Drinking Water Treatment Residual Amendment Lowers Inorganic Arsenic Bioaccessibility in Contaminated soils: A Long-Term Study

Inorganic arsenical pesticides were used widely in agriculture for a long time, leaving large tracts of farmlands and orchards contaminated with high levels of arsenic. These contaminated soils pose a significant public health risk as residential developments encroach on these lands due to urban sprawl. Several studies have documented the effectiveness of iron and aluminum hydroxides in immobilizing arsenic in contaminated soils. Solid residues left from drinking water treatment (Drinking water treatment residuals, or WTRs) have been proposed as a low-cost and effective amendment, as they contain large amounts of aluminum (Al) and iron (Fe) oxides. By conducting in vitro tests after 1 year of equilibration, our group recently documented the effectiveness of two types of WTRs (Fe- and Al-based) in significantly (p∈<∈0.01) lowering As bioaccessibility. However, long-term studies under realistic conditions are necessary to test the ability of WTRs in reducing soil arsenic bioaccessibility. In the current study, the effect of WTRs on inorganic arsenic bioaccessibility and distribution in two soils (Immokalee and Orelia) with variable properties was evaluated over a 3-year time period in a greenhouse setup. Results show that arsenic bioaccessibility decreased significantly (p∈<∈0.001) from 100 to ∼25 % after 3 years of equilibration, compared to the unamended controls. There was a significant (p∈<∈0.001) decline in water-soluble arsenic with time, accompanied by an increase in Fe/Al and Ca/Mg-bound fractions, suggesting that these phases control the mobility of arsenic. The negative correlation between arsenic bioaccessibility Fe/Al- and Ca/Mg-bound fractions can be attributed to the transformation of soluble arsenic to less soluble mineral phases.