title: Mechanism of Arsenic Adsorption on Magnetite Nanoparticles from Water: Thermodynamic and Spectroscopic Studies.
abstract: Removal of arsenic (As) from water supplies is needed to reduce As exposure through drinking water and food consumption in many regions of the world. Magnetite nanoparticles (MNPs) are promising and novel adsorbents for As removal because of their great adsorption capacity for As and easy separation. This study aimed to investigate the adsorption mechanism of arsenate, As(V), and arsenite, As(III), on MNPs by macroscopic adsorption experiments in combination with thermodynamic calculation and microspectroscopic characterization using synchrotron-radiation-based X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). Adsorption reactions are favorable endothermic processes as evidenced by increased adsorption with increasing temperatures, and high positive enthalpy change. EXAFS spectra suggested predominant formation of bidentate binuclear corner-sharing complexes ((2)C) for As(V), and tridentate hexanuclear corner-sharing ((3)C) complexes for As(III) on MNP surfaces. The macroscopic and microscopic data conclusively identified the formation of inner-sphere complexes between As and MNP surfaces. More intriguingly, XANES and XPS results revealed complex redox transformation of the adsorbed As on MNPs exposed to air: Concomitant with the oxidation of MNPs, the oxidation of As(III) and MNPs was expected, but the observed As(V) reduction was surprising because of the role played by the reactive Fe(II).
authors.forename: Cheng-Hua
authors.initials: CH
authors.lastname: Liu
authors.forename: Ya-Hui
authors.initials: YH
authors.lastname: Chuang
authors.affiliation: §Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan.
authors.forename: Tsan-Yao
authors.initials: TY
authors.lastname: Chen
authors.forename: Yuan
authors.initials: Y
authors.lastname: Tian
authors.forename: Hui
authors.initials: H
authors.lastname: Li
authors.affiliation: ∥Department of Agricultural Chemistry, National Taiwan University, Taipei 10617, Taiwan.
authors.forename: Ming-Kuang
authors.initials: MK
authors.lastname: Wang
authors.forename: Wei
authors.initials: W
authors.lastname: Zhang
mesh_terms: D000327:Adsorption; D001151:Arsenic / Q000302:isolation & purification*; D006863:Hydrogen-Ion Concentration; D007526:Isoelectric Point; D058185:Magnetite Nanoparticles / Q000737:chemistry*; D009994:Osmolar Concentration; D010084:Oxidation-Reduction; D056951:Photoelectron Spectroscopy; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*; D014881:Water Supply; D056928:X-Ray Absorption Spectroscopy
chemical_list: D058185:Magnetite Nanoparticles; D014867:Water; D001151:Arsenic