Toxicity removal assessments related to degradation pathways of azo dyes: Toward an optimization of Electro-Fenton treatment

Abstract

The degradation pathway of Acid Orange 7 (AO7) by Electro-Fenton process using carbon felt cathode was investigated via HPLC-UV and LC-MS, IC, TOC analysis and bioassays (Vibrio Fischeri 81.9% Microtox(®) screening tests). The TOC removal of AO7 reached 96.2% after 8 h treatment with the optimal applied current density at -8.3 mA cm(-2) and 0.2 mM catalyst concentration. The toxicity of treated solution increased rapidly to its highest value at the early stage of electrolysis (several minutes), corresponding to the formation of intermediate poisonous aromatic compounds such as 1,2-naphthaquinone (NAPQ) and 1,4-benzoquinone (BZQ). Then, the subsequent formation of aliphatic short-chain carboxylic acids like acetic acid, formic acid, before the complete mineralization, leaded to a non-toxic solution after 270 min for 500 mL of AO7 (1 mM). Moreover, a quantitative analysis of inorganic ions (i.e. ammonium, nitrate, sulfate) produced during the course of degradation could help to verify molar balance with regard to original nitrogen and sulfur elements. To conclude, a clear degradation pathway of AO7 was proposed, and could further be applied to other persistent pharmaceuticals in aquatic environment.

Enhanced Electro-Fenton Mineralization of Acid Orange 7 Using a Carbon Nanotube Fiber-Based Cathode

“…After that, the next decomposition of aromatic by-products forms other low toxic compounds as aliphatic compounds. When TOC removal reached to over 90%, the toxicity of solution will be disappeared which was strongly confirmed in previous study about the correlation between degradation pathway and toxicity of AO7 during EF process (Le et al, 2016b). In this study, there was 96.7% TOC eliminated in 8-h treatment at CNTF@ CF cathode, proving the strength of our material in environment application for wastewater treatment by electrochemical advanced oxidation processes.…”

“…Moreover, this result was actually important to conclude the friendly environmental efficiency for degradation of bio-refractory pollutants by EF process using CNTF@CF cathode. The reason is that the attack of AO7 by hydroxyl radical will generate intermediate compounds more toxic than the parent molecule like 2-naphthol, 1,2-naphthoquinone at the beginning stage of the electrolysis when TOC removal achieves lower 30% (Le et al, 2016b). After that, the next decomposition of aromatic by-products forms other low toxic compounds as aliphatic compounds.…”

“…The EF process using carbon and graphite felt cathodes have been applied to remove successfully many different kinds of POPs including dye pollutants (Panizza and Oturan, 2011), pharmaceutical compounds (Sirés et al, 2007), herbicides and pesticides (Aaron and Oturan, 2001). In addition, the friendly environmental feature of EF treatment is also proved when it can degrade efficiently the very toxic initial contaminants to non-toxic compounds like CO2, H2O, or short-chain carboxylic acids (Le et al, 2016b(Le et al, , 2017.…”

Enhanced treatment of coking wastewater containing phenol, pyridine, and quinoline by integration of an E-Fenton process into biological treatment

“…As it is well known, the electro-Fenton (E-Fenton) process is one of the effective advanced oxidation technologies for the treatment of refractory pollutants in the in aquatic environment in which the hydrogen peroxide (H 2 O 2 ) is electrochemically produced through the oxygen reduction reaction on the cathode (Isarain-Chavez et al 2010, Rosales et al 2012Yu et al 2015;Cotillas et al 2015;Le et al 2016). This E-Fenton process is acknowledged to produce the hydroxyl radicals (·OH) owing to the interaction between electrochemically generated H 2 O 2 and additional Fe 2+ , which are extremely powerful and nonselective to degrade the refractory pollutants (Lei et al 2013).…”

Theoretical investigation on the mechanism of the OH-initiated degradation process of reactive red 2 azo dye

“…10,11 Great efforts have been devoted to tackling the effluent polluted by aromatic azo dyes, and the advanced oxidation process (AOP) is still one of the most common techniques researched and designed for the degradation, decolorization, and mineralization of pollutants under aqueous condition in recent years. [12][13][14][15][16][17][18][19][20][21][22][23][24][25] The AOPs include a variety of effective and sustainable strategies such as homogeneous and heterogeneous photo-catalysis upon near-UV or visible irradiation, ozonation, Fenton, nonthermal plasmas, electrolysis, ultrasound, and wet air oxidation.…”