Free-Radical-Induced Oxidation and Reduction of 1-Arylazo-2-naphthol Dyes:  A Radiation Chemical Study

Abstract

The kinetics and transient optical absorption spectra of the intermediates formed in the reaction of •OH, N3 •, and e- aq with 1-arylazo-2-naphthol and its ortho- and para-substituted chloro, methyl and methoxy derivatives at pH 7.0 have been studied by pulse radiolysis. The rate constants for the reaction of •OH and the nucleophilic hydrated electron (e- aq) with these compounds are (1.0−1.2) × 1010 M-1 s-1 and (0.8−2.5) × 1010 M-1 s-1. The reaction of N3 • with these azo dyes is more selective (ρ+ = −2.0) with the rate constants varying between 0.1 × 109 and 8 × 109 M-1 s-1 depending upon the substituent. The transient optical absorption spectra of the intermediates formed in the reaction of the dyes with •OH are different from the corresponding spectra of the intermediates observed in the reaction of the dyes with N3 •. The •OH radical reacts with the dyes by addition to form the •OH adducts, whereas N3 • reacts with the dyes by direct one-electron transfer to form the one-electron oxidized dye radical. The pK A of the dye radical of the para-methoxy derivative was determined to be 5.2. This radical decays bimolecularly, predominantly by disproportionation, whereas the •OH adducts of the dyes also decay bimolecularly but predominantly by dimerization. The initially formed radical anion in the reaction of e- aq with the unsubstituted dye protonates rapidly to give the hydrazyl radical, which decays bimolecularly by disproportionation. Of the radicals studied, the •OH-induced oxidation of the dyes is the most effective process leading to decolorization of the dyes at pH 7.0.

Ultrasonic irradiation-induced degradation of low-concentration bisphenol A in aqueous solution

“…The synergistic index in US/O 3 system was calculated to be 2.26, indicating a considerably synergistic effect in this coupled system. [26,27], which results in the higher degradation efficiency of BPA in the US/O 3 system. Weavers et al [28] also drew a similar conclusion after degradation of aromatic compounds using a combination of sonolysis and ozonolysis.…”

Electrochemical degradation of azo dye C.I. Reactive Red 195 by anodic oxidation on Ti/SnO2–Sb/PbO2 electrodes

“…When textile effluents are discharged into the aquatic ecosystem without sufficient treatment, the dyes in the wastewater can be transported over long distances with little attenuation because they are high soluble. Moreover, some can be hydrolysed or anaerobically metabolized to form hazardous end-products [3,4]. It is estimated that 10-15% of the dye is lost during the dyeing process and released as effluent [5].…”

Photocatalytic decolorization of soluble dyes by a bis-ions coexistence system of NH4+ and NO3− with high photoreduction ability

“…It is because that the chromophore (-N N-) responsible for the characteristic color of MO is broken up and results in the decreased conjugated number of molecule, as shown in Eq. (9). Similarly, Fig.…”

“…Therefore, the discovery of an effective method to remove the color from the effluents is of great significance. Conventional physicochemical processes such as carbon adsorption, coagulation-flocculation, membrane filtration, ultrafiltration, electrocoagulation, sedimentation [6,7], ultrasonic irradiation, biodegradation [8], sonolysis [9], etc. were used for the treatment of such effluents.…”