Comparison of the Oxidation of 3,5-Dihydroxybenzoic Acid in Rainwater by UV/Fenton-like and UV/H2O2 Processes

Rainwater needs to be recognized as a natural water source for domestic use, but finding viable processes to remove its contaminants is essential. The aim of this work was to compare the UV/H2O2 and UV/Fenton-like processes for the oxidation of 3,5-dihydroxybenzoic acid (3,5-DHBA) in rainwater. The reactions were assessed using ultraviolet-visible (UV-Vis) and molecular fluorescence spectroscopies, and the results showed the formation of new and similar chromophoric compounds in both processes, which were subsequently degraded. At environmentally relevant concentrations of chemical oxidants, namely H2O2 at 10−4 M, the chromophoric organic compounds in solution were degraded within 24 h by the UV/H2O2 process and within 4 h by the UV/Fenton-like process. However, when the concentration of H2O2 was increased by one order of magnitude for the UV/H2O2 process (from 10−4 M to 10−3 M), oxidation rates were similar and nearly complete after 4 h for both UV/H2O2 and UV/Fenton-like processes. These findings highlight that the presence of more oxidizing agents in the oxidation system improves the synergistic effect, leading to a greater contribution of the free radical oxidation pathway, particularly through hydroxyl radicals. Thus, by increasing the concentration of H2O2 in the UV/H2O2 process to 10−3 M, it was possible to achieve a similar level of oxidation (close to 100% after 4 h, as indicated by a decrease in fluorescence intensity) as the UV/Fenton-like process at environmentally relevant concentrations (10−4 M), but using fewer chemical reactants, since UV/H2O2 process does not require Fe(III) as catalyst and oxidant. Therefore, the UV/H2O2 process can be considered a simpler and cleaner process for removing organic contaminants from rainwater.