Single and Coupled Electrochemical Processes and Reactors for the Abatement of Organic Water Pollutants: A Critical Review

Improving the catalytic effect of peroxodisulfate and peroxodiphosphate electrochemically generated at diamond electrode by activation with light irradiation

Boron doped diamond (BDD) anode has been used to oxidatively remove Rhodamine B (RhB), as persistent organic pollutant, from synthetic wastewater by electrolysis, photoelectrolysis and chemical oxidation containing sulfate and phosphate as supporting electrolytes. RhB is effectively oxidized by electrolysis and by chemical oxidation with the oxidants separately produced by electrolyzing sulfate or phosphate solutions (peroxodisulfate and peroxodiphosphate, respectively). The results showed that light irradiation improved the electrolysis of RhB due to the activation of oxidants under irradiation at high current densities. Meanwhile, the efficiency of the chemical oxidation approach by ex situ electrochemical production of oxidants was not efficient to degrade RhB.

The effect of the supporting electrolyte on the electrooxidation of enrofloxacin using a flow cell with a BDD anode: Kinetics and follow-up of oxidation intermediates and antimicrobial activity

The role of the supporting electrolyte - SE (Na₂SO₄; NaCl; Na₂CO₃; NaNO₃; Na₃PO₄ - 0.1 M ionic strength) in the galvanostatic (10 mA cm^-2) electrochemical degradation of the fluoroquinolone antibiotic enrofloxacin (ENRO; 100 mg L^-1) using a filter-press flow cell with a boron-doped diamond anode was investigated (flow rate, solution volume, and temperature were kept fixed at 420 L h^-1, 1.0 L, and 25 °C, respectively). The electrochemical degradation performance with the different SEs was assessed by following up [ENRO], total organic carbon concentration (TOC), oxidation intermediates (detected by LC and LC-QqTOF), and antimicrobial activity towards Escherichia coli as the electrolyses progressed. With NaCl as SE, complete removal of ENRO was attained ∼10 times faster than with the other salts. The determination of terminal oxidation intermediates (short-chain carboxylic acids) produced during the electrolyses allowed concluding that their nature and number is indeed affected by the salt used as SE, most probably due to distinct electrogenerated oxidants. With NaCl, the antimicrobial activity of the electrolyzed solution decreased gradually (to ∼20%) from 8 to 16 h of electrolysis due to the cleavage of the fluoroquinolone structure. On the other hand, with Na₂SO₄, Na₂CO₃ and NaNO₃ as SEs the growth of Escherichia coli cells was observed only after ∼14 h, whereas it was completely inhibited with Na₃PO₄. Clearly, the electrooxidation and mineralization of ENRO is strongly affected by the SEs used, which determine the degradation mechanism and, consequently, the removal rates of the solution's organic load and antimicrobial activity.

Improved blackwater disinfection using potentiodynamic methods with oxidized boron-doped diamond electrodes

Electrochemical disinfection (ECD) has become an important blackwater disinfection technology. ECD is a promising solution for the 2 billion people without access to conventional sanitation practices and in areas deficient in basic utilities (e.g., sewers, electricity, waste treatment). Here, we report on the disinfection of blackwater using potential cycling compared to potentiostatic treatment methods in chloride-containing and chloride-free solutions of blackwater (i.e., untreated wastewater containing feces, urine, and flushwater from a toilet). Potentiodynamic treatment is demonstrated to improve disinfection energy efficiency of blackwater by 24% and 124% compared to static oxidation and reduction methods, respectively. The result is shown to be caused by electrochemical advanced oxidation processes (EAOP) and regeneration of sp²-surface-bonded carbon functional groups that serve the dual purpose of catalysts and adsorption sites of oxidant intermediates. Following 24 h electrolysis in blackwater, electrode fouling is shown to be minimized by the potential cycling method when compared to equivalent potentiostatic methods. The potential cycling current density is 40% higher than both the static oxidative and reductive methods. This work enhances the understanding of oxygen reduction catalysts using functionalized carbon materials and electrochemical disinfection anodes, both of which have the potential to bring a cost-effective, energy efficient, and practical solution to the problem of disinfecting blackwater.

Cost-effective electrogeneration of H2O2 utilizing HNO3 modified graphite/polytetrafluoroethylene cathode with exterior hydrophobic film

Electrochemical 2-electrons oxygen reduction process has been regarded as the effective strategy for H₂O₂ generation in wastewater treatment. However, its large-scale application is still limited by the relatively high cost of the carbon materials and short-term stability. In this study, a nitric acid modified graphite/polytetrafluoroethylene (PTFE) composite cathode with exterior hydrophobic film was fabricated for cost-effective electrogeneration of hydrogen peroxide (H₂O₂). Experimental results show that 2 M HNO₃ modification rendered the introduction of much more defect sites and oxygen/nitrogen-containing groups on graphite. As a result, H₂O₂ electrogeneration was 3.0 times as much as that of virgin graphite counterpart at 3 mA cm^-2. Moreover, the additional introduction of exterior hydrophobic film on the as-prepared graphite/PTFE cathode did not only further promote H₂O₂ electrogeneration, but also endowed the cathode with strong hydrophobic stability. As for the modified cathode with exterior hydrophobic film, the influence of mass graphite/PTFE binder ratio (1:1-4:1) and pH (3.0-9.0) on H₂O₂ electrogeneration was slight, but the current density (3.0-15 mA cm^-2) had evident effect on H₂O₂ electrogeneration. Generally, owing to its low price and being easily available, the modified graphite would be cost-effectively utilized to prepare the gas diffusion cathode for the large-scale electrogeneration of H₂O₂ in industry.

The electrochemical regeneration of granular activated carbons: A review

The electrochemical treatment of exhausted granular activated carbon (GAC) has been identified as an effective alternative to traditional adsorbent regeneration methods (e.g. thermal, chemical, and microbial). However, despite its proven potential and initial investigation over two decades ago, the development of this technology has been progressing slowly, hindering its deployment in industrial applications. Thus, a review has been conducted that aims to present the fundamentals of GAC electrochemical regenerative methods, what research has been conducted to develop the technology to the present day, and lastly, identify limitations and future prospects associated with electrochemical methods. The regenerative mechanism is firstly discussed, followed by a presentation of the varying reactor configurations and operating parameters utilized during the electrochemical treatment of GAC materials exhausted with a broad range of wastewater contaminants. Finally, emerging electrochemical technologies used for the commercial treatment of exhausted adsorbent materials and contaminated soils are discussed.

Treatment of an azo dye effluent by peroxi-coagulation and its comparison to traditional electrochemical advanced processes

Peroxi-coagulation (PC) is an interesting new process that has not been widely studied in the literature. This work presents the application of this technology to treat an azo dye synthetic effluent, studying the effect of different parameters including initial pH, current density (j), initial dye concentration and supporting electrolyte. The two former variables significantly affected the colour removal of the wastewater, followed by the initial dye concentration and the kind of electrolyte, in a lesser extent. The optimum operating conditions achieved were initial pH of 3.0, j = 33.3 mA cm^-2, 100 mg L^-1 of methyl orange (MO) and Na₂SO₄ as supporting electrolyte. The performance of PC was also compared to other electrochemical advanced processes, under similar experimental conditions. Results indicate that the kinetic decay of the MO increases in the following order: electrocoagulation (EC) < electrochemical oxidation (EO) with electrogenerated H₂O₂ << PC < electro-Fenton (EF). This behaviour is given to the high oxidant character of the homogenous OH radicals generated by EF and PC approaches. The EO process with production of H₂O₂ (EO-H₂O₂) is limited by mass transport and the EC, as a separation method, takes longer times to achieve similar removal results. Energy requirements about 0.06 kWh g_COD^-1, 0.09 kWh g_COD^-1, 0.7 kWh g_COD^-1 and 0.1 kWh g_COD^-1 were achieved for PC, EF, EO-H₂O₂ and EC, respectively. Degradation intermediates were monitored and carboxylic acids were detected for PC and EF processes, being rapidly removed by the former technology. PC emerges as a promising and competitive alternative for wastewaters depollution, among other oxidative approaches.

Comparative study for degradation of industrial dyes by electrochemical advanced oxidation processes with BDD anode in a laboratory stirred tank reactor

Comparative degradation of the industrial dyes Blue BR, Violet SBL and Brown MF 50 mg L^-1 has been studied by the electrochemical oxidation (EOx), electro-Fenton (EF), photoelectro-Fenton (PEF) process based on BDD electrode. Each dye was tested in 0.05 mM Na₂SO₄ with 0.5 mM Fe²⁺ at pH 3.0, and electrolyzed in a stirred tank reactor under galvanostatic conditions with 2.0, 5.0, 7.0, 11.0 and 18.0 mA cm^-2. Dyes were oxidized via hydroxyl radicals (OH) formed at the BDD anode from water oxidation coupled with Fenton's reaction cathodically produced hydrogen peroxide (H₂O₂). Under Na₂SO₄ medium close to 100% the decolorization was achieved. Through the color abatement rate the dyes behavior was analyzed at the beginning of the oxidation process. Dissolved Organic Carbon (DOC) was tested to evaluate the degradation. From DOC removal, it was established an increasing relative oxidation power of the EOx < EF < PEF, according with their decolorization trend. This study highlights the potential of the electrochemical/BDD process for the degradation of industrial dyes found in wastewaters under appropriate experimental conditions.

Electrodegradation of naphthalenic amines: Influence of the relative position of the substituent groups, anode material and electrolyte on the degradation products and kinetics

The electrodegradation of the 4-aminonaphthalene-1-sulfonic acid (4AN1S), 5-aminonaphthalene-2-sulfonic acid (5AN2S) and 8-aminonaphthalene-2-sulfonic acid (8AN2S) was studied, using two electrode materials as anode, BDD and Ti/Pt/PbO₂, and two different electrolytes, sodium sulfate and sodium chloride. The highest COD removal rates were obtained at BDD: for 5AN2S and 8AN2S results were similar in both electrolytes; for 4AN1S, results were better in sodium chloride. The lowest COD removal rates were obtained at the system Ti/Pt/PbO₂-sodium sulfate, for all the studied amines. The dissolved organic carbon (DOC) removal was much higher at BDD for all the amines, in sulfate for 5AN2S and 8AN2S and in chloride for 4AN1S. Nitrogen removal was always almost irrelevant in sulfate medium but higher than 60%, after 6-h assays, in chloride. The highest combustion efficiencies were attained at the system BDD-sodium sulfate and were: 4AN1S-75%; 5AN2S-84%; 8AN2S-74%. HPLC results show that total degradation of the studied aminonaphthalene sulfonates is attained at both anode materials, utilizing any of the electrolytes, with a first order kinetics. However, kinetic constants obtained with the variation of the amines concentration in time are 10-40 times higher in chloride, being slightly higher at Ti/Pt/PbO₂ than at BDD. Regarding the presence of carboxylic acids during the degradation assays, it was observed that the electrolysis of the amines 5AN2S and 8AN2S always lead to higher amounts of oxalic acid and lower quantities of acetic acid than the electrolysis of the amine 4AN1S.

Composite wastewater treatment by aerated electrocoagulation and modified peroxi-coagulation processes

Treatment of composite wastewater generating from the industrial estates is a great challenge. The present study examines the applicability of aerated electrocoagulation and modified peroxi-coagulation processes for removing color and COD from composite wastewater. Iron plates were used as anodes and cathodes in both electrochemical processes and experiments were carried out in a working volume of 2 L. Aeration enhanced the efficiency of electrocoagulation process significantly. More than 50% of COD and 60% of color were removed after 1 h of electrocoagulation process operated at pH 3 and applied voltage of 1 V. Efficiency of the modified peroxi-coagulation process was significantly higher than that of aerated electrocoagulation. COD and color removal efficiencies of the modified peroxi-coagulation process were found as 77.7% and 97%, respectively after 1 h of electrolysis operated at 1 V, solution pH 3 and 50 mM hydrogen peroxide addition. This improved efficiency of modified peroxi-coagulation compared to aerated electrocoagulation is mainly due to the attack of in-situ generated hydroxyl radicals.

Efficient electrochemical degradation of multiwall carbon nanotubes

As the production mass of multiwall carbon nanotubes (MWCNT) increases, the potential for human and environmental exposure to MWCNTs may also increase. We have shown that exposing an aqueous suspension of pristine MWCNTs to an intense oxidative treatment in an electrochemical reactor, equipped with an efficient hydroxyl radical generating Boron Doped Diamond (BDD) anode, leads to their almost complete mineralization. Thermal optical transmittance analysis showed a total carbon mass loss of over two orders of magnitude due to the electrochemical treatment, a result consistent with measurements of the degraded MWCNT suspensions using UV-vis absorbance. Liquid chromatography data excludes substantial accumulation of the low molecular weight reaction products. Therefore, up to 99% of the initially suspended MWCNT mass is completely mineralized into gaseous products such as CO₂ and volatile organic carbon. Scanning electron microscopy (SEM) images show sporadic opaque carbon clusters suggesting the remaining nanotubes are transformed into structure-less carbon during their electrochemical mineralization. Environmental toxicity of pristine and degraded MWCNTs was assessed using Caenorhabditis elegans nematodes and revealed a major reduction in the MWCNT toxicity after treatment in the electrochemical flow-by reactor.

Regeneration of Activated Carbon Fiber by the Electro-Fenton Process

An electro-Fenton (EF) based technology using activated carbon (AC) fiber as cathode and BDD as anode has been investigated for both regeneration of AC and mineralization of organic pollutants. The large specific surface area and low intraparticle diffusion resistance of AC tissue resulted in high maximum adsorption capacity of phenol (PH) (3.7 mmol g^-1) and fast adsorption kinetics. Spent AC tissue was subsequently used as the cathode during the EF process. After 6 h of treatment at 300 mA, 70% of PH was removed from the AC surface. The effectiveness of the process is ascribed to (i) direct oxidation of adsorbed PH by generated hydroxyl radicals, (ii) continuous shift of adsorption equilibrium due to oxidation of organic compounds in the bulk, and (iii) local pH change leading to electrostatic repulsive interactions. Moreover, 91% of PH removed from AC was completely mineralized, thus avoiding adsorption of degradation byproducts and accumulation of toxic compounds such as benzoquinone. Morphological and chemical characteristics of AC were not affected due to the effect of cathodic polarization protection. AC tissue was successfully reused during 10 cycles of adsorption/regeneration with regeneration efficiency ranging from 65 to 78%, in accordance with the amount of PH removed from the AC surface.

Photocatalytically-assisted electrooxidation of herbicide fenuron using a new bifunctional electrode PbO2/SnO2-Sb2O3/Ti//Ti/TiO2

The degradation of the herbicide fenuron was investigated using a new porous bifunctional electrode where the electrooxidation takes place on one side and the photocatalysis on the other side. The characterization of the synthetized bifunctional electrode (PbO₂/SnO₂-Sb₂O₃/Ti//Ti/TiO₂) was performed by scanning electron microscopy, energy dispersive X-ray spectrometry and X-ray diffraction analysis and showed that the anodic side (Ti/SnO₂-Sb₂O₃/PbO₂) is covered with a tetragonal β-PbO₂ film and that the photocatalytic side (Ti/TiO₂) consists of an anatase phase of TiO₂. The single application of electrooxidation achieved 87.8% fenuron degradation and 84.1% chemical oxygen demand (COD) removal while heterogeneous photocatalysis resulted in only 59.2% and 39.7% fenuron concentration decay and COD removal, respectively. On the other hand, the photocatalytically-assisted electrooxidation (photo-electrooxidation) performed on the bifunctional electrode provided higher performances of fenuron degradation (97.5%) and mineralization (97.4%). Investigation of operating parameters highlighted the positive effect of increase in current density. Conversely, an increase in fenuron concentration led to a decrease in degradation rate and COD removal. It was also found that the COD removal and mineralization efficiency are higher in a neutral medium.

Bioelectro-Fenton: evaluation of a combined biological-advanced oxidation treatment for pharmaceutical wastewater

Electro-Fenton (EF), an advanced oxidation process, can be combined with a biological process for efficient treatment of wastewater containing refractory pollutants such as pharmaceuticals. In this study, a biological process was implemented in a sequencing batch reactor (SBR), which was either preceded or followed by EF treatment. The main goal was to evaluate the potential of two sequences of a combined electrochemical-biological process: EF/SBR and SBR/EF for the treatment of real wastewater spiked with 0.1 mM of caffeine and 5-fluorouracil. The biological removal of COD and pharmaceuticals was improved by extending the acclimation time and increasing concentration of biomass in the SBR. Hardly biodegradable caffeine and COD were completely removed during the EF post-treatment (SBR/EF). During the EF/SBR sequence, complete removal of pharmaceuticals was achieved by EF within 30 min at applied current 800 mA. With a current of 500 and 800 mA, the initially very low BOD₅/COD ratio increased up to 0.38 and 0.58, respectively, after 30 min. The efficiency of the biological post-treatment was influenced by the biodegradability enhancement after EF pre-treatment. The choice of an adequate sequence of such a combined process is significantly related to the wastewater characteristics as well as the treatment objectives.

Removal of lindane wastes by advanced electrochemical oxidation

The effective removal of recalcitrant organochlorine pesticides including hexachlorocyclohexane (HCH) present in a real groundwater coming from a landfill of an old lindane (γ-HCH) factory was performed by electrochemical oxidation using a BDD anode and a carbon felt cathode. Groundwater (ΣHCHs = 0.42 mg L^-1, TOC₀ = 9 mg L^-1, pH₀ = 7, conductivity = 3.7 mS cm^-1) was treated as received, achieving the complete depletion of the HCH isomers and a mineralization degree of 90% at 4 h electrolysis at constant current of 400 mA. Initial groundwater contains high chloride concentration (Cl₀^- = 630 mg L^-1) that is progressively decreased due to its oxidation to different oxychlorine species: Cl₂, HClO, ClO^-, ClO₂^- ClO₃^- and ClO₄^- some of them (Cl₂, HClO, ClO^-) playing an important role in the oxidation of organic pollutants. The oxidation rate of chloride (and its oxidized intermediates) depends on the applied current value. Although some of the species generated from them are active oxidants, the presence of inorganic salts is detrimental to the efficiency of the electrochemical process when working at current densities above 100 mA due to the high consumption of hydroxyl radicals in wasting reactions. The initial organic carbon content is not crucial for the extension of the process but high organic loads are more profitable for cost effectiveness. The addition of a supporting electrolyte to the solution could be interesting since it increases the conductivity, reducing the cell potential and therefore, decreasing the energy consumption.

Electro-Fenton oxidation of para-aminosalicylic acid: degradation kinetics and mineralization pathway using Pt/carbon-felt and BDD/carbon-felt cells

Degradation of a widely used antibiotic, the para-aminosalicylic acid (PAS), and mineralization of its aqueous solution was investigated by electro-Fenton process using Pt/carbon-felt and boron-doped diamond (BDD)/carbon-felt cells with applied currents in the range of 50-1000 mA. This process produces the highly oxidizing species, the hydroxyl radical (^•OH), which is mainly responsible for the oxidative degradation of PAS. An absolute rate constant of 4.17 × 10⁹ M^-1 s^-1 for the oxidation of PAS by ^●OH was determined from the competition kinetics method. Degradation rate of PAS increased with current reaching an optimal value of 500 mA with complete disappearance of 0.1 mM PAS at 7 min using Pt/carbon-felt cell. The optimum degradation rate was reached at 300 mA for BDD/carbon-felt. The latter cell was found more efficient in total organic carbon (TOC) removal where a complete mineralization was achieved within 240 min. A multi-step mineralization process was observed with the formation of a number of aromatic intermediates, short-chain carboxylic acids, and inorganic ions. Eight aromatic intermediate products were identified using both LC-Q-ToF-MS and GC-MS techniques. These products were the result of hydroxylation of PAS followed by multiple additions of hydroxyl radicals to form polyhydroxylated derivatives. HPLC and GC/MS analyses demonstrated that extended oxidation of these intermediate products conducted to the formation of various short-chain carboxylic acids. Prolonged electrolysis resulted in a complete mineralization of PAS with the evolution of inorganic ions such as NO₃^- and NH₄⁺. Based on the identified intermediates, carboxylic acids and inorganic ions, a plausible mineralization pathway is also deduced. The remarkably high degree of mineralization (100%) achieved by the present EF process highlights the potential application of this technique to the complete removal of salicylic acid-based pharmaceuticals from contaminated water.

Electrochemical treatment of aqueous solutions of organic pollutants by electro-Fenton with natural heterogeneous catalysts under pressure using Ti/IrO2-Ta2O5 or BDD anodes

The treatment of toxic organic pollutants by electro-Fenton (EF) presents some drawbacks such as the necessity to work at low pH and the low solubility of oxygen in water contacted with air or oxygen at room pressure that results often in slow and relatively low abatements. Here, the coupled adoption of natural heterogeneous catalysts and of relatively high pressure was proposed in order to improve the performances of EF for the treatment of organic pollutants. Caffeic acid (CA) and 3-chlorophenol were used as model resistant organic pollutants. EF process was performed using both conventional homogeneous FeSO₄ and natural heterogeneous catalysts (pyrite, chalcopyrite, Fe₂O₃ and Fe₃O₄) as iron catalysts and oxygen at various pressures in the absence or in the presence of BDD anode. The effect of the nature of the catalyst, the oxygen pressure, the current density and the catalyst load was widely investigated in order to optimize the process. It was shown that the coupled utilization of a natural heterogeneous catalyst such as chalcopyrite and a relatively high pressure allows to obtain the total removal of CA and a high removal of the TOC (about 75%) in short times (2 h) with relatively high current efficiencies using an Iridium based anode. In the case of 3-chlorophenol, the utilization of a BDD anode was necessary to achieve a high removal of the pollutant and the TOC. It was shown that the removal of 3-chlorophenol can be effectively performed in different water bodies and with different initial concentrations of 3-chlorophenol.

Pilot-scale evaluation of micropollutant abatements by conventional ozonation, UV/O3, and an electro-peroxone process

The electro-peroxone (E-peroxone) process is an emerging ozone-based advanced oxidation process (AOP) that has shown large potential for micropollutant abatement in water treatment. To evaluate its performance under more realistic conditions of water treatment, a continuous-flow pilot E-peroxone system was developed and compared with conventional ozonation and a UV/O₃ process for micropollutant abatements in various water matrices (groundwater, surface water, and secondary wastewater effluent) in this study. With a specific ozone dose of 1.5 mg O₃/mg DOC, micropollutants that have high and moderate reactivity with ozone (O₃) (diclofenac, naproxen, gemfibrozil, and bezafibrate) could be sufficiently abated (>90% abatement) in the various waters by all three processes. However, ozone-resistant micropollutants (ibuprofen, clofibric acid, and chloramphenicol) were abated only by ∼32-68%, 68-91%, and 73-90% during conventional ozonation of the selected groundwater, surface water, and secondary wastewater effluent, respectively. By electro-generating H₂O₂ or applying UV irradiation to enhance O₃ transformation to •OH during ozonation, the E-peroxone and UV/O₃ processes similarly enhanced the abatement efficiencies of ozone-resistant micropollutants by ∼15-43%, ∼5-15%, and ∼5-10% in the groundwater, surface water, and secondary wastewater effluent, respectively. In addition, the E-peroxone and UV/O₃ processes significantly reduced bromate formation during the treatment of the three waters compared to conventional ozonation. Due to its higher efficiency, the E-peroxone process reduced ∼10-53% of the energy consumption required to abate the concentration of chloramphenicol (the most ozone-resistant micropollutant spiked in the waters) by 1 order of magnitude in the three waters compared to conventional ozonation. In contrast, the UV/O₃ process consumed approximately 4-10 times higher energy than conventional ozonation. This pilot-scale study demonstrates that the E-peroxone process can provide a feasible, effective, and energy-efficient alternative for micropollutant abatement and bromate control in water and wastewater treatment.

Fate of inorganic nitrogen species under homogeneous Fenton combined with electro-oxidation/reduction treatments in synthetic solutions and reclaimed municipal wastewater

The fate of inorganic nitrogen species has been studied for the first time in electro-Fenton (EF) conditions in acid media. A redox cycle is first obtained and validated with a kinetic model in synthetic solution and highlights the removal of nitrite that is quickly oxidized into nitrate while the reduction conditions are sufficient to reduce nitrate into ammonium cation. However, NH₄⁺ and gaseous nitrogen accumulate in such solution. The study in reclaimed municipal wastewater emphasize the removal of NH₄⁺ with formation of chloramines in the presence of initial chloride ions, a species widely present in wastewater effluent. Contrastingly, NO₃^- remain constant all along the electrolysis even after 2.1 Ah L^-1. The oxidation conditions were not sufficient to produce perchlorate while chlorate accumulated in solution. Therefore, it limits the use of EF for direct use for drinking water purpose but could be considered as complementary treatment for wastewater reuse applications.

Current advances and trends in electro-Fenton process using heterogeneous catalysts - A review

Over the last decades, advanced oxidation processes have often been used alone, or combined with other techniques, for remediation of ground and surface water pollutants. The application of heterogeneous catalysis to electrochemical advanced oxidation processes is especially useful due to its efficiency and environmental safety. Among those processes, electro-Fenton stands out as the one in which heterogeneous catalysis has been broadly applied. Thus, this review has introduced an up-to-date collation of the current knowledge of the heterogeneous electro-Fenton process, highlighting recent advances in the use of different catalysts such as iron minerals (pyrite, magnetite or goethite), prepared catalysts by the load of metals in inorganic and organic materials, nanoparticles, and the inclusion of catalysts on the cathode. The effects of physical-chemical parameters as well as the mechanisms involved are critically assessed. Finally, although the utilization of this process to remediation of wastewater overwhelmingly outnumber other utilities, several applications have been described in the context of regeneration of adsorbent or the remediation of soils as clear examples of the feasibility of the electro-Fenton process to solve different environmental problems.

The synergistic effect of nickel-iron-foam and tripolyphosphate for enhancing the electro-Fenton process at circum-neutral pH

A composite nickel-iron-foam (Ni-Fe-F) electrode was used as a cathode in the electro-Fenton (EF) process at circum-neutral pH in the presence of sodium tripolyphosphate (TPP) as supporting electrolyte. It was found that phenol degradation was dramatically improved by the synergistic effect of Ni-Fe-F and TPP, reaching 100% removal in 40 min, with k_app = (8.90 ± 0.12) × 10^-2 min^-1, which was about 18 times higher than that of Ni-Fe-F with sulfate as conventional electrolyte at pH 3.00 (k_app = (5.00 ± 0.14) × 10^-3 min^-1). A (75.00 ± 1.67)% mineralization yield was attained after 4-h treatment time. Ni-Fe-F proved capable of providing the Fe²⁺ ions necessary to catalyze the Fenton's reaction via a controlled chemical/electrochemical redox process. In addition, Ni-Fe-F promoted the chemical and electrochemical generation of H₂O₂. With respect to TPP, its chelation with Fe ions prevented iron precipitation at neutral and higher pH values, extending the pH range of the Fenton's reaction. Furthermore, the TPP ligand promoted the activation of molecular O₂ for the chemical production of OH, enhancing the process efficiency. By overcoming these common limitations of conventional EF in K₂SO₄ electrolyte, the Ni-Fe-F/TPP system represents a more sustainable alternative for practical application of EF. A degradation pathway for phenol mineralization with homogeneous and heterogeneous OH produced by the EF Ni-Fe-F/TPP system is proposed based on the identification of the oxidation by-products.

Performance of (in)active anodic materials for the electrooxidation of phenolic wastewaters from cashew-nut processing industry

This study investigated the anodic oxidation of phenolic wastewater generated by cashew-nut processing industry (CNPI) using active (Ti/RuO2-TiO2) and inactive (boron doped diamond, BDD) anodes. During electrochemical treatment, various operating parameters were investigated, such as current density, chemical oxygen demand (COD), total phenols, O2 production, temperature, pH, as well as current efficiency and energy consumption. After electrolysis under optimized working conditions, samples were evaluated by chromatography and toxicological tests against L. sativa. When both electrode materials were compared under the same operating conditions, higher COD removal efficiency was achieved for BDD anode; achieving lower energy requirements when compared with the values estimated for Ti/RuO2-TiO2. The presence of Cl- in the wastewater promoted the electrogeneration of strong oxidant species as chlorine, hypochlorite and mainly hypochlorous acid, increasing the efficiency of degradation process. Regarding the temperature effect, BDD showed slower performances than those achieved for Ti/RuO2-TiO2. Chromatographic and phytotoxicity studies indicated formation of some by-products after electrolytic process, regardless of the anode evaluated, and phytotoxic action of the effluent. Results encourage the applicability of the electrochemical method as wastewater treatment process for the CNPI, reducing depuration time.

Lindane degradation by electrooxidation process: Effect of electrode materials on oxidation and mineralization kinetics

This study focuses on the effect of electrode materials on abatement of lindane (an organochlorine pesticide) by electrooxidation process. Comparative performances of different anodic (platinum (Pt), dimensionally stable anode (DSA) and boron-doped diamond (BDD)) and cathodic (carbon sponge (CS), carbon felt (CF) and stainless steel (SS)) materials on lindane electrooxidation and mineralization were investigated. Special attention was paid to determine the role of chlorine active species during the electrooxidation process. The results showed that better performances were obtained when using a BDD anode and CF cathode cell. The influence of the current density was assessed to optimize the oxidation of lindane and the mineralization of its aqueous solution. A quick (10 min) and complete oxidation of 10 mg L^-1 lindane solution and relatively high mineralization degree (80% TOC removal) at 4 h electrolysis were achieved at 8.33 mA cm^-2 current density. Lindane was quickly oxidized by in-situ generated hydroxyl radicals, (M(^•OH)), formed from oxidation of water on the anode (M) surface following pseudo first-order reaction kinetics. Formation of chlorinated and hydroxylated intermediates and carboxylic acids during the treatment were identified and a plausible mineralization pathway of lindane by hydroxyl radicals was proposed.

Remarkable improved electro-Fenton efficiency by electric-field-induced catalysis of CeO2

In this study, we designed a novel combined electro-Fenton system for the treatment of wastewater containing biological recalcitrant using electric-field-induced ceria (CeO₂) as the synergistic catalysts. It was found that by applying this CeO₂ electro-Fenton system, the current efficiency improved from 74.49% to 109.82% within 2.5 min; the removal efficiency for dimethyl phthalate (DMP) increased from 85.5% to 94.9% within 20 min; and the mineralization rate increased from 76.01% to 93.58% after 120 min. The effects of parameters such as the applied potential, electrolyte, and concentration of Fe²⁺ on the current efficiency were systematically studied. Investigations by LSV, zeta titration, X-ray photoelectron spectroscopy (XPS), X-Ray Diffraction (XRD)and electron spin resonance (ESR)revealed the reasons for achieving a current efficiency of over 100% in the CeO₂ electro-Fenton system. A mechanism that involved Brønsted acid sites and the redox cycle of sulfate CeO₂ was proposed.

Effective removal of the antibiotic Nafcillin from water by combining the Photoelectro-Fenton process and Anaerobic Biological Digestion

The elimination of the antibiotic Nafcillin (NAF), which is usually used in hospitals and veterinary clinics around the world, was assessed through a combination of three advanced electrochemical oxidation processes followed by anaerobic digestion process. In the first stage different electrochemical advanced oxidation processes (EAOPs) were used: electro-oxidation with hydrogen peroxide (EO-H₂O₂), electro-Fenton (EF) and Photo electro-Fenton (PEF). After PEF, almost complete and highly efficient degradation and elimination of NAF was achieved, with the concomitant elimination of the associated antimicrobial activity. The fast degradation rate produced by PEF is explained by the oxidative action of hydroxyl radicals (•OH) together with the direct UV photolysis of complexes formed between Fe³⁺ and some organic intermediates. Total removal of NAF occurs after 90min of electrolysis by PEF, with the generation of organic intermediates that remain in solution. However, when this post PEF process solution was treated with an anaerobic biological process, the intermediates generated in the electrochemical degradation of NAF were completely eliminated after 24h. The kinetic degradation of NAF as well as the identification/quantification of products and intermediates formed during the degradation of antibiotic, such as inorganic ions, carboxylic acids and aromatic compounds, were determined by chromatographic and photometric methods. Finally, an oxidation pathway is proposed for the complete conversion to CO₂.

Electrochemical degradation of industrial textile dye disperse yellow 3: Role of electrocatalytic material and experimental conditions on the catalytic production of oxidants and oxidation pathway

This study aimed to verify the efficiency of the electrochemical oxidation process for removal the industrial textile Disperse Yellow 3 (DY3) dye in aqueous solutions using different electrocatalytic materials: boron-doped diamond (BDD), Ti/Ru_0.3Ti_0.7O₂ and Ti/Pt anodes. The results were obtained by applying different current densities (40 and 60 mA cm^-2) at 40 °C using different supporting electrolytes (Na₂SO₄ 50 mM and NaCl 50 mM) under values of pH about 2.3, 7.0 and 10.0. Results obtained shown that the process was faster at the beginning of the process for all electrocatalytic materials, using Na₂SO₄ as electrolyte, being more efficient for BDD anode reaching more than 90% of TOC and color decay independently of the current density and pH and supporting electrolyte; while up to 50% of color and TOC was eliminated, using the other anodic materials in sulfate. In NaCl medium a complete mineralization was achieved at Ti/Ru_0.3Ti_0.7O₂ at short electrolysis time, followed by BDD and Ti/Pt. The corresponding kinetic analysis confirms these results. Trends of active chlorine species synthesized at Ti/Ru_0.3Ti_0.7O₂, BDD and Ti/Pt anodes, at different pH conditions, demonstrated that, the concentration of active chlorine species depends on the pH conditions and electrode material. Finally, a cost comparison for each electrocatalytic material under different experimental conditions was realized exhibiting the lowest energy consumption and electrolysis time in NaCl medium. Based on the results obtained, the electrochemical elimination of dye and the profile of the carboxylic by-products formed depend on the nature of material, pH and supporting electrolyte.

Degradation of dye Procion Red MX-5B by electrolytic and electro-irradiated technologies using diamond electrodes

This work focuses on the treatment of synthetic wastewater polluted with dye Procion Red MX-5B by different Electrochemical Advanced Oxidation Processes (EAOP) based on diamond anodes. The influence of the current density and the supporting electrolyte has been studied on dye removal and total mineralization of the organic matter. Results show that electrolysis with diamond electrodes is a suitable technology for an efficient degradation of dye. Nonetheless, the process efficiency increases when using chloride as supporting electrolyte because of the electrochemical generation of hypochlorite in wastewater which significantly contribute to dye removal. On the contrary, the total mineralization of the organic matter is more efficient in sulfate media. In this case, large amounts of peroxodisulfate are electrogenerated, favoring the complete removal of total organic carbon (TOC). On the other hand, lower current densities (10 mA cm^-2) lead to a more efficient removal of both dye and TOC due to the mass transfer limitations of the technology. Finally, the coupling of UV light irradiation or ultrasound to electrolysis significantly improves the process performance, being photoelectrolysis the most efficient technology for the treatment of wastewater polluted with Procion Red MX-5B. This fact is due to the potential production of free chlorine or sulfate radicals that takes place by the activation of the electrogenerated oxidants. These species are more reactive than oxidants and, therefore, they quickly attack the organic matter present in wastewater.

Abatement of the antibiotic levofloxacin in a solar photoelectro-Fenton flow plant: Modeling the dissolved organic carbon concentration-time relationship

The degradation of solutions of the antibiotic levofloxacin (LVN) in sulfate medium at pH 3.0 has been investigated at pre-pilot scale by solar photoelectro-Fenton (SPEF) process. The flow plant included an FM01-LC filter-press cell equipped with a Ti|Pt anode and a three-dimensional-like air-diffusion cathode, connected to a compound parabolic collector as photoreactor and a continuous stirred tank under recirculation batch mode. The effect of volumetric flow rate on H₂O₂ electrogeneration from O₂ reduction was assessed. Then, the influence of initial LVN concentration and Fe²⁺ concentration as catalyst on dissolved organic carbon (DOC) removal was thoroughly investigated. LVN was gradually mineralized by SPEF process, with faster DOC abatement at 0.50 mM Fe²⁺, yielding 100% after 360 min at applied cathodic potential of -0.30 V|SHE. The high mineralization current efficiency (MCE) and low specific energy consumption (EC_DOC) revealed the extraordinary role of homogeneous hydroxyl radicals and natural UV light, which allowed the degradation of the antibiotic and its by-products with MCE values greater than 100%. Five cyclic by-products, N,N-diethylformamide and three short-chain linear carboxylic acids were detected by GC-MS and HPLC analyses. A parametric model to simulate the DOC decay versus electrolysis time was implemented for the SPEF pre-pilot flow plant, showing good agreement with experimental data.

Influence of chelation on the Fenton-based electrochemical degradation of herbicide tebuthiuron

This study describes the performance of electro-Fenton (EF) and photoelectro-Fenton (PEF) processes to degrade the herbicide tebuthiuron (TBH) in 0.050 M Na₂SO₄ at pH = 3.0. Experiments were performed in an undivided cell equipped with a boron-doped diamond (BDD) or Pt anode and an air-diffusion cathode that produces H₂O₂. Physisorbed hydroxyl radicals (M(OH)) generated from water oxidation at the anode and/or free OH formed from Fenton's reaction acted as main oxidants. All processes became much more effective using a BDD anode because of the higher oxidation power of BDD(OH). Sulfate and nitrate were the predominant ions released during TBH destruction. In both, EF and PEF treatments, two distinct kinetic regimes were observed, the first one corresponding to the oxidation of free TBH by OH and the second one to that of the Fe(III)-TBH complex by M(OH). The effect of Fe²⁺ and TBH concentrations on the kinetics of both regions has been examined. Moreover, a poor mineralization was reached with Pt anode, whereas almost total mineralization was attained by EF and PEF with BDD. Both processes showed analogous mineralization rates because the intermediates produced could not be photodegraded by UVA light. Gas chromatography-mass spectrometry analysis of electrolyzed solutions revealed the generation of eight heteroaromatics along with 1,3-dimethylurea, which have been included in a reaction pathway proposed for the initial degradation of TBH.