Effects of sulfite on oxidation and immobilization of arsenite during troilite oxygenation: A mechanistic study

The iron and sulfur species play key roles in the transformation and immobilization of heavy metals (e.g., As(III)) in soil environments, while the underlying mechanism of these processes was not fully explored. In this study, the effects of sulfite (an important intermediate of sulfur species) on As(III) transformation induced by troilite (FeS) were investigated. Results showed that the presence of sulfite not only greatly enhanced As(III) oxidation by FeS, with k_obs increased by approximated 30 times, but also significantly increased the As(III) immobilization on FeS surface. Further analysis reveal that sulfate radicals (SO₄^•−) and hydroxyl radicals (^•OH), derived from the redox reactions of surface Fe(II)/Fe²⁺ with sulfite, were the main contributors to As(III) oxidation. In addition, As(III) oxidation was significantly enhanced in the sulfite/FeS system over a wide pH range, and oxygen was a prerequisite for the generation of free radicals. Besides, the dissolved As can be efficiently immobilized on FeS surface through co-precipitation with amorphous Fe(OH)₃ phases, which was negligibly affected by the presence of anions and organic matters. This study provides new insights into the mechanism by which sulfur species drive the transformation and immobilization of As(III) during oxygenation of iron-sulfide minerals.