CHARACTERIZATION OF ARSENIC CONTAMINATED SOILS FROM POWER STATION FACILITIES

Some soils from electrical substation sites located in densely populated residential areas are heavily contaminated with arsenic because of long-term use of arsenic-containing herbicides in the past two decades. Two soil samples (BH and FW) from representative contaminated sites were selected to characterize the occurrence of arsenic and its environmental leachability. The soils are both sandy loams with varying mineralogical and organic matter contents. Microwave assisted acid digestion (EPA method 3051) of contaminated soils indicates environmentally available arsenic levels of up to 300 and 900 ppm (dry weight basis) for FW and BH soils, respectively. Synthetic acid rain (EPA method 1312) sequential leaching experiments show distinct arsenic leaching behaviors of the two studied soil samples. BH soil, which has the highest arsenic content, shows a slow, steady release of arsenic. In contrast, FW soil, with a lower arsenic content, shows less affinity for arsenic and therefore a much quicker release behavior upon leaching. X-ray powder diffraction analysis (XRD) failed to find any detectable arsenic-bearing phases in either of the studied soil samples. However, scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, and acid hydroxylamine extraction experiments indicate that arsenic is partly associated with amorphous iron oxide coatings on the surface of fine-grained materials in both of the studied soils. Electron microprobe analysis (EMPA) of fine-grained materials separated from the two soil samples further detected a crystalline arsenic-bearing phase in BH soil. This crystalline phase is responsible for the observed difference in the arsenic leaching behaviors of the studied soils. Factors controlling the formation and derivation of this arsenic-bearing phase are discussed.