EFFECT OF EQUILIBRATION TIME ON ARSENIC BIOAVAILABILITY IN TWO FLORIDA SOILS CONTAMINATED WITH ARSENICAL PESTICIDE

Soil investigation of various agricultural lands have revealed elevated concentrations of arsenic due to past arsenical pesticide applications. High residual levels of arsenic threaten public health increasingly as urban growth encroaches on former agricultural lands. Proper evaluation of arsenic bioavailability as a function of soil properties may be critical in establishing cleanup levels, selecting the most appropriate remediation technology and allocating costs for optimal cleanup. So far, little attention has been paid to investigate the reactivity, distribution and stability patterns of the arsenical pesticides in different soil types. Keeping this in mind, an incubation study was designed to determine the effect of pesticide application on geochemical speciation and in-vitro bioavailability of arsenic in chemically variant soils with particular emphasis on the effect of soil aging on arsenic geochemistry. Two agricultural soils, both sandy Spodosols from Florida were selected for these investigations, namely Immokalee (low P, low Fe/Al) and Millhopper (high P, high Fe/Al). Selected soils were amended with sodium arsenite at three rates (45, 225, and 450 mg/kg) and were analyzed at 0 time (immediately after spiking the soil with pesticide), four months, and after 1-year incubation. A sequential extraction scheme was employed to identify the geochemical forms of arsenic in soils (soluble, exchangeable, organic, Fe/Al bound, Ca/Mg-bound, residual). Concentrations of these operationally defined soil arsenic forms were correlated with the “in-vitro” bioavailable fractions of arsenic to identify the arsenic species that are most likely to be bioavailable. Results from this study showed that arsenic is strongly adsorbed onto amorphous iron/aluminum oxides in soils, and the degree of arsenic retention is a direct function of equilibration time. Hence, bioavailability of arsenic was observed to be much lower in the Millhopper soil with high oxalate-extractable Fe/Al content than the Immokalee soil. With increasing soil aging, a greater percentage of total soil arsenic was rendered unavailable to the human gastrointestinal system in both soils.