Decolorization of Methyl Orange Dye at IrO2-SnO2-Sb2O5Coated Titanium Anodes

Dimensionally stable anode (Doped-MMO) mediated electro-oxidation and multi-response optimization study for remediation of urea wastewater

In the present study, the potential application of novel doped-MMO (Ti/IrO₂/Ta₂O₅/SnO₂-Sb₂O₄) anodes as an alternative source to costly electrodes have been visualized for the EO treatment of urea. Parametric optimization for the treatment of urea through the EO process by doped-MMO has been done successfully. The high R² values of both responses i.e. % Degradation and energy consumption for quadratic suggested by BBD under RSM advocates a good correlation between predicted and experimental data. The maximum % Degradation and energy consumption at optimized were found to be 91.2%, 51.53 kWh m^-3 for urea respectively. Additionally, efforts were made to minimize treatment time further by implementing a dual effect, namely photo-electrocatalysis. The anode was found to be relatively stable even after 120 runs. The analysis of treated urea solution was verified in terms of total organic carbon (TOC) 90.0% reduction. The average operating cost of the electro-oxidation treatment process is determined to be 1.91 $ m^-3. The results of this study demonstrate the potential of doped-MMO as a promising concept for the treatment of wastewater that can be successfully applied in real life.

Mineralization of Methyl Orange azo dye by processes based on H2O2 electrogeneration at a 3D-like air-diffusion cathode

This work addresses the mineralization of the widely used Methyl Orange (MO) azo dye by technologies based on H₂O₂ electrogeneration at a 3D-like air-diffusion cathode. These include two Fe²⁺-catalyzed processes such as electro-Fenton (EF) and photoelectro-Fenton (PEF). Bulk electrolyses were performed in a recirculation flow plant, in which the Eco-Cell filter-press electrochemical reactor was connected in series with a UVA photoreactor. The former reactor was equipped with a Ti|Ir-Sn-Sb oxide plate anode alongside a 3D-like air-diffusion cathode made from graphite felt and hydrophobized carbon cloth, aimed at electrogenerating H₂O₂ on site. The influence of current density (j), volumetric flow rate (Q) and initial MO concentration was examined. The greatest oxidation power corresponded to PEF process. The best operation conditions to treat 30 mg L^-1 of total organic carbon of MO in a 50 mM Na₂SO₄ solution by PEF were found at 0.50 mM Fe²⁺, pH 3.0, j = 20 mA cm^-2 and Q = 2.0 L min^-1, obtaining 100% and 94% of color and TOC removals at 30 and 240-300 min, respectively. This accounted for 35% of mineralization current efficiency and 0.12 kWh (g TOC)^-1 of energy consumption at the end of the electrolysis. The oxidation power of EF and PEF was compared with that of anodic oxidation (AO), and the sequence obtained was: PEF > EF > AO. The dye was gradually degraded, yielding non-toxic short carboxylic acids, like maleic, fumaric, formic, oxalic and oxamic, whose Fe(III) complexes were rapidly photolyzed.

Enhanced mass transfer and service time of mesh Ti/Sb-SnO2 electrode for electro-catalytic oxidation of phenol

Titanium-based SnO₂ with Sb dopant (Ti/Sb-SnO₂) was of interest in the field of electro-catalytic oxidation due to its high organic oxidation rates. However, the relatively poor mass transfer performance and short service time limited its practical application. To overcome this problem, Ti/Sb-SnO₂ electrode was fabricated on mesh substrate and used as the anode for electrochemical oxidization of phenol. Compared to the anode prepared on planar Ti, the mesh anode with compact and uniform catalyst surface lowered electron transfer resistance and higher O_ads content (17.41%), which benefited the generation of hydroxyl radicals (·OH) (increment of 24.5%). In addition, this structure accelerated the fluid perturbation around electrode in microscopic scale as the COMSOL simulation result indicated; the electric potential on mesh anode varied regularly along the undulant terrain of electrode so that the mass transfer coefficient was enhanced by 1.67 times. These structure-dependent characteristics contributed to the superior electro-catalytic performance toward degradation of phenol. Experimental results showed that mesh anode had a higher TOC removal efficiency of 90.6% and mineralization current efficiency of 20.1% at current density of 10 mA cm^-2, which was 9.95% and 21.6% higher than the planar anode, and the service lifetime was 1.89 times longer than planar anode. This highly electro-catalytically active and stable Ti/Sb-SnO₂ mesh electrode showed a potential application prospect toward electro-catalytic degradation process.

Experimental study on the optimisation of azo-dyes removal by photo-electrochemical oxidation with TiO2 nanotubes

An experimental investigation is here presented on the photo-electrochemical removal of Methyl Orange (MO), selected as a model of the organic dyes, contained in wastewaters. The process is carried out in an electrochemical flow reactor, in which titania nanotubular electrode is irradiated with a simulated solar light. Design of Experiments (DOE) technique is used to plan the experimental campaign and investigate on the single and combined effects of applied current, electrolyte flow rate, and initial MO concentration, on the specific reaction rate. The results of the DOE analysis, also combined with the study of the distribution of the intermediate products, confirm a reaction mechanism mediated by OH radicals; high applied current and low reactant concentration resulted as favourable conditions to achieve high specific reaction rate of color removal.

Total mineralization of mixtures of Tartrazine, Ponceau SS and Direct Blue 71 azo dyes by solar photoelectro-Fenton in pre-pilot plant

Mixtures of monoazo Tartrazine, diazo Ponceau SS and triazo Direct Blue 71 dyes with 105 mg L^-1 of total organic carbon (TOC) in 0.050 M Na₂SO₄ at pH 3.0 have been treated by solar photoelectro-Fenton (SPEF). Experiments were carried out in a 2.5 L pre-pilot plant with a Pt/air-diffusion cell coupled to a solar planar photoreactor. Comparative trials were made by anodic oxidation with electrogenerated H₂O₂ (AO-H₂O₂) and electro-Fenton (EF) to better understand the role of oxidizing agents. AO-H₂O₂ gave poor degradation due to the low oxidation ability of OH formed at the Pt anode and H₂O₂ produced at the cathode. Similar color removal was achieved in EF and SPEF because the main oxidant was OH formed in the bulk from Fenton's reaction. EF yielded partial mineralization by formation of molecules with high stability against OH. In contrast, these by-products were rapidly photolyzed under sunlight irradiation in SPEF, which was the most powerful treatment. Up to 8 linear final carboxylic acids were detected, along with the release of sulfate and ammonium ions. The effect of Fe²⁺ and azo dye concentrations, and current density over the SPEF performance was assessed. Total mineralization of azo dyes mixtures occurred when operating up to 105 mg L^-1 TOC with 0.50 mM Fe²⁺ at 100 mA cm^-2.

Electrochemical degradation of Mordant Blue 13 azo dye using boron-doped diamond and dimensionally stable anodes: influence of experimental parameters and water matrix

In this work, the electrooxidation as environmentally clean technology has been studied to the degradation of Mordant Blue 13 azo dye (MB13) using boron-doped diamond (p-Si/BDD) and oxide ruthenium titanium (Ti/Ru_0.3Ti_0.7O₂ (DSA)) anodes in various water matrices: distilled water (DW), hot tap water (HTW), and simulated wastewaters with (SWS) and without surfactant (SW). The influence of experimental parameters, such as current density, initial dye concentration, electrolysis time/specific charge, and pH on the MB13 degradation rate, current efficiency, and energy consumption, has been determined. The enhanced rate of both color and chemical oxygen demand (COD) removal in sulfate aqueous solutions with BDD was observed, which indicates that sulfate (SO₄^-•) radicals along with ^•OH ones might be responsible for the degradation process. The MB13 decolorization process obeyed a pseudo-first-order reaction kinetics with the apparent rate constant from 7.36 × 10^-2 min^-1 to 4.39 × 10^-1 min^-1 for BDD and from 9.2 × 10^-3 min^-1 to 2.11 × 10^-2 min^-1 for DSA depending on the electrolysis conditions. The effect of water matrix on the decolorization and COD removal efficiency has been evaluated. Inorganic ions, mordant salt, and surfactant contained in simulated effluents decelerated the COD decay compared to DW and HTW for the both anodes; meanwhile, they differently affected the discoloration process. A comparison of the specific energy consumption for each electrocatalytic material under different experiment conditions has been made. The BDD electrode was more efficient than the DSA to oxidize the MB13 dye in all kinds of water.

Electrochemical oxidation of cyanide on 3D Ti-RuO2 anode using a filter-press electrolyzer

The novelty of this communication lies in the use of a Ti-RuO₂ anode which has not been tested for the oxidation of free cyanide in alkaline media at concentrations similar to those found in wastewater from the Merrill Crowe process (100 mg L^-1 KCN and pH 11), which is typically used for the recovery of gold and silver. The anode was prepared by the Pechini method and characterized by SEM. Linear sweep voltammetries on a Ti-RuO₂ rotating disk electrode (RDE) confirmed that cyanide is oxidized at 0.45 < E < 1.0 V vs SHE, while significant oxygen evolution reaction (OER) occurred. Bulk oxidation of free cyanide was investigated on Ti-RuO₂ meshes fitted into a filter-press electrolyzer. Bulk electrolyzes were performed at constant potentials of 0.85 V and 0.95 V and at different mean linear flow rates ranging between 1.2 and 4.9 cm s^-1. The bulk anodic oxidation of cyanide at 0.85 V and 3.7 cm s^-1 achieved a degradation of 94%, with current efficiencies of 38% and an energy consumption of 24.6 kWh m^-3. Moreover, the degradation sequence of cyanide was also examined by HPLC.

Preparation of IrO2-Ta2O5|Ti electrodes by immersion, painting and electrophoretic deposition for the electrochemical removal of hydrocarbons from water

After intense years of great development, the electrochemical technologies have become very suitable alternatives in niche markets like industrial wastewater reclamation and soil remediation. A key role to achieve a high efficiency in such treatments is played by the characteristics of the coating of the electrodes employed. This paper compares three techniques, namely immersion, painting and electrophoresis, for the preparation of IrO2-Ta2O5ǀTi, so-called dimensionally stable anodes (DSA(®)). The quality of the coatings has been investigated by means of surface and electrochemical analysis. Their ability to generate hydroxyl radicals and degrade aqueous solutions of hydrocarbons like phenanthrene, naphthalene and fluoranthene has been thoroughly assessed. Among the synthesis techniques, electrophoretic deposition yielded the best results, with DSA(®) electrodes exhibiting a homogeneous surface coverage that led to a good distribution of active sites, thus producing hydroxyl radicals that were able to accelerate the degradation of hydrocarbons.

An electrochemical mechanisms study on steel/IrO2–Sb2O3 electrodes for oxidation of phenol in water

Phenol, a known water pollutant, was electrochemically oxidized on a steel/IrO2–Sb2O3 novel anode. Since the oxidation mechanisms vary based on the anode material, a mechanisms study of electrooxidation of phenol on it was conducted. The phenol oxidation was carried out at 20 mA/cm2 constant current density with a pH 11.00 Na2SO4 medium at room temperature. During 6 h of electrolysis, samples were tested for chemical oxygen demand removal efficiency of the anode. The steel/IrO2–Sb2O3anode showed 76.3% chemical oxygen demand removal efficiency. Both 4-nitroso-N,N-dimethylaniline and the HCO3–/CO32– radical scavenger tests confirmed the formation and presence of the hydroxyl radicals in the system. Therefore, it was concluded that the hydroxyl radicals that are generated on the anode surface are the main cause for the oxidation mechanism. Moreover, ICE, HPLC, and UV-vis absorbance and cyclic voltammetry results confirmed the presence of catechol and benzoquinone as intermediates and the reaction mechanism.

Multivariate optimization for electrochemical oxidation of methyl orange: Pathway identification and toxicity analysis

Electrochemical oxidation of methyl orange (Sodium 4-[(4-dimethylamino) phenyldiazenyl] benzenesulfonate) with lead dioxide coated on mild steel was modelled using response surface methodology (RSM) to analyze the influence of pH, NaCl dose and current on color and chemical oxygen demand (COD) removal. Higher current, acidic pH and 0.8-1.2 g L(-1) NaCl dose had an enhancing effect on the removal efficiencies. Interaction effect of the variables highlights the action of (•)OH and HOCl in the oxidation of methyl orange, where HOCl has effect at lower current range. More than 90% COD removal efficiency and ∼100% color removal efficiency was obtained in 5 h at optimum conditions for an initial concentration of 50 mg L(-1). High performance liquid chromatography-mass spectroscopy (HPLC-MS) analysis carried out to identify degradation intermediates revealed the absence of chlorinated intermediates, which was further verified with Fourier transform infrared spectroscopy (FTIR) analysis. The postulated pathway of degradation indicated breakdown through dealkylation, deamination, desulfonation and cleavage of an azo bond and benzene ring. The degradation of methyl orange to smaller compounds was also confirmed by Ion Chromatography (IC). Cytotoxicity analysis on HaCaT cells revealed the intermediates to be more cytotoxic than the dye, possibly due to the aromatic amines and diazines formed during the degradation process.

Sequential electrochemical treatment of dairy wastewater using aluminum and DSA-type anodes

Dairy wastewater is characterized by a high content of hardly biodegradable dissolved, colloidal, and suspended organic matter. This work firstly investigates the performance of two individual electrochemical treatments, namely electrocoagulation (EC) and electro-oxidation (EO), in order to finally assess the mineralization ability of a sequential EC/EO process. EC with an Al anode was employed as a primary pretreatment for the conditioning of 800 mL of wastewater. A complete reduction of turbidity, as well as 90 and 81% of chemical oxygen demand (COD) and total organic carbon (TOC) removal, respectively, were achieved after 120 min of EC at 9.09 mA cm(-2). For EO, two kinds of dimensionally stable anodes (DSA) electrodes (Ti/IrO₂-Ta₂O₅ and Ti/IrO₂-SnO₂-Sb₂O₅) were prepared by the Pechini method, obtaining homogeneous coatings with uniform composition and high roughness. The (·)OH formed at the DSA surface from H₂O oxidation were not detected by electron spin resonance. However, their indirect determination by means of H₂O₂ measurements revealed that Ti/IrO₂-SnO₂-Sb₂O₅ is able to produce partially physisorbed radicals. Since the characterization of the wastewater revealed the presence of indole derivatives, preliminary bulk electrolyses were done in ultrapure water containing 1 mM indole in sulfate and/or chloride media. The performance of EO with the Ti/IrO₂-Ta₂O₅ anode was evaluated from the TOC removal and the UV/Vis absorbance decay. The mineralization was very poor in 0.05 M Na₂SO₄, whereas it increased considerably at a greater Cl(-) content, meaning that the oxidation mediated by electrogenerated species such as Cl₂, HClO, and/or ClO(-) competes and even predominates over the (·)OH-mediated oxidation. The EO treatment of EC-pretreated dairy wastewater allowed obtaining a global 98 % TOC removal, decreasing from 1,062 to <30 mg L(-1).