Degradation mechanism of Direct Red 23 dye by advanced oxidation processes: a comparative study

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Abstract
Textile and dye industries have discharged various kinds of toxic residual dyes particularly azo dyes into the environment. In addition to its resistance to biological degradation, Direct Red 23 (DR-23) azo dye is considered toxic, carcinogenic, teratogenic, and mutagenic to the ecosystem. The aim of this study was to investigate the efficiency of different advanced oxidation processes (AOPs) for DR-23 dye detoxification. Single and combined patterns of AOPs, namely ozonation (O3), O3/ultraviolet (UV), O3/UV/hydrogen peroxide (H2O2), and O3/ultrasonic (US), were applied. The effect of pH (3–11) and initial dye concentration (100-500 mg/L), with implication of UV, H2O2, and US, on dye removal efficiency were studied. Decolorization efficiency was found considerably depends on the initial dye concentration and the pH of the solution. The maximum decolorization was obtained at pH 9. The color removal efficiency reached 100% for 100 mg/L DR-23 dye at 15 min using both O3 and O3/UV processes. The half-life of O3, O3/UV, O3/UV/H2O2, and O3/US were found to be 12.4, 9.0, 15.8, and 10.5 min, respectively. The kinetic of AOPs for DR-23 dye removal followed the pseudo-first-order. For example, the rate constant (kobs) of 400 mg/L dye increased to about 38% using O3/UV as compared to O3 alone. Furthermore, LC-(ESI)-MS/MS analysis of the treated DR-23 dye solution was performed to study the final degradation products. It was shown that a DR-23 dye radical underwent partial and complete decomposition of the dye ring to form final products, namely oxalic acid and formic acid.

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