The Sorption of Roxarsone, an Organoarsenical Animal Feed Additive

Abstract

The organoarsenical roxarsone is added to poultry feed to increase weight gain. Studies have shown that roxarsone does not accumulate in poultry tissue but is excreted, resulting in elevated arsenic concentrations (~40 mg/kg) in poultry litter. However, there is little understanding of the fate of roxarsone once it is introduced into agricultural watersheds.

Using batch experiments, I investigated the sorption characteristics of roxarsone to Ap and Bt soils of the Frederick series, common in the Shenandoah Valley of Virginia, an area of intense poultry production. Results demonstrate that roxarsone sorbs strongly to Bt soils, but only showed moderate to low sorption onto Ap soils. Sorption to the Ap soils demonstrated stronger pH dependence than did sorption to the Bt soils. Removing organic matter (OM) from Ap soils significantly changed the sorption characteristics, suggesting that OM may be coating mineral surfaces in these soils.

Results of this study have implications for roxarsone transport in agricultural watersheds. For soils that have had years of poultry litter application, there will be both sorption and subsequent leaching of roxarsone. In the OM-rich Ap horizon, OM controls sorption. Because roxarsone is loosely bound to OM, it would be rapidly leached into water after a recharge event or field irrigation. Once roxarsone reaches the Bt soils, it is strongly sorbed into iron oxides or clays, decreasing the potential for leaching. However, competition from phosphate or organic acids for sorption sites on mineral surfaces may affect roxarsone retention in the Bt soils.