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

The degradation kinetics and mechanisms of Rhodamine B (RhB) dye by OH and SO based advanced oxidation processes were investigated. The OH and SO radicals were generated by UV photolysis of hydrogen peroxide and persulfate (i.e., UV/HO and UV/PS), respectively. The effects of initial solution pH, RhB concentration, oxidant dosage, Fe 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/HO 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 Fe 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/HO and UV/PS systems, respectively. The presence of NO promoted RhB degradation, while HPO and CO showed an inhibitory effect on both UV/HO and UV/PS processes. Radical scavenging tests revealed the dominant role of SO radicals in the UV/PS system. Furthermore, the evolution of low molecular weight organic acids and NH during the degradation of RhB in these two processes were compared. Both UV/HO and UV/PS systems led to similar formation trends of NH 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 SO-induced oxidation processes.