SOIL CONTAMINATED WITH ARSENIC TRIOXIDE AS A LONG-TERM SOURCE OF ARSENIC TO GROUNDWATER

Arsenic trioxide is one of several arsenic compounds used as a component of herbicides and pesticides. Arsenical pesticide and herbicide applications during the 1950s and 1960s have contaminated soils at many industrial and agricultural sites. Even after 30-50 years of exposure to rainfall and irrigation, many of these soils contain several hundred ppm of As on a dry weight basis.

Characterization of the solid-phase speciation of arsenic in soil is difficult. Conventional electron-beam analytical instruments do not have sufficiently low detection levels to make direct observations of most arsenic-bearing solids possible. Synchrotron micro-spectroscopic techniques allow the speciation and metal associations of arsenic to be determined. XANES spectra of two contaminated soil samples show that the arsenic is in the +5 valence state. Micro-XRF maps demonstrate strong correlations between As, Fe and Ca. In the soil containing 900 ppm total digestible As, trace amounts of phaunoxite were tentatively identified by micro-XRD and XANES. Aluminum, calcium and iron arsenates are insoluble and adsorb to oxide and clay surfaces as inner-sphere complexes. Because the soils studied have low TOC contents and are well-drained, arsenic is unlikely to be in organometallic form.

Sequential chemical extractions performed on the contaminated soils indicate that approximately 50% of the extractable arsenic is associated with the non-specifically adsorbed and the specifically adsorbed soil fractions. Long-term batch sequential leaching of the contaminated soils with SPLP (EPA Method 1351a) fluid removed all of the non-specifically adsorbed and about half of the specifically adsorbed As from the soil. The two soils studied showed distinct leaching behaviors, indicating that the As is held differently in the soils. Sequential chemical extraction results suggest that these differences are caused by different amorphous Al and Fe oxyhydroxide contents. The results of this study indicate that soils contaminated with arsenic trioxide will serve as a source of As to soil and groundwater for more than a century.