Influence of the boron doping level on the electrochemical oxidation of the azo dyes at Si/BDD thin film electrodes

In this study the efficiency of electrochemical oxidation of aromatic pollutants, such as reactive dyes, at boron-doped diamond on silicon (Si/BDD) electrodes was investigated. The level of [B]/[C] ratio which is effective for the degradation and mineralization of selected aromatic pollutants, and the impact of [B]/[C] ratio on the crystalline structure, layer conductivity and relative sp³/sp² coefficient of a BDD electrode were also studied. The thin film microcrystalline electrodes have been deposited on highly doped silicon substrates via MW PE CVD. Si/BDD electrodes were synthesized for different [B]/[C] ratios of the gas phase. Mechanical and chemical stability of the electrodes was achieved for the microcrystalline layer with relatively high sp³/sp² band ratio. Layer morphology and crystallite size distribution were analyzed by SEM. The resistivity of BDD electrodes was studied using four-point probe measurements. The relative sp³/sp² band ratios were determined by deconvolution of Raman and X-ray photoelectron spectra. The efficiency of degradation and mineralization of the reactive azo dye rubin F-2B was estimated based on the absorbance measurements at 545 nm. The influence of commonly used electrolytes NaCl and Na₂SO₄ on the dye removal efficiency was also investigated. The results suggest that, in general, the oxidation occurs indirectly at the anode through generation of hydroxyl radicals •OH, which react with the dye in a very fast and non-selective manner. In NaCl electrolyte the dye was also decomposed by more selective, active chlorine species (Cl₂, HOCl). However the efficiency of this process in BDD depended on the electrode's doping level. Higher amounts of dopant on the surface of BDD resulted in the higher efficiency of dye removal in both electrolytes.     

The improvement of boron-doped diamond anode system in electrochemical degradation of p-nitrophenol by zero-valent iron

Boron-doped diamond (BDD) electrodes are promising anode materials in electrochemical treatment of wastewaters containing bio-refractory organic compounds due to their strong oxidation capability and remarkable corrosion stability. In order to further improve the performance of BDD anode system, electrochemical degradation of p-nitrophenol were initially investigated at the BDD anode in the presence of zero-valent iron (ZVI). The results showed that under acidic condition, the performance of BDD anode system containing zero-valent iron (BDD-ZVI system) could be improved with the joint actions of electrochemical oxidation at the BDD anode (39.1%), Fenton's reaction (28.5%), oxidation–reduction at zero-valent iron (17.8%) and coagulation of iron hydroxides (14.6%). Moreover, it was found that under alkaline condition the performance of BDD-ZVI system was significantly enhanced, mainly due to the accelerated release of Fe(II) ions from ZVI and the enhanced oxidation of Fe(II) ions. The dissolved oxygen concentration was significantly reduced by reduction at the cathode, and consequently zero-valent iron corroded to Fe(II) ions in anaerobic highly alkaline environments. Furthermore, the oxidation of released Fe(II) ions to Fe(III) ions and high-valent iron species (e.g., FeO²⁺, FeO₄²⁻) was enhanced by direct electrochemical oxidation at BDD anode.     

An electrolyte-free system for ozone generation using heavily boron-doped diamond electrodes

An electrolyte-free system for electrochemical ozone generation was developed using boron-doped diamond (BDD) electrodes as the anode and cathodes in combination with Nafion® N117/H⁺ as the separating membrane. Trials using BDD with various B/C ratios suggested that heavily boron-doped BDD with sp² impurities yielded high concentration of ozone. Further experiments by replacement of the feedstock solutions with 0.85 M Na₂SO₄ or 0.85 M NaCl resulted in no significant difference, suggesting that the use of pure water as the feedstock is the most appropriate method for ozone production. A high efficiency could be achieved by applying water as a feedstock for the anode and the cathode chambers. In addition, comparison with Pt electrodes confirmed that the excellent structural stability of BDD was the main factor contributing to this success.     

Degradation of the beta-blocker propranolol by electrochemical advanced oxidation processes based on Fenton's reaction chemistry using a boron-doped diamond anode

The electro-Fenton (EF) and photoelectro-Fenton (PEF) degradation of solutions of the beta-blocker propranolol hydrochloride with 0.5 mmol dm⁻³ Fe²⁺ at pH 3.0 has been studied using a single cell with a boron-doped diamond (BDD) anode and an air diffusion cathode (ADE) for H₂O₂ electrogeneration and a combined cell containing the above BDD/ADE pair coupled in parallel to a Pt/carbon felt (CF) cell. This naphthalene derivative can be mineralized by both methods with a BDD anode. Almost overall mineralization is attained for the PEF treatments, more rapidly with the combined system due to the generation of higher amounts of hydroxyl radical from Fenton's reaction by the continuous Fe²⁺ regeneration at the CF cathode, accelerating the oxidation of organics to Fe(III)-carboxylate complexes that are more quickly photolyzed by UVA light. The homologous EF processes are less potent giving partial mineralization. The effect of current density, pH and Fe²⁺ and drug concentrations on the oxidation power of PEF process in combined cell is examined. Propranolol decay follows a pseudo first-order reaction in most cases. Aromatic intermediates such as 1-naphthol and phthalic acid and generated carboxylic acids such as maleic, formic, oxalic and oxamic are detected and quantified by high-performance liquid chromatography. The chloride ions present in the starting solution are slowly oxidized at the BDD anode. In PEF treatments, all initial N of propranolol is completely transformed into inorganic ions, with predominance of NH₄⁺ over NO₃⁻ ion.     

Treatment of Fenton‐refractory olive oil mill wastes by electrochemical oxidation with boron‐doped diamond anodes

In this work, the electrochemical oxidation of an actual industrial waste with conductive diamond anodes has been studied. The wastewater is the effluent of a wastewater treatment plant consisting of a Fenton reactor followed by a settler and a sand filter, in which the wastes generated in an olive oil mill are treated. These wastes contain a residual chemical oxygen demand of nearly 700 mg dm^?3 which cannot be further oxidized with the Fenton process. The electrolyses were carried out under galvanostatic conditions, using a bench‐scale plant equipped with a single‐compartment electrochemical flow cell. Boron‐doped diamond (BDD) and stainless steel (AISI 304) were use as anode and cathode of the cell, respectively. The complete mineralization of the waste was obtained with high current efficiencies limited only by mass transport processes. This confirms that besides the hydroxyl radical‐mediated oxidation that occurs in the Fenton process, the electrochemical oxidation with conductive diamond electrodes combines other important oxidation processes such as direct electro‐oxidation on the BDD surface and oxidation mediated by other electrochemically formed compounds generated in this electrode. Copyright © 2006 Society of Chemical Industry     

Electrochemical incineration of chloranilic acid using Ti/IrO2, Pb/PbO2 and Si/BDD electrodes

The electrochemical oxidation of chloranilic acid (CAA) has been studied in acidic media at Pb/PbO₂, boron-doped diamond (Si/BDD) and Ti/IrO₂ electrodes by bulk electrolysis experiments under galvanostatic control. The obtained results have clearly shown that the electrode material is an important parameter for the optimization of such processes, deciding of their mechanism and of the oxidation products. It has been observed that the oxidation of CAA generates several intermediates eventually leading to its complete mineralization. Different current efficiencies were obtained at Pb/PbO₂ and BDD, depending on the applied current density in the range from 6.3 to 50 mA cm⁻². Also the effect of the temperature on Pb/PbO₂ and BDD electrodes was studied.UV spectrometric measurements were carried out at all anodic materials, with applied current density of 25 and 50 mA cm⁻². These results showed a faster CAA elimination at the BDD electrode. Finally, a mechanism for the electrochemical oxidation of CAA has been proposed according to the results obtained with the HPLC technique.     

A kinetic study of the electrochemical oxidation of maleic acid on boron doped diamond

Maleic acid (MA) is one of the main intermediates formed during mineralization, by electrooxidation, of aromatic compounds contained in aqueous wastes. This work investigates oxidation of maleic acid with or without the presence of oxalic acid (OA) and formic acid (FA) in aqueous solution by using boron-doped diamond (BDD) electrodes. OA and FA are the main products formed in MA electrooxidation. Voltammetric studies conducted with a BDD electrode of small surface (0.196 cm²) show that MA oxidation takes place at a potential very close to that of the discharge of water. But, under potentiostatic conditions and at concentrations higher than 0.001 M, adsorption of MA blocks its own oxidation. Oxalic and formic acids are before the discharge of water. Again, the presence of maleic acid blocks the oxidation of formic and oxalic acids. Galvanostatic electrolyses of aqueous solutions of MA, OA, FA and mixtures of theses acids were conducted on a BDD electrode. Electrolyses were controlled by measurements of Total Organic Carbon, Chemical Oxygen Demand and by Liquid Chromatography. Results showed that MA was totally mineralized; FA and OA were very low concentration intermediaries. Electrolyses of solutions containing MA, initially in the presence of OA or FA, showed that the OA was oxidized at the same rate as the MA, whereas the FA oxidation began only when the MA had completely disappeared. These results suggest that OA oxidizes by a mass transport limited process coupled with a direct electron transfer with the anode. Under galvanostatic conditions, maleic acid and formic acid are probably oxidized via OH^· radicals generated by water discharge.     

Solar photoassisted anodic oxidation of carboxylic acids in presence of Fe using a boron-doped diamond electrode

This paper reports a comparative study on the anodic oxidation of 2.5 l of 50 mg l⁻¹ TOC of formic, oxalic, acetic, pyruvic or maleic acid in 0.1 M Na₂SO₄ solutions of pH 3.0 with and without 1.0 mM Fe³⁺ as catalyst in the dark or under solar irradiation. Experiments have been performed with a batch recirculation flow plant containing a one-compartment filter-press electrolytic reactor equipped with a 20 cm² boron-doped diamond (BDD) anode and a 20 cm² stainless steel cathode, and coupled to a solar photoreactor. This system gradually accumulates H₂O₂ from dimerization of hydroxyl radical (OH) formed at the anode surface from water oxidation. Carboxylic acids in direct anodic oxidation are mainly oxidized by direct charge transfer and/or OH produced on BDD, while their Fe(III) complexes formed in presence of Fe³⁺ can also react with OH produced from Fenton reaction between regenerated Fe²⁺ with electrosynthesized H₂O₂ and/or photo-Fenton reaction. Fast photolysis of Fe(III)-oxalate and Fe(III)-pyruvate complexes under the action of sunlight also takes place. Chemical and photochemical trials of the same solutions have been made to better clarify the role of the different catalysts. Solar photoassisted anodic oxidation in presence of Fe³⁺ strongly accelerates the removal of all carboxylic acids in comparison with direct anodic oxidation, except for acetic acid that is removed at similar rate in both cases. This novel electrochemical advanced oxidation process allows more rapid mineralization of formic, oxalic and maleic acids, without any significant effect on the conversion of acetic acid into CO₂. The synergistic action of Fe³⁺ and sunlight in anodic oxidation can then be useful for wastewater remediation when oxalic and formic acids are formed as ultimate carboxylic acids of organic pollutants, but its performance is expected to strongly decay in the case of generation of persistent acetic acid during the degradation process.     

Kinetic modelling of the electrochemical mineralization of organic pollutants for wastewater treatment

The electrochemical mineralization of organic pollutants is a new technology for treatment of dilute wastewater (COD < 5 g L⁻¹). In this method, use of the electrical energy can produce complete oxidation of pollutants on high oxidation power anodes. An ideal anode for this type of treatment is a boron-doped diamond electrode (BDD) characterized by a high reactivity towards oxidation of organics. In the present work kinetic aspects of organic mineralization is discussed. The proposed theoretical kinetic model on boron-doped diamond anodes is in excellent agreement with experimental results. In addition economic aspects of electrochemical organic mineralization are reported.     

Mechanistic investigation of the conductive ceramic Ebonex as an anode material

Ebonex^® is a conductive and corrosion-resistant ceramic with the approximate composition Ti₄O₇. Anodic and/or cathodic polarization of a pair of Ebonex electrodes changed their surface composition, as shown by the development of a potential difference between them. In consequence, the activity of an Ebonex anode with respect to oxidation of an organic substrate depends on its past history. The anodic oxidation of p-nitrosodimethylaniline, which has been used as a model compound for the detection and quantitation of hydroxyl radicals, was studied in order to determine whether hydroxyl radicals are produced upon anodic polarization of Ebonex. The results were ambiguous, because direct oxidation of this substrate and oxidation of water to hydroxyl radicals occur at similar potentials. p-Benzoquinone (BQ) was found to be a more satisfactory mechanistic probe because it is resistant to direct oxidation. The rates of both disappearance and overall mineralization of BQ at Ebonex were intermediate between the corresponding rates at boron-doped diamond (BDD) and Ti/IrO₂-Ta₂O₅ anodes, which promote one-electron and two-electron oxidations respectively. However, it is not yet clear whether mineralization is initiated by hydroxyl radicals formed in lower yield than at ‘active’ materials such as BDD, or whether oxidation involves less reactive intermediates such as HO₂ radicals.     

Biological and electrochemical oxidation of naphthalenesulfonates

A biofilm airlift suspension (BAS) reactor and an undivided flow cell equipped with a boron‐doped diamond (BDD) anode and a stainless‐steel cathode were used to investigate the effects of varying operating conditions on process performance in the biological and electrochemical oxidation of a mixture of naphthalenesulfonates contained in the infiltration water of a contaminated industrial site. The experiments were aimed at evaluating the feasibility of process integration and the criteria for optimization (i.e. how to maximize degradation efficiency with minimum energy consumption) in combined biological and electrochemical oxidation of scarcely biodegradable compounds. Because of high reactor biomass concentration and long biomass retention time, the BAS reactor achieved a high degradation capacity (up to 6.8 kg COD m^?3 d^?1). On the other hand, owing to the recalcitrant character of some of the aromatic sulfonates in the leachate, the overall degradation efficiency did not exceed 70% based on COD measurements. All naphthalene‐mono‐ and ‐disulfonates (except naphthalene‐1,5‐disulfonate) were completely degraded in the BAS reactor, whereas more complex molecules (e.g. naphthalenetrisulfonates) were more recalcitrant to biological oxidation. These compounds were completely mineralized by electrochemical oxidation using a boron‐doped diamond anode. The energy consumption and the time required for the complete mineralization of the infiltration water decreased from 80 kWh m^?3 and 4 h to 61 kWh m^?3 and 3 h for the oxidation of raw and biologically pretreated leachate, respectively. Copyright © 2005 Society of Chemical Industry     

Degradation of profenofos in an electrochemical flow reactor using boron-doped diamond anodes

A study of the electrochemical degradation of profenofos in a flow reactor with electrodes comprising boron-doped diamond films deposited on titanium substrate (BDD/Ti) as anodes has been performed. The BDD films were produced at growth times of 7 and 24 h with similar B/C ratios corresponding to acceptor concentrations of around 10²⁰ atoms cm^− 3. The morphological and structural characteristics of the BDD/Ti electrodes were evaluated by scanning electron microscopy and Raman scattering spectroscopy. Degradation experiments were carried out with applied current densities in the range 10 to 200 mA cm^− 2 and flow rates of 50 and 300 L h^− 1. The rates of degradation of profenofos were evaluated by high performance liquid chromatography and variations in total organic carbon (TOC) were monitored during the electrochemical process in order to determine the level of mineralization of organic compounds present in the electrolyte. Under the best conditions (anode comprising a BDD film deposited on titanium for 7 h and reactor operating at a flow rate of 300 L h^− 1) more than 95% of the profenofos was degraded and approximately 87% of TOC was removed within 120 min of reaction time.     

Electro-Fenton, UVA photoelectro-Fenton and solar photoelectro-Fenton degradation of the drug ibuprofen in acid aqueous medium using platinum and boron-doped diamond anodes

The degradation of a 41 mg dm⁻³ ibuprofen (2-(4-isobutylphenyl)propionic acid) solution of pH 3.0 has been comparatively studied by electrochemical advanced oxidation processes (EAOPs) like electro-Fenton, UVA photoelectro-Fenton and solar photoelectro-Fenton at constant current density. Experiments were performed in a one-compartment cell with a Pt or boron-doped diamond (BDD) anode and an O₂-diffusion cathode. Heterogeneous hydroxyl radical (OH) is generated at the anode surface from water oxidation, while homogeneous OH is formed from Fenton's reaction between Fe²⁺ and H₂O₂ generated at the cathode, being its production strongly enhanced from photo-Fenton reaction induced by sunlight. Higher mineralization is attained in all methods using BDD instead Pt, because the former produces greater quantity of OH enhancing the oxidation of pollutants. The mineralization rate increases under UVA and solar irradiation by the rapid photodecomposition of complexes of Fe(III) with acidic intermediates. The most potent method is solar photoelectro-Fenton with BDD giving 92% mineralization due to the formation of a small proportion of highly persistent final by-products. The effect of Fe²⁺ content, pH and current density on photoelectro-Fenton degradation has been studied. The ibuprofen decay always follows a pseudo-first-order kinetics and its destruction rate is limited by current density and UV intensity. Aromatics such as 1-(1-hydroxyethyl)-4-isobutylbenzene, 4-isobutylacetophenone, 4-isobutylphenol and 4-ethylbenzaldehyde, and carboxylic acids such as pyruvic, acetic, formic and oxalic have been identified as oxidation by-products. Oxalic acid is the ultimate by-product and the fast photodecarboxylation of its complexes with Fe(III) under UVA or solar irradiation explains the higher oxidation power of photoelectro-Fenton methods in comparison to electro-Fenton procedures.     

A dual-chamber microbial fuel cell with conductive film-modified anode and cathode and its application for the neutral electro-Fenton process

This study reports on the modification of the anode and the cathode in a dual-chamber microbial fuel cell (MFC) with a polypyrrole (PPy)/anthraquinone-2,6-disulfonate (AQDS) conductive film to boost its performance and the application of the MFC to drive neutral electron-Fenton reactions occurring in the cathode chamber. The MFC equipped with the conductive film-coated anode and cathode delivered the maximum power density of 823 mW cm⁻² that was one order of magnitude larger than that obtained in the MFC with the unmodified electrodes. This was resulted from the enhanced activities of microbial metabolism in the anode and oxygen reduction in the cathode owing to the decoration of both electrodes with the PPy/AQDS composite. The MFC with the modified electrodes resulted in the largest rate of H₂O₂ generation in the cathode chamber by the two-electron reduction of O₂. The increase in the concentration of H₂O₂ was beneficial for the enhancement in the amount of hydroxyl radicals produced by the reaction of H₂O₂ with Fe²⁺, thus allowing an increased oxidative ability of the electro-Fenton process towards the decolorization and mineralization of an azo dye (i.e., Orange II) at pH 7.0.     

Anodic,cathodic and combined treatments for the electrochemical oxidation of an effluent from the flame retardant industry

The electrochemical oxidation of an effluent from the manufacture of phosphorus based flame retardants was studied. To reach a residual concentration of reduced phosphorus lower than 10 mg L⁻¹, in compliance with Italian law for industrial wastewater disposal, anodic oxidation using a boron-doped diamond (BDD) anode and electro-Fenton (EF) treatment were tested. The effects of some factors are optimised and a comparison of the reaction pathways is also presented. A combined treatment using EF with BDD conducted in an undivided cell is shown not to enhance the data obtained with BDD while a novel combined treatment using EF and BDD in a divided cell shows promising results when an anionic membrane is used as separation. In this last case the cell operates as two different batch reactors working with the same current. The anodic compartment, fed with raw effluent, provides partial oxidation, while the cathodic compartment, fed with the partially anodically oxidised solution, completes the treatment. When the effluent is transferred in the cathodic compartment, the anodic one is fed with fresh untreated solution. The advantage of this kind of coupling consists in the simultaneity of the two treatments which allows total oxidation with notable saving of charge and time.     

Catalytic behavior of the Fe/Fe system in the electro-Fenton degradation of the antimicrobial chlorophene

The catalytic behavior of the Fe³⁺/Fe²⁺ system in the electro-Fenton degradation of the antimicrobial drug chlorophene has been studied considering four undivided electrolytic cells, where a Pt or boron-doped diamond (BDD) anode and a carbon felt or O₂-diffusion cathode have been used. Chlorophene electrolyses have been carried out at pH 3.0 under current control, with 0.05 M Na₂SO₄ as supporting electrolyte and Fe³⁺ as catalyst. In these processes the drug is oxidized with hydroxyl radical (OH) formed both at the anode from water oxidation and in the medium from electrochemically generated Fenton's reagent (Fe²⁺ + H₂O₂, both of them generated at the cathode). The catalytic behavior of the Fe³⁺/Fe²⁺ system mainly depends on the cathode tested. In the cells with an O₂-diffusion cathode, H₂O₂ is largely accumulated and the Fe³⁺ content remains practically unchanged. Under these conditions, the chlorophene decay is enhanced by increasing the initial Fe³⁺ concentration, because this leads to a higher quantity of Fe²⁺ regenerated at the cathode and, subsequently, to a greater OH production from Fenton's reaction. In contrast, when the carbon felt cathode is used, H₂O₂ is electrogenerated in small extent, whereas Fe²⁺ is largely accumulated because the regeneration of this ion from Fe³⁺ reduction at the cathode is much faster than its oxidation to Fe³⁺ at the anode. In this case, an Fe³⁺ concentration as low as 0.2 mM is required to obtain the maximum OH generation rate, yielding the quickest chlorophene removal. Chlorophene is poorly mineralized in the Pt/O₂ diffusion cell because the final Fe³⁺–oxalate complexes are difficult to oxidize with OH. These complexes are completely destroyed using a BDD anode at high current thanks to the great amount of OH generated on its surface. Total mineralization is also achieved in the Pt/carbon felt and BDD/carbon felt cells with 0.2 mM Fe³⁺, because oxalic acid and its Fe²⁺ complexes are directly oxidized with OH in the medium. Comparing the four cells, the highest oxidizing power regarding total mineralization is attained for the BDD/carbon felt cell at high current due to the simultaneous destruction of oxalic acid at the BDD surface and in the bulk solution.     

Anodic oxidation of isothiazolin-3-ones in aqueous medium by using boron-doped diamond electrode

Electrochemical degradation of biocide compound, isothiazolin-3-ones, was studied in aqueous medium, with Na₂SO₄ supporting electrolyte using boron-doped diamond (BDD) anode. The redox response of isothiazolin-3-ones at BDD was examined by cyclic voltammetric study. The degradation of isothiazolin-3-ones and its mineralization trend were monitored by UV–vis spectrophotometric method and total organic carbon (TOC) analyzer, respectively. The effect of operating parameters such as applied current density, biocide concentration, electrolyte pH and nature of supporting electrolytes (Na₂SO₄, NaNO₃ and NaCl) on degradation rate was studied in detail. It was established that the hydroxyl radicals (OH) generated at BDD surface were responsible for the degradation and the mineralization of the biocide contaminant. The rate of degradation was almost independent of electrolyte pH but became faster as the applied current density increased and the biocide concentration decreased. The kinetic studies revealed that the biocide decay follows a pseudo-first-order rate. The apparent rate constant for the oxidation of isothiazolin-3-ones was determined to be 2.65 × 10^− 4 s^− 1 at an applied current density of 25 mA cm^− 2 in the presence of 0.1 mol dm^− 3 Na₂SO₄ at pH 6.0. A poor mineralization efficiency was observed in the case of NaCl as supporting electrolyte which cause in-situ generation of chlorine based mediated oxidants such as Cl₂ and OCl⁻ which have negligible influence in mineralizing the isothiazolin-3-ones compared to peroxodisulfate (S₂O₈^2 −) oxidants that formed in the case of Na₂SO₄. The oxidizing ability of the BDD anode was compared with those of Pt and glassy carbon anodes under similar experimental conditions.     

Response surface methodology as a tool to optimize the electrochemical incineration of bromophenol blue on boron-doped diamond anode

This study investigated the electrochemical incineration of bromophenol blue (BPB) at boron-doped diamond (BDD) anode. The individual and interaction effects of three control parameters (applied current density, flow rate and supporting electrolyte concentration) on process efficiency were estimated by central composite rotatable design. Among the independent variables, supporting electrolyte concentration displayed the most interesting roles on BPB degradation. The optimal conditions obtained by response surface methodology were: applied current density of 7.36 mA cm^− 2, 2.6 mM Na₂SO₄ and flow rate of 568 ml min^− 1, which gave a decolorization rate of 91.7% and a mineralization rate of 47.3%, as well as an energy consumption of 3518.89 kWh kg^− 1 TOC (7.0 kWh m^− 3) and a mineralization current efficiency of 15.1% at 120 min of electrolysis. The results presented here demonstrated the high efficiency of BDD technology in mineralizing BPB under mild conditions, as well as the usefulness and capability of the experimental design strategy for successful investigation and modeling of the electrocatalytic processes.     

Photoelectrocatalysis reactivity of independent titania nanotubes

Two types of independent titania nanotube arrays with the separated tube wall structure have been fabricated by a controlled anodization process and used for photoelectrocatalysis (PEC) applications. The photocatalysis degradation efficiency of the organic pollutant is improved from 6.0 to 9.2% through increasing tube length and inter-tube space. The PEC degradation efficiency is 20.4% at an applied potential of 2.885 V for titania long nanotube array. An electro-Fenton-assisted PEC reaction system has been developed using titania long nanotube array and an iron sheet as two anodes in a parallel connection and a multiporous carbon as one cathode. The current distribution among three functional electrodes is conducted to optimize titania PEC reaction and electro-Fenton reaction. Accordingly, the degradation efficiency is improved from 20.4% in PEC to 60.2% in electro-Fenton-assisted PEC, and the mineralization efficiency is also improved from 8.1 to 37.4%. The corresponding reaction rate constant of 5.19 × 10⁻³ min⁻¹ is even higher than that of 3.98 × 10⁻³ min⁻¹ for the sum of individual oxidation reactions of titania PEC, electro-Fenton, and anodic electrolysis.     

Electrochemical treatment of water containing chlorides under non-ideal flow conditions with BDD anodes

In this work a undivided parallel plate cell equipped with boron doped diamond (BDD) anode was tested as electrochemical reactor for disinfection of water. Two configurations were adopted: a single pass configuration (SPC) and a recirculated configuration (RC) in which also a reservoir was inserted in the hydraulic circuit. In both the experimental configurations the system worked in continuous mode with a flow rate ranging from 0.05 to 0.42 dm³ min⁻¹; in the RC the recirculating flow rate ranged from 0.45 to 6 dm³ min⁻¹. Thermostated (25 °C) galvanostatic electrolyses were carried out with aqueous solutions containing 100 mg dm⁻³ of chloride ions: values of current density from 2.5 to 5.0 mA cm⁻² were used. Steady state data revealed that active chlorine and chlorate ions were the main oxidation products. Particular attention was paid to the hydrodynamics both for SPC and RC: pulse-response curves were experimentally obtained with an inert tracer, and the behaviour of the system was interpreted by models based on a combination of ideal flow reactors, bypass flow elements, and dead zones. The hydrodynamic models were utilized to predict the outlet concentration of the electrolysis products. A good agreement between model predicted and experimental data was obtained for a wide range of experimental conditions. Preliminary disinfection tests were then performed using Escherichia coli as model microorganism. Results were discussed in terms of both disinfection efficiency and by-products formation.