Degradation of azo dye C.I. Acid Red 18 using an eco-friendly and continuous electrochemical process

Critical review on electrooxidation and chemical reduction of azo dyes: Economic approach

Effective degradation technologies have been extensively investigated and used to remove azo dyes from wastewater for decades. However, no review dealing with both electrooxidation and chemical reduction of azo dyes from an economic and, therefore, application-relevant perspective has been found in the current literature. A novelty of this review article consists not only in the brief summarization and comparison of both methods but mainly in the evaluation of their economic side. Based on the literature survey of the last 15 years, the costs of treatment approaches published in individual research articles have been summarized, and the missing data have been calculated. A broad spectrum of advanced electrode materials and catalysts have been developed and tested for the treatment, specifically aiming to enhance the degradation performance. An outline of the global prices of electrode materials, reducing agents, and basic chemicals is involved. All additional costs are described in depth in this review. The advantages and disadvantages of respective methods are discussed. It was revealed that effective and cheap treatment approaches can be found even in advanced degradation methods. Based on the collected data, electrooxidation methods offer, on average, 30 times cheaper treatment of aqueous solutions. Concerning chemical reduction, only ZVI provided high removal of azo dyes at prices <100 $ per kg of azo dye. The factors affecting total prices should also be considered. Therefore, the basic diagram of the decision-making process is proposed. In the conclusion, challenges, future perspectives, and critical findings are described.

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

Heterogeneous electro-Fenton mineralization of Acid Red 18 (AR18) in aqueous solution was studied with magnetite Fe₃O₄ (MNPs) and haematite Fe₂O₃ (HNPs) nanoparticles as catalysts. High mineralization yields of AR18 were obtained with magnetite, 81% TOC removal after 180 min of electrolysis in 40 mg L^-1 Fe₃O₄, pH 3.0, at 50 mA of current intensity and in 50 mM Na₂SO₄. In order to explain the obtained mineralization yield achieved with MNPs, the quantification of hydrogen peroxide (H₂O₂), hydroxyl radical (•OH) and iron leaching were performed at 50 and 100 mA. From the high iron concentration found in the bulk solution and the slight impact of the catalyst mass concentration on TOC removal, the formation of hydroxyl radicals occurs mainly through homogeneous process. In the presence of hydroxyl radical scavenger, degradation remained total after 15 min showing the involvement of a direct electrochemical oxidation of the dye at the electrode surface. The hydroxyl radical oxidation is responsible for at least 50% of mineralization.

Homogeneous and heterogeneous catalytic oxidation of some azo dyes using copper(II) ions

The kinetics of the homogeneous and heterogeneous catalytic oxidation processes of three azo dyes in presence of copper (II) - ions, copper (II) - ions - supported on alumina and on zinc oxide as well as copper ammonia complex supported on alumina were investigated in aqueous solutions. The dyes are Chromotrope 2B (C2B), Chromotrope 2R (C2R) and Chrysophenen (CRY). The reaction progress was followed by monitoring the decrease in absorbance at ℷmax 512, 511 and 401 nm, respectively. The rate of reaction increased with increasing either the concentration of the dye or the catalyst, giving a plateau at high concentrations of the catalyst. On the other hand, the rate of reaction increased gradually with increasing hydrogen peroxide concentration attaining a maximum then decreased thereafter. The reaction rate was also increased with increasing pH and temperature and was found to be entropy controlled. This work could be applied for dye baths water reuse to reduce costs in textile and dyeing factories as will as for environmental purposes.

Enhanced removal of Eriochrome Black T in wastewater by zirconium-based MOF/graphene oxide

The zirconium-based MOF/graphene oxide (UiO-66-NH2/GO) composites were prepared by ultrasonic dispersing different amounts of graphene oxide (GO) in a well-dissolved zirconium tetrachloride/H2BDC-NH2 mixture, obtaining 2 wt% (UiO-66-NH2/GO-1), 5 wt% (UiO-66-NH2/GO-2), and 10 wt% (UiO-66-NH2/GO-3) GO composites. The products were characterized by XRD, FTIR, SEM, BET, Raman, UV, XPS, and Zeta potential. Adsorption experiments on simulated Eriochrome Black T (EBT) printing and dyeing wastewater were carried out using UiO-66-NH2/GO, and the optimal conditions for adsorption were obtained by exploring the effects of initial EBT concentration, time, pH, and salt ionic strength. Adsorption isotherms, kinetics, mechanism, and regeneration were also researched. The adsorption behavior was consistent with the Langmuir isotherm and fully compliant with pseudo secondary dynamics model. The adsorption capacity of UiO-66-NH2/GO-2 was found to be the highest of the three products, which was 263.158 mg/g. Therefore, the UiO-66-NH2/GO-2 composite was considered to be an excellent adsorbent for the adsorption of EBT from organic dye wastewater.

Effects of Electrical Stimulation on the Degradation of Azo Dye in Three-Dimensional Biofilm Electrode Reactors

Three-dimensional biofilm electrode reactors (3D-BERs) were constructed to degrade the azo dye Reactive Brilliant Red (RBR) X-3B. The 3D-BERs with different influent concentrations and external voltages were individually studied to investigate their influence on the removal of X-3B. Experimental results showed that 3D-BERs have good X-3B removal efficiency; even when the influent concentration was 800 mg/L, removal efficiency of 73.4% was still achieved. In addition, the X-3B removal efficiency stabilized shortly after the influent concentration increased. In 3D-BERs, the average X-3B removal efficiency increased from 52.8% to 85.4% when the external voltage rose from 0 to 2 V. We further identified the intermediate products via UV-Vis and gas chromatography-mass spectrometry (GC-MS) analyses, and discussed the potential mechanism of degradation. After the conjugate structure of X-3B was destroyed, all of the substances generated mainly consisted of lower-molecular-weight organics.

The degradation of azo dye with different cathode and anode structures in biofilm electrode reactors

In this study, biofilm electrode reactors (BERs) were constructed to degrade the azo dye Reactive Brilliant Red (RBR) X-3B.

Electrodeposition of PANI/MWCNT nanocomposite on stainless steel with enhanced electrocatalytic activity for oxygen reduction reaction and electro-Fenton process

A more efficient and cost-effective cathode material for in situ electrogeneration of H₂O₂ and electro-Fenton process is prepared.