Crystal face-dependent pyrite oxidation: An electrochemical study

Abstract

The oxidation of pyrite has attracted significant attention due to its predominant contribution to the serious environmental issue of acid mine drainage (AMD). However, relatively few studies have been conducted investigating the oxidation characteristics of individual pyrite crystal faces. The oxidation rates, under acidic, neutral and alkaline conditions, of three typical pyrite surfaces have been examined with their contrasting behaviors explored in terms of surface structure. Electrochemical and surface analytical measurements were conducted across the pH range 1 to 11, showing surface formation of Fe(OH)3, S0 and SO42−. Tafel plots, electrochemical impedance spectroscopy and surface analyses demonstrate that the oxidation rate follows the order of {210} > {111} > {100}. This rate trend is directly related to the breakage of bonds on surface formation with formation of the {210} surfaces requiring the greatest bond breakage; hence this surface is the most unstable, and the {100} surface owns the least bond breakage and the greatest stability. The oxidation rate of all pyrite surfaces under strongly acidic conditions (pH = 1) is greater than for strongly alkaline conditions (pH = 11), with the slowest rates being observed under neutral condition, since S0, iron oxyhydroxide (Fe-OOH) and Fe(OH)3 formed under neutral or alkaline conditions cover the pyrite surface, thus inhibiting further oxidation. However, a more aggressive oxidant of hydroxo–Fe(III) complex is present under alkaline conditions, resulting in greater oxidation rate compared to that under neutral conditions that determined by the oxidation of Fe2+ by O2. DFT calculations suggest that H2O has the greatest affinity for the {210} surface as compared to the {100} and {111} surfaces but does not dissociate. However, after the dissociative adsorption of O2, H2O can dissociate on all the pyrite surfaces, enhancing further oxidation. Under acidic conditions this process is likely to be rate determining. Under alkaline and neutral conditions the oxidation process is dependent on the transfer of hydroxyls to the surface S group and surface cycling of Fe(II) − Fe(III).