Speciation and geochemical cycling of lead, arsenic, chromium, and cadmium in a metal-contaminated histosol.

摘要

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. Speciation is the key factor in controlling mobility and bioavailability and information on the mineralogy and geochemistry of contaminant metals is important for developing in-situ remediation strategies. We sampled a Histosol that received runoff and seepage water from the site of a former lead smelter. 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 (mu-XRD) and micro X-ray fluorescence patterns (mu-XRF) for soil aggregates ∼100--200 mum in diameter. Arsenic and chromium X-ray absorption near edge structure (XANES) spectra were then obtained for aggregates with significant concentrations of arsenic or chromium. Results show a clear pattern of metal speciation changes with depth. The oxidized yellow surface layer (0--10 cm depth) is dominated by goethite and poorly crystalline akaganeite. Lead and arsenic are highly associated with these Fe oxides by forming stable inner-sphere surface complexes. The occurrence of akaganeite in a natural soil is reported for the first time in this thesis. Gypsum, schwertmannite, and jarosite were identified in the surface layer as well, particularly for samples collected during dry periods. Fe(II)-containing minerals, including magnetite, siderite, and possibly wustite occur in the intermediate layers (10--30 cm depth). The unusual presence of hematite and wustite in the subsurface horizons is probably the results of a burning event at this site. Iron, lead, and arsenic sulfide minerals predominate at depths > ∼30 cm and phases included realgar, greigite, galena, and sphalerite, alacranite, and others. Most of these minerals occur as almost pure phases in sub-millimeter aggregates and appear to be secondary phases that have precipitated from solution. Mineralogical and chemical heterogeneity and the presence of phases stable under different redox conditions make this a challenging soil for in-situ remediation.