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

Abstract

Pyrite oxidation by hydrogen peroxide (H2O2) occurs in both natural and engineered systems. Hydroxyl radical ([rad]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 [rad]OH production, whereas here we show that H2O2 decomposition by surface Fe(II) sites contributes considerably to [rad]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 [rad]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 [rad]OH production, 1 mM 2, 2′-bipyridine (BPY) was added to inhibit H2O2 decomposition by aqueous Fe2+. The rate constants of [rad]OH production decreased by 44.4–65.6% with addition of 1 mM BPY, which suggests that both surface and aqueous reactions contributed to [rad]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 [rad]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 [rad]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 [rad]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 [rad]OH production from pyrite oxidation by both H2O2 and O2 in natural and engineered systems.