EDTA enhances the photodegradation of p-arsanilic acid in the presence of iron at near-neutral pH

• EDTA is capable of accelerating the iron-mediated photodegradation of ASA. • The effect of EDTA is mainly dependent on its dosage and the solution pH. • Fe(III)-EDTA complex is responsible for the enhanced photodegradation of ASA. • Products of ASA in photodegradation differ from that in direct photolysis. The widespread used organic arsenic pesticides and veterinary drugs have drawn attention for decades. Its photochemical transformation in the surface water contributes to its degradation and the formation of products. The effect of co-existent components like anthropogenic reagents (e.g. ethylenediaminetetraacetatic acid, EDTA) on the photochemical transformation has still to be investigated in depth. Fe(III)-EDTA complex is a common and abundant complex in surface water. In this work, UVA light and simulated sunlight have been used as light sources to explore the effect of Fe(III)-EDTA on the photodegradation of p -arsanilic acid (ASA), used as an organic arsenic feed additive. Whereas ASA hardly absorbs UVA light and can only be directly photolyzed by simulated sunlight, the Fe(III)-EDTA complex is photoactive under either light source. At pH 6, the presence of Fe(III)-EDTA complex enhances the photodegradation efficiency of ASA from 0% to ca. 23% after 180 min reaction under UVA light irradiation, and from 30% to ca. 58% after 45 min reaction under simulated sunlight irradiation. Appropriate amount of EDTA enhances the photodegradation efficiency, whereas excessive EDTA suppresses the initial reaction rate. Mechanistic study has revealed contributions from direct photolysis and photochemical reactions of Fe(III)-EDTA and Fe(III)–OH complexes. Generated •OH has been confirmed as the important contribution. Photodegradation products include inorganic arsenic, organic arsenic, and other organic by-products, and the proportions of these products vary in different light-induced systems due to the diverse reaction pathways. The results of this work improve our understanding of the risks of residual ASA in the environment, allow prediction of its migration and transformation, and show how organic arsenic livestock and poultry feed additives might be removed.