Functional group influences on the reactive azo dye decolorization performance by electrochemical oxidation and electro-Fenton technologies

Abstract

Electrochemical water treatment technologies are highly promising to achieve complete decolorization of dyebath effluents, as demonstrated by several studies reported in the literature. However, these works are focused on the treatment of one model pollutant and generalize the performances of the processes which are not transposable since they depend on the pollutant treated. Thus, in the present study, we evaluate, for the first time, the influence of different functional groups that modify the dye structure on the electrochemical process decolorization performance. The textile azo dyes Reactive Orange 16, Reactive Violet 4, Reactive Red 228, and Reactive Black 5 have been selected because they present the same molecular basis structure with different functional groups. The results demonstrate that the functional groups that reduce the nucleophilicity of the pollutant hinder the electrophilic attack of electrogenerated hydroxyl radical. Thereby, the overall decolorization efficiency is consequently reduced as well as the decolorization rate. Moreover, the presence of an additional chromophore azo bond in the molecule enhances the recalcitrant character of the azo dyes as pollutants. The formation of a larger and more stable conjugated π system increases the activation energy required for the electrophyilic attack of OH, affecting the performance of electrochemical technologies on effluent decolorization.

Understanding the electrochemical oxidation of dyes on platinum and boron–doped diamond electrode surfaces: experimental and computational study

“…However, in the current work, it is possible to understand that the oxidation mechanisms are also dependent on the anode surfaces studied. In fact, the order followed by the BDD is E (RV4) < E (RR228) < E (RB5) < E (RO16) which differs from the order of discoloration process reported in our previous study [32].…”

“…In a previous work, the decolorization study of the same dyes was carried out [32], concluding that the effect of the molecular structure is the responsible of the difference on the color removal efficiencies during bulk electrolysis, following the order RV4 > RO16 > RB5 > RR228. The different color removal rate, which depended mainly on the action of mediated oxidation (reaction of oxidants with dyes in solution), was clearly influenced by the functional groups, which are the main structural difference on the treated reactive azo dyes.…”

“…It has been successfully used for removing pharmaceuticals [26,27], dyes [28], petroleum [29], pesticides, and herbicides [30]. Nevertheless, few reports about the study of AO of dyes with similarities in their basic chemical structure have been published [31][32][33][34], but no detail studies understanding the oxidation mechanisms at anode surfaces associating electrochemical measurements with theoretical chemistry have been published yet. Therefore, this work presents the results obtained by the applicability of electrochemical potentiodynamic study of four different reactive azo dyes (Reactive Orange 16 (RO16), Reactive Violet 4 (RV4), Reactive Red 228 (RR228), and Reactive Black 5 (RB5)) with an identical basic structure but differ in their functional groups (see Table 1) by using Ti/Pt and BDD anodes, which were explained by theoretical calculations on the morphological studies of the frontier orbitals where the electrons are more energetic and then, the electron-transfer to electrode surface is achieved.…”

“…It is important to consider that even though the decolorization percentage for the treatment of different reactive azo dyes under similar experimental conditions is a good indicative of the oxidation mechanism [32], this process could be completely different from the mineralization point of view. It can be due to the participation of specific oxidizing species which are electrogenerated at Ti/Pt or BDD surfaces, and consequently acting either close to the anode surface or in the reaction cage.…”

Iron oxide nanoparticles as heterogeneous electro-Fenton catalysts for the removal of AR18 azo dye

“…Current intensity is an important parameter considering its direct implication in the process operating cost, the regeneration of ferrous catalyst at the cathode (reaction 1) and the generation of H2O2 (reaction 5) [47]. An increase in both Fe (II) and H2O2 as a result of an increase of the current intensity leads to a higher production of • OH radicals, inducing a higher mineralization of the dye.…”