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, birnessite (δ-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 δ-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 rinit=2.36×10(-5)[ASA]0.8[MnO2]0.9[H+]0.7. p-ASA first forms a surface precursor complex on δ-MnO2 during degradation, followed by formation of p-ASA radicals through single-electron transfer to δ-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 δ-MnO2. The fast transformation of p-ASA to arsenite and arsenate mediated by δ-MnO2 significantly increases the risk of soil arsenic pollution and deserves significant attention in the animal farming zones still using this feed additive.