Effect of HO, SO4− and CO3−/HCO3 radicals on the photodegradation of the herbicide amitrole by UV radiation in aqueous solution

• The effectiveness of UV/H2O2, S2O82− or Na2CO3 for amitrole degradation was studied.
• Amitrol photodegradation and mineralization rates depended on the system used.
• The UV/S2O82− system was the most effective for the herbicide photodegradation.
• Addition of small amounts of Fe2+ to the UV/H2O2 system increased TOC removal.
• The effectiveness of these water treatments depended on the type of water used.

This study analyzed the photodegradation of the herbicide amitrole (AMT) in water by advanced oxidation processes (AOPs) based on ultraviolet (UV) radiation (UV, UV/H2O2, UV/Fe2+/H2O2, UV/K2S2O8, and UV/Na2CO3) and examined the influence of the initial concentration of radical generators, the solution pH, and the chemical composition of the water. The values of AMT reaction rate constants with HO, SO4− and CO3−/HCO3 radicals obtained at pH 7 were 5.85 ± 0.25 × 108 M−1 s−1, 3.77 ± 0.17 × 108 M−1 s−1, and 1.37 ± 0.12 × 106 M−1 s−1, respectively. The AMT degradation rate constant in the absence of radical generators (0.013 ± 0.001 min−1) was considerably increased by their addition, yielding values of 0.037 ± 0.005, 0.059 ± 0.009, or 0.024 ± 0.002 min−1 with only 0.15 × 10−3 mol L−1 H2O2, K2S2O8, or Na2CO3, respectively. In all cases, these constants increased with greater amounts of reagent, reaching values of 0.309 ± 0.032, 0.748 ± 0.091, and 0.043 ± 0.004 min−1, respectively, with the addition of 3.47 × 10−3 mol L−1 of the corresponding radical generator. The percentage of total organic carbon removal showed the same trend as the AMT degradation rate, reaching 86.8% with the addition of 3.47 × 10−3 mol L−1 H2O2 and increasing to 99.0% when 0.04 × 10−3 mol L−1 Fe2+ was added to the system. We analyzed the effectiveness of three systems (UV/H2O2, UV/K2S2O8, and UV/Na2CO3) for AMT photodegradation in different types of water (ultrapure, tap, and treated urban wastewaters), verifying their high effectiveness, especially UV/K2S2O8. The high efficacy of these systems is related to the generation of both SO4− and HO radicals and the greater selectivity of SO4− radical in its reaction with AMT.