The anodic oxidation reaction can be written as: CuFeS₂ → Cu²⁺ + Fe²⁺ + 2S⁰ + 4e⁻ The dominant cathodic reaction in ferric sulfate systems is ferric reduction: Fe³⁺ + e⁻ → Fe²⁺ Ferric ions are regenerated through oxidation of ferrous ions by oxygen: 4Fe²⁺ + O₂ + 4H⁺ → 4Fe³⁺ + 2H₂O In bioleaching environments this ferric regeneration reaction is catalyzed by iron-oxidizing microorganisms such as Acidithiobacillus ferrooxidans. Under mixed-potential conditions, the dissolution rate of chalcopyrite is determined by the intersection of anodic and cathodic polarization curves (Figure 2). Experimental measurements show that the exchange current density for ferric reduction on chalcopyrite surfaces is low compared with noble metals, contributing to slow overall reaction kinetics (Ghahremaninezhad et al., 2012; Yue & Asselin, 2014). Figure 2 illustrates the conceptual mixed-potential framework governing chalcopyrite dissolution. The anodic polarization behaviour of chalcopyrite (solid blue curve) exhibits an active–passive transition. This process is often associated with the formation of surface reaction products and it can be measured experimentally at slow potentiodynamic scan rates or via stepped potentiostatic experiments (Viramontes-Gamboa et al., 2007; Ghahremaninezhad et al., 2013). The cathodic polarization curve (dashed red line) represents the reduction of ferric ions to ferrous ions in acidic solution. The intersection of the two curves defines the mixed potential (EM) and corresponding dissolution current density (rate of the electrochemical reaction), id, for the system. Under typical sulfate heap leaching conditions, solution potentials often fall within or near the passive region of the CuFeS2 polarization curve, resulting in slow dissolution kinetics despite relatively high bulk solution oxidation–reduction potentials (Ren et al., 2022; Viramontes-Gamboa et al., 2007).
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