Degradation of Dyes by UV/Persulfate and Comparison with Other UV-based Advanced Oxidation Processes: Kinetics and Role of Radicals

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 center dot OH, SO4 center dot-, and CO3 center dot- were calculated and found to be: k(center dot OH,MeB) = 5.6 x 10(9) M-1 s(-1), k(SO)(4 ,MeB)(center dot-) = 3.3 x 10(9) M-1 s(-1), k(CO)(-3)(center dot) (,MeB) = 6.9 x 10(7) M-1 s(-1) k(center dot OH,MeO) = 3.2 x 10(9) M-1 s(-1), k(SO)(-4)(center dot) (,MeO) = 13 x 10(9) M-1 s(-1), k(CO)(center dot-) x 10(6) M-1 s(-1) k(center dot OH,RhB) = 14.8 x 10(9) M-1 s(-1), k(SO)(-4 ,RhB)(center dot) = 5 x 10(9) M-1 s(-1), k(CO)(-3)(center dot) (,MeO) = 1 x 10(7) M-1 s(-1). The steady-state concentrations of center dot OH and SO(center dot-)4 (including other reactive species) were determined using both chemical probes and modeling methods (Kintecus (R) V6.8). In the UV/PS, the dye degradation depends on the pH of the solution with the order: k(dye) (at pH of 7) > k(dye) (in acidic conditions) > k(dye) (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 center dot OH and SO4 center dot-. 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 center dot- and RCS contributed to dye degradation more than center dot OH, while center dot 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.