Simultaneous oxidation and reduction of arsenic by zero-valent iron nanoparticles:Understanding the significance of the core-shell structure

Increasing evidence suggests that nanoscale zerovalent iron (nZVI) is effective for the removal of arsenic from contaminated water, but the immobilization mechanism is unclear. In particular, the existence of As(0) on the nanoparticle surface has been proposed but not substantiated in prior studies. By using high-resolution X-ray photoelectron spectroscopy (HR-XPS), we report clear evidence of As(0) species on nZVI surfaces after reactions with As(III) or As(V) species in solutions. These results prove that reduction to elemental arsenic by nZVI is an important mechanism for arsenic immobilization. Furthermore, reactions of nZVI with As(III) generated As(0), As(III), and As(V) on the nanoparticle surfaces, indicating both reduction and oxidation of As(III) take place with nZVI treatment. The dual redox functions exhibited by nZVI are enabled by its core-shell structure containing a metallic core with a highly reducing characteristic and a thin amorphous iron (oxy)hydroxide layer promoting As(III) coordination and oxidation. Results demonstrated here shed light on the underlying mechanisms of arsenic reactions with nZVI and suggest nZVI as a potential multifaceted agent for arsenic remediation.