Birnessite (δ-MnO2) Mediated Degradation of Organoarsenic Feed Additive p-Arsanilic Acid

p-Arsanilic acid (p-ASA), is a widely used animal feed additive in many developing countries, and is often introduced to agricultural soils with animal wastes. A common soil metal oxide, bimessite (delta-MnO2), was found to mediate its degradation with fast rates under acidic conditions. Experimental results indicate that adsorption and degradation of p-ASA on the surface of delta-MnO2 were highly pH dependent, and the overall kinetics for p-ASA degradation and formation of precursor complex could be described by a retarded first-order rate model. For the reaction occurring between pH 4.0 and 6.2, the initial rate equation was determined to be r(init) = 2.36 x 10(-5)[ASA](0.8)[MnO2](0.9)[H+](0.7). p-ASA first forms a surface precursor complex on delta-MnO2 during degradation, followed by formation of p-ASA radicals through single-electron transfer to delta-MnO2. The p-ASA radicals subsequently undergo cleavage of arsenite group (which is further oxidized to arsenate) or radical-radical self-coupling. Instead of full mineralization (with respect to arsenic only), about one-fifth of the p-ASA couples to form an arsenic-bearing azo compound that binds strongly on delta-MnO2. The fast transformation of p-ASA to arsenite and arsenate mediated by delta-MnO2 significantly increases the risk of soil arsenic pollution and deserves significant attention in the animal farming zones still using this feed additive.