Mobility and Bioaccessibility of Arsenic In Soil Contaminated by Arsenic Trioxide

A sandy loam soil contaminated by a one-time massive application of arsenic trioxide approximately 50 years ago has been extensively studied to determine the speciation and mobility of arsenic. These studies show that 200-900 ppm of environmentally available arsenic, as As(V), remains in the near-surface soil after more than 5 decades of natural weathering. Between 30%-65% of the total acid-digestible arsenic is mobilized by long-term sequential leaching by synthetic acid rain. Batch and column experiments demonstrate that ferrous sulfate treatment of the contaminated soil significantly reduces the mobility of arsenic during synthetic acid rain leaching compared to the untreated soil. Arsenic mobility is decreased by co-precipitation with, and adsorption onto, ferric hydroxide.

The bioaccessibility of arsenic in the <250 micron fraction of the untreated soil, and in the <250 micron fraction of soil treated at a variety of soil to treatment solution ratios, has been estimated using a modified in vitro gastrointestinal (IVG) method. This method, in which arsenic-contaminated soil is placed in contact with simulated gastrointestinal solutions, has been shown to be linearly correlative with an in vivo bioavailability model. Comparison of IVG bioaccessible arsenic with arsenic extracted by a variety of other techniques show similar trends with increasing treatment solution to soil mass ratio. IVG bioaccessible arsenic is decreased by increased treatment solution to soil mass ratios and is not correlated with IVG bioaccessible iron, indicating that the in vitro test extracts arsenic that is not chemically fixed by the ferrous sulfate treatment.