Accelerated Fe2+ Regeneration in an Effective Electro-Fenton Process by Boosting Internal Electron Transfer to a Nitrogen-Conjugated Fe(III) Complex

The regeneration rate of Fe²⁺ from Fe³⁺ dictates the performance of the electro-Fenton (EF) process, represented by the amount of produced hydroxyl radicals (·OH). Current strategies for the acceleration of Fe²⁺ regeneration normally require additional chemical reagents, to vary the redox potential of Fe²⁺/Fe³⁺. Here, we report an attempt at using the intrinsic property of the electrode to our advantage, i.e., nitrogen-doped carbon aerogel (NDCA), as a reducing agent for the regeneration of Fe²⁺ without using foreign reagents. Moreover, the pyrrolic N in NDCA provides unpaired electrons through the carbon framework to reduce Fe³⁺, while the graphitic and pyridinic N coordinate with Fe³⁺ to form a C-O-Fe-N₂ bond, facilitating electron transfer from both the external circuit and pyrrolic N to Fe³⁺. Our Fe²⁺/NDCA-EF system exhibits a 5.8 ± 0.3 times higher performance, in terms of the amount of generated ·OH, than a traditional Fenton system using the same Fe²⁺ concentration. In the subsequent reaction, the Fe²⁺/NDCA-EF system demonstrates 100.0% removal of dimethyl phthalate, 3-chlorophenol, bisphenol A, and sulfamethoxazole with a low specific energy consumption of 0.17-0.36 kW·h·g^-1. Furthermore, 90.1 ± 0.6% removal of dissolved organic carbon and 83.3 ± 0.9% removal of NH₃-N are achieved in the treatment of domestic sewage. The purpose of this work is to present a novel strategy for the regeneration of Fe²⁺ in the EF process and also to elucidate the role of different N species of the carbonaceous electrode in contributing to the redox cycle of Fe²⁺/Fe³⁺.

Carbonaceous cathode materials for electro-Fenton technology: Mechanism, kinetics, recent advances, opportunities and challenges

Electro-Fenton (EF) technique has gained significant attention in recent years owing to its high efficiency and environmental compatibility for the degradation of organic pollutants and contaminants of emerging concern (CECs). The efficiency of an EF reaction relies primarily on the formation of hydrogen peroxide (H₂O₂) via 2e^─ oxygen reduction reaction (ORR) and the generation of hydroxyl radicals (^●OH). This could be achieved through an efficient cathode material which operates over a wide pH range (pH 3-9). Herein, the current progresses on the advancements of carbonaceous cathode materials for EF reactions are comprehensively reviewed. The insights of various materials such as, activated carbon fibres (ACFs), carbon/graphite felt (CF/GF), carbon nanotubes (CNTs), graphene, carbon aerogels (CAs), ordered mesoporous carbon (OMCs), etc. are discussed inclusively. Transition metals and hetero atoms were used as dopants to enhance the efficiency of homogeneous and heterogeneous EF reactions. Iron-functionalized cathodes widened the working pH window (pH 1-9) and limited the energy consumption. The mechanism, reactor configuration, and kinetic models, are explained. Techno economic analysis of the EF reaction revealed that the anode and the raw materials contributed significantly to the overall cost. It is concluded that most reactions follow pseudo-first order kinetics and rotating cathodes provide the best H₂O₂ production efficiency in lab scale. The challenges, future prospects and commercialization of EF reaction for wastewater treatment are also discussed.

Highly stable binder free CNTs/rGO aerogel electrode for decolouration of methylene blue & palm oil mill effluent via electro-Fenton oxidation process

In this study, single wall carbon nanotubes (CNTs)/reduced graphene oxides (rGO) aerogels were prepared by a one-pot hydrothermal process without using a binder.

High oxygen reduction reaction performance nitrogen-doped biochar cathode: A strategy for comprehensive utilizing nitrogen and carbon in water hyacinth

In this study, a novel nitrogen-doped biochar oxygen reduction reaction cathode-water hyacinth carbon, was prepared by ZnCl₂ molten salt carbonization without additional nitrogen source, which displayed a high performance in electro-Fenton (E-Fenton) process. The BET result shows that water hyacinth carbon achieved a much larger specific surface area (829 m²·g^-1) than non-melt salt carbonized one (323 m²·g^-1) and graphite powder (28 m²·g^-1). Furthermore, characterization by XPS and EIS shows that both pyridinic-N (43.24%) and graphitic-N (56.75%) existed in water hyacinth carbon and Warburg constant was only 0.051. Because of a high H₂O₂ producing yield 1.7 mmol·L^-1 and corresponding current efficiency 81.2 ± 2.5% in molten salt carbonized water hyacinth biochar, a high kinetic constant 0.318 min^-1 in DMP degradation was achieved, which was 4 times higher than graphite powder (0.076 min^-1). The TOC removal achieved 86.8% in 30 min and the corresponding energy consumption reached a low level 60.15 kW·h·kgTOC^-1.

Redox cycling of iron by carbon dot enhanced chemiluminescence: mechanism of electron-hole induction in carbon dot

The chemiluminescence (CL) of the Fenton system with nitrogen doped carbon dots (N-CDs) was significantly enhanced. The introduction of N-CDs improved the utilization of H₂O₂ and drastically enhanced the generation of ˙OH, which resulted in enhanced CL emission of the Fenton system through energy and electron transfer processes. The oxidation of N-CDs by ˙OH led to rapid incorporation of oxygen into N-CDs. The mechanism relied on the production of ^•OH radicals through the Fenton reaction and clearly indicated the important role of peroxide-induced redox cycling of Fe²⁺ ⇔ Fe³⁺ in the presence of N-CDs. The CL intensity of the system containing Fe²⁺ was higher than that containing Fe³⁺ because the rate of the Fe³⁺ reaction was much slower than that of the Fe²⁺ reaction. The CL signal remained constant after some time due to redox cycling, which established equilibrium, irrespective of the form of iron. This study provides a feasible approach to greatly enhance the weak CL of the Fenton system with the introduction of environmentally friendly N-CDs, and initiates an inspiring research in the domain of catalysis, CL and the mechanism of the Fenton system, which will be helpful in various applied research areas.