DSA electrochemical treatment of olive mill wastewater on Ti/RuO<Subscript>2</Subscript> anode

The electrochemical oxidation of olive mill wastewater (OMW) over a Ti/RuO₂ anode was studied by means of cyclic voltammetry and bulk electrolysis and compared with previous results over a Ti/IrO₂ anode. Experiments were conducted at 300–1,220 mg L⁻¹ initial chemical oxygen demand (COD) concentrations, 0.05–1.35 V versus SHE and 1.39–1.48 V versus SHE potential windows, 15–50 mA cm⁻² current densities, 0–20 mM NaCl, Na₂SO₄, or FeCl₃ concentrations, 80 °C temperature, and acidic conditions. Partial and total oxidation reactions occur with the overall rate being near first-order kinetics with respect to COD. Oxidation at 28 Ah L⁻¹ and 50 mA cm⁻² leads to quite high color and phenols removal (86 and 84%, respectively), elimination of ecotoxicity, and a satisfactory COD and total organic carbon reduction (52 and 38%, respectively). Similar performance can be achieved at the same charge (28 Ah L⁻¹) using lower current densities (15 mA cm⁻²) but in the presence of various salts. For example, COD removal is less than 7% at 28 Ah L⁻¹ in a salt-free sample, while addition of 20 mM NaCl results in 54% COD reduction. Decolorization of OMW using Ti/RuO₂ anode seems to be independent of the presence of salts in contrast with Ti/IrO₂ where addition of NaCl has a beneficial effect on decolorization.     

Preparation and evaluation of RuO2–IrO2, IrO2–Pt and IrO2–Ta2O5 catalysts for the oxygen evolution reaction in an SPE electrolyzer

IrO₂–RuO₂, IrO₂–Pt and IrO₂–Ta₂O₅ electrocatalysts were synthesized and characterized for the oxygen evolution in a Solid Polymer Electrolyte (SPE) electrolyzer. These mixtures were characterized by XRD and SEM. The anode catalyst powders were sprayed onto Nafion 117 membrane (catalyst coated membrane, CCM), using Pt catalyst at the cathode. The CCM procedure was extended to different in-house prepared catalyst formulations to evaluate if such a method could be applied to electrolyzers containing durable titanium backings. The catalyst loading at the anode was about 6 mg cm⁻², whereas 1 mg cm⁻² Pt was used at the cathode. The electrochemical activity for water electrolysis was investigated in a single cell SPE electrolyzer at 80 °C. It was found that the terminal voltage obtained with Ir–Ta oxide was slightly lower than that obtained with IrO₂–Pt and IrO₂–RuO₂ at low current density (lower than 0.15 A cm⁻²). At higher current density, the IrO₂–Pt and IrO₂–RuO₂ catalysts performed better than Ir–Ta oxide.     

Structural analyses of RuO<Subscript>2</Subscript>–TiO<Subscript>2</Subscript>/Ti and IrO<Subscript>2</Subscript>–RuO<Subscript>2</Subscript>–TiO<Subscript>2</Subscript>/Ti anodes used in industrial chlor-alkali membrane processes

The morphology and composition of RuO₂–TiO₂/Ti and IrO₂–RuO₂–TiO₂/Ti anodes, which have been used for the production of chlorine for more than 10 years, were analyzed by various methods; such as high-resolution scanning electron microscopy, high-resolution Auger electron spectroscopy, electron probe X-ray emission microanalysis and X-ray diffraction analysis. Drastic changes in the surface morphology, including partial exfoliation of a small amount of the oxide layer and a reduction in the content of ruthenium species through dissolution, were observed for the RuO₂–TiO₂/Ti anode. For the IrO₂–RuO₂–TiO₂/Ti anode, on the other hand, there were moderate changes in the surface morphology and moderate dissolution of iridium and ruthenium species.     

Electrochemical treatment of bisphenol-A using response surface methodology

The decomposition of bisphenol-A (BPA) in synthetic solution and in municipal effluent was investigated using an electro-oxidation process. Electrolysis was conducted using a cylindrical electrolytic cell containing two circular anodes (expanded metal) and two circular cathodes (stainless steel) alternated in the electrode pack. Different anode materials (Ti/SnO₂, Ti/IrO₂ and Ti/PbO₂) were tested, and Ti/PbO₂ was found to be the most effective electrode for BPA degradation. An experimental design methodology (2³ Box–Behnken design) was applied to determine the optimal experimental conditions in terms of cost effectiveness. The BPA concentration (C ₀ = 1.0 mg l⁻¹) could be optimally diminished by up to 90% by applying a current intensity of 2.0 A for a 100-min reaction period in the presence of 250 mg Na₂SO₄ l⁻¹ (used as a supporting electrolyte). Then, the optimal conditions were applied on a municipal wastewater effluent (sampled after secondary treatment) artificially contaminated with 1 mg BPA l⁻¹ without the addition of a supporting electrolyte. The treatment was more effective with the municipal effluent due to the presence of a high concentration of chloride ions that could easily be transformed into active chlorine. BPA could be oxidized by both direct anodic electrochemical oxidation (by means of OH·) and indirect electrochemical oxidation via mediators, such as hypochlorous acid generated by chloride oxidation. Both actions (direct and indirect effects) lead to the formation of powerful oxidizing agents capable of rapidly oxidizing BPA.     

Effects of the geometry and operating temperature on the stability of Ti/IrO<Subscript>2</Subscript>–SnO<Subscript>2</Subscript>–Sb<Subscript>2</Subscript>O<Subscript>5</Subscript> electrodes for O<Subscript>2</Subscript> evolution

Ternary IrO₂–Sb₂O₅–SnO₂ anode has shown its superiorities over IrO₂ and many other electrocatalysts for O₂ evolution, in terms of electrochemical stability, activity and cost. The performance of IrO₂–Sb₂O₅–SnO₂ anodes is affected by its electrochemical properties and operating conditions. In this paper, the electrochemical stability and activity of the Ti/IrO₂–Sb₂O₅–SnO₂ anodes prepared with three different geometries were investigated under different operating conditions. It was found that anodes with large mean curvature have high electrochemical stability. Although increasing temperature results in a decrease in the stability of Ti/IrO₂–Sb₂O₅–SnO₂, the anode with a mean curvature of 200 m⁻¹ still shows acceptable service life even at 70 °C. This tolerance of high temperature was attributed to the thermal expansion difference between the substrate and the coating layer, the redox window for Ir(V)/Ir(IV) conversion, and the redox reversibility of Sb and Sn species in the coating layer.     

A novel IrO<Subscript>2</Subscript> electrode with iridium–titanium oxide interlayers from a mixture of TiN nanoparticle and H<Subscript>2</Subscript>IrCl<Subscript>6</Subscript> solution

A novel IrO₂ anode on titanium substrate with iridium–titanium oxide interlayer (Ti/IrO _x –TiO₂/IrO₂) was prepared and investigated for oxygen evolution. IrO _x –TiO₂ interlayer was coated on titanium substrate by impregnation-thermal decomposition method from a mixture of TiN nanoparticles and H₂IrCl₆ solution at 500 °C. The results showed that the service life of Ti/IrO _x –TiO₂/IrO₂ was a factor of six times longer than that of Ti/IrO₂, which was attributed to the IrO _x –TiO₂ interlayer, it could form a metastable solid solution between IrO _x and thin titanium oxide layer on titanium substrate during calcination. The interlayer contributed to the decrease in migration rate of oxygen atom or molecule toward substrate and the increase in bonding force among IrO₂ layer, interlayer, and substrate. Therefore, besides keeping high electrocatalytic activity, the service life of Ti/IrO _x –TiO₂/IrO₂ electrode was greatly improved, and its overall electrocatalytic performance for oxygen evolution was increased as well.     

Performance of sol–gel Titanium Mixed Metal Oxide electrodes for electro-catalytic oxidation of phenol

Mixed metal oxides SnO₂–RuO₂–IrO₂, Ta₂O₅–IrO₂ and RhO₂–IrO₂ were immobilised on a Ti substrate using sol–gel techniques. The Ti mixed metal oxides were characterized in terms of morphology using scanning electron microscopy. Cyclic voltammetric responses of phenol at Ti/SnO₂–RuO₂–IrO₂, Ti/Ta₂O₅–IrO₂ and Ti/RhO₂–IrO₂ electrodes were evaluated and showed significantly low potentials for Ti/Ta₂O₅–IrO₂ (+100 mV), Ti/SnO₂–RuO₂–IrO₂ (+200 mV) and Ti/RhO₂–IrO₂ (−100 mV). The degradation of phenol in aqueous solution and its intermediates were investigated by bulk electrolysis and quantitatively assessed by HPLC analysis to elucidate the decomposition pathways and to develop a kinetic model for the electro-catalytic oxidation of phenol over Ti mixed metal oxides. Ring compounds, benzoquinone/hydroquinone, catechol, and short chain organics, carboxylic acids, have been identified as intermediate products for the electro-catalytic oxidation of phenol. Fundamental kinetic data were obtained for the conversion of phenol at these electrodes and was found to proceed in accordance with the pseudo-first-order kinetics with respect to the phenol concentration.     

Supercapacitive properties of polyaniline/hydrous RuO2 composite electrode

A composite electrode based on polyaniline (PANI) and hydrous RuO₂ is prepared by electrochemical deposition of PANI onto hydrous RuO₂ (PANI/RuO₂) and its supercapacitive properties are investigated using cyclic voltammetry. The specific capacitances of PANI/RuO₂ and hydrous RuO₂ electrodes are determined to be 708 and 517 F g⁻¹ at 5 mV s⁻¹, respectively. Simple electrodeposition of PANI on the hydrous RuO₂ can achieve comparatively greater capacitance values.     

Electrocatalytic oxidation of coal on Ti-supported metal oxides coupled with liquid catalysts for H2 production

Electrocatalytic oxidation of coal on Ti-supported metal/metal oxides coupled with liquid catalysts is systematically investigated as a method of producing hydrogen at the cathode. The composition of the liquid catalyst was varied to determine its effect on the coal electrolysis. A spectrum of byproducts from the coal oxidation at the anode was analyzed. The Ti-supported metal oxide electrodes were prepared by thermal decomposition and characterized by scanning electron microscopy (SEM). X-ray diffraction results show that the composition of the electrodes was Ti/Pt, Ti/RuO₂, Ti/IrO₂ and Ti/IrO₂–RuO₂. Coal oxidation tests on these electrodes indicate that Ti/IrO₂ has the best electrocatalytic activity. Polarization curves reveal that redox catalysts, such as Fe³⁺, K₃Fe(CN)₆, KBr and V₂O₅, bridge the coal particles and the solid electrode surface, thus increasing the rates of coal oxidation. The dynamic transition of Fe³⁺/Fe²⁺ is proven by a KMnO₄ titration experiment, and the possible catalytic mechanism is discussed. Product analysis shows that pure H₂ is generated at the cathode and that CO₂ is the main product at the anode.     

Effect of IrO2 loading on RuO2–IrO2–TiO2 anodes: A study of microstructure and working life for the chlorine evolution reaction

RuO₂–IrO₂–TiO₂/Ti anodes are widely used in chlor-alkali and chlorate industry. The working life is of particular importance for anodes. The relationship between the content of IrO₂ and working life was investigated. The microstructure of the anode with 0.5 mg/cm² IrO₂ is much more homogeneous than those with lower IrO₂ contents. Both the size of particles containing RuO₂ and IrO₂ and the content of rutile TiO₂ phase in anodes decrease with increased IrO₂ content. The working life of anode with 0.5 mg/cm² IrO₂ is almost double that of the anode with 0.3 mg/cm² IrO₂.     

Electrochemical treatment of ammonia in wastewater by RuO<Subscript>2</Subscript>–IrO<Subscript>2</Subscript>–TiO<Subscript>2</Subscript>/Ti electrodes

This study investigated the removal of ammonia in wastewater by an electrochemical method using titanium electrodes coated with ruthenium and iridium (RuO₂–IrO₂–TiO₂/Ti) with low chlorine evolution over-voltage. The effects of operating parameters, including chloride ion concentration, current density and initial pH, were also investigated. The results were evaluated primarily by considering the efficiency of the elimination of NH₄⁺-N. The removal of ammonia by electrochemical oxidation mainly resulted from the indirect oxidation effect of chlorine/hypochlorite produced during electrolysis. The direct anodic oxidation efficiency of ammonia was less than 5%, and the current efficiency was less than 10%. The ammonia removal followed pseudo-first-order kinetics. The electrochemical process can be applied successfully as a final polishing step, or as an alternative method to biological nitrification. The process seems to be most beneficial for small coastal cities     

Oxidation of unsaturated and hydroxy fatty acids by ruthenium tetroxide and ruthenium oxyanions

The reactions of ruthenium VIII tetroxide (RuO₄) and the ruthenium VII and VI oxyanions, perruthenate (RuO₄ ⁻) and ruthenate (RuO₄ ⁼) with hydroxy substituted and unsaturated fatty acids have been studied. At a 1:1 molar ratio, ruthenium tetroxide (RuO₄) and both oxyanions (RuO₄ ⁻ and RuO₄ ⁼) oxidized 12-hydroxystearic acid to 12-ketostearic acid. With 9, 10-dihydroxystearic acid, the type of oxidation products obtained depended on the amount of ruthenium oxidant used. At high ratios of oxidant to substrate, cleavage to pelargonic and azelaic acids occurred whereas at lower ratios, partial oxidation to diketo and acyloin derivatives predominated. The oxidation of oleic acid with excess ruthenium tetroxide (RuO₄) or perruthenate anion (RuO₄ ⁻) gave the cleavage products pelargonic and azelaic acid through the intermediate formation of dihydroxy and diketo intermediates. Ruthenate anion (RuO₄ ⁼) did not react with the double bond of oleic acid. Agricultural Research Service, U.S. Department of Agriculture.     

Electrochemical waste water treatment: Electrooxidation of acetaminophen

Oxidation of acetaminophen at boron-doped diamond (BDD) and at Ti/SnO₂ anodes in a plug-flow divided electrochemical reactor led to electrochemical combustion, whereas at Ti/IrO₂ benzoquinone was the exclusive product except at very long electrolysis times. The difference is explicable in terms of the different mechanisms of oxidation: direct oxidation at the anode for Ti/IrO₂ vs. indirect oxidation involving electrogenerated hydroxyl radicals at BDD and Ti/SnO₂. At BDD, at which the efficiency of degradation of acetaminophen was greatest, the rate of electrolysis at constant concentration was linearly dependent on the current, and at constant current linearly dependent on the concentration. Current efficiencies for mineralization up to 26% were achieved without optimization of the cell design.     

In situ active chlorine generation for the treatment of dye-containing effluents

This study examined the possibility to remove colour causing-compounds from synthetic effluent by indirect electrochemical oxidation using iridium oxide anode electrodes. Using a high concentration of chloride ions (17.1 mM) and various current densities, it was possible to produce high concentration of active chlorine with a specific production rate of 2.8 mg min⁻¹ A⁻¹. The best performance for acid methyl violet 2B dye (MV2B) decomposition was obtained using Ti/IrO₂ anodes operated at a current density of 15 mA cm⁻² during 40 min of treatment in the presence of 3.42 mM of chloride ions. Under these conditions, more than 99% of MV2B was removed (with a reaction rate apparent constant of 0.20 min⁻¹), whereas COD and TOC removal were 51% and 75%, respectively. The electrolytic cell was then used for the degradation of three other synthetic dye solutions: Eosin yellowish (EOY), Trypan Blue (TRB), Acridine Orange (ACO). TRB was the most difficult dye to remove from solution with a value reaction rate constant of 0.12 min⁻¹, compared to 0.19 min⁻¹ and 0.24 min⁻¹ recorded for ACO and EOY dyes, respectively. More than 99% of these dyes were removed by electrochemical oxidation.     

Thermodynamics of chromate replacements by various homologous transition metal oxyanions

The free energy change is calculated for the interaction of 19 different oxyanions (metalates) with iron (steel) or aluminum surfaces. The oxyanions considered here are those of the transition metals in the fourth through sixth periods of the periodic table. The oxyanions which produce more negative values of ΔG ^o (per mole of oxyanion) than that of chromate (CrO₄ ⁻²) are permanganate (MnO₄ ⁻), nickelate (NiO₄ ⁻²), ruthenate (RuO₄ ⁻ or RuO₄ ⁻²), and rhodate (RhO₄ ⁻²). The oxyanions which produce values of ΔG ^o (per mole of oxyanion) similar to CrO₄ ⁻² are osmate (OsO₄ ⁻²), and iridate (IrO₄ ⁻²).     

Pt film electrodes prepared by the Pechini method for electrochemical decolourisation of Reactive Orange 16

Electrochemical decolourisation of Reactive Orange 16 was carried out in an electrochemical flow-cell, using as working electrodes a Pt thin film deposited on a Ti substrate (Pt/Ti) prepared by the Pechini method and a pure platinum (Pt) foil. Using the Pt/Ti electrodes better results for dye decolourisation were obtained under milder conditions than those used for pure Pt. For the Pt electrode, colour removal of 93 % (λ = 493 nm) was obtained after 60 min, at 2.2 V vs. RHE, using 0.017 mol L⁻¹ NaCl + 0.5 mol L⁻¹ H₂SO₄ solution. For the Pt/Ti electrode there was better colour removal, 98%, than for the Pt electrode. Moreover, we used 0.017 mol L⁻¹ NaCl solution and the applied potential was 1.8 V. Under this condition after 15 min of electrolysis, more than 80% of colour was removed. The rate reaction constant, assuming a first order reaction, was 0.024 min⁻¹ and 0.069 min⁻¹, for Pt and Pt/Ti electrodes, respectively.     

Mutagenic activity removal of selected disperse dye by photoeletrocatalytic treatment

The degradation of black dye commercial product (BDCP) composed of C.I. Disperse Blue 373, C.I. Disperse Orange 37, C.I. Disperse Violet 93 dyes was investigated by photoelectrocatalysis process. The dyes have shown high mutagenic activity with Salmonella strain YG1041 and TA98 with and without S9. Samples of BCPD dye submitted to conventional chlorination and photoelectrocatalytic oxidation were compared monitoring its products by HPLC using a diode array detector, spectrophotometry UV–vis, TOC removal, and mutagenicity potency. The photoelectrocatalytic method operating with Ti/TiO₂ as anode at +1.0 V and UV illumination presented fast oxidation of test solutions containing 10 mg L⁻¹ of dye in 0.1 mol L⁻¹ NaCl pH 4.0 leading to 100% of discoloration, 67% of mineralization, and negative response to all tested Salmonella strains. The formation of Cl^•, CL₂ ^• on photoelectrocatalytic medium improved the efficiency of the method in relation to conventional chlorination method that promoted 100% of discoloration, but only 8% of TOC removal and more mutagenic product.     

Supercapacitive properties of composite electrodes consisting of polyaniline, carbon nanotube, and RuO2

Three types of composite supercapacitor electrodes were prepared; electroactive polyaniline (PANI), PANI/multi-walled carbon nanotube (CNT), and PANI/CNT/RuO₂. Specifically, the PANI and PANI/CNT were prepared by polymerization, and PANI/CNT/RuO₂ was prepared by electrochemical deposition of RuO₂ on the PANI/CNT matrix. Cyclic voltammetry between −0.2 and 0.8 V (vs. Ag/AgCl) at various scan rates was performed to investigate the supercapacitive properties in an electrolyte solution of 1.0 M H₂SO₄. The PANI/CNT/RuO₂ electrode showed the highest specific capacitance at all scan rates (e.g., 441 and 392 F g⁻¹ at 100 and 1,000 mV s⁻¹, respectively). In contrast, the PANI/CNT electrode demonstrated the best capacitance retention (66%) after 10⁴ cycles. Additional analysis including morphology and complex impedance spectroscopy suggested that with small loading of RuO₂, an increase in capacitance was observed, but dissolution and/or detachment of RuO₂ species from the electrode might occur during cycling to reduce the cycle performance.     

Electrochemical process for the treatment of landfill leachate

In this paper, the anodic oxidation of a real leachate from an old municipal solid waste landfill has been studied using an electrolytic flow cell equipped with a lead dioxide (PbO₂) anode and stainless steel as the cathode. The influence of several operation parameters such as (i) the applied current (from 0.5 to 3 A), (ii) liquid flow rate (from 50 to 420 L h⁻¹), (iii) temperature (from 25 to 50 °C), and (iv) pH (from 3.5 to 8.2) on the COD removal rate, current efficiency, and energy consumption has been evaluated. The galvanostatic electrolyses always yielded COD values below the discharge limit (COD <160 mg L⁻¹); the COD removal rate increased with rising applied current, solution pH, and temperature, whereas it remained almost unaffected by the recirculation flow rate. These results indicate that the organic compounds were mainly removed by their indirect oxidation by the active chlorine generated from chlorides oxidation. The specific energy consumption necessary to reduce the organic load to below the disposal limit was 90 kWh m⁻³.     

Studies on the oxidation rate of metallic inert anodes by measuring the oxygen evolved in low-temperature aluminium electrolysis

The rate of oxygen evolution on metallic inert anodes was measured as a function of current density during electrolysis of a low-melting NaF(12)–KF–AlF₃ bath ([NaF + KF]/[AlF₃] = 1.5 mol mol⁻¹) at 800 °C. The oxidation rate of the anode substrate (A cm⁻²) was calculated. The anode oxidation process was depressed at the potentials of oxygen evolution. The dynamics of the decrease in the oxidation rate, which were obtained in previous study by the change in geometrical size of the metallic part of the specimen, was reproduced both by the technique proposed and also in potentiostatic electrolysis at potentials below that of oxygen evolution, in some cases, depending on prepolarisation.