Environ. Sci. Technol. 2010, 44, 109–115
Evidence for Different Surface Speciation of Arsenite and Arsenate on Green Rust: An EXAFS and XANES Study Y U H E N G W A N G , * ,† G U I L L A U M E M O R I N , † GEORGES ONA-NGUEMA,† FARID JUILLOT,† FRANC ¸ OIS GUYOT,† GEORGES CALAS,† AND G O R D O N E . B R O W N , J R . ‡,§ Institut de Mine´ralogie et de Physique des Milieux Condense´s (IMPMC), UMR 7590, CNRS, Universite´ Paris 6 (UPMC), Universite´ Paris 7 (UPD), IPGP, 140, rue de Lourmel, 75015 Paris, France, Surface & Aqueous Geochemistry Group, Department of Geological & Environmental Sciences, Stanford University, Stanford, California 94305-2115, and Stanford Synchrotron Radiation Lightsource, SLAC, National Accelerator Laboratory, 2575 Sand Hill Road, MS 69, Menlo Park, California 94025
Received June 3, 2009. Revised manuscript received August 9, 2009. Accepted September 14, 2009.
The knowledge of arsenic speciation at the surface of green rusts (GRs), [FeII(1-x)FeIIIx (OH)2]x+ (CO3, Cl, SO4)x-, is environmentally relevant because arsenic sorption onto GRs could contribute to arsenic retention in anoxic environments (hydromorphic soils, marine sediments, etc.). The nature of arsenic adsorption complexes on hydroxychloride green rust 1 (GR1Cl) at nearneutral pH under anoxic conditions was investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy at the As K-edge. Sorption data indicate that As(V) sorbs more efficiently than As(III) at the studied As loadings (0.27 µmol m-2 and 2.7 µmol m-2). EXAFS results indicate that arsenite [As(III)] and arsenate [As(V)] form inner-sphere complexes on the surface of GR1Cl at arsenic surface coverages of 0.27 and 2.70 µmol m-2, with distinct types of As(III) and As(V) sorption complexes, which change in relative concentration as a function of arsenic loading. For As(V), the EXAFS-derived As-Fe distances (3.34 ( 0.02 and 3.49 ( 0.02 Å) suggest the presence of binuclear bidentate double-corner complexes (2C) and monodentate mononuclear corner-sharing complexes (1V). For As(III), EXAFS-derived As-As distance (3.32 ( 0.02 Å) and As-Fe distances (3.49 ( 0.02 and 4.72 ( 0.02 Å) are consistent with the presence of dimers of As(III) pyramids binding to the edges of the GR1Cl layers by corner sharing with FeO6 octahedra. However, 2C and 1V As(III) complexes cannot be excluded. These results improve our knowledge of the mode of As(V) and As(III) inner-sphere adsorption on green rusts, which will help to constrain sorption modeling of arsenic in soils, sediments, and aquifers.
* Corresponding author e-mail:
[email protected]. † Institut de Mine´ralogie et de Physique des Milieux Condense´s (IMPMC). ‡ Stanford University. § Stanford Synchrotron Radiation Lightsource. 10.1021/es901627e
2010 American Chemical Society
Published on Web 10/01/2009
Introduction Iron and arsenic bioreduction in anoxic media is thought to be one of the processes responsible for arsenic contamination of groundwaters in various localities, especially in Southeast Asia (1, 2). Fe(II)-bearing minerals such as green rusts (GRs) that commonly form in hydromorphic soils (3, 4) during bioreduction reactions can influence the mobility of toxic trace elements such as arsenic via sorption and coprecipitation processes (5-7). GRs are mixed valence Fe(II)-Fe(III) layered double hydroxides, whose crystal structure is best described as consisting of brucite-like layers [FeII(1-x)FeIIIx (OH)2]x+ and whose positive charge is compensated by interlayer anions; hydrogen bonding brought by water molecules ensures additional cohesion (8). Under laboratory conditions (9, 10), GRs can be formed via bioreduction of lepidocrocite (γ-FeOOH) by Shewanella sp., which are common dissimilatory iron-respiring bacteria (DIRB) occurring in a wide range of habitats. Recent studies (11, 12) of As-containing high-iron sediments in North Haiwee Reservoir (Olancha, CA) have shown that green rust-like solids may play an important role in controlling arsenic release during the bioreduction of ferric oxides. Moreover, arsenic sorption onto products of iron metal corrosion such as magnetite and GR has been shown to play an important role in a recently proposed water treatment process for arsenic (13-15). These environmental implications have emphasized the need for improved knowledge of the nature of arsenic sorption complexes at the surface of these Fe(II)-containing minerals. In the case of Fe-spinel minerals, several recent studies have reported spectroscopic evidence for a new type of As(III) sorption complex at the magnetite-water (16, 17) and maghemite-water interfaces (18) with tridentate bonding geometry. In the case of GR, Ona-Nguema et al. (19) proposed for the first time that multinuclear As(III) sorption complexes form on nano-Fe(OH)2 and GR particles based on X-ray absorption spectroscopic analysis, while Jo¨nsson and Sherman (20) suggested that monomeric As(III) complexes form on GR particles. In this context, the present study aimed at determining the nature of the As(III) surface complexes on GR, which may help further the progress in modeling arsenic sorption to common soil minerals, as recently demonstrated for goethite by Stachowicz et al. (21). We investigated the reactivity of As(III) and As(V) with respect to fine particles of synthetic hydroxychloride GR1 (GR1Cl) using X-ray absorption fine structure (XAFS) spectroscopy in order to determine the nature of the As(III) and As(V) surface complexes as a function of surface coverage. Under pH and As-loading conditions representative of natural systems, XAFS data show that As(III) surface complexes on GR1Cl differ in binding mode from those of As(V), which has also been found for As(III) and As(V) sorption on ferric (oxyhydr)oxides (13, 22, 23).
Experimental Section Sorption Experiments. GR1Cl (general formula: FeII4(1-x)FeIII4xOH8Cl · nH2O) was synthesized by aerial oxidation of a ferrous hydroxide suspension in the presence of a slight excess of dissolved ferrous chloride (3). X-ray powder diffraction (XRD) data indicated that the samples consisted of GR1Cl, with halite (NaCl) used as the internal standard. XRD patterns of 24 h sorption samples indicated that GR1Cl had not undergone any mineralogical transformation, and no significant peak shift was observed after arsenic sorption. No measurable amount (
