Electrochemical characterization of pyrrhotite reactivity under simulated weathering conditions

Abstract

A previously reported methodology applied to evaluate the factors affecting the reactivity pyrite and pyritic samples under simulated weathering conditions is now utilized to analyze pyrrhotite reactivity at different environmental alteration stages. The methodology is based on the comparison of the voltammetric responses of the mineral obtained through the alteration process; changes in the electrochemical behavior are associated with changes in the surface state of the mineral and finally are related with changes of the mineral reactivity. The results of an initial study of the electrochemical behavior of pyrrothite before alteration suggest that its alteration involves the formation of 3 surface layers (in agreement with previous reports): (1) in immediate contact with pyrrhotite corresponding to a metal-deficient sulfide; (2) an intermediate layer corresponding to elemental S, and; (3) the most external layer, consisting of precipitates of Fe oxy-hydroxides, like goethite. The pyrrhotite reactivity seems to be controlled by the formation of oxidation product layers that coat and passivate the pyrrhotite surface, where the elemental S layer has most significance. The results confirm the advantage of incorporating cyclic voltammetry as an auxiliary method for acid rock drainage prediction, due to its demonstrated capacity to describe the factors that influence sulfide mineral reactivity which are not evaluated by other predictive techniques.