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In this research, different electrochemical advanced oxidation processes (EAOPs), such as electro-oxidation (EO), electro-Fenton (EF) and photoelectro-Fenton (PEF) were aimed to enhance the degradation efficiency of two azo dyes: Chocolate Brown HT (CB) and Eriochrome Black (EB), as model industrial dyes. Boron doped Diamond (BDD) electrodes were used as an anode and a cathode for EAOPs production. The capacity of individual effects of each process and coupling were evaluated as functions of the decolorization and mineralization of industrial dye solutions. The azo dye solutions were completely decolorized under acidic conditions pH 2.8–3.0. The mineralization of the mixture of solutions was improved in the following order: EO OH), PEF is more potent due to the photolysis of the oxidation products. Chemical oxygen demand abatement of 91% for EO, 95% for EF and 98% for PEF processes were obtained, when using an applied current density (j) of electrolyzed solution of 20 mAcm−2. The azo dye decolorization for all treatments followed pseudo-first-order kinetics, suggesting the attack of a constant production of OH radicals in the solution under EF and PEF, and EO at the anode surface.

This project assessed the mineralization of Black NT2 (BNT2), a diazo dye frequently used in tanneries, using electrochemical technologies based on boron-doped diamond electrodes (BDD). The experimental trials included adding an Fe2+-catalyzed process known as the electro-Fenton (EF) reaction. The process was carried out in a recirculating pre-pilot plant that used a filter-press electrochemical reactor connected to a reservoir. The pre-pilot reactor was equipped with a BDD plate anode coupled with a BDD cathode to electrogenerate hydrogen peroxide (H2O2) in situ. The effects of experimental variables such as current density (j) and initial BNT2 concentration were tested. The best conditions to mineralize an initial BNT2 concentration equivalent to 250 mg L−1 of total organic carbon (TOC) in a 50 mM Na2SO4 solution using the EF process were 0.3 mM Fe2+, pH 3.0, j = 30 mA cm2, and Q = 12 L min−1. After 90 min of treatment, 100% TOC abatement was achieved. In all cases, TOC decreased following a pseudo-first-order kinetics. For the highest initial dye concentration, 100% TOC was achieved after 120 min, with 70% average current efficiency and 0.01088 KWh (g TOC)−1 energy consumption at the end of the process. For experiments without complete mineralization, the evolution of some nontoxic, short-chain carboxylic acids—such as oxalic, maleic, succinic, acetic, and formic—were quantified using high performance liquid chromatography (HPLC) analysis. These findings suggest that the EF reaction is an interesting, feasible alternative for enhanced dye degradation in tannery water treatment processes.

A 4 cm2 BDD/TiO2 electrode has been synthesized by a novel method in which nano-particulated TiO2 (P25, Degussa) was electrophoretically deposited onto a boron-doped diamond (BDD) substrate, followed by sintering at 450 °C. Its surface characteristics were analyzed by SEM-EDX. The electrocatalytic behavior of the BDD/TiO2 electrode was clarified from the cyclic voltammetry of a 0.50 M K4[Fe(CN)₆] solution. The much higher electroactive area of the nano-structured TiO2 deposit over the hybrid electrode gave much higher currents of the Fe(CN)63–/Fe(CN)64– pair than using the raw BDD electrode. When BDD/TiO2 was used in the anodic oxidation of 50 mg L− 1 Acid Blue 80 solution at pH 3.0 and 5.0 at 22.5 mA cm− 2 in the dark, it decolorized much more rapidly the dye solution compared to anodic oxidation with BDD. This was related to the production of greater amounts of hydroxyl radical at the high electroactive area of the nano-structured TiO2. Quicker color removal was obtained by photoelectrocatalysis by irradiating the BDD/TiO2 anode with a 75 mW cm− 2 UVA light (λmax = 365 nm). The dye concentration decay was always much faster than decolorization due to the formation of more recalcitrant colored products from the attack of oxidizing species. In 60 min, photoelectrocatalysis removed 96% of Acid Blue 80. The use of nano-structured TiO2 deposited onto BDD as anode in photoelectrocatalysis then exceeds the efficiency of anodic oxidation with the best anode known of BDD.

Three different advanced oxidation processes, electrochemical oxidation (EO), electro-Fenton (EF) and photoelectro-Fenton (PEF) were tested for the treatment of individual and mixed solutions of Bismark Brown G, Bismark Brown R and Brown DGI dyes. The experiments were performed in a 500 mL stirred undivided cell with a 25 cm2 BDD anode, using 0.050 M Na2SO4 as electrolyte and pH 3.0. The cathodes used for the different processes were a stainless-steel plate in EO and another BDD electrode in EF and PEF. In the two latter processes, pumped air was bubbled for H2O2 generation and 0.50 mM Fe2+ was added as the Fenton's reagent. For PEF, the solution was irradiated using an UVA lamp. The oxidation power of the processes increased in the order: EO

In this paper, the quantification of Pb2+ and Cd2+ at trace levels was carried out by differential pulse anodic stripping voltammetry technique, using a paste carbon electrode modified with spent coffee grounds (SCG) as working electrode. Different percentages of SCG (40, 50, 60, 70% of SCG) were tested in the experiments; the best electrochemical response was obtained with the electrode modified with 50% of SCG. An experimental design was performed in order to maximize the intensity of the oxidation current, using the accumulation time and the reduction potential as factors. The best experimental conditions for the electrodeposition of the cadmium ions were obtained at an accumulation time of 76 s, and the reduction potential of −1115 mV; while for the electrodeposition of lead ions, the best performance was obtained at an accumulation time of 120 s and a reduction potential of −1200 mV. The detection limit of the carbon paste electrode modified with 50% of SCG were 89 and 90 μM for Cd(II) and Pb(II), respectively, showing the potential of these electrodes as a viable and sustainable alternative for the determination of heavy metals in aqueous solutions.

Electrochemical oxidation of the effluent of a slaughterhouse facility by electrolysis under galvanostatic control was studied using Ti-RuO 2 /carbon felt (CF), Ti-PdPtOx/CF and BDD/CF cells by electro-Fenton at constant current density. Electrode material and the applied current density are significant parameters on oxidative degradation of pollutants in wastewater. It was found that pollutants of the effluent were oxidized to simple substrates and CO 2 with different results, obtaining good removal efficiencies of chemical oxygen demand of 83.5%, 77.9% and 74.3%, for the Ti-RuO 2 /CF, Ti-PdPtOx/CF and BDD/CF systems, respectively. Trends in degradation depend on the production of hydroxyl radicals ( OH) on the anodic materials and coagulation as confirmed by this study. Efficiency of electrochemical oxidation process was tested by carrying out chemical oxygen demand (COD) determinations. Additional operating parameters such as pH, initial ferrous concentration and electrolysis time were also evaluated.

Here, 2.5 L of solutions of 2,4-dichlorophenoxyacetic acid (2,4-D) herbicide of pH 3.0 have been degraded by electrochemical oxidation (EO) and electro-Fenton (EF) using a recirculation flow plant equipped with a single Pt/air-diffusion or BDD/BDD cell containing electrodes of 20 cm2 area. In EO, organics were mainly oxidized by OH formed at the anode surface from water oxidation, whereas in EF they were also destroyed in the bulk by OH generated from Fenton’s reaction between added Fe2+ and H2O2 produced at the cathode. In both treatments, the use of a single BDD/BDD cell always allowed larger decontamination due to the higher ability of OH at the BDD surface to mineralize organic intermediates. The most potent EF process with this cell gave 59% mineralization with 23% efficiency and 0.42 kW h g−1 TOC specific energy after 300 min at 0.5 A. The coupling of Pt/air-diffusion and BDD/BDD cells with equal individual current in EF led to similar mineralization degree to that found for the Pt/air-diffusion one at the same total current, but in much shorter time. The effect of current on the mineralization rate, mineralization current efficiency and specific energy of each method was examined. The influence of herbicide concentration on the EF performance of the coupled cell at 2.0 A/2.0 A was also studied and 83% mineralization was obtained as maximal for 276 mg L−1 2,4-D. The herbicide decay always obeyed a pseudo-first-order reaction. A mixture of short-linear carboxylic acids was detected in all final electrolyzed solutions, which were the major organic by-products generated using the single BDD/BDD cell but formed in minor proportion in the EF processes with the Pt/air-diffusion and coupled ones.

Comparison of tannery wastewater treatment results of electrocoagulation (EC), electro-oxidation (EOx), electro-Fenton (EF) and photoelectro-Fenton (PEF) processes using two current densities was performed and employed no only to assess the best electrochemical approach but also to have elements to suggest a combined and cost effective process. In this way, iron plates were used as the anode and cathode in the EC tests. For the EOx process, a boron-doped diamond (BDD) thin-film electrode was used as the anode, and an iron plate was employed as the cathode. For the EF and PEF treatments, both electrodes were of BDD and H2O2 was produced by O2 reduction at the cathode surface. Electrolytic trials were carried out in stirred open tank reactors containing 250 mL of wastewater applying either 65 or 111 mA cm−2. In PEF trials, the wastewater was irradiated using a 6 W UVA light source. The characterization of the sludge generated in the EC process showed that it was not dangerous for the environment. Total Organic Carbon (TOC) removal values were larger at the highest density. Higher mineralization on the other hand, was found in EF and PEF when the concentration of Fe2+ was 3.0 instead 1.0 mM. Comparison of the methods revealed that their efficiency to remove the organic pollutant increased in the order EOx < EC ∼ EF < PEF after 180 min of electrolysis. Furthermore, the information obtained from the study, also allowed designing a combined strategy aimed to produce an effluent suitable for discharge. The combination of EC followed by PEF was also studied. The results showed that the EC/PEF treatment gave higher organic removal than EC and was more economic than PEF, being able to yield 90% TOC reduction of the tannery wastewater.