Bioaccessibility and Speciation of An Organoarsenical in Drinking-Water Treatment Residual Amended Soil: A Long Term Greenhouse Study

Until recently organoarsenical-based pesticides were allowed to be used for agricultural purposes under the assumption that they are non-carcinogenic. However, there has always been a serious concern regarding potential release of arsenic (As) in the environment from such pesticide application. Associated evidences of organoarsenical transformation and degradation to carcinogenic inorganic As species in agricultural and golf sites demand further consideration. Earlier work in our lab showed that the drinking-water treatment residuals (WTRs) were effective sorbents of inorganic forms of As. Hence, we hypothesized that WTR could also reduce the bioaccessibility for organoarsenical in soils. A 3 year greenhouse experiment was set up to investigate the long-term WTR-amendment effects on As bioaccessibility in two soils, one acidic (Immokalee series) and one basic (Orelia series) spiked with dimethylarsinic acid or DMA (1500 mgAs kg-1) and amended with an Al-WTR and Fe-WTR at two rates (5% and 10% by wt.). Soil sampling was done immediately after spiking (time zero), 0.25, 0.5, 1, and 3 years of equilibration. Soil samples were subjected to an in-vitro gastric phase bioaccessibility test followed by aqueous speciation using a coupled HPLC-ICPMS setup. Results showed that As bioaccessibility in the WTR-amended soils significantly (p<0.001) decreased by 40-70% within 3 years compared to unamended control. The Fe-WTR was more effective than Al-WTR in decreasing soil As bioaccessibility. The in-vitro and water extracted samples were subjected for As speciation at time final (3rd year) and results showed routine transformation of DMA into As (V), irrespective of the form of WTR. The Orelia soil showed significantly (p<0.001) higher transformation compared to Immokalee soil. This is the first greenhouse study documenting WTR as an effective remedial agent for organoarsenical contaminated soils.