Hydroxyl and sulfate radical-based oxidation of RhB dye in UV/H2O2 and UV/persulfate systems: Kinetics, mechanisms, and comparison.

The degradation kinetics and mechanisms of Rhodamine B (RhB) dye by •OH and SO4•- based advanced oxidation processes were investigated. The •OH and SO4•- radicals were generated by UV photolysis of hydrogen peroxide and persulfate (i.e., UV/H2O2 and UV/PS), respectively. The effects of initial solution pH, RhB concentration, oxidant dosage, Fe2+ concentration, and water matrices were examined. The results showed that the degradation of RhB followed pseudo-first-order kinetics in both processes, with the UV/H2O2 process exhibiting better performance than that of the UV/PS process. Acidic conditions were favorable to the degradation of RhB in both systems. Increasing the oxidant dosage or decreasing the contaminant concentration could enhance the degradation of RhB. Photo-Fenton-like processes accelerated the performance when Fe2+ was added into both systems. The removal efficiency of RhB was inhibited upon the addition of humic substances. The addition of Cl- displayed no significant effect and promoted RhB degradation in UV/H2O2 and UV/PS systems, respectively. The presence of NO3- promoted RhB degradation, while H2PO4- and C2O42- showed an inhibitory effect on both UV/H2O2 and UV/PS processes. Radical scavenging tests revealed the dominant role of SO4•- radicals in the UV/PS system. Furthermore, the evolution of low molecular weight organic acids and NH4+ during the degradation of RhB in these two processes were compared. Both UV/H2O2 and UV/PS systems led to similar formation trends of NH4+ and some ring-opening products (e.g., formic acid, acetic acid, and oxalic acid), suggesting some analogies in the decay pathways of RhB by •OH and SO4•--induced oxidation processes.