Electro-generated H2O2 Research Articles

Comparison of electrochemical advanced oxidation processes for the treatment of complex synthetic dye mixtures

Many industries use complex synthetic organic dye mixtures in their dyeing processes, producing wastewater effluents with complex dye mixtures. In this study, electrochemical advanced oxidation processes like electrochemical oxidation (ECO), ECO with electrogenerated H2O2 (ECO-H2O2), electro-Fenton (EF), and photo electro-Fenton (PEF) with UVC irradiation were compared to discolor and mineralize a mixture of Methyl Orange, Methyl Red, and Toluidine Blue O with a boron-doped diamond anode and a gas-diffusion cathode. While ECO and ECO-H2O2 required long times to achieve a complete discoloration, the dyes were quickly degraded by EF and PEF due to the oxidation with ∙OH formed from Fenton’s reaction. Almost complete mineralization was rapidly achieved by PEF for a solution with 424 mg O2 L−1 of chemical oxygen demand (COD) at a current density of 66.6 mA cm−2 due to the synergistic action of the UVC irradiation. Additionally, the treated effluent was evaluated using the DFZ (Deutsche Farb Zah) parameter for water condition, offering a new perspective and distinguishing this work from previous studies focused on electrified technologies for dye removal. The same behavior has been found for the mixture of such dyes in an industrial effluent upon PEF treatment, even showing faster discoloration and COD reduction as result of the additional removal with active chlorine formed from Cl− oxidation. Ion-exclusion HPLC analysis of the treated synthetic effluent revealed the formation of final carboxylic acids like malic, acetic, oxalic, and formic. Released nitrate, ammonia, and nitrite were detected as nitrogenated by-products from the initial N of the dyes.

Ciprofloxacin degradation in microbubble ozonation combined with electro-generated H2O2 process: Operational parameters and oxidation mechanism

Microbubble ozonation combined with electro-generated H2O2 process (EH2O2-MB-O3) was applied to degrade ciprofloxacin (CIP) as a typical antibiotic in wastewater in this study. The results showed that the performance of CIP degradation and mineralization in EH2O2-MB-O3 process was more efficient than other related processes. Microbubble ozonation and electro-generated H2O2 showed obvious synergistic effect on enhanced CIP removal in EH2O2-MB-O3 process. The O3 dosage, applied current and initial pH in EH2O2-MB-O3 process were optimized as 12.5 mg/min, 50 mA and 7.0 (uncontrolled). Under the optimal conditions, CIP could be degraded completely within 60 min and the TOC removal efficiency of 28.85% could be achieved within 90 min in EH2O2-MB-O3 process. The corresponding ozone utilization efficiency was close to 100%. More efficient ROS generation was observed in EH2O2-MB-O3 process, especially •OH, compared with other related processes. The contribution rates of •OH, 1O2, O3 and O2.- to CIP removal were determined as 66.79%, 23.57%, 6.67% and 2.98% in EH2O2-MB-O3 process, respectively. The electro-generated H2O2 was clarified to contribute to not CIP removal but ROS generation. Three CIP degradation pathways were proposed according to identified degradation intermediates, which indicated that pyridone structure of quinoline and piperazine ring in CIP molecule could be destroyed more easily than benzene ring of quinoline, and the hydroxylation seemed to prevail in these CIP degradation pathways due to •OH electrophilic addition reaction probably. The removal of other antibiotics in EH2O2-MB-O3 process also depended on not ozone direct oxidation but •OH oxidation mainly.

Construction of bubbles enrichment sites and gas-diffusion-microchannel to assist Cr(VI) electrochemical reduction in a flow-through electrochemical system

Due to characteristics of the forced convection and enhanced mass transfer, graphite felt cathode based flow-through electrochemical system presented high electroreduction efficiency for aqueous Cr(VI). However, disadvantage is that bubbles generated by water electrolysis cannot pass through the liquid filled microchannel in time, which is easy to cause gas accumulation and an unstable electrochemical operation. Herein, we demonstrated graphite felt flow-through cathode designed with hydrophobic polytetrafluoroethylene (PTFE) as bubbles enrichment sites and construction of gas-diffusion-microchannel to assist in Cr(VI) electrochemical reduction. Cr(VI) reduction kinetics constant by graphite felt flow-through cathode increased 4.6 times after loading PTFE and was 2.6 ∼ 5.7 times higher than parallel-plate electrodes system. Typical electrochemical analysis suggested that PTFE can enrich and accelerate the dissolved oxygen mass transfer rate at PTFE-electrode-solution micro-interface, realizing oxygen digestion via two-electron reduction reaction. The unconsumed bubbles were transported out of electrochemical cell along gas-diffusion-microchannel to avoid gas accumulation and maintained a stable operation. Batch experiments indicated the ∼ 99.9% Cr(VI) reduction efficiency when the initial pH 2 and 100 mg/L Cr(VI) feed was treated under a single-pass pattern with flow rate of 1.5 mL/min and current density of 12.74 mA/cm2. Mechanism study revealed that faraday reaction was the main driving force for Cr(VI) reduction, while Cr(VI)-induced activation of in-situ electro-generated H2O2 contributed to formations of •OH and intermediate chromium species. This work also shows the feasibility of remediation for Cr(VI)-organic coexistence environment by a catalyst-free flow-through electrochemical system.

Strategy and mechanism for strong and stable electrochemiluminescence of graphitic carbon nitride

Electrochemiluminescence (ECL) of graphitic carbon nitride nanosheets (CNNSs) is showing extensive interest to researchers. However, the poor ECL stability of CNNSs has always been an urgent problem, which severely limits its application. Herein, a systemic investigation is carried out for the exact mechanism affecting the ECL stability of CNNSs. It is found experimentally that dissolved oxygen in the solution is selectively reduced into H2O2 through a two-electron pathway on the surface of CNNSs. The enriched electrogenerated H2O2 on the electrode surface leads to the progressive inhibition of ECL of CNNSs due to its competition with negatively charged CNNSs (CNNSs•–) for sulfate radical ions (SO4•–). On the basis, the pathway of eliminating O2 by N2, composing CNNSs with other nanomaterials to realize the four-electron reduction, or improving the concentration of K2S2O8 is proved to be effective means for improving ECL stability successfully. Proofing the concept of CNNSs with high ECL stability, an ECL sensor is developed for the detection of H2O2. This work not only exploits the intrinsic mechanism of poor ECL stability of CNNSs, but also provides efficient methods to improve the ECL stability. Thus, we believe it has promising attraction in the research of CNNSs-based assays.

In-situ electrochemical synthesis of H2O2 for p-nitrophenol degradation utilizing a flow-through three-dimensional activated carbon cathode with regeneration capabilities

The growing ubiquity of recalcitrant organic contaminants in the aqueous environment poses risks to effective and efficient water treatment and reuse. A novel three-dimensional (3D) electrochemical flow-through reactor employing activated carbon (AC) encased in a stainless-steel (SS) mesh as a cathode is proposed for the removal and degradation of a model recalcitrant contaminant p-nitrophenol (PNP), a toxic compound that is not easily biodegradable or naturally photolyzed, can accumulate and lead to adverse environmental health outcomes, and is one of the more frequently detected pollutants in the environment. As a stable 3D electrode, granular AC supported by a SS mesh frame as a cathode is hypothesized to 1) electrogenerate H2O2 via a 2-electron oxygen reduction reaction on the AC surface, 2) initiate decomposition of this electrogenerated H2O2 to form hydroxyl radicals on catalytic sites of the AC surface 3) remove PNP molecules from the waste stream via adsorption, and 4) co-locate the PNP contaminant on the carbon surface to allow for oxidation by formed hydroxyl radicals. Additionally, this design is utilized to electrochemically regenerate the AC within the cathode that is significantly saturated with PNP to allow for environmentally friendly and economic reuse of this material. Under flow conditions with optimized parameters, the 3D AC electrode is nearly 20% more effective than traditional adsorption in removing PNP. 30 grams of AC within the 3D electrode can remove 100% of the PNP compound and 92% of TOC under flow. The carbon within the 3D cathode can be electrochemically regenerated in the proposed flow system and design thereby increasing the adsorptive capacity by 60%. Moreover, in combination with continuous electrochemical treatment, the total PNP removal is enhanced by 115% over adsorption. It is anticipated this platform holds great promises to eliminate analogous contaminants as well as mixtures.

Highly efficient electro-Fenton process on hollow porous carbon spheres enabled by enhanced H2O2 production and Fe2+ regeneration

Electro-Fenton (e-Fenton) is a promising method for wastewater treatment that relies on powerful ·OH generated via the decomposition of electro-generated H2O2 catalyzed by Fe2+. In this regard, developing a catalyst capable of simultaneously producing H2O2 and accelerating Fe2+ regeneration is of considerable importance; however, this remains a challenge because of the difficulty in modulating the electronic microenvironment. Herein, a hollow porous carbon sphere catalyst (HPCS) is developed to synchronously enhance H2O2 generation and accelerate Fe3+/Fe2+ cycling by constructing an electron-rich microenvironment via surface curvature regulation. The Fe2+ regeneration efficiency reaches 35.5% on HPCS featuring a larger curvature structure (HPCS-TPOS), which is 1.6 times higher than the smaller curvature HPCS-S catalyst (22.8%). Density functional theory reveals that the electron-rich microenvironment on the outer surface of high curvature structure promotes Fe2+ regeneration. The H2O2 production rate on HPCS-TPOS is 47.2 mmol L−1 h−1, exceeding the state-of-the-art e-Fenton catalysts reported. Benefiting from the concurrent high-efficiency of H2O2 production and Fe2+ regeneration, HPCS-TPOS e-Fenton is demonstrated to be efficient for sulfamethoxazole removal with the kinetic rate of 0.30–0.72 min−1 at pH 3–7. This work offers new insight into the design of efficient catalysts by rationally regulating curvature structures for wastewater treatment.

Highly-efficient cathodic lysis of biofilm on conductive carbon nanotube membranes

Biofouling is a major problem observed with membrane-based water treatment systems. In this work, cathodic lysis of bacteria and biofilms on carbon nanotube (CNT) membrane was investigated as a novel mechanism for biofouling reduction. Considering the biofilm growth cycle, electrolysis of sessile bacteria and established bacterial biofilms were separately investigated. The results showed that cathodic electrolysis at 3.0 V for 3 h effectively eliminated 99.23% of the sessile bacteria on the CNT membrane. An extended treatment of mature biofilm for 21 h led to 93.92% reduction in viable bacterial density and 98.6% decrease in biofilm coverage. Besides, analyses of the membrane permeate revealed that the biofilm lysis released cell contents into the CNT matrix, which were subsequently removed by electrolysis. As a result, the normalized transmembrane pressure for the fouled membrane decreased from 4.23 to 2.06 after 21 h of electrolysis. Moreover, the cathodic lysis of bacteria and biofilm by the low-voltage electric field was mainly ascribed to the electro-generated H2O2 and the associated bio-electrochemical effects. This work provided a facile approach for in-situ mitigation of biofouling caused by chlorine-resistant bacteria, which was conducive to energy-efficient operation of membrane-based water treatment facilities.

A novel bifunctional cathode for the generation and activation of H2O2 in electro-Fenton: Characteristics and mechanism

External addition of Fe2+ is commonly required for the sake of activating H2O2 in electro-Fenton, which limits the reaction continuity and causes the potential secondary pollution. Herein, a novel bifunctional cathode (rGO/MIL-88A/CF) was fabricated for achieving the in-situ H2O2 generation and activation simultaneously. The rotating disc electrode (RDE) test showed that the average electron transfer number was about 2.01, indicating that selective two-electron reduction of oxygen molecule (2e– ORR) occurred on the as-prepared cathode. The quenching experiments and EPR tests proved that the oxygen-containing reactive species such as OH• and O2•– could be efficiently generated on rGO/MIL-88A/CF cathode, which contributed to the pollutant degradation. The obtained rGO/MIL-88A/CF cathode not only obtained the improvement of its charge transfer rate owing to the modification by the reduced graphene (rGO), but also acted as a stable activator for the electro-generated H2O2 without additional reagents. Moreover, rGO/MIL-88A/CF cathode effectively facilitated the circulation of Fe2+/Fe3+ through cathodic reduction, which consequently enhanced the performance of the electro-Fenton. Our current investigation presented a feasible example of a novel bifunctional cathode applied in electro-Fenton towards efficient pollutant removal without exogenous H2O2 activator.

Treatment of beauty salon effluents using advanced oxidative electrochemical processes with boron-doped diamond anode

Beauty salons constitute a growing economic sector that is responsible for the release of effluents containing a diversity of pollutant compounds. This work proposes the local treatment of beauty salon effluent (BSE), with the aim of decentralizing the treatment process and avoiding increasing the organic load at municipal effluent treatment stations. The processes of electrochemical oxidation (EO) and simultaneous EO with electrogenerated H2O2 (EO/H2O2) were applied using different anode/cathode electrode pairs: BDD/stainless steel and BDD/GDE. Investigation was made of the influence of salts on the generation of oxidants in different support electrolytes, using the N,N-dimethyl-4-nitrosoaniline (pNDA) decoloration technique. The presence of active chlorine species was shown to influence the global oxidizing power. The BSE was characterized and the conversion rate of the organic load was monitored during 180 min of the processes. For the EO/H2O2 process, without adjustment of the initial pH, the COD conversions in 120 min were 43%, 63% and 69%; with the initial pH adjusted to 3.0 were 53%, 73%, and 78%, for the three different current densities evaluated. Regarding EO/H2O2 process, without pH adjustment, DOC have also a greater reduction reaching in 120 min 33%, 48%, and 73%, for 15, 38 and 62 mA cm−2, respectively. The lowest energy consumptions were 0.002, 0.005 and 0.006 kWh g COD−1, applying 15, 38, and 62 mA cm−2, respectively, with pH adjustment in the EO/H2O2 process. The results showed that the lowest energy consumption and the greatest reduction of the BSE organic load were achieved using the EO/H2O2 process.

Electro-peroxone enables efficient Cr removal and recovery from Cr(III) complexes and inhibits intermediate Cr(VI) generation in wastewater: Performance and mechanism

Available oxidation processes for removing Cr(III) complexes from water/wastewater usually encounter the formation of highly toxic Cr(VI) and the generation of Cr enriched waste sludge, posing challenges on the subsequent disposal. Herein, we achieve efficient removal of Cr(III)-organic complexes and simultaneous recovery of Cr from wastewater with enhanced curtailment of intermediate Cr(VI), by using an electrochemically driven peroxone (i.e., electro-peroxone) process with activated carbon fiber (ACF) electrodes. For Cr(III)-EDTA, electro-peroxone could remove ∼90% total Cr from 11.50 mg/L to 1.20 mg/L and ∼80% total organic carbon, with a strong curtailment of Cr(VI) to less than 0.2 mg/L. Additionally, the process could obtain a complete recovery of the removable Cr, of which 78.3% are enriched at ACF cathode as amorphous Cr(OH)3 deposits and the remaining 21.7% are adsorbed at the anode, thus avoiding the generation of Cr laden sludge. Mechanism studies show the electro-generated H2O2 reacts with O3 to generate abundant HO· for decomplexation, which sequentially oxidizes Cr(III) to Cr(VI), and degrades the released EDTA via stepwise decarboxylated process, as confirmed by HPLC analysis. Multiple pathways including electro-reduction, H2O2 reduction and electro-adsorption synergistically curtail and immobilize the formed intermediate Cr(VI). ACF characterizations and continuous 5-cycle experiments substantiate the excellent reusability of the ACF electrodes. Moreover, this process exhibits satisfactory effectiveness to Cr(III) complexed with other ligands (e.g., citrate and oxalate), and complexed Cr(III) in the real electroplating wastewater. We believe this study would provide an efficient and eco-friendly alternative for Cr(III) complexes removal from wastewater.

Anthraquinone (AQS)/polyaniline (PANI) modified carbon felt (CF) cathode for selective H2O2 generation and efficient pollutant removal in electro-Fenton

A novel binder-free anthraquinone (AQS)/polyaniline (PANI) modified carbon felt (CF) cathode for selective H2O2 generation and efficient pollutant removal in electro-Fenton was fabricated by CV electro-deposition method. AQS, the oxygen reduction reaction (ORR) catalyst, was immobilized by the PANI film, which contributed to the obtained high stability of the AQS/PANI@CF cathode. The concentration of the electro-generated H2O2 on AQS/PANI@CF cathode (83.3 μmol L−1) was about 10 times higher than that of the bare CF cathode. And the high yield of H2O2 was attributed to the catalytic reduction of O2 by AQS to generate more superoxide radical (O2•-), which combined with H+ to form H2O2. Additionally, the rhodamine B (RhB) degradation efficiency reached 98.8% within 60 min with the AQS/PANI@CF served as the cathode with high stability and good repeatability. The main generated reactive radicals were determined by the quenching experiments and the electron paramagnetic resonance (EPR) tests. Besides, a plausible mechanism of the AQS/PANI@CF cathode applied electro-Fenton process was proposed. This work provided a reliable reference for the subsequent investigations of the binder-free cathode with high performance and stability.

Role of in-situ electro-generated H2O2···bridge in tetracycline degradation governed by mechanochemical Si-O anchoring Cu2+ as electron shuttle during E-peroxone process

The efficient mineralization of tetracycline (TC) and reaction mechanism were investigated by designing heterogeneous H2O2···bridge linked with active sites in higher valency species. CuSi-BM60 was synthesized by anchoring Cu2+ on Si-O functional groups of MCM-48 by mechanochemical technology to assist electro-peroxone treatment (E-peroxone). Results suggested that catalysts as electron shuttle enhanced electron transfer and reactive oxygen species (ROSs) generation, showing a complete tetracycline (TC) degradation and excellent mineralization (73.2%, 60 min). Cu sites significantly raised the interaction with in-situ electro-generated H2O2, generating uneven electron cloud distribution by Si−O···Cu···HO−OH* bonding bridge, which enlarged H-O bonding length/angle near Cu and decreased orbital energy level gap. This work provided new insights into the complex of ball milling mediated Cu2+ with H2O2 accompanied by the cycle of H-O bonding cleavage and regeneration for efficient ROSs production during E-peroxone.

Onsite Graywater Treatment By Decoupled Electro-Peroxone Process with Recycled Effluent

This study investigates the efficacy of a decoupled electro-peroxone (E-peroxone) process for the lab-scale treatment of synthetic graywater. The E-peroxone process generally integrates conventional ozonation with in situ production of hydrogen peroxide (H2O2) in a single reactor for the transformation of ozone (O3) to hydroxyl radicals (*OH). In this study, however, the electrolysis of H2O2 generation via cathodic oxygen reduction was performed separately in order to finely control the H2O2 utilization for the E-peroxone process and to prevent the degradation of polytetrafluoroethylene (PTFE) coated carbon cathodes by highly reactive *OH. The dosage of the electro-generated H2O2 into a main reactor was optimized to enhance the performance of synthetic graywater treatment by facilitating the *OH formation. Under the optimal condition, the decoupled E-peroxone system treated synthetic graywater with initial chemical oxygen demand (COD) of 358 mg O2/L, total organic carbon (TOC) of 96.9 ppm, biochemical oxygen demand (BOD) of 162 mg O2/L, and turbidity of 11.2 NTU; the system consistently achieved removal of 89% COD, 86% TOC, 91% BOD, and 95% turbidity after 90-minute treatment, producing an effluent acceptable for discharge and non-potable applications such as toilet flushing. Part of the effluent was also recycled back into the system for H2O2 generation to minimize water consumption. This recycling process was successfully repeated consecutively for four cycles. Although the duration to reach the desirable concentration of H2O2 slightly increased from cycle to cycle, the system maintained comparable treatment performance with COD and TOC removal above 85% and 73%, respectively. Given these promising results in single-pass treatment and closed-loop recycling of graywater, the decoupled E-peroxone system can provide effective decentralized wastewater treatment for remote or water-sensitive areas. Figure 1

Carbon felt modified with N-doped rGO for an efficient electro-peroxone process in diuron degradation and biodegradability improvement of wastewater from a pesticide manufacture: Optimization of process parameters, electrical energy consumption and degradation pathway

In this study, diuron abatement in aqueous medium and the remediation of wastewater from a pesticide manufacture were studied by the E-peroxone process, using carbon felt modified with N-doped rGO (N-rGO/CF) as cathode and Ti/PbO2 anode. Compared with the unmodified CF, the N-rGO/CF displayed a higher response current, improved oxygen reduction activity, and larger BET specific surface area. Also, the modified CF provided a higher H2O2 production rate. The influence of several effective parameters on the diuron degradation rate in aqueous medium by the E-peroxone process was systematically explored by central composite design-response surface methodology; the approach suggested the optimal operating condition at pH of 9, applied current of 500 mA, Na2SO4 of 0.05 M, ozone flow of 1 L min−1, and operating time of 15 min to attain the maximum removal of diuron. At the optimal condition, N-rGO/CF cathode increased diuron degradation rate and decreased electrical energy consumption rather than CF cathode, in the E-peroxone system. Radical quenching experiment and degradation pathway of diuron revealed that hydroxyl radicals were the dominant reactive oxidants. A possible degradation mechanism was proposed based on hydroxylated and demethylated products identified during diuron degradation. No significant difference of the electro-generated H2O2 after 10 successive recycles confirms the admirable reusability of N-rGO/CF. The application of the process for pesticide wastewater increased BOD5/COD ratio significantly from 0.041 (for non-treated wastewater) to 0.4 during 90 min treatment, showing a substantial improvement of the biodegradability of the wastewater. Accordingly, the E-peroxone system can be utilized as a promising pretreatment step before a biological process to facilitate and shorten the length of the biological treatment. The findings of this study could offer valuable information for practical applications of the E-peroxone system in the treatment of pesticide-manufacturing wastewater.

Electrocatalytic degradation of oxytetracycline using three-dimensional electrode and optimization via fuzzy logic modeling

ABSTRACT The present paper focuses on a hybrid system combining electrochemical oxidation (EO) and activated carbon (AC) adsorption in a concept known as three-dimensional (3D) electrocatalytic technology for the treatment of oxytetracycline from aqueous solution; the conventional one-factor-at-a-time variation approach (AC dosage, OTC concentration, and current density) was used for determining the effects of operating parameters on the 3D electrochemical process and optimized via fuzzy logic modeling. The process’s superior performance was assessed based on comparative studies of the degradation efficiency of the 3D electrochemical process with AC adsorption and conventional 2D electrochemical process. The study showed that the AC particle electrode enhanced the performance of the 3D system by up to 21% in terms of OTC degradation efficiency compared to both adsorption and the conventional two-dimensional (2D) electrochemical system. When 2D and 3D processes were compared in required energy and electricity budgets, 3D electrode process with 0.75 g/L activated carbon requires 27.60 kWh/m3 and it perceived 2 times lower than 2D process; accordingly, the electricity budget is €5.38 and €2.81 for electrooxidation and 3D electrode process, respectively. The obtained results suggest that under the electric field, polarized AC worked as both adsorbent and a catalyst in the 3D electrochemical system, enhancing subsequently the decomposition of electro-generated H2O2 to more hydroxyl radicals (·OH) and promote the capacity of OTC mineralization.

Electrochemical oxidation of Naphthol Blue Black with different supporting electrolytes using a BDD /carbon felt cell

The electrochemical oxidation of Naphthol Blue Black (NBB) solution by means of anodic oxidation with electro-generated H2O2 (AO-H2O2) and Electro-Fenton (EF) was studied, using boron doped diamond (BDD)/ carbon felt (CF) cell. The experiments were carried out in NaCl and Na2SO4 as supporting electrolytes with initial concentration of 0.1 mM of NBB. The studied parameters were pH, applied current, concentration of Fenton catalyst, concentration of supporting electrolytes, and Cl-/SO42- mixture. The degradation of NBB was almost total when NaCl was used compared to Na2SO4, thanks to the electro-generated active chlorine (HClO/ClO-). The higher degradation is found with EF compared to AO-H2O2 process, the kinetic of degradation of NBB always follows a pseudo first-order reaction. The optimum conditions for the mineralization of NBB (i.e., 0.1 mM NBB, 50 mM Na2SO4 at pH 3.0, 0.1 mM Fe2+, and a current of 300 mA) were determined. These conditions yielded a total color removal in less than 10 min and 98% of total organic carbon (TOC) removal at 120 min electrolysis time. The biochemical oxygen demand/ Chemical oxygen demand (BOD/COD) ratio was decreased from 0.5 to 0.3, during the same timescales. Whereas, the mineralization current efficiency (MCE%) dropped from 21.5% to 0.05% in the electrolysis time range from 15-120 min suggesting the concomitant parasitic reactions. The evolution of nitrite NO2-, nitrate NO3-, ammonium NH4+, and sulfate SO42- concentrations were also followed as the end-products during the electrolysis.

Electrocatalytic activities of engineered carbonaceous cathodes for generation of hydrogen peroxide and oxidation of recalcitrant reactive dye

Abstract The electrocatalytic activity, H2O2 generation capacities and kinetics, and energy consumption of engineered carbonaceous cathodes including graphite plate, graphite felt, carbon felt, activated carbon felt and carbon fiber felt (CFF) was systematically evaluated in a double-chamber electrolysis system. The CFF cathode exhibited an excellent electrocatalytic activity in 2-electron oxygen reduction for H2O2 generation in both acidic and alkaline solutions. Furthermore, Reactive Red X-3B oxidation efficacy of carbonaceous cathodes via electro-generated H2O2 was compared in synthetic dyeing wastewater. The nature of carbonaceous materials dominated the degradation of Reactive Red X-3B in the cathodic chamber. Decolorization rate of more than 99% was achieved in all the cathodes tested, but there was a significant difference in the rate of mineralization. It was found that CFF was outclassed by 24.54% COD removal and high H2O2 generation ability in long runs. The degradation rate constant of Reactive Red X-3B in the CFF cathode chamber was 3.3 times faster than that of the graphite felt with rate constants of 1.27 × 10−3 min−1 and 3.82 × 10−4 min-1, respectively. The desirable stability and electrocatalytic activity of CFF were attributed to the better graphitic structure, higher conductivity and electroactive surface area with N-containing functional groups. Our findings demonstrate the potential of CFF to be used as a high-efficiency cathode in the electrochemical treatment of recalcitrant pollutants in practical applications.

Revealing Decay Mechanisms of H2O2-Based Electrochemical Advanced Oxidation Processes after Long-Term Operation for Phenol Degradation.

Hydrogen peroxide (H2O2)-based electrochemical advanced oxidation processes (EAOPs) have been widely attempted for various wastewater treatments. So far, stability tests of EAOPs are rarely addressed and the decay mechanism is still unclear. Here, three H2O2-based EAOP systems (electro-Fenton, photoelectro-Fenton, and photo+ electro-generated H2O2) were built for phenol degradation. More than 97% phenol was removed in all three EAOPs in 1 h at 10 mA·cm-2. As a key component in EAOPs, the cathodic H2O2 productivity is directly related to the performance of the system. We for the first time systematically investigated the decay mechanisms of the active cathode by operating the cathodes under multiple conditions over 200 h. Compared with the fresh cathode (H2O2 yield of 312 ± 22 mg·L-1·h-1 with a current efficiency of 84 ± 5% at 10 mA·cm-2), the performance of the cathode for H2O2 synthesis alone decayed by only 17.8%, whereas the H2O2 yields of cathodes operated in photoelectro-generated H2O2, electro-Fenton, and photoelectro-Fenton systems decayed by 60.0, 90.1, and 89.6%, respectively, with the synergistic effect of salt precipitation, •OH erosion, organic contamination, and optional Fe contamination. The lower current decay of 16.1-32.3% in the electrochemical tests manifested that the cathodes did not lose activity severely. Therefore, the significant decrease of H2O2 yield was because the active sites were altered to catalyze the four-electron oxygen reduction reaction, which was induced by the long-term erosion of •OH. Our findings provided new insights into cathode performance decay, offering significant information for the improvement of cathodic longevity in the future.

A new method to fabricate the cathode by cyclic voltammetric electrodeposition for electro-Fenton application

To develop a stable and efficient cathode applied in electro-Fenton is indispensable. In current study, the carbon felt (CF) was modified by the cyclic voltammetric (CV) electrodeposited anthraquinone 2-sulfonate (AQS) and polypyrrole (PPy). The newly fabricated CF/AQS/PPy cathode by CV electrodeposition method was applied in electro-Fenton, which showed higher stability and efficiency compared with that of the traditional potentiostatic electrodeposited (TPED) method. The yield of the electro-generated H2O2 and the degradation efficiencies of the target pollutant (RhB) in electro-Fenton with the newly fabricated cathode were around 10 times and 5 times higher than those of the unmodified CF cathode respectively. Recyclability evaluation showed that the degradation efficiency kept a good stability and could reach 100% within 1 h reaction time. The significantly enhanced performance and stability were attributed to the reduced charge transfer resistance (Rct) of the fabricated CF/AQS/PPy cathode decorated by the high conductive PPy and catalytic activity of AQS for oxygen reduction. The modification could result in an elevated extent of oxygen reduction to generate more O2•-, which subsequently combined with the protons to produce H2O2, the indispensable reactant to form reactive OH• in the Fenton’s process. The quenching experiment and EPR determination further confirmed the existence of OH• and O2•- free radicals in electro-Fenton equipped with the newly fabricated cathode.

Evaluation of the stability of polyacrylonitrile-based carbon fiber electrode for hydrogen peroxide production and phenol mineralization during electro-peroxone process

This study evaluated the stability of polyacrylonitrile-based carbon fiber cathode for hydrogen peroxide (H2O2) production and phenol mineralization during multiple cycles of electro-peroxone (E-peroxone) process. Results show that the oxidation of bulk carbon fiber by electro-generated H2O2 and bubbled ozone (O3) is negligible during the E-peroxone process. Nevertheless, the carbon fiber surface was oxidized to some extent as the cathode was repeatedly used in the multi-cycle E-peroxone process. Due to the oxidation by H2O2 and O3, nitrogen-containing groups on the carbon fiber surface were converted from pyridinic-N to pyridonic-N during the E-peroxone process. These changes resulted in an increase in the activity of the cathode for oxygen reduction reaction (ORR), but a decrease in the selectivity of the cathode for two-electron ORR to H2O2. After the carbon fiber cathode was used for 30 cycles of the E-peroxone treatment of phenol solutions, the cathodic potentials for ORR shifted positively by ~450 mV, which is beneficial to reduce the energy consumption of electrochemical H2O2 production. Nevertheless, the apparent current efficiency (ACE) for H2O2 production decreased from ~91.5% for the virgin cathode to ~48.2% for the used cathode. Despite the decrease in the ACE for H2O2 production, sufficient amounts of H2O2 could still be produced during the E-peroxone process with the used cathode. Therefore, complete phenol mineralization was maintained during all 30 cycles of the E-peroxone treatment of phenol solutions. These results suggest that the polyacrylonitrile-based carbon fiber is a promising cathode material for long-term E-peroxone operations.