Peroxodisulfate generation on boron-doped diamond microelectrodes array and detection by scanning electrochemical microscopy

Innovative and efficient electroanalytical approach for determining persulfate in aqueous solutions using a gold electrode

Persulfate (PDS), peroxodisulfate, peroxydisulfate, peroxodisulfuric acid, is an oxidant that can be generated by direct oxidation of sulfate ions or indirectly via reaction with hydroxyl radicals in anodes with high oxygen overpotential. Quantitative methods for determining/quantifying PDS in the presence of other strong oxidants or other anions in eco-friendly applications do not give reliable results because of these interferents. Therefore, an additional method is needed to improve the efficacy to determine/quantify the PDS concentration in oxidative environments. In this frame, an alternative sensing approach was developed based on the electroreduction of PDS in the polycrystalline gold electrode using the square wave voltammetry (SWV) technique for its detection and quantification. Then, the procedure was evaluated in terms of its effectiveness for determining PDS in complex matrices, such as in the electrolysis of sulfate ion precursor solutions using anodes with high oxygen overpotential (e.g.: diamond electrode) capable of generating other strong oxidants. Based on the results obtained, it was confirmed that only the direct electron transfer step is attained when PDS is electrochemically synthetized at the surface of the polycrystalline gold electrode, contributing to its detection and quantification by SWV. It was also observed that at acidic conditions, the PDS electroreduction process is controlled by mass transfer while that the sensitivity for PDS detection is improved, achieving detection limits of about 14 and 19 μM for perchloric and sulfuric acids medium, respectively. When the electrolysis of sulfate-based solution at acidic conditions was performed to determine the electrochemical production of PDS by SWV approach with Au sensor, the concentration of PDS was effectively determined and no interferences were assessed by other strong oxidants generated during the electrolysis. Conversely, the spectrophotometric method showed that, the results of the PDS concentration were overestimated and other strong oxidants significantly interfere with its determination during the electrolysis of sulfuric acid solutions. Therefore, the electroanalytical method presented here is a suitable alternative for determining PDS during the applicability of the environmental-electrochemical technologies.

Diamond-Based Electrodes for Detection of Metal Ions and Anions

Diamond electrodes have long been a well-known candidate in electrochemical analyte detection. Nano- and micro-level modifications on the diamond electrodes can lead to diverse analytical applications. Doping of crystalline diamond allows the fabrication of suitable electrodes towards specific analyte monitoring. In particular, boron-doped diamond (BDD) electrodes have been reported for metal ions, anions, biomolecules, drugs, beverage hazards, pesticides, organic molecules, dyes, growth stimulant, etc., with exceptional performance in discriminations. Therefore, numerous reviews on the diamond electrode-based sensory utilities towards the specified analyte quantifications were published by many researchers. However, reviews on the nanodiamond-based electrodes for metal ions and anions are still not readily available nowadays. To advance the development of diamond electrodes towards the detection of diverse metal ions and anions, it is essential to provide clear and focused information on the diamond electrode synthesis, structure, and electrical properties. This review provides indispensable information on the diamond-based electrodes towards the determination of metal ions and anions.

Research on the mechanism and reaction conditions of electrochemical preparation of persulfate in a split-cell reactor using BDD anode

Through a cyclic voltammetry (CV) curve, electron spin resonance spectroscopy (ESR) characterization and a free radical competitive trapping experiment, an analysis was performed on the mechanism of persulfate (PDS) electro-synthesis by sulfate at a boron-doped diamond (BDD) anode. It had been found that there were two pathways of PDS formation. The first was to form PDS through the interaction of sulfate radicals, which were generated from the oxidation reaction mediated by hydroxyl radicals, where the protonized bisulfate ions and sulfuric acid were oxidized by hydroxyl radicals to sulfate radicals. The second was to produce PDS by generating sulfate radicals through the direct loss of electrons from sulfate and bisulfate ions on the electrode surface. In addition, the effects of initial pH, temperature, current density and electrolyte concentration on the synthesis of PDS were investigated in the slotted anode cycle electrolysis mode. As indicated by the results, despite the small effect of the initial pH on PDS synthesis, acidic pH was slightly beneficial to the synthesis of PDS; in electrolysis, the temperature should be controlled below the thermal decomposition temperature of PDS; and in practical application, the increase of impressed current or voltage contributed little to the increase of PDS synthesis concentration and current efficiency. In the case of the impressed current exceeding the limiting current, the adoption of concentrated electrolyte solution shall improve the PDS output and current efficiency.

Through cyclic voltammetry (CV) curve, electron spin resonance spectroscopy (ESR) characterization and free radical competitive trapping experiment, an analysis was performed on the mechanism of persulfate (PDS) electro-synthesis by sulfate at boron-doped diamond (BDD) anode.

Conductive diamond: synthesis, properties, and electrochemical applications

This review summarizes systematically the growth, properties, and electrochemical applications of conductive diamond.

Oxidation of hydroxide ions in weak basic solutions using boron-doped diamond electrodes: effect of the buffer capacity

The electrochemical oxidation of hydroxide ions using boron-doped diamond (BDD) electrodes in weak basic solutions was examined.

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.

Boron-doped diamond electrode: Preparation, characterization and application for electrocatalytic degradation of m-dinitrobenzene

Boron-doped diamond (BDD) electrode was successfully prepared via microwave plasma chemical vapor deposition method and it was used in electrocatalytic degradation of m-dinitrobenzene (m-DNB). The electrocatalytic degradation efficiency of m-DNB was evaluated under different experimental parameters including current density, temperature, pH, Na₂SO₄ concentration and initial m-DNB concentration. Under optimal parameters, degradation efficiency of m-DNB reached up to 82.7% after 150min. The degradation process of m-DNB was fitted well with pseudo first-order kinetics. Moreover, UV and HPLC analyses implied that m-DNB was totally destroyed and mineralized after 240min degradation, and the proposed mechanism during the electrocatalytic degradation process was analyzed. All these results demonstrated that BDD electrode possessed excellent electrocatalytic property and showed a great potential application in wastewater treatment.

Oscillatory Changes of the Heterogeneous Reactive Layer Detected with the Motional Resistance during the Galvanostatic Deposition of Copper in Sulfuric Solution

Metallic copper was galvanostatically deposited on quartz|gold resonant electrodes by applying a constant current in a 0.5 M CuSO4/0.1 M H2SO4 aqueous solution. Galvanostatic copper deposition is one of the best methodologies to calibrate the electrochemical quartz crystal microbalances (EQCM), a gravimetric sensor to evaluate changes in mass during the electrochemical reactions through the Sauerbrey equation. The simultaneous measurement of mass, current density, and motional resistance by an EQCM with motional resistance monitoring allows us to characterize the processes occurring on the electrode surface and at the interfacial regions with unprecedented detail. During the galvanostatic copper deposition, Cu(H2O)4(OH)2 is accumulated close to the copper surface, generating a passive layer. This passive layer can act as Cu(2+) reservoir for the Cu(2+) → Cu process since the copper deposition is not affected. The analysis of motional resistance evolution in different experimental conditions reveals that the passive layer is formed by the reaction of oxidizing agents generated at the counter electrode with the metallic copper surface. The simplistic Cu(2+) → Cu process is completed with a more detailed mechanism, which includes the passive layer formation/dissolution and the transport of species from the counter electrode surface (Pt) to the working electrode surface. The results further support the calibration procedure of EQCM by the galvanostatic deposition of copper in sulfuric solutions. However, we suggest applying high current densities, separating the counter electrode and quartz|gold resonant electrode about 0.5 cm, and keeping oxygen in solution for the EQCM calibration. Moreover, the better interval time to calculate the Sauerbrey's constant from charge and resonant frequency data is between 150 and 300 s.

Decolorization and mineralization of Allura Red AC aqueous solutions by electrochemical advanced oxidation processes

The decolorization and mineralization of solutions containing 230 mg L(-1) of the food azo dye Allura Red AC at pH 3.0 have been studied upon treatment by electrochemical oxidation with electrogenerated H2O2 (EO-H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF). Experiments were performed with a stirred tank reactor containing a boron-doped diamond (BDD) or Pt anode and an air-diffusion cathode to generate H2O2. The main oxidants were hydroxyl radicals formed at the anode surface from water oxidation and in the bulk from Fenton's reaction between H2O2 and added Fe(2+). The oxidation ability increased in the sequence EO-H2O2 < EF < PEF and faster degradation was always obtained using BDD. PEF process with BDD yielded almost total mineralization following similar trends in SO4(2-), ClO4(-) and NO3(-) media, whereas in Cl(-) medium, mineralization was inhibited by the formation of recalcitrant chloroderivatives. GC-MS analysis confirmed the cleavage of the −N=N− bond with formation of two main aromatics in SO4(2-) medium and three chloroaromatics in Cl(-) solutions. The effective oxidation of final oxalic and oxamic acids by BDD along with the photolysis of Fe(III)-oxalate species by UVA light accounted for the superiority of PEF with BDD. NH4(+), NO3(-) and SO4(2-) ions were released during the mineralization.

Critical review of electrochemical advanced oxidation processes for water treatment applications

Electrochemical advanced oxidation processes (EAOPs) have emerged as novel water treatment technologies for the elimination of a broad-range of organic contaminants. Considerable validation of this technology has been performed at both the bench-scale and pilot-scale, which has been facilitated by the development of stable electrode materials that efficiently generate high yields of hydroxyl radicals (OH˙) (e.g., boron-doped diamond (BDD), doped-SnO2, PbO2, and substoichiometic- and doped-TiO2). Although a promising new technology, the mechanisms involved in the oxidation of organic compounds during EAOPs and the corresponding environmental impacts of their use have not been fully addressed. In order to unify the state of knowledge, identify research gaps, and stimulate new research in these areas, this review critically analyses published research pertaining to EAOPs. Specific topics covered in this review include (1) EAOP electrode types, (2) oxidation pathways of select classes of contaminants, (3) rate limitations in applied settings, and (4) long-term sustainability. Key challenges facing EAOP technologies are related to toxic byproduct formation (e.g., ClO4(-) and halogenated organic compounds) and low electro-active surface areas. These challenges must be addressed in future research in order for EAOPs to realize their full potential for water treatment.

Electrochemical oxidation of electrodialysed reverse osmosis concentrate on Ti/Pt-IrO2, Ti/SnO2-Sb and boron-doped diamond electrodes

Reverse osmosis concentrate from wastewater reclamation contains biorefractory trace organic contaminants that may pose environmental or health hazard. Due to its high conductivity, electrochemical oxidation of brine requires low voltage which is energetically favourable. However, the presence of chloride ions may lead to the formation of chlorinated by-products, which are likely to exert an increased toxicity and persistence to further oxidation than their non-chlorinated analogues. Here, the performance of Ti/Pt-IrO(2), Ti/SnO(2)-Sb and Si/BDD anodes was evaluated for the electrochemical oxidation of ROC in the presence of chloride, nitrate or sulfate ions (0.05 M sodium salts). In order to investigate the electrooxidation of ROC with nitrate and sulfate ions as dominant ion mediators, chloride ion concentration was decreased 10 times by electrodialytic pretreatment. The highest Coulombic efficiency for chemical oxygen demand (COD) removal was observed in the presence of high chloride ions concentration for all anodes tested (8.3-15.9%). Electrooxidation of the electrodialysed concentrate at Ti/SnO(2)-Sb and Ti/Pt-IrO(2) electrodes exhibited low dissolved organic carbon (DOC) (i.e. 23 and 12%, respectively) and COD removal (i.e. 37-43 and 6-22%, respectively), indicating that for these electrodes chlorine-mediated oxidation was the main oxidation mechanism, particularly in the latter case. In contrast, DOC removal for the electrodialysed concentrate stream was enhanced at Si/BDD anode in the presence of SO(4)(2-) (i.e. 51%) compared to NO(3)(2-) electrolyte (i.e. 41%), likely due to the contribution of SO(4)(·-) and S(2)O(8)(2-) species to the oxidative degradation. Furthermore, decreased concentration of chloride ions lead to a lower formation of haloacetic acids and trihalomethanes at all three electrodes tested.

Multichannel boron doped nanocrystalline diamond ultramicroelectrode arrays: design, fabrication and characterization

We report on the fabrication and characterization of an 8 × 8 multichannel Boron Doped Diamond (BDD) ultramicro-electrode array (UMEA). The device combines both the assets of microelectrodes, resulting from conditions in mass transport from the bulk solution toward the electrode, and of BDD's remarkable intrinsic electrochemical properties. The UMEAs were fabricated using an original approach relying on the selective growth of diamond over pre-processed 4 inches silicon substrates. The prepared UMEAs were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results demonstrated that the electrodes have exhibited a very fast electrode transfer rate (k(0)) up to 0.05 cm·s(-1) (in a fast redox couple) and on average, a steady state limiting current (in a 0.5 M potassium chloride aqueous solution containing 1 mM Fe(CN)(6)(4-) ion at 100 mV·s(-1)) of 1.8 nA. The UMEAs are targeted for electrophysiological as well as analytical applications.