Degradation of 1,4-dioxane via controlled generation of radicals by pyrite-activated oxidants: Synergistic effects, role of disulfides, and activation sites

The controlled generation of radicals is an effective way to improve the stoichiometric efficiency of oxidants, particularly when treating highly recalcitrant contaminants. In this study, an innovative oxidation system was used to degrade extremely recalcitrant 1,4-dioxane. This system was based on a combination of pyrite, an abundant mineral used as a slow-release source of iron, and peroxymonosulfate (PMS). In addition, other oxidants, including hydrogen peroxide and peroxydisulfate, and their conditions were examined. PMS had the highest degradation performance of the oxidants tested. Near-100% degradation of 1,4-dioxane (50 mg L-1) was achieved after 40 min using PMS; the corresponding degradation rates with peroxydisulfate and hydrogen peroxide were around 50% and 15%, respectively. The production of hydroxyl radicals and sulfate radicals by pyrite-activated PMS was confirmed by electron paramagnetic resonance. Neither solid pyrite nor dissolved Fe2+ or Fe3+ was directly involved in the degradation. Instead, the Fe2+ generated from pyrite oxidation activated PMS homogeneously. Quenching tests and the rapid degradation of nitrobenzene show that hydroxyl radicals were the major active species produced by pyrite-PMS. In contrast to the significant scavenging effect of Cl- (3.0 mM) on Co2+-PMS, no significant Cl- (<= 10 mM) scavenging was observed on pyrite-PMS. These results suggest that the use of pyrite-PMS mitigates the scavenging effects of Fe2+ and Cl-, and, therefore, may increase the application of iron-based materials and PMS in the water treatment industry.