Radicals (•OH, Cl•, ClO• and Cl2•-) concentration profiles in the intensified degradation of reactive green 12 by UV/chlorine process: Chemical kinetic analysis using a validated model

Understanding the production of reactive species in chlorine photolysis is critical for applying advanced oxidation processes (AOPs) in future. This paper developed a chemical kinetic model consisting of 66 chemical reactions, including many radicals and non-radical intermediates/products, to elucidate the intensified removal of reactive green 12 (RG12) dye by UV/chlorine AOP. The generated kinetic model fits our experimental data satisfactorily across a broad range of solution pH and initial chlorine concentration. The simulation process was accomplished by COPASI (R) software coupled with the deterministic Nelder-Mead optimization method for determining some unavailable rate constants, including the reaction of RG12 with (OH)-O-center dot (k = 1.2 x 10(10) M-1 s(-1)), Cl-center dot (k = 2.1 x 10(9) M-1 s(-1)), Cl-2(center dot-) (k = 1.26 x 10(7) M-1 s(-1)) and ClO center dot (k = 6.12 x 10(4) M-1 s(-1)). As well, the first-order rate constants of chlorine photolysis, HOCl + hv -> (OH)-O-center dot + Cl-center dot and OCl- + hv -> O center dot- + Cl-center dot, were respectively (0.81-6.90) x 10(-3) s(-1) and (0.87-8.29) x 10(-3) s(-1), depending on the solution pH and chlorine dosage. The radicals concentration (and RG12 degradation rate) increased with the increase of initial chlorine dosage, whereas a solution pH of 5 is recorded for the best process efficiency (i.e., radicals' yield and dye destruction rate). Even though the steady-state concentration of ClO center dot (similar to 10(-8) M) and Cl-2(center dot-) (similar to 10(-11) M) are highest compared to (OH)-O-center dot (similar to 10(-12) M) and Cl-center dot (similar to 10(-12) M), the selectivity analysis results showed that (OH)-O-center dot radicals contributed majorly in the degradation (similar to 85 %) for the different investigated scenarios of pH and initial chlorine concentration. The remaining portion (similar to 15 %) is ensured by Cl-center dot and ClO center dot nearly equitably. These findings were attributed to the high reactivity of RG12 toward (OH)-O-center dot rather than reactive chlorine radicals, as reflected by the initial rates of the different RG12-degradation by-products.

Automated summary

This study developed a chemical kinetic model to investigate the intensified removal mechanism of reactive green 12 dye using UV/chlorine AOP. The model fits well with experimental data across different pH values and initial chlorine concentrations. The results showed that (OH)-O• radicals contributed majorly (about 85%) to the degradation, while Cl• and ClO• contributed about 15%.