Mechanisms of hydroxyl radicals production from pyrite oxidation by hydrogen peroxide: Surface versus aqueous reactions

Pyrite oxidation by hydrogen peroxide (H2O2) occurs in both natural and engineered systems. Hydroxyl radical (OH) is a key reactive intermediate for pyrite and coexisting substances oxidation. In acidic H2O2/pyrite systems, H2O2 decomposition by aqueous Fe2+ is documented to predominate for OH production, whereas here we show that H2O2 decomposition by surface Fe(II) sites contributes considerably to OH production under certain conditions. Pyrite oxidation by H2O2 under anoxic conditions was performed under different conditions (2-12 g/L pyrite, 0.025-1 mM H2O2 and pH 2-4), and OH and aqueous Fe2+/Fe3+ production as well as H2O2 consumption were measured during the oxidation. In order to evaluate the contribution of surface reaction to OH production, 1 mM 2, 2′-bipyridine (BPY) was added to inhibit H2O2 decomposition by aqueous Fe2+. The rate constants of OH production decreased by 44.4-65.6% with addition of 1 mM BPY, which suggests that both surface and aqueous reactions contributed to OH production. Regarding the surface reaction, density functional theory (DFT) calculation reveals that H2O2 was adsorbed onto the Fe(II) sites on pyrite surface and transformed to surface adsorbed OH which desorbed subsequently into the aqueous solution. On the basis of mechanistic understanding, a kinetic model was developed to assess the relative contributions of surface and aqueous reactions to OH production. The relative contribution of surface reaction is dependent on the ratio of pyrite surface concentration to aqueous Fe2+ concentration, which decreases with the progress of pyrite oxidation due to the increase in aqueous Fe2+. When the ratio is higher than the threshold value of 1.6 × 103 m2/mM, surface reaction becomes predominant for OH production. Typical systems necessitating consideration of surface reaction involve pyritic rocks and shale leaching and pollutants treatment by H2O2/pyrite. The mechanisms unraveled in this study supplement the fundamental of OH production from pyrite oxidation by both H2O2 and O2 in natural and engineered systems.