Development of a free radical-based kinetics model for the oxidative degradation of chlorazol black in aqueous solution using periodate photoactivated process

Abstract

This paper presents the first modeling study on UV/periodate (IO4–) advanced oxidation process. A reaction mechanism consisting in 45 chemical reactions including a number of radicals (IO3•, IO4•, •OH, O•–, HO2•, O2•–, O3•–) and non-radical intermediates/products (O(3 P), O3, IO3–, H2O2, HO2–, I2O6 and I2O8) has been developed to assess the reactive species generation, use and distribution during the oxidation of one model azo dye, chlorazol black (CB), by the periodate photoactivated process. The model fitted excellently the CB degradation data over a wide range of solution pH and initial IO4– concentration. Unavailable second-order rate constants of several important reactions were optimized using the genetic algorithm. Those include O3 + IO3• → IO4• + O2 (k = 1 × 108 M−1s−1), 2IO4• → I2O8 (k = 4.68 × 108 M−1s−1), reaction of CB with •OH (k = 1 × 109 M−1s−1), IO3• (k = 2.63 × 108 M−1s−1), IO4• (k = 0 M−1s−1), O(3 P) (k = 0 M−1s−1) and O3 (k = 0.574 M−1s−1). Besides, the first order kinetic constant of periodate photolysis, IO4– + hv → IO3• + O•– and IO4– + hv → IO3– + O(3 P), were respectively (0.5–4.16)×10-3 s−1 and ∼2 × 10-4 s−1, indicating that the radical pathway for IO4– photolysis is predominately the IO3–-releasing path. OH and IO3 were found to play the key role in the CB degradation. The concentration of radicals increased with pH decrease and initial periodate concentration rise, favoring higher degradation rate at acidic conditions and higher IO4– dosages. The selectivity analysis showed that the contribution of these species is ∼ 79 % for •OH and ∼ 21 % for IO3•, under various conditions of pH and [IO4–]0. This study would helpfully provide some practical indications for the application of UV/periodate AOP