THE KINETICS AND ELECTROCHEMICAL RATE-DETERMINING STEP OF AQUEOUS PYRITE OXIDATION

Rate data available in the literature have been compiled for the react ion 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 [GRAPHICS] where r is the rate o f pyrite destruction in units of mol m(-2) s(-1). A series of batch an d mixed flow reactor experiments were performed to determine the effec t of SO42-, Cl-, ionic strength, and dissolved oxygen on the rate of r eaction of pyrite with ferric iron. Of these, only dissolved oxygen wa s found to have any appreciable effect. Experimental results of the pr esent study were combined with kinetic data reported in the literature to formulate rate laws that are applicable over a six order of magnit ude range in Fe3+ and Fe2+ concentration for the pH range similar to 0 .5-3.0. In N-2-purged solution, the rate law is [GRAPHICS] and when di ssolved oxygen is present, [GRAPHICS] where r is the rate of pyrite de struction in mol m(-2) s(-1). Experiments were also performed in which a single pyrite sample was repeatedly reacted with ferric iron soluti ons of the same composition and identical surface area to mass of solu tion ratio (AIM). For each subsequent experiment, the rate of reaction slowed and the original behavior of the pyrite could not be reestabli shed by washing the pyrite with concentrated HNO3 or EDTA. This behavi or was interpreted as representative of a change in the electrochemica l properties of the solid pyrite. Pretreating pyrite samples with aque ous solutions of ferrous iron and EDTA did not change the reaction rat e with ferric iron; however, pretreatment with hydroxylamine hydrochlo ride lowered the rate significantly. The data presented are best model ed by a nonsite-specific Freundlich multilayer isotherm. Good correlat ion 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 catho dic-anodic electrochemical mechanism is favored to explain the transfe r of the electron from pyrite to the aqueous oxidant. Mechanistically, the results of this study suggest a nonsite specific interaction betw een dissolved oxidants and the pyrite surface. Rate correlates strongl y with Eh (Fe3+/Fe2+ ratio or DO concentration) and is consistent with an electrochemical mechanism where anodic and cathodic reactions occu r at different places on the pyrite surface.