Optimization and mechanism study of CI Acid Blue 25 wastewater degradation by ozone/Fenton oxidation process: response surface methodology, intermediate products and degradation pathway

Degradation process of C.I. Acid Blue 25 by the combined ozone and Fenton (O-3/Fenton) system was investigated. The Box-Behnken design and response surface methodology were applied in designing the experiments for evaluating the interactive effects of the three most important operating variables. The results demonstrated that the optimal operating conditions were pH of 5.48, [H2O2]/[Fe2+] of 45.47, c(0) of 55.08 mg.L-1 and q(O-3) of 42.06 L.h(-1). Under conditions above, 96.67% decolorization rate of C.I. Acid Blue 25 could be obtained after 4 min reaction time. By validating the results through parallel experiment, the decolorization rate reached 96.39%, and the deviation with the model prediction was only 2.90%. In addition, the highest removal rate of chemical oxygen demand was higher than 85% after 20 min. Main reaction intermediate products involving p-benzoquinone, aniline, catechol, hydroquinone etc. were determined by analyses of ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy and gas chromatography/mass spectrometry (GC/MS). Based on the results of analyses above, the possible degradation pathways of C.I. Acid Blue 25 by O-3/Fenton system were proposed.