p-Arsanilic acid(p-ASA) is widely used in China as livestock and poultry feed additive for promoting animal growth.The use of organoarsenics poses a potential threat to the environment because it is mostly excreted by animals in its original form and can be transformed by UV–Vis light excitation.This work examined the initial rate and efficiency of p-ASA phototransformation under UV-C disinfection lamp.Several factors influencing p-ASA phototransformation,namely,p H,initial concentration,temperature,as well as the presence of Na Cl,NH4+,and humic acid,were investigated.Quenching experiments and LC–MS were performed to investigate the mechanism of p-ASA phototransformation.Results show that p-ASA was decomposed to inorganic arsenic(including As(Ⅲ) and As(V))and aromatic products by UV-C light through direct photolysis and indirect oxidation.The oxidation efficency of p-ASA by direct photosis was about 32%,and those by HOU and1O2 were 19% and 49%,respectively.Cleavage of the arsenic–benzene bond through direct photolysis,HOU oxidation or1O2 oxidation results in simultaneous formation of inorganic As(Ⅲ),As(IV),and As(V).Inorganic As(Ⅲ) is oxidized to As(IV) and then to As(V) by1O2 or HOU.As(IV) can undergo dismutation or simply react with oxygen to produce As(V) as well.Reactions of the organic moieties of p-ASA produce aniline,aminophenol and azobenzene derivatives as main products.The photoconvertible property of p-ASA implies that UV disinfection of wastewaters from poultry and swine farms containing p-ASA poses a potential threat to the ecosystem,especially agricultural environments. p-Arsanilic acid (p-ASA) is widely used in China as livestock and poultry feed additive for promoting animal growth. The use of organoarsenics poses a potential threat to the environment because it is mostly excreted by animals in its original form and can be transformed by UV-Vis light excitation. this work examined the initial rate and efficiency of p-ASA phototransformation under UV-C disinfection lamp. Seral factors influencing p-ASA phototransformation, namely, p H, initial concentration, temperature, as well as the Presence of Na Cl, NH4 +, and humic acid, were investigated. Quenching experiments and LC-MS were performed to investigate the mechanism of p-ASA phototransformation. Results show that p-ASA was decomposed to inorganic arsenic (including As As (V)) and aromatic products by UV-C light through direct photolysis and indirect oxidation. The oxidation efficency of p-ASA by direct photosis was about 32%, and those by HOU and 1O2 were 19% and 49%, respectively. Cleavage of the arseni c-benzene bond through direct photolysis, HOU oxidation or 1O2 oxidation results in simultaneous formation of inorganic As (III), As (IV), and As (V) (V) by1O2 or HOU.As (IV) can undergo dismutation or simply react with oxygen to produce As (V) as well. Reactions of the organic moieties of p-ASA produce aniline, aminophenol and azobenzene derivatives as main products. Photoconvertible property of p-ASA implies that UV disinfection of wastewaters from poultry and swine farms containing p-ASA poses a potential threat to the ecosystem, especially agricultural environments.