Electro-generated H2O2 Research Articles

Lignin depolymerization via an integrated approach of anode oxidation and electro-generated H2O2 oxidation

Lignin is a natural aromatic macromolecule in huge quantity and might serve as sustainable resources for the chemical industry after being depolymerized. An electrochemical approach combining anode oxidation and electro-generated H2O2 oxidation has been developed for converting lignin into value-added aromatic chemicals in this study. Lignin in alkali solution was electrolyzed in an undivided electrolytic cell with a cylindrical graphite felt cathode and a RuO2–IrO2/Ti mesh anode, in which the by-product O2 on the anode could be efficiently reduced to H2O2 on the cathode in situ. Results display that the depolymerization productivity via the integrated approach obviously surpassed the sum of that by separate H2O2 oxidation and anode oxidation. Moreover, the analysis results of GC-MS, GPC, and C9 expanded formula confirmed that C–C bonds and C–O–C bonds in lignin were cleaved synergistically by direct anodic oxidation and indirect H2O2 oxidation, and the macromolecules are gradually depolymerized into final products of monomers and dimers.

Study of the photo-electrochemical activity of cobalt- and nickel-doped TiO2 photo-anodes for the treatment of a dye-contaminated aqueous solution

The photo-electrochemical activity of Co- and Ni-doped TiO2 electrodes was studied using an aqueous solution containing an organic dye (Orange II) as a model polluted effluent. The results showed not only that both dopant species increased the activity of the photo-anode toward the dye color degradation but also that a specific proportion of the dopant materials maximized this effect. Interestingly, the best performance in the photo-electrochemical dye degradation process for the two doped materials was obtained using a 20 wt% doping synthetic proportion; achieving in both cases close to 90 % color removal during the first 10 min of reaction time. Employing the Langmuir–Hinshelwood formalism, the dopant effect was measured in terms of an increase in the kinetic rate coefficient and a coupled decrease in the dye adsorption equilibrium constant. In this way, while the surface rate constant (k c) for the doped materials were found to be 3.3 (Co) and 2.7 (Ni) times higher than that of the un-doped TiO2, the L–H adsorption constant were calculated as 0.083, 0.021, and 0.036 (mgL−1)−1 for the TiO2, Co- and Ni-doped materials, respectively. The results from additional experiments that followed the temporal evolution of the concentration of electro-generated H2O2 suggested a Fenton-like effect with regard to the anode performance caused by the doping species in the semiconductor anode.

Design of a visible light driven photo-electrochemical/electro-Fenton coupling oxidation system for wastewater treatment

In this study, we report on a photo-electrochemical/electro-Fenton oxidation (PEC/EF) system by coupling visible light driven photo-electrochemical oxidation (PEC) and electro-Fenton oxidation (EF) in an undivided cell. Bi2WO6 nanoplates deposited on FTO glass (Bi2WO6/FTO) and Fe@Fe2O3 core–shell nanowires supported on activated carbon fiber (Fe@Fe2O3/ACF) were used as the anode and the cathode in the PEC/EF system, respectively. This novel PEC/EF system showed much higher activity than the single PEC and EF systems on degradation of rhodamine B in aqueous solution at natural pH. Moreover, the degradation and the instantaneous current efficiencies of the PEC/EF system were increased by 154% and 26% in comparison with the sum of those of single PEC and EF systems, respectively. These significant enhancements could be attributed to the synergetic effect from better separation of photo-generated carriers in the photo-anode and the transfer of photo-electrons to the oxygen diffusion cathode to generate more electro-generated H2O2 and hydroxyl radicals on the Fenton cathode. The better separation of photo-generated carriers contribute more to the overall degradation enhancement than the photo-electrons generated H2O2 and the subsequent Fenton reaction on the cathode during the PEC/EF process.

Electrochemical mineralization of the azo dye Acid Red 29 (Chromotrope 2R) by photoelectro-Fenton process

The degradation of 100mL of 244mgL−1 of the azo dye Acid Red 29 (AR29) has been studied by photoelectro-Fenton (PEF) using an undivided cell containing a boron-doped diamond (BDD) anode and an air-diffusion cathode under UVA irradiation. The effect of current density, concentration of catalytic Fe2+ and pH on the process was examined. Quick decolorization and almost total mineralization were achieved due to the synergistic action of UVA light and oxidant hydroxyl radicals formed in the bulk from Fenton’s reaction between electrogenerated H2O2 at the cathode and added Fe2+, as well as in the BDD surface from water oxidation. Optimum PEF conditions were found for 0.5–1.0mM Fe2+ and pH 3.0. Comparable electro-Fenton (EF) degradations in the dark yielded much poorer mineralization. The decay kinetics of AR29 followed a pseudo-first-order reaction with similar rate for EF and PEF. The azo dye disappeared much more rapidly than solution color, suggesting the formation of colored conjugated products with λmax similar to that of AR29. Ion-exclusion HPLC allowed the detection and quantification of tetrahydroxy-p-benzoquinone, oxalic, oxalacetic, tartronic, tartaric, oxamic, malonic and fumaric acids as intermediates in the PEF process. Oxalic acid, accumulated in large extent, was quickly destroyed by the efficient photolysis of Fe(III)–oxalate complexes with UVA light, whereas tartronic and oxamic acids were the most persistent byproducts because of the larger stability of their Fe(III) complexes. The mineralization of the initial N of the azo dye yielded NH4+ ion and NO3- ion in smaller proportion.

Biodegradability enhancement of wastewater containing cefalexin by means of the electro-Fenton oxidation process

The comparative degradation behavior of cefalexin (CLX) using advanced oxidation processes (AOPs) with the aim of improving CLX biodegradability was studied. Among the AOPs used, RuO2/Ti anodic oxidation (AO), AO in the presence of electro-generated H2O2 (AO-H2O2), and the electro-Fenton (EF) process, the EF process was the most effective. In the EF process an activated carbon fiber (ACF) was used as a cathode. Different input variables, including catalyst concentration, pH, and current density were evaluated to find the optimum conditions for the EF. The most suitable operational conditions were as follows: a current density of 6.66mA/cm2, a pH of 3, and a concentration of 1mM of Fe2+ as the catalyst. Different CLX concentrations were analyzed as well of different reaction times to assess the degree of mineralization. The change in biodegradability was evaluated by the BOD5/COD and the BOD14/COD ratios. The EF did not effectively remove the COD, but removed enough to achieve suitable biodegradability for a further biological process.

Degradation of disperse azo dyes from waters by solar photoelectro-Fenton

Solutions of the azo dyes Disperse Red 1 (DR1) and Disperse Yellow 3 (DY3), commonly used in the Chilean textile industry, in 0.1moldm−3 Na2SO4 and 0.5mmoldm−3 Fe2+ of pH 3.0 were comparatively degraded by electro-Fenton (EF) and solar photoelectro-Fenton (SPEF) using a 2.5dm3 recirculation flow plant containing a BDD/air-diffusion cell coupled with a solar photoreactor. Organics were oxidized in EF with hydroxyl radicals formed at the anode surface from water oxidation and in the bulk from Fenton's reaction between electrogenerated H2O2 and added Fe2+. The oxidizing power of SPEF was enhanced by the additional production of hydroxyl radicals from the photolysis of Fe(III) hydrated species and the photodecomposition of Fe(III) complexes with intermediates by UV light of solar irradiation. Total decolorization, complete dye removal and almost overall mineralization for both dye solutions were only achieved using the most potent SPEF process, yielding higher current efficiencies and lower energy consumptions than EF. Final carboxylic acids like pyruvic, acetic, oxalic and oxamic were detected during the SPEF treatments. NO3− ion was released as inorganic ion. The use of a solution pH of 2.0–3.0 at 50mAcm−2 was found preferable for SPEF. Synthetic textile dyeing solutions containing the dyes were treated under these conditions yielding lower decolorization rate, slower dye removal and smaller mineralization degree than only using 0.1moldm−3 Na2SO4 due to the parallel oxidation of organic dyeing components. However, lower energy consumptions were obtained by the destruction of more amounts of total organic carbon, indicating that SPEF is a useful and viable method for the remediation of textile industrial wastewaters with high contents of disperse azo dyes.

Study of oxygen reduction on copper applied to the peroxidase-mediated oxidation of methylene blue

The kinetics of oxygen reduction was studied on copper in tartarate solutions using a rotating disc electrode. The effect of pH on the cathodic reduction was examined. AC impedance and cathodic polarization curves showed that the reaction is partially controlled by mass transport. At high current densities, the limiting current values showed that O2 is reduced mainly through the four electron pathway. From measurements of density and viscosity and data taken from Levich and Tafel plots, diffusion coefficients for oxygen in tartarate medium were calculated. Peroxidase-mediated oxidation of methylene blue (MB) results using electrogenerated H2O2 on copper electrode showed that O2 reduction reaction occurs in two steps producing the adsorbed intermediate species H2O2 by k 2. In the absence of peroxidase, the hydrogen peroxide elimination has to be rapid with k 3 ≫ k M. The MB oxidation by lignin peroxidase using electrogenerated H2O2 was studied. After 24 h LiP is able to oxide the MB, producing different oxidized forms: azure C and thionine, with yield of 23% and 66%, respectively.

Oxidative degradation of direct orange 61 by electro-Fenton process using a carbon felt electrode: Application of the experimental design methodology

The degradation of an azo dye, the direct orange 61 (DO 61), has been studied by electro-Fenton process using a Pt anode and a carbon felt cathode. The great oxidation ability of this process is due to the large production of hydroxyl radicals (OH) in the medium from electrochemically assisted Fenton’s reaction which takes place between electrogenerated H2O2 and Fe2+ formed by cathodic reduction of O2 and Fe3+, respectively. These radicals are able to oxidize any organics until their mineralization. A factorial experimental design was used for determining the influent parameters on the DO 61 degradation in aqueous medium by electrochemically generated OH radicals. The results obtained have shown that the current intensity and the dye initial concentration were the main influent parameters on the degradation rate. Thus, under the obtained conditions, kinetic analysis of DO 61 degradation showed a pseudo-first order reaction. The optimal experimental parameters for DO 61 mineralization have been investigated by the use of Doehlert matrix. It has been demonstrated that under the optimal conditions determined by this method, electro-Fenton process can lead to a complete mineralization of the dye solution after 6 h (98% of TOC removal) giving carboxylic acids and inorganic ions as final end-products before mineralization. The results show the efficiency of the electro-Fenton process to remove organic pollutants from aqueous medium.

A Novel Composite Electrode of Photocatalyst‐TiO2/C Loading on the Surface of the Air (Oxygen) Electrode

AbstractThe methods for preparing the H2O2 generating air (oxygen) electrode and the composite electrode of photocatalyst‐TiO2/C loading on the surface of the air (oxygen) electrode were introduced. In the case of the composite electrode, the current efficiency of electro‐generated H2O2 is higher than 80% (J ≤ 15 mA/cm2). The degradation of aniline was used as an example to measure the influence of the composite electrode and compared with the system in which the air (oxygen) electrode and the photocatalyst‐TiO2 were separated. The results confirmed that the composite electrode played an active role on accelerating the degradation rate of aniline. According to the measurement of the polarization curves of composite electrode and TiO2 photo anode, and of the adsorbing amount of aniline on the surface of the composite electrode, the principle of descending the recombination rate of photo‐generated electron and hole and of enhancing the oxidation rate of organic molecule was described. The mechanism about the degradation of aniline was also discussed.