Suppression of pyrite oxidation by ferric-catecholate complexes: An electrochemical study

Pyrite oxidation is the main cause of acid mine drainage (AMD) formation, a very serious environmental problem in numerous mining areas worldwide. To manage AMD, a promising approach called carrier-microencapsulation has been proposed to suppress pyrite oxidation via the formation of a protective coating on the mineral using redox sensitive catecholate complexes of Fe3+, Al3+, Si4+ and Ti4+. In this study, the mechanisms involved in the suppression of pyrite oxidation by Fe-based CME were investigated by conducting a series of electrochemical studies (i.e., linear sweep voltammetry (LSV), electrochemical impedance spectrometry (EIS), and chronoamperometry). The results of LSV showed that the sequential decomposition of Fe3+-catecholate complexes on pyrite (tris-catecholate -> bis-catecholate -> mono-catecholate -> Fe3+) occurred at different electrode potentials that corresponded to HOMO energy levels of the three Fe3+-catecholate complexes. Moreover, the oxidative decomposition of Fe3+-catecholate complexes formed a passivating coating as illustrated by chronoamperometry and EIS results using a rotating disk platinum (Pt) electrode. With longer decomposition time and at higher anodic potentials, defects in the coat decreased, resulting in the formation of a more resistant and evenly distributed coating. A comparison between chronoamperometry results of coated and uncoated pyrite electrodes suggests that the coating formed by Fe3+-catecholate complexes suppressed both anodic and cathodic half-cell reactions of pyrite oxidation by limiting the diffusion of reactants and products between pyrite and bulk solution phase.