Role of Activated Carbon in Hydroxyl Radical Production and Secondary Mineral Formation during Iron Sulfide (FeS) Oxidation

Anthropogenic inputs of engineered carbon materials into aquatic and terrestrial environments potentially influence electron transfer processes in redox biogeochemical reactions due to their electron-shuttling capacities. However, the impact of carbonaceous materials on the oxidative transformation of iron sulfide (FeS), a widespread ferrous mineral in sediments and soils, and the underlying mechanisms remain insufficiently understood. In this study, we demonstrated that activated carbon (AC) facilitated FeS oxidation, resulting in enhanced hydroxyl radicals (•OH) production. Electron transfer processes were facilitated by AC through aqueous phase electron shuttling and solid-solid interaction via the voltaic effect. Electrons derived from S(-II) were utilized to regenerate Fe(II), thereby promoting the formation of elemental sulfur (S0) and •OH. While crystalline lepidocrocite formed on the surface of FeS after oxidation, the presence of AC induced the formation of low-crystalline Fe(III) (oxyhydr)oxides (i.e., ferrihydrite) and a new Fe3S4 phase. These findings highlight the role of AC in facilitating electron transfer, enhancing •OH production, and altering mineral phase transformation under redox fluctuations, thus providing new insights into the biogeochemical behavior of iron sulfide minerals in the presence of carbonaceous electron shuttles.