Duet Fe3C and FeNx Sites for H2O2 Generation and Activation toward Enhanced Electro-Fenton Performance in Wastewater Treatment

In situ anchoring strategy to enhance dual nonradical degradation of sulfamethoxazole with high loading manganese doped carbon nitride

A low-cost catalyst with high metal loading and unique catalytic activities is highly desired for peroxymonosulfate (PMS) activation in environmental remediation. Herein, in situ anchoring strategy using 1,10-phenanthroline is reported to construct manganese doped carbon nitride (PMCN) with 8.2 wt% manganese loading and dramatically enhanced PMS adsorption and sulfamethoxazole (SMX) removal efficiency. Our study revealed that the PMCN/PMS system readily reacted with contaminants with electron-rich groups, where complete degradation of sulfamethoxazole (SMX) was achieved within 60 min. Combining quenching experiments, EPR tests, and electrochemical analysis, we proposed a dual nonradical pathway dominated by high-valent manganese oxygen species (Mn(V) = O) and electron transfer. Systematic investigation elucidated that the introduction of 1,10-phenanthroline constructed denser catalyst active sites, and identified the manganese center and pyridine nitrogen as the active sites for PMS activation. Furthermore, PMCN exhibited excellent pH anti-interference ability and good reusability, achieving more than 90% SMX degradation efficiency after four cycles. This study provides new insights into the regulation of Mn-N active sites and promotes the mechanistic understanding of the synergistic effect of manganese and pyridine nitrogen in PMS activation.

Cu/Fe oxide integrated on graphite felt for degradation of sulfamethoxazole in the heterogeneous electro-Fenton process under near-neutral conditions

In the heterogeneous electro-Fenton (EF) system, high-efficiency and durable materials have attracted widespread attention as cathodes for degradation of refractory organic pollutants. In this study, a stable Cu/Fe oxide modified graphite felt electrode (Cu_0.33Fe_0.67NBDC-300/GF) was fabricated via a one-step hydrothermal method and subsequent thermal treatment, which used a bimetallic metal-organic framework (MOF) with 2-aminoterephthalic acid (NH₂BDC) ligand as the precursor. The Cu_0.33Fe_0.67NBDC-300/GF electrode was used as the cathode for sulfamethoxazole (SMX) degradation in the heterogeneous EF process. The coexistence of the Fe^II/Fe^III and Cu^I/Cu^II redox couples significantly accelerates the regeneration of Fe^II and promotes the generation of active free radicals (•OH and •O₂^-). Fe^IV was detected during the process, which indicates that the high-valent iron-oxo species was produced in near-neutral pH conditions. The removal efficiency of SMX (10 mg L^-1) can reach 100.0% within 75 min over a wide pH range (4.0-9.0). After five cycles, the electrode retained a high stability and an outstanding catalytic capacity. Furthermore, the mechanisms and pathways for SMX degradation were proposed, the products and intermediates of SMX were analyzed, and the toxicity was evaluated. It was found that the toxicity decreased after degradation. This study displays a novel strategy for building an efficient and stable self-supporting electrode for treating antibiotic wastewater.

Trimetallic carbon-based catalysts derived from metal-organic frameworks for electro-Fenton removal of aqueous pesticides

Pesticide overuse has posed a threat to agricultural community as well as aquatic animals. Heterogeneous electro-Fenton (HEF) processes have received considerable attention for aqueous contaminants removal, and metal-organic frameworks (MOFs) serve as promising templates for fabrication of carbon-based HEF catalysts with low Fe leaching and enhanced stability. Herein, multimetallic MOF-derived HEF catalysts CMOFs@PCM have been demonstrated as efficient and stable HEF catalysts for aqueous pesticide degradation and mineralization. The porous carbon monolith (PCM) substrate effectively catalyzed 2-electron oxygen reduction reaction (ORR) over the pH range of 4-10 to in situ generate H₂O₂, which was then activated by the anchored Fe₃O₄, Fe₃C and NiO into OH for pesticide degradation. Fe₈Al₇Ni₅-CMOF@PCM achieved over 90% napropamide degradation within 60 min in the pH range of 4-10, and 96% degradation at neutral condition, 39% higher than monometallic CMIL-88(Fe)@PCM. Meanwhile, the embedded NiO and γ-Al₂O₃ showed synergistic effect in promoting the catalytic activity of Fe sites, resulting in substantially enhanced performance of trimetallic Fe_xAl_yNi_z-CMOF@PCM compared to the monometallic counterparts. On the other hand, the unique core-shell structure and Fe₃C interlayer formed by co-pyrolyzing Fe-containing MOFs-NH₂ with PCM greatly minimized the metal leaching and enhanced the stability of the electrocatalysts.

Iron-based materials for simultaneous removal of heavy metal(loid)s and emerging organic contaminants from the aquatic environment: Recent advances and perspectives

The existence of heavy metals and emerging organic contaminants in wastewater produces serious toxic residues to the environment. Developing cheap and efficient materials to remove these persistent pollutants is crucial. Iron-based materials are cost-effective and environmentally friendly catalysts, and their applications in the environmental field deserve attention. This paper critically reviewed the removal mechanisms of heavy metals and emerging organic pollutants by different influencing factors. The removal of pollutants (heavy metals and emerging organic pollutants) in a multi-component system was analyzed in detail. The mechanisms of synergism, antagonism and non-interference were discussed. This paper had a certain reference value for the research of wastewater remediation technology which could simultaneously remove various pollutants by iron-based materials.

Fe-N-C catalyst with Fe-NX sites anchored nano carboncubes derived from Fe-Zn-MOFs activate peroxymonosulfate for high-effective degradation of ciprofloxacin: Thermal activation and catalytic mechanism

Developing high-efficient catalysts is crucial for activating peroxymonosulfate (PMS). Fe-N-C catalysts exhibit excellent performance for PMS activation because of the contribution of doped N, Fe-Nx and Fe₃C sites. In our work, a series of Fe-N-C catalysts with high-performance was obtained by pyrolyzing Fe-Zn-MOFs precursors. During pyrolysis process, the change of chemical bonds and formation of active sites in the precursor were elucidated by characterization analysis and related catalytic experiments. Graphitic N, Fe-Nx and Fe₃C were confirmed to activate PMS synergistically for ciprofloxacin (CIP) degradation. Besides, the catalytic performance was proportional to the amount of doped iron and calcination temperature. Moreover, the Fe-N-C-3-800/PMS system not only displayed good recycling performance, but also had high anti-interference ability. Integrated with quenching and electron paramagnetic resonance (EPR) experiments, a non-radical pathway dominated by ¹O₂ was proposed. Furthermore, PMS could bond to Fe-N-C-3-800 to form intermediate for charge transfer, thus accelerate electron transfer between CIP and PMS to realize degradation of CIP. Six main pathways of CIP degradation were proposed, which include bond fission of N-C on piperazine ring and direct oxidation of CIP. This study provided a new idea for the design of heterogeneous carbon catalysts in advanced oxidation field.

Photo-Fenton degradation of carbamazepine and ibuprofen by iron-based metal-organic framework under alkaline condition

Metal-organic frameworks have been widely used as photocatalytic materials. In this paper, a novel photocatalyst HSO₃-MIL-53(Fe) with acidity regulating groups was successfully synthesized by the solvothermal method and applied to remove carbamazepine (CBZ) and ibuprofen (IBP). The photodegradation efficiency of vis/H₂O₂/HSO₃-MIL-53(Fe) can reach 100% when the pH value is 8 or 9. The free radical capture experiment and electron paramagnetic resonance analysis proved that hole (h⁺), hydroxide radical (·OH), singlet oxygen (¹O₂), and superoxide Radical (·O₂^-) are the main active species for pollutants degradation. In the vis/H₂O₂/HSO₃-MIL-53(Fe) system, the high pollutant degradation efficiency under alkaline conditions was attributed to two factors: (1) the acidity adjusting group -HSO₃ adjusts the pH value of the whole system, which is beneficial to the photo-Fenton process. (2) The photogenerated electrons of HSO₃-MIL-53(Fe) can be captured by Fe (III), H₂O₂ and O₂ to accelerate the reduction of Fe (III) and generate ·OH, ¹O₂, and ·O₂^-. Besides, H₂O₂ can also be activated by Fe (II) and Fe (III). The above processes synergistically improved the photocatalytic efficiency. Based on liquid chromatography-mass spectrometry (LC-MS) analysis, the possible degradation pathways of the two pollutants were proposed.

High-density dispersion of CuNx sites for H2O2 activation toward enhanced Photo-Fenton performance in antibiotic contaminant degradation

In this study, a copper-based catalyst (CuCN) with CuN_x active sites highly dispersed in a porous carbon nitride matrix was synthesized and applied to a heterogeneous photo-assisted Photo-Fenton (PF) system to degrade tetracycline (TET). The results showed that the CuCN/PF system degraded up to 93.6% of TET within 60 min for ultrapure water matrix under the best experimental conditions, and more than 70% of TET for both river and lake water matrix. Toxicological tests suggested that the environmental risk caused by TET can be effectively inhibited by the CuCN/PF system. The good visible-light response and charge transport abilities of CuCN catalyst were identified in photoelectrochemical experiments. Free radical scavenging experiments and electron paramagnetic resonance (EPR) spectroscopy indicated that the active species in the degradation process were·OH, h⁺,·O₂^- and ¹O₂. Density functional theory (DFT) calculations revealed the positive effect of CuN_x sites in CuCN on the formation of hydroxyl radicals by activating H₂O₂. This work will provide a new insight for the development of high-efficiency heterogeneous catalysts in wastewater environmental remediation.

How to Build a Microplastics-Free Environment: Strategies for Microplastics Degradation and Plastics Recycling

Microplastics are an emergent yet critical issue for the environment because of high degradation resistance and bioaccumulation. Unfortunately, the current technologies to remove, recycle, or degrade microplastics are insufficient for complete elimination. In addition, the fragmentation and degradation of mismanaged plastic wastes in environment have recently been identified as a significant source of microplastics. Thus, the developments of effective microplastics removal methods, as well as, plastics recycling strategies are crucial to build a microplastics-free environment. Herein, this review comprehensively summarizes the current technologies for eliminating microplastics from the environment and highlights two key aspects to achieve this goal: 1) Catalytic degradation of microplastics into environmentally friendly organics (carbon dioxide and water); 2) catalytic recycling and upcycling plastic wastes into monomers, fuels, and valorized chemicals. The mechanisms, catalysts, feasibility, and challenges of these methods are also discussed. Novel catalytic methods such as, photocatalysis, advanced oxidation process, and biotechnology are promising and eco-friendly candidates to transform microplastics and plastic wastes into environmentally benign and valuable products. In the future, more effort is encouraged to develop eco-friendly methods for the catalytic conversion of plastics into valuable products with high efficiency, high product selectivity, and low cost under mild conditions.

An Eco-friendly Iron Cathode Electro-Fenton System Coupled With a pH-Regulation Electrolysis Cell for p-nitrophenol Degradation

The high consumption of salt reagents and strict pH control are still bottlenecks for the full-scale application of the Fenton reaction. In this work, a novel eco-friendly iron cathode electrochemical Fenton (ICEF) system coupled with a pH-regulation divided electrolysis cell was developed. In a pH-regulation divided electrolysis system, the desired pH for an effective Fenton reaction and for a neutral treated media could be obtained by H₂O splitting into H⁺ and OH⁻ at the anode and cathode, respectively. In an ICEF system, an iron plate was used as the cathode to inhibit the release of iron ions and promote the reduction of Fe³⁺ to Fe²⁺. It was found that when a potential of 1.2 V/SCE was applied on the iron cathode, 98% of p-nitrophenol was removed in the combined system after 30 min with continuously adding 200 mg/L of H₂O₂. Meanwhile, a COD and TOC removal efficiency of 79 and 60% was obtained, respectively. In this case, the conductivity just slightly increased from 4.35 to 4.37 mS/cm, minimizing the increase of water salinity, as compared with the conventional Fenton process. Generally, this combined system was eco-friendly, energy-efficient, and has the potential of being a promising technology for the removal of bio-refractory organic pollutants from wastewaters.

Fast Generation of Hydroxyl Radicals by Rerouting the Electron Transfer Pathway via Constructed Chemical Channels during the Photo-Electro-Reduction of Oxygen

A strategy for the fast generation of hydroxyl radicals (HO·) via photo-electro-reduction of oxygen by rerouting the electron transfer pathway was proposed. The rate-determining step of HO· production is the formation of H₂O₂ and the simultaneous reduction of H₂O₂. Engineering of F-TiO₂ with single atom Pd bonded with four F and two O atoms favored the electrocatalytic 2-electron oxygen reduction to H₂O₂ with as high as 99% selectivity, while the additional channel bond HO-O···Pd-F-TiO₂ facilitates the photogenerated electron transfer from the conduction band to single atom Pd to reduce Pd···O-OH to HO·. The optimized HO· production rate is 9.18 μ mol L^-1 min^-1, which is 2.6-52.5 times higher than that in traditional advanced oxidation processes. In the application of wastewater treatment, this proposed photoelectrocatalytic oxygen reduction method, respectively, shows fast kinetics of 0.324 and 0.175 min^-1 for removing bisphenol A and acetaminophen. Around 93.2% total organic carbon and 99.3% acute toxicity removal were achieved. Additionally, the degradation efficiency was less affected by the water source and pH value because of the evitable usage of metallic active sites. This work represents a fundamental investigation on the generation rate of HO·, which would pave the way for the future development of photoelectrocatalytic technologies for water purification.

Enhanced electron transfer and hydrogen peroxide activation capacity with N, P-codoped carbon encapsulated CeO2 in heterogeneous electro-Fenton process

Designing catalysts that can effectively activate oxygen and hydrogen peroxide is a huge challenge in electro-Fenton (EF) process. Considering the superior ability of electrons transport and activation of H₂O₂, ceria encapsulated with N, P-codoped carbon material was a promising catalyst for EF reaction. Herein, CeO₂-NPC_T^X (where T and X represented the calcination temperature and the initial mass of CeO₂, respectively) materials were synthesized via pyrolysis process and used as catalysts to degrade ciprofloxacin (CIP) in EF process. The results indicated that CeO₂-NPC₁₀₀₀¹⁰⁰ catalyst had good degradation performance under the optimal conditions. Compared with CeO₂ and CeO₂-NC₁₀₀₀¹⁰⁰ catalysts, CeO₂-NPC₁₀₀₀¹⁰⁰ catalyst had more content of graphite N and more oxygen vacancies, which were beneficial to activation of oxygen and hydrogen peroxide. Scavenging experiments and electron paramagnetic resonance analysis confirmed ·O₂^- and ·OH were the main reactive oxygen species in the CIP degradation process. And three logical degradation routes of CIP were given. In addition, CeO₂-NPC₁₀₀₀¹⁰⁰ catalyst still had good stability after three times of continuous operation, and presented good universality for the treatment of a variety of antibiotic wastewaters. Finally, a convincing mechanism in the EF system with CeO₂-NPC₁₀₀₀¹⁰⁰ for CIP degradation was proposed.

Synthesis and characterization of iron-nitrogen-doped biochar catalysts for organic pollutant removal and hexavalent chromium reduction

Fe, N atoms deposited on porous biochar (Fe-N@BC) composites were synthesized and employed as an efficient catalyst for organic pollutant removal and Cr^VI reduction. Naturally abundant, renewable and N-rich pomelo peel as a carbon and nitrogen source and unsubstituted phthalocyanine/iron phthalocyanine complexes as a Fe and nitrogen resource were used to develop the Fe-N@BC via a carbonization process. It is found that Fe-N@BC hybrids have homogeneous dispersion of Fe and N atoms on 3D hierarchically porous biochar, which significantly improves the performance toward the detoxification of organic pollutants using peroxymonosulfate as an oxidant, as well as the reduction of hexavalent chromium by formic acid as a reductant. Furthermore, the effects of Fe loading and pyrolytic temperature on catalysis were comprehensively analyzed and optimized. The excellent activity of Fe-N@BC in acid media can be attributed to the high dispersion of Fe species, high content of doped nitrogen as well as hierarchical micro-mesopores, which induce to expose more active sites for catalysis. Owing to the structure-enabled acidic stability, Fe-N@BC efficiently retains its activity as well as its structural stability after several cycles of reactions. This work provides a new approach to construct Fe, N-doped biochar as an effective catalyst for the detoxification of organic and inorganic pollutants.

Enhanced Chlorinated Pollutant Degradation by the Synergistic Effect between Dechlorination and Hydroxyl Radical Oxidation on a Bimetallic Single-Atom Catalyst

Chlorinated organic pollutants are highly toxic and widespread in the environment, which cause ecological risk and threaten the human health. Chlorinated pollutants are difficult to degrade and mineralize by the conventional advanced oxidation process as the C-Cl bond is resistant to reactive oxygen species oxidation. Herein, we designed a bifunctional Fe/Cu bimetallic single-atom catalyst anchored on N-doped porous carbon (FeCuSA-NPC) for the electro-Fenton process, in which chlorinated pollutants are dechlorinated on single-atom Cu and subsequently oxidized by the ·OH radical produced from O₂ conversion on single-atom Fe. Benefitting from the synergistic effect between dechlorination on single-atom Cu and ·OH oxidation on single-atom Fe, the chlorinated organic pollutants can be efficiently degraded and mineralized. The mass activity for chlorinated organic pollutant degradation by FeCuSA-NPC is 545.1-1374 min^-1 g_metal^-1, excessing the highest value of the reported electrocatalyst. Moreover, FeCuSA-NPC is demonstrated to be pH-universal, long-term stable, and environment friendly. This work provides a new insight into the rational design of a bifunctional electrocatalyst for efficient removal of chlorinated organic pollutants.

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⁺.