Enhanced removal of roxarsone by Fe3O4@3D graphene nanocomposites: synergistic adsorption and mechanism

Roxarsone (ROX) is an emerging arsenic pollutant due to its potential degradation into highly toxic inorganic arsenic species in the environment. Adsorbents which can capture ROX with both high capacity and affinity are urgently needed. Herein, a nanocomposite of nano-Fe3O4 and three-dimensional graphene (Fe3O4@RGO)was designed, aiming to simultaneously attract arsenate and benzene groups in ROX. Characterization of the nanocomposite revealed that Fe3O4 nanoparticles (20-50 nm) with exposed (400) planes were highly dispersed on the graphene support. Adsorption experiments showed that Fe3O4@RGO had higher adsorption capacity, affinity, and adsorption rate towards ROX than pristine materials and efficiently removed ROX from both simulated natural and waste waters. The adsorption mechanism was confirmed as a synergetic interaction of As-Fe coordination, hydrogen bonding and pi-pi interaction. X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) results suggested that the As-Fe complexes and hydrogen bonds between Fe3O4@RGO and ROX were stronger than those in pristine nano-Fe3O4, due to the greater number of surface hydroxyls and shorter As-Fe atomic distance in Fe3O4@RGO. The pi-pi interaction between ROX and the graphene part in Fe3O4@RGO was also enhanced. This study provided a novel idea for designing materials to remove pollutants with both inorganic and organic moieties, such as phenylarsonic acid compounds, from water.