Surface adsorption of organoarsenic roxarsone and arsanilic acid on iron and aluminum oxides

Aromatic organoarsenicals roxarsone (ROX) and p-arsanilic acid (ASA) are common feed additives for livestock and could be released into the environment via animal manure and agricultural runoff. To evaluate their environmental fate, the adsorption behavior of ROX and ASA was investigated with two common soil metal oxides, goethite (FeOOH) and aluminum oxide (Al ₂O ₃), under different reactant loading, water pH and competing ion conditions. ROX and ASA exhibit essentially identical adsorption characteristics. FeOOH and Al ₂O ₃ exhibit similar adsorption trends for both organoarsenicals; however, the adsorption efficiency on the surface site basis was about three times lower for Al ₂O ₃ than for FeOOH. The adsorption reaction is favorable at neutral and acidic pH. Phosphate and natural organic matter significantly interfere with aromatic arsenical adsorption on both metal oxides, whereas sulfate and nitrate do not. Pre-adsorbed aromatic arsenicals can be quickly but not completely displaced by phosphate, indicating that ion exchange is not the only mechanism governing the adsorption process. The adsorption envelope was successfully modeled by a diffuse double layer surface complexation model, identifying the critical role of di-anionic organoarsenic species in the adsorption. Results of this research can help predict and control the mobility of aromatic arsenicals in the environment.