ADSORPTION CHARACTERISTICS OF ROXARSONE, AN ORGANOARSENIC POULTRY FEED ADDITIVE

Roxarsone, an organic arsenic compound, is used in chicken feed at concentrations of 22.7 to 45.5 g/ton to aid in growth stimulation, pigmentation, and increase egg production (Anderson et al, 1983). According to previous research, roxarsone is not absorbed by the feathers or tissue (Morrison, 1969), but is excreted in poultry manure (Andreae, 1986). Poultry litter, a combination of poultry manure and bedding material, contains arsenic at concentrations of 10-50 mg/kg (Garbarino et al., 2003). Poultry litter is produced and applied throughout the United States, with Virginia producing 33x10⁷ kg of poultry litter each year (Mullins, 2000). Application of poultry litter to agricultural fields may introduce arsenic to soil and groundwater. One possible retention mechanism for arsenic in soils and aquifers is adsorption to mineral surfaces. The purpose of this study is to investigate the adsorption properties of the roxarsone to several iron hydroxides and clay minerals to determine what role mineral surfaces play in controlling the mobility of roxarsone in poultry-dominated watersheds. The minerals that will be used include the common soil minerals goethite, ferrihydrite, and kaolinite. Adsorption isotherm experiments will be conducted to examine concentration dependence of roxarsone and As(V) adsorption to the three minerals. Adsorption edge experiments will be conducted by varying pH. Experiments will be conducted in a Brinkmann Titrino 419 pH-controlled reactor. Isotherm and edge data will yield important information on roxarsone adsorption characteristics. Because the binding mechanisms of As(V) to iron hydroxides are known, comparison of the adsorption data for roxarsone to those of As(V) will provide insight into the adsorption mechanisms of roxarsone, which will aid in understanding how litter-derived arsenic behaves in agricultural watersheds.