Adsorption Properties of Roxarsone and Arsenate on Goethite and Kaolinite

This study 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 dependence on pH 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 show surface saturation for As(V), but not for roxarsone. The overall adsorption patterns show that As(V) and roxarsone adsorption is similar, suggesting that the arsenate functional group is the dominant control on roxarsone adsorption. However, there are some subtle differences between adsorption of As(V) and roxarsone, which can be explained by the relative sizes of the molecules, the presence of functional groups, differences in solubility, and differences in the type of adsorption (monolayer versus multilayer). Comparison of roxarsone adsorption to goethite and kaolinite reveals 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 are important controls on roxarsone mobility.