Electrochemical generation of hydrogen peroxide using carbon black-, carbon nanotube-, and carbon black/carbon nanotube-coated gas-diffusion cathodes: effect of operational parameters and decolorization study

All-in-one continuous electrochemical monitoring of 2-phenylphenol removal from water by electro-Fenton treatment

The biggest allure of heterogeneous electro-Fenton (HEF) processes largely fails on its high efficiency for the degradation of a plethora of hazardous compounds present in water, but still challenging to search for good and cost-effective electrocatalyst. In this work, carbon black (CB) and oxidised carbon black (CBox) materials were investigated as cathodes in the electrochemical production of hydrogen peroxide involved in HEF reaction for the degradation of 2-phenylphenol (2PP) as a target pollutant. The electrodes were fabricated by employing carbon cloth as support, and the highest H2O2 production yields were obtained for the CBox, pointing out the beneficial effect of the hydrophilic character of the electrode and oxygen-type functionalization of the carbonaceous surface. HEF degradation of 2PP was explored at -0.7 V vs. Ag/AgCl exhibiting the best conversion rates and degradation grade (total organic carbon) for the CBox-based cathode. In addition, the incorporation of an electrochemical sensor of 2PP in line with the HEF reactor was accomplished by the use of screen-printed electrodes (SPE) in order to monitor the pollutant degradation. The electrochemical sensor performance was evaluated from the oxidation of 2PP in the presence of Fe2+ ions by using square wave voltammetry (SWV) technique. The best electrochemical sensor performance was based on SPE modified with Meldola Blue showing a high sensitivity, low detection limit (0.12 ppm) and wide linear range (0.5-21 ppm) with good reproducibility (RSD 2.3 %). The all-in-one electrochemical station has been successfully tested for the degradation and quantification of 2PP, obtaining good recoveries analysing spiked waters from different water matrices origins.

Electrochemically enhanced iron oxide-modified carbon cathode toward improved heterogeneous electro-Fenton reaction for the degradation of norfloxacin

In this work, different iron-based cathode materials were prepared using two different approaches: a novel one-step approach, which involved the incorporation of iron oxide with Printex® L6 carbon/PTFE (PL6C/PTFE) on bare carbon felt (CF) and a two-step approach, where iron oxide is deposited onto CF previously modified with PL6C/PTFE. The results obtained from the physical characterization indicated that the presence of iron oxide homogeneously dispersed on the felt fibers with the CF 3-D network kept intact in the one-step approach; whereas the formation of iron oxide aggregates between the felt fibers for material obtained using the two-step approach. Among the iron oxide-based cathodes investigated, the iron-incorporated electrode exhibited the greatest efficiency in terms of the removal and mineralization of norfloxacin (NOR) under neutral pH (complete NOR removal in less than 30 min with around 50% mineralization after 90 min). The findings of this study show that the low cost and simple-to-prepare iron-modified carbon-based materials in HEF process led to the enhanced degradation of organic contaminants in aqueous solutions.

Electrochemical transformations catalyzed by cytochrome P450s and peroxidases

Cytochrome P450s (Cyt P450s) and peroxidases are enzymes featuring iron heme cofactors that have wide applicability as biocatalysts in chemical syntheses. Cyt P450s are a family of monooxygenases that oxidize fatty acids, steroids, and xenobiotics, synthesize hormones, and convert drugs and other chemicals to metabolites. Peroxidases are involved in breaking down hydrogen peroxide and can oxidize organic compounds during this process. Both heme-containing enzymes utilize active Fe^IVO intermediates to oxidize reactants. By incorporating these enzymes in stable thin films on electrodes, Cyt P450s and peroxidases can accept electrons from an electrode, albeit by different mechanisms, and catalyze organic transformations in a feasible and cost-effective way. This is an advantageous approach, often called bioelectrocatalysis, compared to their biological pathways in solution that require expensive biochemical reductants such as NADPH or additional enzymes to recycle NADPH for Cyt P450s. Bioelectrocatalysis also serves as an ex situ platform to investigate metabolism of drugs and bio-relevant chemicals. In this paper we review biocatalytic electrochemical reactions using Cyt P450s including C-H activation, S-oxidation, epoxidation, N-hydroxylation, and oxidative N-, and O-dealkylation; as well as reactions catalyzed by peroxidases including synthetically important oxidations of organic compounds. Design aspects of these bioelectrocatalytic reactions are presented and discussed, including enzyme film formation on electrodes, temperature, pH, solvents, and activation of the enzymes. Finally, we discuss challenges and future perspective of these two important bioelectrocatalytic systems.

Recent advances in carbonaceous catalyst design for the in situ production of H2O2 via two-electron oxygen reduction

The electrochemical oxygen reduction reaction has received increasing attention as a relatively green, safe and sustainable method for in situ hydrogen peroxide (H₂O₂) production. Recently, significant achievements have been made to explore carbon-based (noble metal-free) low-cost and efficient electrocatalysts for H₂O₂ electroproduction, which could potentially replace the traditional anthraquinone process. However, to realize industrial-scale implementation, a highly active and selective catalytic material is needed. In this review paper, we first expound on the oxygen reduction reaction (ORR) mechanism, which is the origin of in situ H₂O₂ production. Then, the recent progress in the development of modified carbon-based catalysts is reviewed and classified, corresponding to their physical or chemical modulation. Furthermore, an overview is provided of the available examples from pilot/large-scale applications. Finally, an outlook on the current challenges and future research prospects to transfer the lab-developed catalysts into pilot or industrial-scale reactors is briefly discussed.

A microbubble-assisted rotary tubular titanium cathode for boosting Fenton's reagents in the electro-Fenton process

To improve cathodic H₂O₂ accumulation and Fe³⁺ reduction synchronously in the electro-Fenton (EF) process, a microbubble-assisted rotary tubular titanium cathode (MRTTC) was designed for the first time. By utilizing this MRTTC, H₂O₂ accumulation improved by 4.05-fold, along with a 200% enhancement in iron reduction compared to the conventional EF process. This promotion is mainly attributed to a considerably higher oxygen mass transfer, which reduces the thickness of the adhered diffusion layer. The oxygen mass transfer coefficient (K_La) also improved from 0.0073 s^-1 to 0.012 s^-1 at a rotational speed of 300 rpm. In addition, the microbubble-assisted cathode further improved the K_La to 0.047 s^-1. The synergistic effect between the rotating and microbubble-assisted cathodes further intensified H₂O₂ accumulation in MRTTC. Apart from H₂O₂ promotion, the iron reduction rate was elevated because the newly formed O₂^-• provided an additional reduction pathway for Fe³⁺ reduction in addition to the cathodic path. The effectiveness of MRTTC was confirmed by treating a benchmark organic pollutant, sulfamerazine (SMR), where approximately 100% SMR decay was obtained in 3 h. The results show that MRTTC is a novel and promising design in EF for antibiotic wastewater treatment.

Heterogeneous Electro-Fenton-like Designs for the Disposal of 2-Phenylphenol from Water

The hunt for efficient and environmentally friendly degradation processes has positioned the heterogeneous advanced oxidation processes as an alternative more interesting and economical rather than homogenous processes. Hence, the current study lies in investigating the efficiency of different heterogeneous catalysts using transition metals in order to prevent the generation of iron sludge and to extend the catalogue of possible catalysts to be used in advanced oxidation processes. In this study, nickel and zinc were tested and the ability for radical-generation degradation capacity of both ions as homogeneous was evaluated in the electro-Fenton-like degradation of 2-phenylphenol. In both cases, the degradation profiles followed a first-order kinetic model with the highest degradation rate for nickel (1 mM) with 2-phenylphenol removal level of 90.12% and a total organic reduction near 70% in 2 h. To synthesise the heterogeneous nickel catalyst, this transition metal was fixed on perlite by hydrothermal treatment and in a biochar or carbon nanofibers by adsorption. From the removal results using the three synthesized catalysts, it is concluded that the best catalysts were obtained by inclusion of nickel on biochar or nanofibers achieving in both with removal around 80% before 1 h. Thus, to synthetize a nickel electrocatalyst, nickel doped nanofibers were included on carbon felt. To do this, the amount of carbon black, nickel nanofibers and polytetrafluoroethylene to add on the carbon felt was optimized by Taguchi design. The obtained results revealed that under the optimised conditions, a near-complete removal was achieved after 2 h with high stability of the nickel electrocatalyst that open the applicability of this heterogeneous system to operate in flow systems.

Electro-generation of hydrogen peroxide using a graphite cathode from exhausted batteries: study of influential parameters on electro-Fenton process

In this work, the study of hydrogen peroxide (H₂O₂) electro-generation using graphite from exhausted batteries (Gr-Bat) was conducted. Linear sweep voltammetry and electrolysis experiments were carried out in a single compartment electrochemical cell. Study of the possibility to use this electrode revealed that it presents, as vitreous carbon (VC) electrode, a reduction of oxygen with two successive waves (bi-electronic reduction). The first wave corresponds to the reduction of O₂ to H₂O₂, while the second one corresponds to the reduction of H₂O₂ to H₂O. The cathodic potentials for electro-generation of H₂O₂ appeared at -600 and -700 mV vs. Ag/AgCl for Gr-Bat and VC electrodes, respectively. Subsequently, electrolysis experiments were conducted by imposing the potentials required for H₂O₂ formation. The effect of several operating parameters on H₂O₂ production, such as the nature and concentration of the electrolyte, the pH, the presence of ferrous ions and O₂ injection were studied using Gr-Bat and VC electrodes, respectively. For both electrodes, the acidic medium was more favorable for H₂O₂ electro-generation. The oxygen injection in solution promoted an increase of H₂O₂ concentration, but its effect was more pronounced in the case of VC electrode. Application for crystal violet degradation by electro-Fenton revealed that Gr-Bat had the best purification performance. A removal rate of 73.18% was obtained with Gr-Bat electrode against 62.27% with VC electrode for an electrolysis time of 120 min. This study has demonstrated the possibility of recycling Gr-Bat by using them as cathode materials in the electro-Fenton process.

Template-free microspheres decorated with Cu-Fe-NLDH for catalytic removal of gentamicin in heterogeneous electro-Fenton process

Nano-layered double hydroxide (NLDH) decorated with Fe and Cu was applied as a novel heterogeneous catalyst for catalytic degradation of gentamicin by the electro-Fenton (EF) process. The EF process was equipped with graphite plate under aeration to electrochemically generate hydrogen peroxide in the solution. The characterization analyses confirmed the suitable structure of as-synthesized Cu-Fe-NLDH to be acted as catalyst for treating the target pollutant. The comparative study showed the highest removal efficiency of 91.3% when the Cu-Fe-NLDH-equipped EF process was applied in comparison with the Fenton (50%) and the electro-oxidation alone (25.6%). The acidic pHs favored the degradation of gentamicin. Increasing the current resulted in the enhanced degradation of gentamicin, while the excessive electrolyte concentration (0.1 mol/L) and catalyst dosage (1.5 g/L) led to the tangible drop in the reactor performance. At a specified reaction time, the injection of O₃ gas enhanced the efficiency of the Cu-Fe-NLDH-equipped EF process. The presence of ethanol led to more suppressing effect than benzoquinone, indicating the dominant role of OH radical in the degradation of gentamicin compared with other free radical species such as O₂^- radical. Only 10% drop in the degradation efficiency of gentamicin was observed within 10 operational runs. The mineralization efficiency of about 77% was achieved after 300 min in terms of chemical oxygen demand (COD) removal. The intermediate byproducts generated during the destructive removal of gentamicin were also identified.

Rolling-made gas diffusion electrode with carbon nanotube for electro-Fenton degradation of acetylsalicylic acid

H₂O₂ production plays an important role in electro-Fenton process for pharmaceutical and personal care products (PPCPs) degradation. In this work, carbon nanotube (CNT) was attempted to make a gas diffusion electrode (GDE) by rolling method to achieve a high H₂O₂ production and current efficiency, and it was further used as electro-Fenton cathode for the degradation of acetylsalicylic acid (ASA) as one kind of PPCPs. The optimal amount of catalyst layer was 0.15 g CNT and 93.75 μL PTFE, obtaining the production of H₂O₂ of 805 mg L^-1 in 0.05 mM Na₂SO₄ solution at 100 mA after 180 min. The degradation of ASA by electro-Fenton on such a CNT-GDE cathode was studied, and some important parameters such as current, pH as well as the dosage of Fe²⁺ were optimized. The degradation ratio of ASA could achieve almost 100% after 10 min and the TOC removal ratio was 62% at 1 h under the condition of 100 mA and pH 3, showing a great potential for the treatment of PPCPs.

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.

Decomposition of ibuprofen in water via an electrochemical process with nano-sized carbon black-coated carbon cloth as oxygen-permeable cathode integrated with ultrasound

The main aim of the present investigation was the treatment of ibuprofen (IBP)-polluted aquatic phase using a novel oxygen-permeable cathode (OPC)-equipped electrochemical process (ECP) integrated with ultrasound (US). According to kinetic modeling, the decomposition rate of IBP by the integrated process was 3.2 × 10^-2 min^-1 which was significant in comparison with the OPC-equipped ECP (1.4 × 10^-2 min^-1) and US alone (2.4 × 10^-3 min^-1). Increasing the current resulted in the enhanced generation of H₂O₂ and consequently, improved the degradation of IBP in the solution. Excessive concentrations of Na₂SO₄ as supporting electrolyte led to no significant enhancement in the reactor efficiency. At initial IBP concentration of 1 mg L^-1, complete removal of IBP with reaction rate of 1.7 × 10^-1 min^-1 was happened within a short reaction time of 30 min. The pulse mode of US led to more than 10% increase in the removal efficiency compared with the normal mode. The presence of scavenging compound of methanol caused the highest drop in the efficiency of the integrated treatment process, indicating the substantial role of free hydroxyl radicals in the degradation of IBP. Intermediate byproducts generated in the solution during the decomposition were also identified and interpreted.

Enhanced hydroxyl radical generation in the combined ozonation and electrolysis process using carbon nanotubes containing gas diffusion cathode

Combination of ozone together with electrolysis (ozone-electrolysis) is a promising wastewater treatment technology. This work investigated the potential use of carbon nanotube (CNT)-based gas diffusion cathode (GDC) for ozone-electrolysis process employing hydroxyl radicals (·OH) production as an indicator. Compared with conventional active carbon (AC)-polytetrafluoroethylene (PTFE) and carbon black (CB)-PTFE cathodes, the production of ·OH in the coupled process was improved using CNTs-PTFE GDC. Appropriate addition of acetylene black (AB) and pore-forming agent Na2SO4 could enhance the efficiency of CNTs-PTFE GDC. The optimum GDC composition was obtained by response surface methodology (RSM) analysis and was determined as CNTs 31.2 wt%, PTFE 60.6 wt%, AB 3.5 wt%, and Na2SO4 4.7 wt%. Moreover, the optimized CNT-based GDC exhibited much more effective than traditional Ti and graphite cathodes in Acid Orange 7 (AO7) mineralization and possessed the desirable stability without performance decay after ten times reaction. The comparison tests revealed that peroxone reaction was the main pathway of ·OH production in the present system, and cathodic reduction of ozone could significantly promote ·OH generation. These results suggested that application of CNT-based GDC offers considerable advantages in ozone-electrolysis of organic wastewater.