Intermediates of Hydrogen Peroxide-Assisted Photooxidation of Salicylic Acid: Their Degradation Rates and Ecotoxicological Assessment.

Accelerated photooxidation of salicylic acid (SA) was performed using UV radiation and hydrogen peroxide. HPLC-MS analysis showed that the primary intermediates are 2,5-dihydroxybenzoic acid, 2,3-dihydroxybenzoic acid, pyrocatechol, and phenol. Deeper oxidation leads to low molecular weight aliphatic acids, such as maleic, fumaric, and glyoxylic. The photooxidation of the main intermediates was carried out in the same conditions. The degradation of SA and its main intermediates follows first-order reaction kinetics. In the case of UV irradiation alone, photodegradation of 2,5-dihydroxybenzoic acid is slightly faster (reaction rate constant is 0.007 min) compared to SA (0.0052 min). Other products degrade more slowly than SA. Hydrogen peroxide, in concentrations of 1.8-8.8 mM, accelerates the photodegradation of salicylic acid and intermediate products. An ecotoxicological evaluation of SA and the main products was performed using the EPI Suite software. The overall persistence (P) and long-range transport potential (LRTP) of all transformation products were assessed using OECD P and the LRTP screening tool. Salicylic acid and its transformation products have low toxicity. Due to their high solubility, these contaminants can travel considerable distances in the aquatic environment. SA and phenol have LRTP values of 156-190 km. Other products can travel shorter distances (less than 100 km).