Kinetic study on methylene blue removal from aqueous solution using UV/chlorine process and its combination with other advanced oxidation processes

This study investigated the kinetic degradation of methylene blue (MB) by a UV/chlorine process and its combination with other advanced oxidation processes. The center dot OH and reactive chlorine species (RCS: Cl center dot, ClO center dot, etc.) were the primary reactive species, which accounted for 56.7% and 37.6% of MB degradation at pH 7, respectively. The second-order rate constant of Cl center dot towards MB was calculated to be 2.8 x 10(9) M-1 s(-1). When the pH increased from 3 to 7, k(MB) by center dot OH increased from 0.15 to 0.21 min(-1) before being reduced to 0.11 min(-1) at pH 11. k(MB) by RCS continuously reduced from 0.16 to 0.13 min(-1) when the pH was increased to 11. Humic acid (HA), Br-, and Cl- inhibited the degradation with k(MB) in the order: k(MB) (in HA) < k(MB) (in Br-) < k(MB) (in Cl-). HCO3- increased k(MB) from 0.37 to 0.48 min(-1). The experimental and modeling methods fit well, indicating the effectiveness of using Kintecus (R) in predicting concentrations of free radicals in complex water matrices. TOC removal was achieved at 60% after 30 min in a control process and it was strongly inhibited by the presence of HA, with 22% removal achieved at 5 mg(c) L-1 HA. UV/chlorine/electrochemical oxidation (UV/chlorine/EO) significantly improves k(MB) from 0.37 to 0.94 min(-1) at a high current (240 mA), while UV/chlorine/H2O2 decreased k(MB) at a low concentration of 0.01 mM H2O2 (k(MB) decreased by 6.1%). The results indicate that the energy cost for UV irradiation was the main cost in MB treatment in both UV/chlorine and UV/persulfate (UV/PS) processes, accounting for 91% and 84%, respectively.