Degradation of Dyes by UV/Persulfate and Comparison with Other AOPs: Kinetics and Role of Radicals

This study investigated the degradation of several dyes (MeB, MeO, RhB) by the UV/PS process. The kinetic degradation of the dyes by this process was also compared with the UV/chlorine and UV/H₂O₂ processes. The results showed that the dyes were eliminated efficiently using the UV/PS process and the dye degradation in the investigated UV-AOPs followed the first-order kinetic model. The second-order rate constant of the dyes with •OH, SO₄^•-, and CO₃^•- were calculated and found to be: <i>k</i>_{•<i>OH</i>, <i>MeB</i>} = 5.6×10⁹ M^-1s^-1, <i>k</i>_{<i>SO</i>₄^•-}, <i>MeB</i> = 3.3×10⁹ M^-1s^-1, <i>k</i>_{<i>CO</i>₃^•-, <i>MeO</i>}= 6.9×10⁷ M^-1s^-1; <i>k</i>_{•<i>OH</i>, <i>MeO</i>} = 3.2×10⁹ M^-1s^-1, <i>k</i> _{<i>SO</i>₄^•-, <i>MeO</i>} = 13×10⁹ M^-1s^-1, <i>k</i>_{CO₃^•-, <i>MeO</i>} = 4.4×10⁶ M^-1s^-1; <i>k</i>_{•<i>OH, RhB</i>} = 14.8×10⁹ M^-1s^-1, <i>k</i>_{SO₄^•-, <i>RhB</i>} = 5×10⁹ M^-1s^-1, <i>k</i>_{<i>CO</i>₃^•-, <i>MeO</i>} = 1×10⁷ M^-1s^-1. The steady-state concentrations of •OH and SO₄^•- were determined using both chemical probes and modeling methods (<i>Kintecus V6.8®</i>), indicating the usefulness of Kintecus <i>V6.8</i>® for estimating the concentration of species in a complex system. The other reactive species were estimated using <i>Kintecus V6.8</i>®, while •OH played a major role for the decomposition of the dyes by UV/H₂O₂. The presence of other aqueous components had different impacts on dye degradation: The HCO₃^- and Cl^- can promote the degradation efficiency of one dye, but also inhibit the degradation of other dyes. The UV/PS process is more beneficial for TOC removal compared to that of the UV/Chlorine and UV/H₂O₂ processes for MeO and RhB. In contrast, the TOC removal can be observed higher in the UV/Chlorine and UV/H₂O₂ processes in the case of MeB degradation. Based on the total cost for electrical energy and oxidant, the UV/PS system showed lower cost than the UV/Chlorine and UV/H₂O₂ systems for MeO, RhB degradation but higher cost for MeB removal.