ADSORPTION PROPERTIES OF ROXARSONE ON GOETHITE AND KAOLINITE

We investigated the adsorption properties of roxarsone, an organoarsenic poultry feed additive, to goethite and kaolinite in order to determine what role mineral surfaces play in controlling the mobility of roxarsone in watersheds where poultry litter is applied. Adsorption edge experiments for goethite and kaolinite showed a pH dependence for both As(V) and roxarsone. This pattern can be explained by the pH-dependent changes in the mineral surface charge and protonation of the aqueous arsenic species. Isotherms for As(V) and roxarsone on goethite and kaolinite suggest surface saturation for As(V), but not for roxarsone. The overall adsorption patterns demonstrate that As(V) and roxarsone adsorption is similar, suggesting that the As(V) functional group is the dominant control on roxarsone adsorption. EXAFS analysis of the post-reaction slurry is being conducted to evaluate binding mechanisms of roxarsone to the two minerals and to soils collected from a site where poultry litter has been historically applied. Our results indicate that at the low concentrations of roxarsone that are expected to leach from poultry litter into soil water, goethite adsorbs roxarsone more strongly then kaolinite. However, due to the abundance of kaolinite, both minerals are important controls on roxarsone mobility.