EFFECT OF RADICAL SCAVENGERS AND PROPOSED PATHWAYS FOR AZO DYE DEGRADATION IN A PERSULPHATE-BISULFITE SYSTEM

Abstract

Introduction. The textile industry is one of the most significant manufacturing sectors that produce large volumes of highly polluted and toxic wastewater. Along with the light industry, pigments and dyes industry, domestic service, dry-cleaning, etc., it contributes significantly to water pollution, where dyes are among the top pollutants. Methods. In this study, the degradation of Acid Orange 7 (AO7) dye in a persulfate-bisulfite system under visible (Vis) light (wavelength ≥ 420 nm) was performed. Based on the electron spin resonance spin-trapping technologies and radical scavenger measurements, the produced hydroxyl radical (·OH) is regarded as the predominant reactive oxidant for AO7 decolorization also involving the sulfate radical (SO4· –). The formation of shortlived radicals during AO7 decolorization was detected by the ESR spin-trapping technique at room temperature using a Bruker ESR A-300 spectrometer with the following parameters: center field 3516 G, sweep width 100 G, microwave frequency 9.86 G, modulation frequency 100 kHz, microwave power 1 mW. The intermediate products of the AO7 degradation reaction were analyzed by mass spectrometry. The experiments were performed using an Esquire LC ion trap mass spectrometer (Bruker Daltonics, Bremen, Germany) equipped with an orthogonal geometry ESI source. Nitrogen was used as a drying (3 L/min) and nebulizing (6 psi) gas at 300 °C. The spray shield voltage was 4.0 kV and the capillary cap voltage was 4.5 kV. Scanning was performed from m/z 90 to 400 in the standard resolution mode at a scan rate of 13 kDa/s. Before the analysis, each sample was diluted ten-fold. The intermediates were determined by electrospray ionization-mass spectrometry (ESI-MS) analysis and, as a result, a plausible degradation pathway was proposed. Results. The results of the study can be useful in designing a simple, effective, and economically sound system for the treatment of non-biodegradable azo dyes.