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.

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, nZVIlimiting 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.

Multi-tiered arsenic distributions within nanoscale zero-valent iron (nZVI) were observed upon reactions of nZVI with aqueous As(iii) species, suggesting arsenite oxidation and reduction may occur in parallel at different sub-domains of the nanoparticles owing to the particle's core-shell structure. New insights into arsenic remediation with iron nanomaterials are obtained.

Brushite samples doped with Ni(II) in different concentrations, from 5% to 20%, were prepared in aqueous solution at pH ¼ 7 and at two temperatures: 25 and 37 C. The solid samples were characterized by chemical analysis, infrared spectroscopy (FTIR) and x-ray powder diffraction (XRPD). Chemical analysis has shown Ni(II) almost complete incorporation to the solid phase up to 15%. X-ray diffraction patterns have allowed to identify brushite phase with almost no modification of the line breadth and only small shifts of lines positions with increasing Ni(II) incorporation up to 15%. For larger Ni(II) concentration, in solution, a mixture of phases has been detected. Infrared spectra have supported diffraction results. For Ni(II) 20% and over the characteristic bands of HPO 4 2anions tend to vanish, and the typical shaped PO 4 3À bands are observed. These results have allowed to establish that the presence of low levels of Ni in the synthetic process not only helps brushite formation; but, also prevents brushite from apatite conversion and, in addition, preserves brushite crystallinity. According to these findings, it is possible to propose that nickel traces present in the urinary system might be a trigger to brushite stone formation and/or growth, rather than the expected brushite conversion to hydroxyapatite. This outcome would explain the recurrent detection of difficult to treat brushite stones, observed in the last three decades.

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