A novel transformation pathway of p-arsanilic acid in water by colloid ferric hydroxide under UVA light

Iron species that occur in natural surface water could affect the photochemical behavior of pollutants. Complexation between iron species and polycarboxylate or heavy metals has been widely reported, where the ligands could be oxidized via ligand-to-metal charge transfer (LMCT) by light inducement. Such complexation and photochemical reactions might also occur for low valance metal-containing organic compounds, which is worthy of investigation. This work studied the phototransformation of p-arsanilic acid (ASA), an organic arsenic compound that is widely used as a feed additive in the poultry industry, by colloidal ferric hydroxide (CFH) using black light lamps (λ = 365 nm) as the light source. The results revealed the contribution to ASA transformation at circumneutral conditions by CFH through an LMCT process, which is the same as that for As(III). The complexation between ASA and CFH was investigated using UV-vis spectroscopy. The estimated equilibrium constant for the CFH-ASA complex was log K_f271 = 4.22. The analysis of the photoproducts found the generation of both inorganic and organic arsenic. Our findings confirmed the similarities in the photochemical mechanisms of ASA and As(III) in the presence of CFH. The results help in further understanding the fate of organoarsenicals in the surface water environment.