Mechanisms of hydroxyl radical production from abiotic oxidation of pyrite under acidic conditions

Hydroxyl radicals (OH) produced from pyrite oxidation by O2 have been recognized, but mechanisms regarding the production under anoxic and oxic conditions are not well understood. In this study, the mechanisms of OH production from pyrite oxidation under anoxic and oxic conditions were explored using benzoic acid (BA) as an OH probe. Batch experiments were conducted at pH 2.6 to explore OH production under anoxic and oxic conditions. The cumulative OH concentrations produced under anoxic and oxic conditions increased linearly to 7.5 and 52.2 μM, respectively within 10 h at 10 g/L pyrite. Under anoxic conditions, OH was produced from the oxidation of H2O on the sulfur-deficient sites on pyrite surface, showing an increased production with the increase of pyrite surface exposure due to oxidation. Under oxic conditions, the formation of OH proceeds predominantly via the two-electron reduction of O2 on pyrite surface along with a minor contribution from the oxidation of H2O on surface sulfur-defects and the reactions of Fe2+/sulfur intermediates with O2. For both O2 reduction and H2O oxidation on the surface sulfur-defects, H2O2 was the predominant intermediate, which subsequently transformed to OH through Fenton mechanism. The OH produced had a significant impact on the transformation of contaminants in the environment. Anoxic pyrite suspensions oxidized 13.9% As(III) (C0 = 6.67 μM) and 17.6% sulfanilamide (C0 = 2.91 μM) within 10 h at pH 2.6 and 10 g/L pyrite, while oxic pyrite suspensions improved the oxidation percentages to 55.4% for As(III) and 51.9% for sulfanilamide. The ratios of anoxic to oxic oxidation are consistent with the relative contribution of surface sulfur-defects to OH production. However, Fe2+ produced from pyrite oxidation competed with the contaminants for OH, which is of particular significance with the increase of time in a static environment. We conclude that OH can be produced from abiotic oxidation of pyrite under acidic conditions, and the production dramatically increases in the the presence of O2. The oxidation induced by the OH produced should be taken into account for substance transformation due to its highly reactive nature.