Thursday, 13 July 2006

Speciation and Geochemical Cycling of Pb, As, Cr, and Cd in a Contaminated Histosol: X-ray Absorption Near-edge Structure (XANES) Spectroscopy.

Xiadong Gao and Darrell G. Schulze. Purdue Univ, Agronomy Dept, 915 W. State St., West Lafayette, IN 47907-2054

Metal contamination of soils is a widespread problem at many current and former industrial and military sites. Lead, As, Cr, and Cd are of particular concern because of their toxicity and potentially harmful effects on the environment. In situ immobilization is a desirable strategy for reducing metal bioavailability through precipitation or adsorption by adding chemical amendments to contaminated soils. Most current investigations of soil heavy metal contamination emphasize the importance of speciation of metals rather than just the total amounts present. Speciation is the key factor in controlling mobility and bioavailability of metals in soils, and information on the mineralogy and geochemistry of contaminant metals can provide important information for risk assessment and remediation strategies. Nondestructive techniques, such as x-ray absorption near edge structure (XANES) spectroscopy, synchrotron micro x-ray fluorescence (SXRF) spectroscopy, and micro x-ray diffraction (μ-XRD) are useful to investigate metal speciation. We sampled a Histosol in a peat bog that receives runoff and seepage water from the site of a former lead smelter. The water table is at or slightly above the soil surface for most of the year and the site is covered with common reed (Phragmites australis) and cattail (Typha sp.). There is an intense redox gradient with depth that probably varies as the water table fluctuates during the year. Distinctly different layers were observed from the surface to a depth of 65 cm. We collected soil samples from different depths during both wet and dry seasons. Along with major elements such as Fe and S, the soil contains 1,700 mg kg-1 Pb, 2,200 mg kg-1 As, 930 mg kg-1 Cr, 210 mg kg-1 Cd. We used a binocular microscope to select aggregates ~100 to 200 mm in diameter that had distinctive morphologies and appeared to be inorganic. We used the synchrotron x-ray microprobe on beamline X26A at the National Synchrotron Light Source at Brookhaven National Laboratory to obtain micro x-ray diffraction patterns and micro x-ray fluorescence patterns of each aggregate. Arsenic K-XANES spectra were then obtained for aggregates with high levels of arsenic. Arsenic occurs mainly as inorganic +3 and +5 oxidation states in soils, with As(III) generally being more mobile and more toxic than As(V). XANES spectra show distinct differences in As oxidation state with depth (see figure below). Soil particles from the surface layer contained predominately goethite and schwertmannite and contained only As(V), probably as stable inner-sphere surface complexes between the arsenate anion and the Fe oxyhydroxide surface. The particles from subsurface layers that contained mainly magnetite contained predominately As(V) with some As(III). Particles that contained mainly siderite contained predominately As(III) with some As(V). Particles from the most reduced layer contained As(II), which is consistent with the presence of realgar as shown by μ-XRD. From the XANES results we postulate that arsenic mobility is low in the oxidized surface layer and in the deepest, most reduced subsurface layer. Arsenic is controlled by the adsorption to iron oxyhydroxides in the oxidized zone at the soil surface, and by metal sulfides in the most reduced zone. Substantial mobilization of arsenic may occur, however, under shifting redox conditions such as in the layers that contain magnetite and siderite and high contents of As(III).  

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