Electrogenerated Hydroxyl Radicals Research Articles

Cobalt single-atom catalyst as a multifunctional electrocatalyst for boosting radical generation

Electro-generated hydroxyl radicals (•OH) are of fundamental importance in electrochemical advanced oxidation processes (EAOPs). Herein, a cobalt single-atom (Co1) dispersed on N-doped carbon black (NCB) was synthesized and used forthefirsttime as a multifunctional electrocatalyst on both the anode and cathode to boost the generation of •OH. Specifically, the Co1–NCB anode electrochemically generates •OH through water oxidation reaction (WOR), whereas the Co1–NCB cathode generates hydrogen peroxide (H2O2) through oxygen reduction reaction (ORR), followed by its subsequent electrochemical/catalytic activation by Co1–NCB, which synergistically promotes the formation of •OH. In an electrochemical system with the optimized coordination structure and operating conditions, the Co1–NCB electrode-based EAOP exhibited high phenol-removal efficiency (>95%) over a wide pH range (pH = 3–9). This study provides a new strategy for maximizing radical production and suggests its application in the electrochemical oxidation of recalcitrant organic pollutants.

Role of anodically electrogenerated hydroxyl radicals in minimizing mineral cathodic electroprecipitation in the presence of hard water

Electrochemical systems are attracting increasing interest in environmental protection as relatively sustainable processes, particularly in wastewater treatment and reuse. However, cathode scaling in electrochemical processes for wastewater treatment is a major issue that is often overlooked. It is proposed for the first time to investigate the anodic contribution towards CaCO3 electroprecipitation phenomena under the advanced electrooxidation conditions applied to remove organic biorecalcitrant pollutants. The contribution of the reaction of the hydroxyl radical (•OH) with carbonates, which reduces cathodic scaling at a micrometric interelectrode distance (500 µm) and at a high current density (16 mA cm−2), is described in detail. In addition, the anti-scaling effect of local anodic acidification should be considered. A new kinetic model of electroprecipitation fits the experimental curves well (root mean square error (RMSE) < 0.19 for Ca2+) and confirms this anodic role, combined with the gas hindrance and scale detachment induced by gas bubble electrogeneration at sufficiently high current intensities.

Electrochemical removal of tetrabromobisphenol A by fluorine-doped titanium suboxide electrochemically reactive membrane

This study reports fluorine-doped titanium suboxide anode for electrochemical mineralization of hydrophobic micro-contaminant, tetrabromobisphenol A. Fluorinated TiSO anode promoted electro-generated hydroxyl radicals (•OH) with higher selectivity and activity, due to increased O2 evolution potential and more loosely interaction with hydrophobic electrode surface. For electro-oxidation process, fluorine doping had an insignificant impact on outer-sphere reaction and exerted inhibition on inner-sphere reaction, as indicated by cyclic voltammogram performed on Ru(NH3)63+/2+, Fe(CN)63−/4− and Fe3+/2+ redox couple. This facilitated electrochemical conversion of TBBPA and intermediates via more efficient outer-sphere reaction and hydroxylation route. Additionally, generated O2 micro-bubbles could be stabilized on hydrophobic F-doped TiSO anode, which extended the three-phase boundary available for interfacial enrichment of TBBPA and subsequent mineralization. Under action of these comprehensive factors, 0.5% F-doped TiSO electrochemically reactive membrane could achieve 99.7% mineralization of TBBPA upon energy consumption of 0.52 kWh m−3 at current density of 7.8 ± 0.24 mA cm−2 (3.75 V vs SHE) and flow rate of 1628 LHM based on flow-through electrolysis. The modified anode exhibited superior performances compared with un-modified one with more efficient TBBPA removal, less toxic intermediate accumulation and lower energy consumption. The results may have important implications for electrochemical removal and detoxification of hydrophobic micro-pollutants.

Electron Spin Resonance Evidence for Electro-generated Hydroxyl Radicals

Electro-generated hydroxyl radicals (•OH) are of fundamental importance to the electrochemical advanced oxidation process (EAOP). Radical-specific electron spin resonance (ESR) evidence is still lacking in association with the direct electron transfer (DET) reaction of spin trap (e.g., 5,5-dimethyl-1-pyrroline-N-oxide; DMPO) and side reactions of the DMPO-OH adduct in the strongly oxidative environment offered by anodic polarization. Herein, we showed ESR identification of electro-generated •OH in EAOP based on the principle of kinetic selection. Excessive addition of a DMPO agent and fast spin trapping allowed suitable kinetic conditions to be set for effective spin trapping of electro-generated •OH and subsequent ESR identification. Otherwise, interferential triplet signals would emerge due to formation of paramagnetic dimer via dehydrogenation, DET oxidation, and dimerization reactions of the DMPO-OH adduct. The results demonstrate that •OH formation during spin-trapping on the titanium suboxide (TiSO) anode could be quantified as 47.84 ± 0.44 μM at current density of 10 mA cm-2. This value revealed a positive dependence on electrolysis time, current density, and anode potential. The effectiveness of ESR measurements was verified by the results obtained with the terephthalic acid probe. The ESR identification not only provides direct evidence for electro-generated •OH from a fundamental point of view, but also suggests a strategy to screen effective anode materials.

Trends in Synthetic Diamond for Electrochemical Applications

Trends in Synthetic Diamond for Electrochemical Applications

Effective mineralization of anti-epilepsy drug carbamazepine in aqueous solution by simultaneously electro-generated H2O2/O3 process

A novel electro-generated hydroxyl radical (OH) technique was constructed to effectively degrade the anti-epilepsy carbamazepine (CBZ) representing typical recalcitrant pharmaceutical only through electrochemical method as an alterative peroxone process. The technique involves simultaneously electro-generated H2O2 using a carbon-polytetrafluoroethylene (C/PTFE) cathode to reduce the injected O2 and electro-generated O3 using H2O oxidation with a Ti mesh/PbO2 anode in membrane electrode assembly (MEA). The significant OH formation through the synergetic system of in-situ produced H2O2 and O3 electrochemically could achieved carbamazepine decomposition and total organic carbon (TOC) removal as compared to electro-generated H2O2 or O3 alone. The main influential factors including current density and aqueous pH were investigated to reveal their effect on CBZ removal efficiencies. Under optimal parameters, CBZ decomposition and TOC mineralization could be completely accomplished after 10 min and 90 min electrolysis, respectively. According to intermediates distribution (e.g. hydroxylated CBZ, phenolics, and carboxylic acids) identified by HPLC-MS/MS, the degradation pathways were proposed for CBZ removal by the synergetic system of C/PTFE-O2 and MEA. The above results indicated that this electrochemically synergetic system is a potential technique to further purify the water contaminated by recalcitrant pharmaceuticals.

A novel approach towards multivariate optimization of graphite/PbO2 anode synthesis conditions: Insight into its enhanced oxidation ability and physicochemical characteristics

Electrochemical oxidation has drawn great interest for its potential application in degrading persistent organic pollutants (POPs) through electrogenerated hydroxyl radical at the surface of anode. This study aims at the preparation of an inexpensive graphite/PbO2 anode. For enhancing oxidation performance, preparation process parameters viz. Pb(NO3)2 concentration, potential, and time of electrodeposition of a graphite/PbO2 anode, electrodeposited from acidic electrolyte bath, were optimized targeting a POP, 2,4-dinitrophenol removal. The changes in morphological properties of the developed oxide films were analyzed using scanning electron microscopy (SEM) which manifested significant impacts of selected anode preparation process parameters. Furthermore, PbO2 film prepared at optimum conditions were characterized using SEM, atomic force microscopy (AFM), energy dispersive X-ray spectroscope (ESD) elemental mapping, and X-ray diffraction (XRD) for thorough investigation of crystal structures, elemental distribution over surface, and phase of PbO2. Angular structures in both SEM and AFM analysis and appearance of β-PbO2 characteristic peaks in XRD analysis confirmed formation of electrocatalytically active phase of PbO2. For further enhancing the oxidation ability, influencing experimental parameters viz. current intensity, pH, and NaCl concentration were optimized. After 2 h of electrolysis at optimum experimental conditions, COD and total organic carbon removal efficiency of 93.6 ± 0.63% and 71.7 ± 1.52% were obtained respectively.

Study of the degradation of an organophosphorus pesticide using electrogenerated hydroxyl radicals or heat-activated persulfate

The use of pesticides and their release into the natural environment constitutes a direct threat for the environment and the living beings especially the human health. Consequently, the development of technics to detoxify the pesticide residues to reduce at least areas and contaminated matrix is needed. In the present work, a comparative study was performed on the chemical oxidation of an organophosphorus compound, the dimethoate (DIM), with sulfate radicals and hydroxyl radicals. Both oxidants are generated in situ, the sulfate radicals were produced by heat-activation of persulfate (PS) and hydroxyl radicals were generated by water electrooxidation using a boron doped diamond anode (BDD). For both cases, the target molecule has disappeared but the selective reaction of sulfate radicals with organics led to the production of intermediates which are less biodegradable than DIM since the ratio between the biological oxygen demand (BOD5) and the chemical oxygen demand (COD) has been divided by 2 within the first hour of the process. Whereas the BOD5/COD ratio during the electrochemical oxidation of DIM via hydroxyl radicals showed that it was possible to render the solution biodegradable without reaching a complete mineralization. However, it has been shown that the presence of chlorides in the solution must be avoided because of the formation of undesired organochlorides during the process.

ELECTROCHEMICAL DEGRADATION OF CLINDAMYCIN BY ANODIC OXIDATION ON SnO2-Sb COATED TITANIUM ANODES

Degradation of Clindamycin phosphate (CMP) was studied in aqueous solutions by an anodic oxidation process under galvanostatic conditions. The electrolysis cell consisted of a Ti/SnO2-Sb anode, prepared by dip-coating technique, and a 316 stainless steel cathode, both of which had a surface area of 6 cm2. The effects of critical factors, including CMP concentration, current density, initial pH, and the supporting electrolyte were evaluated. The electrochemical oxidation of CMP was controlled by mass transport within the studied range. The kinetic analysis indicated that the degradation reactions followed pseudo-first-order equation. The rate of CMP decay, as well as that of COD removal, decreased with increasing initial concentrations. Better Instantaneous Current Efficiency (ICE) values were obtained at higher organic concentrations, due mainly to the increased mass transfer flux towards non-active anode surface. Increasing current density and initial pH led to the faster removal of CMP, primarily because of enhanced electrogenerated hydroxyl radicals (�OH) on the anode surface. However, COD reduction was found to be pH independent. The removal rates of CMP and COD were also discussed with respect to the supporting electrolyte in different types and concentrations. The GC-MS analysis indicated that tert-butyl compounds were the major organic intermediates for the electrochemical degradation of Clindamycin. © 2018, Gheorghe Asachi Technical University of Iasi, Romania. All rights reserved.

Functional group influences on the reactive azo dye decolorization performance by electrochemical oxidation and electro-Fenton technologies.

Electrochemical water treatment technologies are highly promising to achieve complete decolorization of dyebath effluents, as demonstrated by several studies reported in the literature. However, these works are focused on the treatment of one model pollutant and generalize the performances of the processes which are not transposable since they depend on the pollutant treated. Thus, in the present study, we evaluate, for the first time, the influence of different functional groups that modify the dye structure on the electrochemical process decolorization performance. The textile azo dyes Reactive Orange 16, Reactive Violet 4, Reactive Red 228, and Reactive Black 5 have been selected because they present the same molecular basis structure with different functional groups. The results demonstrate that the functional groups that reduce the nucleophilicity of the pollutant hinder the electrophilic attack of electrogenerated hydroxyl radical. Thereby, the overall decolorization efficiency is consequently reduced as well as the decolorization rate. Moreover, the presence of an additional chromophore azo bond in the molecule enhances the recalcitrant character of the azo dyes as pollutants. The formation of a larger and more stable conjugated π system increases the activation energy required for the electrophyilic attack of •OH, affecting the performance of electrochemical technologies on effluent decolorization.

Use of Sub-stoichiometric Titanium Oxide as a Ceramic Electrode in Anodic Oxidation and Electro-Fenton Degradation of the Beta-blocker Propranolol: Degradation Kinetics and Mineralization Pathway

Oxidative degradation of aqueous acidic solutions of the beta-blocker propranolol (PPN) has been studied by anodic oxidation (AO) and for the first time by electro-Fenton (EF) process using sub-stoichiometric titanium oxide (Ti4O7) anode elaborated by plasma deposition. The oxidative degradation of the PPN by Ti4O7(OH) formed at the surface of the Ti4O7 anode and OH generated via electrochemically assisted Fenton’s reaction was investigated. Decay of PPN concentration followed pseudo-first order reaction kinetics with degradation rates influenced by both applied current and initial PPN concentration. The absolute rate constant of the reaction between PPN and OH/Ti4O7(OH) was determined by competition kinetics and found to be (2.99±0.02) × 109Lmol−1s−1. Relatively high mineralization efficiency of PPN solution (82% TOC removal) was achieved by AO with Ti4O7 anode at 120mA after 480min of treatment, whereas almost complete mineralization (96% TOC removal) was reached at similar conditions in EF process. Analogous EF treatment with DSA anode showed lower mineralization (89% TOC removal) compared to Ti4O7 anode. The initial N content of PPN was mainly released as NH4+, with smaller proportion of NO3−. Aromatic intermediates such as 1-naphtol; hydroxylated 1-naphtol and phthalic acid were identified by both reversed-phase HPLC and GC-MS analyses. Oxalic, oxamic, maleic and glycoxylic acids were the main short-chain carboxylic acid detected. Based on the identified intermediates, carboxylic acids and inorganic end-products as well as TOC removal results, a plausible reaction sequence for mineralization of PPN by electrogenerated hydroxyl radicals is proposed.

Effect of electrogenerated hydroxyl radicals, active chlorine and organic matter on the electrochemical inactivation of Pseudomonas aeruginosa using BDD and dimensionally stable anodes

Abstract In this work, the disinfection of 100 mL of 106 CFU mL−1 Pseudomonas aeruginosa suspensions at pH 5.8 by electrochemical oxidation at 33.3 mA cm−2 is reported. The undivided electrolytic cell was equipped with either a boron-doped diamond (BDD) or an IrO2-based or RuO2-based dimensionally stable anode and a stainless steel cathode. Physisorbed hydroxyl radicals M( OH) formed from anodic water oxidation and active chlorine generated from anodic Cl− oxidation were the main oxidizing species in pure Na2SO4 medium and in the presence of NaCl, respectively. A faster inactivation was always found using the dimensionally stable anodes. In 7 mM Na2SO4, this behavior was associated to the much larger adsorption of the bacteria onto the anode, which accelerated the M( OH)-mediated oxidation and inactivation of the cells. The inactivation rate was strongly enhanced in 7 mM Na2SO4 + 1 mM NaCl due to the larger oxidation power of active chlorine compared to that of M( OH). The effect of NaCl concentration and current density on the disinfection process was examined with BDD and the best performance was obtained in 7 mM Na2SO4 + 7 mM NaCl at 8.3 mA cm−2, with total inactivation in 2 min and energy consumption of 0.059 kW h m−3. The addition of paracetamol in 7 mM Na2SO4 medium inhibited the disinfection at short electrolysis time regardless of the anode, owing to the preferential action of M( OH) on this pollutant. For BDD, the inactivation rate rose over time at higher drug content due to the generation of greater amounts of toxic by-products. For the IrO2-based anode, the progressive formation of toxic and less adsorbable by-products enhanced the process over time, giving rise again to a quicker total disinfection compared to that with BDD.

Correlation between degradation pathway and toxicity of acetaminophen and its by-products by using the electro-Fenton process in aqueous media

The evolution of the degradation by-products of an acetaminophen (ACE) solution was monitored by HPLC-UV/MS and IC in parallel with its ecotoxicity (Vibrio fischeri 81.9%, Microtox® screening tests) during electro-Fenton (EF) oxidation performed on carbon felt. The aromatic compounds 2-hydroxy-4-(N-acetyl) aminophenol, 1,4-benzoquinone, benzaldehyde and benzoic acid were identified as toxic sub-products during the first stage of the electrochemical treatment, whereas aliphatic short-chain carboxylic acids (oxalic, maleic, oxamic, formic, acetic and fumaric acids) and inorganic ions (ammonium and nitrate) were well identified as non-toxic terminal sub-products. Electrogenerated hydroxyl radicals then converted the eco-toxic and bio-refractory property of initial ACE molecule (500 mL, 1 mM) and subsequent aromatic sub-products into non-toxic compounds after 2 h of EF treatment. The toxicity of every intermediate produced during the mineralization of ACE was quantified, and a relationship was established between the degradation pathway of ACE and the global toxicity evolution of the solution. After 8 h of treatment, a total organic carbon removal of 86.9% could be reached for 0.1 mM ACE at applied current of 500 mA with 0.2 mM of Fe2+ used as catalyst.

Boron-doped diamond electrooxidation of ethyl paraben: The effect of electrolyte on by-products distribution and mechanisms

Ethyl paraben (EP), a representative emerging pollutant of the parabens family, was subject to electrochemical oxidation over a boron-doped diamond (BDD) anode. Experiments were carried out in a single-compartment cell at 10–70 mA cm−2 current density, 200–600 μg L−1 EP concentration, initial solution pH 3–9 and 0.1 M electrolyte concentration. The degradation rate is favored at increased current densities and in the presence of NaCl as the supporting electrolyte, while the pH effect is inconsiderable. For instance, the first order rate constant for the degradation of 200 μg L−1 EP at 30 mA cm−2 was 0.25, 0.1 and 0.07 min−1 with NaCl, Na2SO4 and HClO4, respectively. Degradation in secondary treated wastewater was faster than in pure water presumably due to the action of chloride ions present in the effluent.Liquid chromatography time-of-flight mass spectrometry (LC-TOF-MS) was employed to determine major transformation by-products (TBPs). The route of EP degradation with Na2SO4 involves hydroxylation and demethylation reactions, signifying the role of electrogenerated hydroxyl radicals in the process. Twenty one TBPs were identified with NaCl as the electrolyte, including several chlorinated and non-chlorinated dimers and trimers; these findings suggest that indirect oxidation mediated by chlorine radicals and other chlorine active species also takes place. In this view, the role of the supporting electrolyte is crucial since it can influence both reaction kinetics and pathways.

Crosslinking of poly(vinylpyrrolidone) activated by electrogenerated hydroxyl radicals: A first step towards a simple and cheap synthetic route of nanogel vectors

A facile electrosynthesis route for the preparation of polymer nanogels based on the in situ production of hydroxyl radicals is reported for the first time. Electro-Fenton process with continuous H2O2 electrogeneration and Fe2+ regeneration performs better than electro-oxidation with a boron-doped diamond or dimensionally stable anode for promoting crosslinking of poly(vinylpyrrolidone).

Electrochemical degradation of organic dye by electrochemical oxidation on a Ti/Cu2O electrode

This work investigated the electrocatalytic degradation of an azo dye by electrochemical oxidation on copper oxide anode. The influence current density, time of electrolysis, temperature, the conductive salt concentration and the initial dye concentrations were critically examined. The results of these influences are expressed in terms of Chemical Oxygen Demand (COD removal). Also, both of the current efficiency and power consumption values are calculated. In this paper we report that, the highest electrocatalytic activity was achieved in the presence of NaCl (0.1 M), 50 mA/cm2, pH of 3 and temperature of 30 °C. The highest electroactivity could be attributed to the contribution from direct oxidation via reaction of this dye with the electrogenerated hydroxyl radicals adsorbed on the copper oxide surface.

Electrochemical advanced oxidation of 2-chlorobenzoic acid using BDD or Pt anode and carbon felt cathode

Electrochemical advanced oxidation processes, anodic oxidation and electro-Fenton, have been applied to oxidative degradation of 2-chlorobenzoic acid (2-CBA) in acidic aqueous medium by using Pt or boron-doped-diamond (BDD) anode. Electrolyses were conducted in an open and cylindrical cell with a carbon-felt cathode for H2O2 generation. The main oxidizing species is hydroxyl radical (OH) formed at the BDD surface in both processes and in the bulk from the Fenton’s reaction between initially added and then regenerated Fe2+ (catalyst) and electrogenerated H2O2. Decay of 2-CBA concentration followed a pseudo-first order reaction kinetics. The absolute rate constant of the hydroxylation of 2-CBA was determined by using the competition kinetics method and found to be 2.04×109M−1s−1. The comparative study of TOC removal measurements during electro-Fenton treatment showed a higher mineralization rate with BDD anode than Pt anode due to the higher oxidizing power of the former anode. The chlorine present in 2-CBA is completely released in the form of chloride ion at Pt anode, which is slowly oxidized to Cl2 at the BDD anode. Aromatic oxidation intermediates such as 2-chlorophenol, catechol and benzoquinone are identified by reversed-phase liquid chromatography. Oxalic acid is detected by ion-exclusion-chromatography as the main carboxylic acid. This ultimate end product is finally oxidized into CO2 by hydroxyl radicals. Based on the identified intermediates and inorganic end products, and TOC removal results, a plausible mineralization pathway of 2-CBA by electrogenerated hydroxyl radicals was proposed.

Electrochemical degradation of a chlorophenoxy propionic acid derivative used as an herbicide at boron-doped diamond

The electrochemical degradation of diclofop-methyl (DM), an herbicide deriving from aryloxy propionic acid, was carried out by galvanostatic electrolysis at boron-doped diamond electrode. The oxidation process leads in an early step to the cleavage of the aryloxy propionic ester bond and the formation of 4-(2,4-dichlorophenoxy) phenol (P1). The subsequent oxidation of P1 resulted in a quantitative mineralization of DM. Measuring the reduction in chemical oxygen demand and total organic carbon during the electrolysis shows that the mineralization efficiency increases with decreasing current densities. As a result, two mechanistic pathways were proposed for DM electrochemical degradation. The first one is a direct electro-oxidation of the starting molecule leading to the breakdown of aromatic ether bonds. A second evidenced competitive pathway uses electrogenerated hydroxyl radicals as mediators in the mineralization process of DM.

Reactivity of hydroxy-containing aromatic compounds towards electrogenerated hydroxyl radicals

A kinetic study on the oxidation of hydroxy-containing aromatic compounds by electrogenerated HO radical and simultaneous by direct electron transfer is presented. First order kinetics are used to describe consumption rates of hydroquinone, benzoic acid and hydroxybenzoic acid derivatives by galvanostatic electrolysis with simultaneous oxygen evolution at a Pt electrode. Linear correlations were established from the effect of electrolyses current density on kapp. The meaning of the intercept and slope is analyzed. A good agreement is found between intercept values and the apparent rate constants from potentiostatic electrolysis without O2 evolution. Simultaneously, the slopes magnitude corroborate the relative reactivity order of species that was established considering the occurrence of positive charge densities on carbon atoms of the aromatic ring. Therefore, the present analysis provides kinetic information concerning both, the direct electron-transfer and the reaction with HO radical.

Tris(2,2′-bipyridyl) ruthenium(II) electrochemiluminescence of glyoxal, glyoxylic acid, methylglyoxal, and acetaldehyde

Tris(2,2′-bipyridyl)ruthenium(II) electrochemiluminescence (ECL) of glyoxal, glyoxylic acid, methylglyoxal and acetaldehyde has been investigated for the first time, and was compared with ECL of the well-known coreactant oxalate. Glyoxal, glyoxylic acid and methylglyoxal are 4.4-times, 2.5-times and 2.3-times the ECL intensity of oxalate at glassy carbon electrode, respectively. Based on their high ECL efficiencies, glyoxal, methylglyoxal, glyoxylic acid and acetaldehyde have been sensitively detected by ECL methods without the need of derivatization. The ECL intensities increase linearly with the concentrations of glyoxal, glyoxylic acid, methylglyoxal and acetaldehyde over the ranges of 1μM–10mM, 1μM–5mM, 1μM–5mM and 0.3mM–5mM, respectively. Electrochemistry studies show that electrogenerated hydroxyl radicals play important roles in ECL, and coreactant family can be expanded by taking advantage of the strong oxidation ability of electrogenerated hydroxyl radicals.