As(III) sequestration by iron nanoparticles: Study of solid-phase redox transformations with X-ray photoelectron spectroscopy

Nanoscale zerovalent iron (nZVI) has shown a high efficacy for removing arsenite (As(III)), a groundwater contaminant of great concern, yet the chemical transformations of As(III) enabled by nZVI during the sequestration process are not well understood. Using high-resolution X-ray photoelectron spectroscopy (HR-XPS), arsenic in multiple valence states was observed for nZVI particles reacted with aqueous As(III), which establishes that nZVI is capable of inducing As(III) oxidation and reduction, a unique attribute imparted by the core-shell nature of nZVI particles. Time-dependent analysis shows that As(III) oxidation was a facile and reversible reaction taking place at the surface of the iron oxide shell, whereas As(III) reduction occurred at a slower rate and led to gradual diffusion and accumulation of reduced arsenic at a subsurface layer near the Fe(0) core. Long-term (146 days) exposure of the arsenic-laden nZVI in an aqueous environment caused progressive depletion of the Fe(0) cores; however, arsenic was retained in the native oxide shell without leaching into the aqueous phase. The speciation of arsenic in the nanoparticles is strongly dependent on the loading of nZVI. While a large proportion of the arsenic was bound in a reduced state in the presence of ample nZVI, nZVI-limiting conditions resulted in rapid depletion of the Fe(0) cores and enclosure of arsenic within the oxide formation. These results show that the mechanism of nZVI-mediated arsenite removal is substantially different from that of conventional iron oxide-based adsorbents. Encapsulation of arsenic into the bulk of the solid phase suggests nZVI a potentially more capacious and robust sequestration agent for arsenic abatement.