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Highly efficient electrochemical advanced oxidation processes (EAOPs) based on carbon catalysts are promising and green technologies for environmental remediation. Herein, for the purpose of cost-effectiveness, wide pH suitability and excellent reusability, graphite felt modified with regulatable N-doped graphene was developed as a cathode to electrochemically generate H2O2 with high yield and selectivity, and efficiently catalyze H2O2 to form OH for organic pollutants degradation by in-situ metal-free EAOPs. Particularly, the catalytic mechanism of N-doped graphene for enhanced performance was explored. Optimized N-doped graphene showed a very high H2O2 generation rate of 8.6 mg/h/cm2, low electric energy consumption (9.8 kW h/kg) and high H2O2 selectivity of 78.02% in neutral pH solution. Compared with electro-Fenton (EF), this in-situ metal-free EAOPs on N-doped graphene displayed significant improvement on the degradation performance of organic pollutants in neutral and alkaline solutions, and was certified to be less affected by initial pH. The pyridinic N and C C in N-doped graphene enhanced the onset potential while graphite N determined the disk current of oxygen reduction reaction (ORR) process. Most importantly, it proved that the introduction of graphite N could promote the 2e- ORR process for H2O2 generation, and the presence of pyridinic N could catalyze H2O2 to the production of OH. Taken phenol as target pollutant, OH generated by N-doped graphene accounted for 80.72% while O2− contributed 19.28% to its degradation, based on which a possible mechanism for phenol degradation was proposed. Moreover, in-situ metal-free EAOPs showed excellent stability, reusability and performance for various organic pollutants degradation. This work would shed light on the catalytic mechanism for metal-free EAOPs, and thus promote its application for organic pollutants degradation.

Herein, ZnO/CdS hierarchical composite was prepared through a hydrothermal and chemical bath deposition (CBD) process. Its photocatalytic H2-production performance was tested. Mass ratio of CdS acted a pivotal part in light absorption and photocatalytic properties. Noticeably, promoted photocatalytic H2-production activity of 4134 μmol g−1 h-1 was achieved by the sample with optimal CdS content. Significantly, the photoluminescence (PL) detection of hydroxyl radicals, as well as the in-situ XPS measurements was selected to verify the direct Z-scheme charge migration mechanism. This mechanism endowed the composite with strong capability for hydrogen evolution and elucidated the improved photocatalytic performance. The improvement of photocatalytic activity was due to hierarchical structure, extended visible light response and direct Z-scheme mechanism. This work will give an innovative vision in constructing direct Z-scheme photocatalytic system with great photocatalytic H2-production activity.

This exhaustive review focuses on the fundamental principles and applications of heterogeneous electrochemical wastewater treatment based on Fenton’s chemistry reaction. The elementary equations involved in formation of hydroxyl radical in homogeneous electro-Fenton (EF) and photo electro-Fenton (PEF) processes was presented and the advantages of using insoluble solids as heterogeneous catalyst rather than soluble iron salts (heterogeneous electro-Fenton process) (Hetero-EF) was enumerated. Some of the required features of good heterogeneous catalysts were discussed, followed by the mechanisms of catalytic activation of H2O2 to reactive oxygen species (ROS) especially hydroxyl radical ( OH) by heterogeneous catalyst in Hetero-EF system. Extensive discussion on the two configuration of Hetero-EF system vis-a-vis added solid catalysts and functionalized cathodic materials were provided along with summaries of some relevant studies that are available in literature. The solid catalysts and the functionalized cathodic materials that have been utilized in Hetero-EF wastewater treatment were grouped into different classes and brief discussion on their synthesis route were given. Besides, the use of solid catalysts and iron-functionalized cathodic materials in bioelectrochemical system (BES) especially bioelectro-Fenton technology (BEF) using microbial fuel cells (MFCs) with concurrent electricity generation for Hetero-EF treatment of biorefractory organic pollutants was discussed. In the final part, emphasis was made on the challenges and future prospects of the Hetero-EF for wastewater treatment.

The implementation of nanostructured materials in electrochemistry implied the enhancement of conversion yield in fuel cell, in electrosynthesis of oxidants and electrolytic treatment for environmental protection, while it allowed reducing the detection limit in electroanalysis. Nanostructured materials are becoming a hot topic of research, especially in electrochemical treatment for environmental applications that is strongly related to the rise of graphene and subsequent 2D materials that emerged in the last ten years. Nano-structuration allows bringing new properties of the materials such as number of active sites and conductivity improvement. It can therefore enhance the heterogeneous catalysis mechanism at electrode surface. This is primordial since it makes increase the rate of electrochemical reactions that can be the rate limiting steps in electrocatalytic treatment. Such advanced materials contribute to make advanced electrochemical processes as “greener” processes than the conventional ones. This paper aims to be a comprehensive, critical, and accessible review of general interest. The literature covers mainly the last ten years’ period due to the recent topic, especially the last five years with the considerable increase of number of publications in this period. The contents particularly devote efforts to establish links between the nanostructured-based electrode properties and electrochemical treatment efficiency through the mechanisms involved. The perspectives about mechanisms understanding and electrodes stability improvement are especially discussed.

Catalytic oxidation of formaldehyde (HCHO) at room temperature is one of the most viable approaches to indoor HCHO pollution abatement. Herein, three-dimensional (3D) carbon foam decorated with Pt/MnO2 nanosheets (Pt/MnO2-CF) was in-situ synthesized for room-temperature catalytic oxidation of HCHO. The 3D Pt/MnO2-CF with a low platinum content of 0.3 wt% exhibited excellent catalytic activity because of its hierarchically porous structure facilitating the diffusion of reactant molecules. Moreover, an abundance of active oxygen species resulting from oxygen vacancies are favorable for HCHO oxidation. In addition, the carbon foam substrate exhibited a very good HCHO adsorption capability, which helps achieve prompt reduction in HCHO concentration in the gas-phase and subsequent complete oxidation of adsorbed HCHO. The combination of adsorption and oxidation was more favorable for oxidative decomposition of HCHO. This work demonstrates that such 3D nanocomposites with low noble metal loading have promising application for indoor HCHO removal and air purification.

A novel sequential electrochemical treatment consisting in an electrocoagulation (EC) pre-treatment and the subsequent advanced oxidation by photoelectro-Fenton (PEF) process with in-situ H 2 O 2 electrogeneration and UVA light irradiation has been envisaged for the removal of organic pollutants from water, showing a high performance for the decolorization and mineralization of Tartrazine (Acid Yellow 23) solutions. EC has a dual purpose as a coagulation and catalyst source step since it allows the formation of Fe(OH) n ( n = 2 or 3) coagulant and Fe 3+ /Fe 2+ ions. The influence of the anode material (Fe and Al), supporting electrolyte, pH and current during the individual EC on the abatement of color and total organic carbon (TOC) has been assessed. EC with Fe in 0.05 M NaCl yielded the best results. Next, various single electrochemical advanced oxidation processes (EAOPs) such as electro-oxidation (EO), electro-Fenton (EF) and PEF with a Pt or boron-doped diamond (BDD) anode and an air-diffusion cathode (ADE) have been tested. PEF with BDD/ADE yielded the quickest degradation among all the EAOPs in NaCl, due to the action of oxidants like active chlorine as well as • OH formed at the anode surface from H 2 O oxidation and in the bulk from UV-enhanced Fenton's reaction between cathodic H 2 O 2 and added Fe 2+ . Therefore, the two-step EC (Fe/steel)/PEF (BDD/ADE) process was the best EC/EAOP combination. The EC treatment in 0.05 M NaCl at natural pH 6.3 and 200 mA, followed by PEF treatment of the supernatant at pH 3.0 and 200 mA, yielded the best conditions with a total decolorization in 15 min of EC and total mineralization in ca. 300 min of PEF. GC–MS analysis showed the formation of several benzenic by-products during the application of EC/PEF. Independent electrolyses revealed the ability of EC to accumulate soluble chlorobenzene derivatives, which can be completely destroyed in the PEF step under the action of the mixture of oxidants, particularly by successive hydroxylation via • OH largely promoted in the bulk by the Fe 2+ ions generated in EC.

Efficient anode materials are essential to electrochemical advanced oxidation processes (EAOPs) for organic wastewater treatment. In this regard, blue TiO2 nanotube arrays (Blue-TNA) anode was prepared for the first time in formic acid electrolyte by electrochemical self-doping and applied for electrochemical degradation of contaminants. Characterized by XPS, Raman and Mott-Schottky curves, the formation of Ti3+ on Blue-TNA was confirmed. This anode was more efficient and had a higher hydroxyl radical production activity (1.7 × 10−14 M) than BDD (9.8 × 10-15 M), inducing a higher TOC and COD removal of 100 mg/L phenol with a lower energy consumption of 9.9 kW h/(kg COD) at current density 2.5 mA/cm2, pH 5 in 0.1 M Na2SO4, account for the lower accumulation of degradation intermediates. Both •OH and SO4•- were responsible for the degradation on Blue-TNA anode, while their contributions differed greatly with that of BDD, and could be affected and regulated by the operating parameters like current density, initial pH and Na2SO4 concentration. Blue-TNA anode represented a relative stable performance for 5 cycles degradation of 100 mg/L phenol for each cycle of 300 min, and such an oxidation capacity could be easily regenerated by electrochemical reduction in formic acid. Blue-TNA anode had an excellent performance on the TOC removal and MCE especially at low current density of 2.5 mA/cm2 when compared with other anodes. Therefore, Blue-TNA anode is hopeful a promising and cost-effective anode for electrochemical oxidation.

A nickel-foam cathode modified by a self-nitrogen-doped biochar derived from waste giant reed was synthesized. The fabricated cathode (B@Ni-F) proved to be with high oxygen reaction reactive (ORR) reactivity and H2O2 selectivity (70.41%) owing to the enrichment of oxygen functional groups and pyridinic N when low-temperature pyrolyzed biochar was incorporated. The charge transfer resistance of B@Ni-F decreased to 7.18 Ω, which was 95.7 Ω for the original nickel-foam, proving by electrochemical impedance spectroscopy (EIS). Expectedly, Its H2O2 accumulation improved 14 times, thus making it comparable with commonly used electrodes like carbon cloth and graphite plate. Subsequently, B@Ni-F cathode and iron-foam (Fe-F) catalyst were firstly used in the electro-Fenton (EF) process for sulfamerazine (SMR) degradation. Double-functional polyphosphate electrolytes including tetrapolyphosphate (4-TPP), tripolyphosphate (3-TPP), pyrophosphate (PP) and Na3PO4 were compared with the conventional Na2SO4 electrolyte in EF for SMR degradation. The absolute rate constant for oxidation of SMR by OH was determined to be (3.4 ± 0.09) × 109 M−1 s−1. SMR degradation enhancement in the presence of polyphosphate-based electrolytes is associated with bulk OH generation from Fe2+- polyphosphate ligand complexes via O2 activation. The Fe2+-3-TPP complexes have relatively higher oxidation ability compared to Fe2+-PP, Fe2+-PO4 species. A plausible SMR oxidation pathway is proposed based on the by-products detected by UPLC-MS/MS and density functional theory (DFT) calculations. The dominant SMR degradation pathway was hydroxylation of aniline residue of SMR, followed with the cleavage of S N and then breakage of aromatic rings.

Hydrogen is considered an ideal alternative energy source to replace fossil energy. Herein, a novel graphdiyne (GD)/graphitic carbon nitride (g-C3N4) nanocomposite was successfully synthesized via a facile calcination approach, and display excellent H2-generation performance under visible light. When the mass ratio of GD reaches 0.5 wt% in GD/g-C3N4 nanocomposite, it shows a maximum hydrogen evolution rate, exceeding that of g-C3N4 by 6.7-fold. After systematic characterization, a new C N bond is confirmed to form between GD and g-C3N4 following heat treatment, and this bond serves as a charge carrier channel that facilitates the migration of photogenerated electrons from g-C3N4 to GD. Positive effects, such as a prolonged photogenerated charge carrier lifetime, intensified electron density, decreased reaction overpotential and improved charge carrier mobility, also contribute to the enhanced photocatalytic performance of the nanocomposites. The proposed technique provides a promising approach for modifying photocatalysts in future applications.

Fe-doped TiO 2 (Fe-TiO 2 ) nanorods were prepared by an impregnating-calcination method using the hydrothermally prepared titanate nanotubes as precursors and Fe(NO 3 ) 3 as dopant. The as-prepared samples were characterized by scanning electron microscope, transmission electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy, N 2 adsorption–desorption isotherms and UV–vis spectroscopy. The photocatalytic activity was evaluated by the photocatalytic oxidation of acetone in air under visible-light irradiation. The results show that Fe-doping greatly enhance the visible-light photocatalytic activity of mesoporous TiO 2 nanorods, and when the atomic ratio of Fe/Ti ( R Fe ) is in the range of 0.1–1.0%, the photocatalytic activity of the samples is higher than that of Degussa P25 and pure TiO 2 nanorods. At R Fe = 0.5%, the photocatalytic activity of Fe-TiO 2 nanorods exceeds that of Degussa P25 by a factor of more than two times. This is ascribed to the fact that the one-dimensional nanostructure can enhance the transfer and transport of charge carrier, the Fe-doping induces the shift of the absorption edge into the visible-light range with the narrowing of the band gap and reduces the recombination of photo-generated electrons and holes. Furthermore, the first-principle density functional theory (DFT) calculation further confirms the red shift of absorption edges and the narrowing of band gap of Fe-TiO 2 nanorods.

Mesoporous titania nanospheres with high specific surface area and good photocatalytic activity were fabricated on a large scale by a simple spray-hydrolytic method at 90 °C. The as-prepared TiO2 spherical powders were characterized by X-ray diffraction, N2 adsorption–desorption measurements, field emission scanning electron microscope, transmission electron microscopy, UV–visible diffuse reflectance spectra and photocurrent measurements. The photocatalytic activity was evaluated by photocatalytic decomposition of toluene in air. The effects of calcination temperatures on the microstructures and photocatalytic activity of the TiO2 powders were investigated and discussed. The results revealed that a large amount of mesoporous titania nanospheres could spontaneously form by self-assembly of countless tiny TiO2 primary nanoparticle during the spray hydrolysis at 90 °C. The calcination temperature exhibited a strong effect on the microstructures and photocatalytic activity of the prepared titania. The 400 °C-calcined sample showed the highest photocatalytic activity and was about two times higher than that of Degussa P25. At 600 °C, the photocatalytic activity decreased because of the destruction of bimodal mesoporous structure of the titania and the drastic decrease of specific surface areas.

Bimodal nanocrystalline mesoporous TiO2 powders with high photocatalytic activity were prepared by a hydrothermal method using tetrabutylorthotitanate (TiO(C4H9)4, TBOT) as precursor. The as-prepared TiO2 powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and N2 adsorption–desorption measurements. The photocatalytic activity of the as-prepared TiO2 powders was evaluated by the photocatalytic degradation of acetone (CH3COCH3) under UV-light irradiation at room temperature in air. The effects of hydrothermal temperature and time on the microstructures and photocatalytic activity of the TiO2 powders were investigated and discussed. It was found that hydrothermal treatment enhanced the phase transformation of the TiO2 powders from amorphous to anatase and crystallization of anatase. All TiO2 powders after hydrothermal treatment showed bimodal pore-size distributions in the mesoporous region: one was intra-aggregated pores with maximum pore diameters of ca. 4–8 nm and the other with inter-aggregated pores with maximum pore diameters of ca. 45–50 nm. With increasing hydrothermal temperature and time, the average crystallite size and average pore size increased, in contrast, the Brunauer-Emmett-Teller (BET) specific surface areas, pore volumes and porosity steadily decreased. An optimal hydrothermal condition (180 °C for 10 h) was determined. The photocatalytic activity of the prepared TiO2 powders under optimal hydrothermal conditions was more than three times higher than that of Degussa P25.