Received for review July 1, 2005

Revised manuscript received November 10, 2005

Accepted November 16, 2005

Abstract:

The arsenic (As) solid-state speciation (i.e., oxidation state, precipitates, and adsorption complexes) is one of the most important factors controlling dissolved As concentrations at As contaminated sites. In this case study, two representative subsurface samples (i.e., oxidized and semi-reduced sites) from former lead arsenate contaminated soils in the northeastern United States were chosen to investigate the effects of aging on As retention mechanisms using multiscale spectroscopic techniques. X-ray powder diffraction (XRD), synchrotron based microfocused () XRD, in situ -synchrotron based X-ray fluorescence spectroscopy (SXRF), and -X-ray absorption near edge structure (XANES) spectroscopy were used to compliment the final bulk X-ray absorption spectroscopy (XAS) analyses. In the sample from an oxic area, As is predominantly (~71%) present as As(V) adsorbed onto amorphous iron oxyhydroxides with a residue (~29%) of an original contaminant, schultenite (PbHAsO₄). Contrarily, there is no trace of schultenite in the sample from a semi-reduced area. Approximately 25% of the total As is present as adsorbed phases on amorphous iron oxyhydroxide and amorphous orpiment (As₂S₃). The rest of the fractions (~46%) were identified as As(V)-Ca coprecipitates. This study shows that aging effects can significantly alter the original chemical constituent (schultenite) in soils, resulting in multi and site-specific As solid-state speciation. The variability in spatial and temporal scale may be important in assessing the environmental risk and in developing in situ remediation technologies.

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