Environ. Sci. Technol. 1998, 32, 1444-1452
X-ray Absorption Fine-Structure Spectroscopy Study of Photocatalyzed, Heterogeneous As(III) Oxidation on Kaolin and Anatase A . L . F O S T E R , * ,† G . E . B R O W N , J R . , †,‡ A N D G . A . P A R K S † Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, and Stanford Synchrotron Radiation Laboratory, Stanford, California 94309
We have used X-ray absorption fine-structure spectroscopy (XAFS) to investigate As(III) adsorption on bulk Georgia kaolin (KGa-1b), Wyoming smectite (SWy-2), anatase (TiO2), gibbsite [Al(OH)3], and amorphous silica (am-SiO2) with the goal of identifying the oxidants responsible for As(III) oxidation in KGa-1b. After 30 h of reaction, g30% of the added As(III) was oxidized to As(V) in KGa-1b and anatase experiments, whereas only ≈10% of the added As(III) was oxidized in experiments using bulk SWy-2 and gibbsite. As(III) oxidation was negligible in experiments using amSiO2. The proportion of As(III) oxidized by KGa-1b increased with increasing equilibration time, decreased significantly with increasing pH (up to pH 8), and decreased slightly with increasing ionic strength. Although Mn has been proposed as the potential oxidant of As(III) in KGa-1b, it can account for only ≈2% of the As(V) produced under the conditions of our experiments because of its low concentration in this material (3 ppm). Proof that a Ticontaining phase rather than a Mn-containing phase is the primary oxidant of As(III) in KGa-1b is provided by experimental results showing that As(III) oxidation in slurries of KGa-1b and anatase is strongly dependent on both light and the concentration of oxygen, whereas the extent of As(III) oxidation by Mn(IV) is independent of oxygen concentration. The reaction of As(III) with natural or synthetic Ti-containing phases could provide new remediation strategies for arsenic. The high Ti content of the standard reference kaolins KGa-1 and KGa-1b is a result of the unique genesis of this clay deposit and may not be representative of soil kaolins in general.
Introduction Clay minerals represent a significant proportion of the reactive surface area in soils, where they can adsorb aqueous inorganic and organic species and participate in reductionoxidation (redox) reactions. In particular, electron-transfer reactions between structural or mineralogical impurities in * Corresponding author address at Stanford University, Bldg. 320, Rm. 118, Stanford, CA 94305-2115; telephone: (650) 723-4152; fax: (650) 725-2199; e-mail:
[email protected]. † Stanford University. ‡ Stanford Synchrotron Radiation Laboratory. 1444
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 10, 1998
clay minerals and toxic species sensitive to redox can affect the bioavailability and environmental fate of these species. Inorganic arsenic (As), which exists as the oxyanion species [HnAs(III)O3]n-3 and [HnAs(V)O4]n-3 in the near-surface aqueous environment, is particularly sensitive to both adsorption and electron-transfer reactions. The large-scale conversion of reduced As species in solid phases [As(0) and As(-III)] to dissolved As(III) and As(V) anionic species has been implicated in the contamination of drinking water supplies in India (1) and is also of concern in the United States (2-4). Remediation strategies for As often employ a conversion step from the highly mobile As(III) species to As(V). Arsenic(V) adsorbs strongly on mineral surfaces and forms precipitates with several common cations [e.g., Mg(II), Ca(II), and Fe(III)] under ambient conditions (5-10). Using batch uptake and chromatographic methods, Manning and Goldberg (11) determined that bulk Georgia kaolin (KGa-1) and Montana illite (IMt-1) oxidized a significant quantity of As(III), but that bulk Wyoming smectite (SWy-1) did not. Their results suggest that As(III) may not persist in surface environments, because one or more of these common clay minerals is found in almost every type of sediment and as suspended particles in lakes and oceans. However, pure kaolinite contains only Al(III) and Si(IV) cations, which do not readily participate in electron-transfer reactions under ambient conditions. For this reason, the identity of the oxidant in KGa-1 remains unclear. Since no kaolin deposit is compositionally or mineralogically pure, it is reasonable to assume that one or more additional mineral phases present in the KGa-1 or elements substituted into the kaolinite structure can oxidize As(III). Several common transition metals present in KGa-1 could conceivably be responsible for the observed oxidation of As(III). Arsenic(III) is readily oxidized in laboratory experiments using either manganese(IV) oxide (12) or Mn(III)doped goethite (13), suggesting that Mn present in impurity phases in KGa-1 could potentially oxidize As(III). Equilibrium thermodynamic calculations suggest that under acidic conditions solution iron(III) should oxidize As(III) (12), but these calculations have not been confirmed by direct observations, to our knowledge, nor has As(III) oxidation been reported in studies of As(III) adsorption on pure Fe(III) oxides and hydroxides (7, 8). Titanium (Ti), although usually considered a geochemically immobile and unreactive element (14), can also participate in electron-transfer reactions with water and adsorbed organic and inorganic constituents (15-18). Of particular relevance to this study, Gruebel (18) reported the photocatalyzed oxidation of selenite [Se(IV)] to selenate [Se(VI)] by anatase (TiO2) in the presence of oxygen and light. An analogous oxidation reaction of As(III) by Ticontaining phases in KGa-1 is plausible and may also be oxygen- and light-dependent. We have used X-ray absorption near-edge structure (XANES) spectroscopy to investigate As(III) sorption and oxidation on bulk KGa-1b kaolinite, SWy-2 smectite (clays from the same deposits as the KGa-1 and SWy-1 clays used in previous studies), anatase, gibbsite [Al(OH)3], and amorphous silica (am-SiO2) with the goal of identifying oxidants responsible for As(III) oxidation in KGa-1 and KGa-1b. XANES spectroscopy is uniquely suited for this investigation because As oxidation state can be quantitatively determined in complex heterogeneous solid and liquid samples without the need for chemical preparations that could change the chemical state of species of interest (19-25). S0013-936X(97)00846-8 CCC: $15.00
1998 American Chemical Society Published on Web 04/08/1998
TABLE 1. Compositional Analyses of Sorbents oxide/element KGa-1b SWy-2 anatase gibbsite silica XRF (wt % oxide) (kaolin) (smectite) (TiO2) [Al(OH)3] (am-SiO2) SiO2 (0.01) Al2O3 (0.01) CaO (0.01) MgO (0.01) Na2O (0.01) K2O (0.01) Fe2O3 (0.01) TiO2 (0.001) P2O5 (0.01) LOI (0.01)b sum ICP-AES (mg/kg) V (2) Sc (0.5) Cr (1) Mn (2) Ni (1) Cu (0.5) Zn (0.5)
43.9 40.1
