Degradation of dyes by UV/Persulfate and comparison with other UV-based advanced oxidation processes: Kinetics and role of radicals

Abstract

This study investigated the degradation of methylene blue (MeB), methyl orange (MeO), and rhodamin B (RhB) by the UV/Persulfate (UV/PS) process. The dye degradation in the investigated UV-based Advanced Oxidation Processes (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: k•OH,MeB = 5.6 × 109 M−1 s−1, kSO4•-,MeB = 3.3 × 109 M−1 s−1, kCO3•-,MeB = 6.9 × 107 M−1 s−1; k•OH,MeO = 3.2 × 109 M−1 s−1, kSO4•-,MeO = 13 × 109 M−1 s−1, kCO3•-,MeO = 4.4 × 106 M−1 s−1; k•OH,RhB = 14.8 × 109 M−1 s−1, kSO4•-,RhB = 5 × 109 M−1 s−1, kCO3•-,MeO = 1 × 107 M−1 s−1. The steady-state concentrations of •OH and SO4•− (including other reactive species) were determined using both chemical probes and modeling methods (Kintecus® V6.8). In the UV/PS, the dye degradation depends on the pH of the solution with the order: kdye (at pH of 7) > kdye (in acidic conditions) > kdye (in alkaline conditions). The presence of water matrices had different impacts on dye degradation: 1) The HCO3− and Cl− promoted the degradation efficiency of one dye, but also inhibited the degradation of other dyes; 2) Humic acid (HA) inhibited dye degradation as it scavenged both •OH and SO4•−.The degradation of the dyes by UV/PS was also compared with the UV/Chlorine (UV/HOCl) and UV/H2O2 and it was established that: 1) In UV/PS and UV/HOCl, SO4•− and RCS contributed to dye degradation more than •OH, while •OH played a major role in dye degradation by UV/H2O2; 2) The calculated toxicity in UV/PS was the lowest probably due to the low toxicity of by-products; 3) For MeO and RhB, the UV/PS process is more beneficial for the total organic carbon (TOC) removal compared to that of the UV/HOCl and UV/H2O2 processes; 4) The UV/PS showed lower cost than the UV/HOCl and UV/H2O2 systems for MeO, and RhB degradation but higher cost for MeB removal.