A Study On Treatment Of Azo Dye C.I. Reactive Black5by Ozonation:Degradation Kinetics And Degradation Mechanism

Azo dyes play an important role in the printing and dyeing industry, C.I. Reactive Black5(RB5) is one of the most representative black dyes. Because of the extensive applications, RB5widely exist in all kinds of the aquatic environment, may cause toxicity to human body and aquatic organism. Ozonation treatment of RB5as advanced oxidation processes (AOPs) which is characterized with the ability to use the high reactivity of hydroxyl radicals in driving oxidation process was investigated in aqueous solution at the initial pH8.0with an initial RB5concentration of200mg/L, using ozone dose of3.2g/h with reaction temperature of25℃, by our study of the influence of various operating parameters on the degradation rate. Reversed-phase high performance liquid chromatography coupled with negative-ion electrospray ionization mass spectrometry is applied for the identification of the major ozone degradation products. In addition, a diode-array detector (DAD) was placed before the mass analyzer to provide UV-vis spectra of the products in the same run. With retention times, mass spectra, UV-vis spectra and the knowledge of the structure of the original dye, it is feasible to propose probable structures of the degradation products by the ozonation treatment. RB5degradation pathways mainly included the conversion of the initial dye to simpler molecules with m/z281,546,201,350,286and222, indicating that the cleavage of the azo bond and introduction of OH groups in the corresponding positions are the predominant reaction mechanism, meanwhile, suggesting the loss of sulfate esters or sulfonic acid groups is a common reaction during the ozonation treatment. The probable degradation mechanism of RB5in aqueous solution was proposed on the basis of the experimental results and verified by the molecular orbital calculation of frontier electron densities (FEDs) by DFT/B3LYP method with6-31G*basis set using Gaussian09program package. According to frontier orbital theory, those atoms with the highest FED2HOMO+FED2LIMO value were the most vulnerable to hydroxyl radical attack, thus we were able to conjecture the configuration of the degradation products and furthermore the ozone oxidation pathway of RB5. The theoretical prediction is in satisfactory agreement with the LC-MS results, indicating there is the feasibility and operability to predict the major events during the whole ozone oxidation of RB5, no longer just the initial oxidation stage. Moreover, sulfate ion, nitrate ion and three carboxylic acids, i.e., oxalic acid, formic acid, and acetic acid were identified using ion exchange chromatography by comparison with authentic standards. In order to assess the extent of mineralization, the reduction of total organic carbon (TOC) during ozone degradation of RB5was also investigated. In order to evaluating the toxicity change of dye solution after ozone treatment, we measured the luminescent bacteria inhibition rate in the dye solution, in accordance with the national standard of determination of acute toxicity by luminescent bacteria method (GB/T15441-1995).