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

Abstract

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® software coupled with the deterministic Nelder-Mead optimization method for determining some unavailable rate constants, including the reaction of RG12 with OH (k = 1.2 × 1010 M−1 s−1), Cl (k = 2.1 × 109 M−1 s−1), Cl2− (k = 1.26 × 107 M−1 s−1) and ClO (k = 6.12 × 104 M−1 s−1). As well, the first-order rate constants of chlorine photolysis, HOCl + hv → OH + Cl and OCl− + hv → O− + Cl, were respectively (0.81–6.90) × 10−3 s−1 and (0.87–8.29) × 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 (∼10−8 M) and Cl2− (∼10−11 M) are highest compared to OH (∼10−12 M) and Cl (∼10−12 M), the selectivity analysis results showed that OH radicals contributed majorly in the degradation (∼85 %) for the different investigated scenarios of pH and initial chlorine concentration. The remaining portion (∼15 %) is ensured by Cl and ClO nearly equitably. These findings were attributed to the high reactivity of RG12 toward OH rather than reactive chlorine radicals, as reflected by the initial rates of the different RG12-degradation by-products.