Adsorption and reduction of roxarsone on magnetic greigite (Fe3S4): Indispensable role of structural sulfide

Roxarsone (ROX) is an organoarsenic compound that is widely used as an additive in swine and poultry feed to inhibit disease and promote growth. Due to its low metabolism in animals, it is excreted in animal urine and feces leading to its widespread contamination of soils and aquatic ecosystems. Herein, we demonstrated that ROX can be adsorbed and subsequently reduced by a common iron sulfide (greigite - Fe3S4) in the pH range of 3.6 to 8.6. ROX removal processes followed a pseudo-second-order kinetic model that was strongly pH dependent. The nitro group of ROX was reduced by structural sulfide rather than dissolved sulfide or ferrous iron to generate an amino-group containing product, 4-hydroxy-3-aminophenylarsonic acid (HAPA). At neutral to alkaline pH values ROX and HAPA are preferentially adsorbed rather than reduced on the Fe3S4 surface. The interaction between ROX and Fe3S4 was minimally affected by interactions with coexisting cations, anions and natural organic matter (humic acid). These novel findings provide new insights for understanding the transformation mechanism of ROX by iron sulfides minerals, and have practical application for designing efficient systems for advanced treatment of ROX.