Extractability and bioavailability of arsenic in soils and the effect of iron remediation efforts

Scope and method of study. Objective for part one was to examine the relationship between soil properties (pH, clay, organic matter and Fe-oxide) and arsenic phyto-availability and extractability and develop mathematical models to describe the relationship. Objectives of part two and three were to evaluate four iron-containing materials for remediation in arsenic contaminated soils and waste medium. Evaluation includes multiple pathways: extractability, plant bioavailability, earthworm bioavailability and human bioaccessibility. Twenty-two soils were selected based on their soil properties for part one. The soils were spiked with 250-mg/kg arsenic and evaluated for extractability, plant bioavailability and earthworm bioavailability. Part two used iron-containing amendments (added at Fe:As, 20:1 molar ratio) to remediate four arsenic spiked soils and one slag waste material. Amendments included FeCl₃, Fe₂(SO₄)₃, zero-valence Fe and Fe-water treatment residual (Fe-WTR). Evaluation was done first for ecological parameter (extractability, plant bioassays, and earthworm bioassays) then human bioaccessibility was addressed using a bench scale in vitro gastrointestinal procedure.; Findings and conclusions. The ranges in plant yield and germination rates were the results of differences in arsenic bioavailability due to arsenic interaction with soil properties. Models and regressions indicate that Fe-oxide and clay content are the two most important soil parameters in predicting plant bioavailability. Significant relationships were found with pore water (P = 0.001), Bray-1 (P = 0.001) and Na-phosphate (P = 0.01) extraction of arsenic and lettuce response. Pore water and Bray-1 were also predictive for earthworm mortality (P = 0.004). The amount of extractable arsenic varied widely with sandy, low Fe-oxide soils having high amounts of extractable arsenic and soils high in clay and Fe-oxides had little extractable arsenic. In part two all Fe-treatments were successful in reducing the amount water-soluble and plant available arsenic. Germination rates and lettuce shoot growth increased in all soils and slag with the addition of Fe as compared to the untreated arsenic spiked soils. Fe-WTR had the best overall performance in reduction of extractable arsenic and increasing plant viability. Furthermore Fe-treatments were able to reduce the bioaccessible fraction of arsenic from the soils. The slag waste has reductions in bioaccessible arsenic in all Fe-treatments except the zero-Fe.