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Recently, the electrochemical treatment of wastewater polluted by recalcitrant organics and containing chlorides was shown to be significantly improved adopting proper operative conditions such as suitable anodes and cathodes and low current density. In particular, the use of suitable cathodes such as carbon felt or silver could increase the removal of TOC and reduce the formation of chlorinated by‐products. In this study, attention was focused on the utilization of carbon felt cathodes with the main aim to evaluate the effect of air pressure on the process using phenol as model organic pollutant. It was shown that, in the presence of carbon felt cathode, the use of pressurized air allows both to increase the removal of TOC and to reduce the formation of chlorate. Moreover, the effect of the nature of the anode and of the presence of FeSO4, in order to study the involvement of electro‐Fenton process, was evaluated. [ABSTRACT FROM AUTHOR]

Highlights • The electrogeneration of H 2 O 2 in undivided cell was studied. • The generation of H 2 O 2 dramatically depends on the ration between cathode and anode surfaces. • The nature of the anode dramatically affect the final concentration of H 2 O 2. • The generation of H 2 O 2 depends on current density value and mixing rate. Abstract In the last years, the electrochemical conversion of oxygen to hydrogen peroxide at carbon felt has been largely studied in order to define a new route for the production of H 2 O 2 and to optimize the electro-Fenton process, which is based on the cathodic generation of H 2 O 2. In particular, many studies regarding electro-Fenton process were carried out in undivided cells in order to avoid the costs of the separator and to reduce the cell potentials. Hence, in order to optimize the cathodic conversion of oxygen to H 2 O 2 in undivided cells, the effect of many parameters linked to the anodic process were here evaluated. In particular, it was demonstrated that the performances of the process strongly depends on the ratio between the cathode and the anode surfaces, the nature of the anode, the mixing rate and the current density. [ABSTRACT FROM AUTHOR]

In the last years, large attention has been devoted to the electrochemical treatment of wastewater polluted by recalcitrant organics and containing chlorides. However, the utilization on an applicative scale of this route is limited by the fact that usually it is not possible to achieve high removals of organics without the generation of significant amounts of chlorinated by-products. Here, the role of several operative parameters on the performances of the process was systematically evaluated using phenol as a model organic pollutant. It was shown that the removal of phenol and TOC and the generation of many by-products including chlorophenols, chloroacetic acids, chlorate and perchlorate dramatically depend on the adopted operative conditions (i.e., nature of anode and cathode, current density and concentration of chlorides). The use of boron doped diamond (BDD) anode gave the highest removal of TOC but also the highest final concentrations of chlorate and perchlorate and the lowest concentrations of chloroacetic acids. Conversely, the use of Ti/IrO 2 -Ta 2 O 5 gave the lowest generation of chlorate and no perchlorate. The use of lower current densities allowed as well to reduce the concentrations of these compounds. It is worth to mention that lower amounts of chlorinated by-products were achieved using suitable cathodes. Carbon felt cathodes, as previously reported for the water disinfection, allowed to reduce the generation of chlorate. Moreover, it was shown, for the first time, that the use of silver cathodes minimized the concentrations of chloroacetic acids. In addition, it was found that the use of a system equipped with both silver and carbon felt cathodes and proper current densities allowed to increase remarkably the removals of TOC and the current efficiency and to achieve very low concentrations of chlorinated by-products. [ABSTRACT FROM AUTHOR]

[Display omitted] • The first study to apply NPC and CB to fabricate a novel rolled carbonaceous GDE. • Application of NPC/CB cathode in heterogeneous EF with metal sulfides as co-catalysts, the removal of the tetracycline reach 100%. • The study showed metal sulfides can accelerate the conversion of Fe3+ to Fe2+ by exposing reducible metal active sites. This work utilizes nitrogen-doped porous carbon (NPC) and carbon black (CB) to fabricate a novel rolled carbonaceous gas-diffusion cathode by polytetrafluoroethylene (PTFE) as adhesive, which greatly improves the production of H 2 O 2 to 7.27 mg h−1 cm−2 by optimization in near neutral pH condition. Further application of NPC/CB cathode in heterogeneous electro-Fenton (EF) experiments with synthetic pyrite (FeS 2) as catalyst, metal sulfides (MoS 2 , WS 2) as co-catalysts, the removal efficiency of the tetracycline (50 mg L−1) could reach 100% and the total organic carbon (TOC) removal efficiency of counterpart could reach 81.3%, respectively. Because of the FeS 2 produces H+ and Fe2+ in the presence of H 2 O 2 , which provides pH value and catalytic ion that suitable for EF. Meanwhile, the metal sulfides can accelerate the conversion of Fe3+ to Fe2+ by exposing reducible metal active sites on the surface of the catalysts, thus drastically increased the efficiency of H 2 O 2 decomposition and pollutant degradation, also optimized the disadvantage of little Fe2+ leaching from FeS 2 in heterogeneous EF. Further tests of this system indicated its potential for the treatment of tannery effluents. It provided an effective and feasible scheme for promoting the improvement of the heterogeneous EF system and the development of practical application. [ABSTRACT FROM AUTHOR]

• H 2 O 2 production with 0.15 g-ZIF-8/ACF as cathode was much larger than with ACF. • H 2 O 2 production could achieve high state of H 2 O 2 production in consecutive 5 runs. • Tetracycline could be completely removed within 20 min in the EF system. • EF system with ZIF-8/ACF cathode is a good alternative process for pollutant removal. In this work, the active carbon fibers (ACFs) was modified by zeolitic imidazolate framework-8 (ZIF-8) to prepare a novel modified electrode as cathode. The effect of different amount of ZIF-8 modified ACFs (0–0.30 g) on H 2 O 2 production and electric energy consumption (EEC) were investigated and the optimal ZIF-8 amount was 0.15 g. Characterized by linear sweeping voltammetry (LSV), electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM), contact angle, and X-ray photoelectron spectroscopy (XPS) after modification were observed considerably changed. Some nanoparticles and oxygen-contained functional groups appeared on the ZIF-8/ACF cathode surface, which significantly enhanced the hydrophilic property and the electrochemical performance. It was observed that H 2 O 2 production with 0.15 g-ZIF-8/ACF as cathode (1545.1 mg L−1) was much larger than with raw ACF as cathode (64.1 mg L−1). Meanwhile, H 2 O 2 production could also achieve a balance in a high state of H 2 O 2 production (around 1500 mg L−1) with low EEC (<35.0 kWh kg−1) in consecutive 5 runs. Moreover, 50 mg L−1 tetracycline could be completely removed within 20 min in the electro-Fenton (EF) system with 0.15 g-ZIF-8/ACF as cathode and the TOC removal could achieve 82.6% which was also much higher with raw ACF as cathode (15.5%). According to the ESR tests also indicating much more OH radicals could be generated in the EF system with 0.15 g-ZIF-8/ACF. Therefore, EF system with ZIF-8/ACF cathode is a good alternative process for pollutant removal. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Mild tartaric acid can be used as both leaching and precipitating reagent. • About 98% Co and 97% Li can be simultaneously recovered. • Leaching of Co and Li fits well to logarithmic rate law model. • Cobalt tartrate with purity of 96.4% can be directly recovered. • This process can be a short-cut and sustainable alternative for Co and Li recovery. Abstract Sustainable recovery of metals from spent lithium-ion batteries (LIBs) may be of great significance regarding the conservation of metal resources and alleviation of potential risk towards eco-system. Herein an environmentally benign process was proposed for the recovery of high value-added metals (Co and Li) from typical waste cathode materials (LiCoO 2) of spent LIBs using mild tartaric acid as both leaching and precipitating reagent. Leaching results indicate that Co and Li can be effectively separated under experimental conditions of reaction temperature- 80 °C, retention time- 30 min, pulp density- 30 mL/g, reducatant dosage- 3 vol% H 2 O 2 and acid concentration- 0.6 mol/L. After leaching, 98% Co and 97% Li can be recovered as precipitate and Li+ enriched solution, respectively. And the leaching of Co and Li fits well to logarithmic rate model, with apparent activation energy of 34.5 and 29.7 KJ/mol, respectively. In addition, characterization results (i.e. FT-IR, SEM-EDS and TGA-DSC) suggest that Co can be directly recovered as relatively pure cobalt tartrate (C 4 H 4 O 6 Co, with a purity of 96.4%) in leaching residues. This whole process can be a sustainable alternative for the simultaneous recovery of Li and Co from waste cathode materials of spent LIBs. [ABSTRACT FROM AUTHOR]

• Stoichiometric ratios of acids are used for Li and transition metals leaching. • Dilute tartaric acid shows high selectivity towards Li. • Transition metals can be completely dissolved in dilute sulfuric acid. • NCM cathode materials can be re-fabricated from the obtained products. • A closed-loop process with high selectivity and efficiency is established. Sustainable recycling value-added metals from spent lithium-ion batteries (LIBs) has been supposed to be a promising alternative to alleviate the current environmental and resource issues. Reduced reagents consumption and closed-loop reutilization are still challenging in current prevailing recycling processes. This study proposed a novel recycling strategy involved with gradient extraction of valuable metals and closed-loop re-fabrication of cathode materials. Lithium was selectively recovered as lithium enriched lixivium in mild tartaric acidic medium with a high yield of 99.7 % and little co-extraction of transition metals (Ni, Co and Mn) under optimized leaching conditions. Then transition metals enriched residues can be completely dissolved in facile sulfuric acidic medium without the contamination of Li. Li 2 CO 3 and ternary precursors were recovered from Li enriched lixivium and transition metal enriched lixivium, respectively. Finally, cathode materials of LiNi 1/3 Co 1/3 Mn 1/3 O 2 are refabricated using obtained products to close the recycling loop. It can be concluded that it is possible for the gradient recycling of Li and transition metals based on their inherent properties with minimized consumption of acids under facile leaching conditions, which can also facilitate metals separation process for closed-looped re-fabrication of new cathode materials. [ABSTRACT FROM AUTHOR]