As(III) Sequestration by Iron Nanoparticles: Study of Solid-Phase Redox Transformations with X-ray Photoelectron Spectroscopy Weile Yan, †,⊥ Mauricio A. V. Ramos, ‡ Bruce E. Koel, ‡,§ and Wei-xian Zhang* ,†,∥ † Center for Advanced Materials and Nanotechnology Department of Civil and Environmental Engineering and ‡ Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015, United States § Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544, United States ∥ State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai, P. R. China 20092 * S Supporting Information ABSTRACT: 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. ■ INTRODUCTION High levels of arsenic in groundwater pose a serious health threat to millions of people around the world. 1,2 The situation is of particular concern in rural areas of developing countries such as Bangladesh, India, Vietnam, and Cambodia, where there is no centralized water treatment facility and the contaminated groundwater is heavily utilized for drinking and irrigation of food crops. 3,4 Severe health implications including cancers have been traced to long-term arsenic intake, 5 and the WHO guideline imposes a stringent limit of 10 μg/L for arsenic in drinking water. 6 Many chemical treatment options have been explored for arsenic removal, and these include coagulation, adsorption, ion exchange, and membrane processes. 7−11 Coagulation with alum or ferric chloride is commonly used in large-scale water treatment plants, and the process requires careful operation control. 7 Sorbents, such as activated carbon and metal oxides, or filtration units with an ion-exchange capability can be tailored for household or small community use, which offer a more practical solution in rural regions of the affected countries. 8−11 The principal forms of arsenic in the aqueous environments are arsenite (As(III), predominantly as H 3 AsO 3 ) and arsenate (As(V), predominantly as H 2 AsO 4 − or HAsO 4 2‑ ). 12,13 Amorphous or crystalline iron oxides possess strong affinity for both As(V) and As(III) species. Under neutral pH, both As(III) and As(V) adsorb strongly onto iron oxide surfaces via surface complex formation. 13,14 The molecular structures of arsenic−iron oxide complexes have been characterized by various spectroscopic techniques. 14−18 Recent studies have shown that zerovalent iron (ZVI) is an effective remediation agent for treating arsenic-laden ground- water or drinking water. 19−23 It is generally conceived that As(III) and/or As(V) is removed by adsorbing on the iron oxide layer enclosing the ZVI particles 19,21 or forming coprecipitates with iron hydroxide produced during in situ iron corrosion. 20,23 However, spectroscopic investigations of Received: September 6, 2011 Revised: February 8, 2012 Published: February 9, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 5303 dx.doi.org/10.1021/jp208600n | J. Phys. Chem. C 2012, 116, 5303−5311