Kinetics and mechanisms of pH-dependent direct photolysis of p-arsanilic acid under UV-C light

The usage of p-arsanilic acid (ASA) in livestock and poultry farms leads to its release into the environment. The phototransformation of ASA generates toxic inorganic arsenic species and organic by-products, which affects the water security of the peripheral river basin. In this work, we have investigated the pH-dependent kinetics and mechanisms of ASA photolysis under Ultraviolet-C (UV-C) light (254 nm) irradiation in the pH range 1-11 in the absence of oxygen. The degradation rate of ASA, the ratio of the generated inorganic arsenic species, and the amount of primary organic by-products were determined under the aforesaid conditions. Our results clearly showed the pH-dependent photochemistry of ASA. The photolysis rate was low under acidic conditions, and As (III) and p-aminophenol were the main products. With increasing pH, the photolysis rate increased significantly, and As(V) and aniline became the main products. A pH-based kinetic model has been established that allows prediction of the generation rates of As(III), As(V), p-aminophenol, and aniline according to the photolysis rate of ASA.