxidation of pyrite with DO and ferric iron. Because the fractional orders of reaction are difficult to explain with a purely molecular-based mechanism, a cathodic-anodic electrochemical mechanism is favored to explain the transfer of the electron from pyrite to the aqueous oxidant.Mechanistically, the results of this study suggest a nonsite specific interaction between dissolved oxidants and the pyrite surface. Rate correlates strongly with Eh (Fe3+Fe2+ ratio or DO concentration) and is consistent with an electrochemical mechanism where anodic and cathodic reactions occur at different places on the pyrite surface.