A Highly Stable Metal-Organic Framework-Engineered FeS2/C Nanocatalyst for Heterogeneous Electro-Fenton Treatment: Validation in Wastewater at Mild pH

Stability challenges of transition metal-modified cathodes for electro-Fenton process: A mini-review

Electro-Fenton (EF) process with transition-metal (TM) modified cathode has been regarded as a green and promising technology for wastewater treatment. Recently, breakthroughs in boosting catalyst activity for both two-electron oxygen reduction reaction (2e- ORR) and Fenton's reaction have gained intensive attention. However, achieving long-term stability of catalysts remains challenging, but is decisive for large-scale applications. This minireview provides fundamental understanding on the activity-stability correlation and the deactivation mechanisms of TM-based catalysts in EF systems, focusing on physical and chemical evolution, metal dissolution, catalyst detachment and structure collapse during long-term electrolysis. Subsequently, ongoing efforts from catalyst design to electrode engineering to stabilize the metal active sites are highlighted. Finally, the challenges and future perspectives in developing active and durable TM-modified cathodes are discussed, serving as a roadmap towards the large-scale application of EF process for wastewater treatment.

Efficient generation of singlet oxygen (1O2) by CoP/Ni2P@NF for degradation of sulfamerazine through a heterogeneous electro-Fenton process at circumneutral pH

In electro-Fenton (EF), the development of a catalytic material with wide pH application range and high interference resistance is more suitable for practical wastewater treatment. In this study, the nanoneedle-shaped CoP/Ni2P heterostructure loaded onto a nickel foam substrate (CoP/Ni2P@NF) was successfully fabricated, which was used as a cathode material for heterogeneous electro-Fenton (Hetero-EF) to degrade sulfamerazine (SMR) at circumneutral pH. The SMR degradation efficiency within 90 min went to 100% and 87% at initial pH of 6.8 and 11, respectively. Experiments and theoretical calculations demonstrated that the heterostructure of CoP/Ni2P redistributed the interfacial charge and accelerated the electron transfer, resulting in different two-electron oxygen reduction (2e-ORR) selectivity and activity than CoP and Ni2P. The ion interference and complex water quality experiment exhibited that the degradation performance remained almost unchanged, showing better anti-interference ability and complex water quality applications. Through quenching experiments and EPR tests, it is confirmed that singlet oxygen (1O2) was the major reactive oxygen species (ROS) and 1O2 was converted from hydroxyl radical (·OH) adsorbed on the catalyst surface. This study provides an efficient catalyst for the application of Hetero-EF to remove organic compounds in complex water at circumneutral pH.

FeS-based nanocomposites: A promising approach for sustainable environmental remediation - Focus on adsorption and photocatalysis - A review

Population expansion, industrialization, urban development, and climate changes increased the water crisis in terms of drinking water availability. Among the various nanomaterials for nanoremediation towards water treatment, FeS-based nanocomposites have emerged as promising candidates in the adsorptive and photocatalytic removal of contaminants. This paper, therefore, evaluates the potential of FeS-based nanocomposites for environmental applications, more specifically the combined use of adsorption and photocatalysis. Pyrite and mackinawite structures outcompeted the other FeS configurations due to their large surface areas, numerous active sites, and enhanced conductivity, factors that enhance the adsorption and photovoltaic processes. To improve photocatalytic performance FeS requires modification with additional materials. Various fabrication strategies (including hydrothermal method, co-precipitation, electrochemical anodization, electrospinning, impregnation, green synthesis, mechanochemical approach/ball milling) of FeS-based composites and their efficacy and the mechanisms for removing organic and inorganic pollutants are reviewed in this paper. The structural characteristics of FeS scaffolds play a crucial role in the effective removal of heavy metals, such as Hg and Cr ions, primarily through ion exchange and surface complexation. Organic pollutants such as methylene blue and tetracycline were effectively degraded by advanced oxidation processes (AOPs). A large scale applications of FeS include industrial wastewater treatment, groundwater remediation towards trichloroethylene and other organic solvents removal, municipal wastewater, oil spills cleanup, pre-treatment for seawater desalination. Current challenges relate to catalysts stability, their removal after treatment stage, recycling, metals leaching and up-scaling as well as high effectiveness in real case-scenario and costs optimization. In summary, this review will help to advance research in the field of environmental remediation using FeS-based nanocomposites.

Overlooked pyrite-mediated heterogeneous Fenton processes: Mechanisms of surface hydroxyl radical generation and associated decontamination performance

Pyrite-based Fenton-like processes have been extensively studied for wastewater decontamination; however, most relevant studies placed excessive emphasis on the homogeneous Fenton reaction mediated by aqueous Fe2+, resulting in the proposed technologies facing issues such as additional acid requirements for pH adjustment and excessive iron sludge production. Herein, through in situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), custom dual-chamber reactor experiments, and a series of control experiments, significant hydroxyl radical generation was identified during the pyrite/H2O2 process, while the dominant reactive iron species was verified to the structural Fe sites on the pyrite surface, rather than structural Fe(II) in secondary iron minerals and surface adsorbed Fe2+. Consequently, even with significant suppression of the homogeneous Fenton pathway, the pyrite/H2O2 process exhibited significant degradation efficiency for sulfamethoxazole (SMX) at pH 4. Moreover, the pyrite/H2O2 process was found to selectively remove 50 μM of pollutants with high affinity for pyrite (bisphenol A, carbamazepine, nitrobenzene, and SMX), even in the presence of 50-100 mM methanol. Compared to the typical iron-based reductive catalyst (zero-valent iron, ZVI), pyrite mediated a Fenton process with greater potential for practical applications at pH 4, achieving a 43.75-fold reduction in iron sludge production and almost doubling the H2O2 utilization efficiency. Additionally, in contrast to ZVI, minimal iron oxide formed on the pyrite surface during the oxidation process. Thus, after seven cycles of degradation experiments, the decontamination efficiency of the pyrite/H2O2 process remained stable. These findings are crucial for understanding the complex environmental behavior of pyrite in both natural and engineering processes and provide a new perspective for the efficient utilization of pyrite resources as well.

Low-toxicity natural pyrite on electro-Fenton catalytic reaction in a wide pH range

The resource utilization of natural pyrite not only reduces secondary pollution but also brings certain environmental benefits. However, the green and efficient use of pyrite presents certain challenges. In this study, a novel electro-Fenton (EF) system was constructed utilizing copper modified graphite felt (GF/Cu) as cathode and natural pyrite (com-FeS2) as catalyst. The results demonstrated that the system exhibited a remarkable stability over an extensive pH range (3.0-10.0) and remained effective even under adverse environmental conditions, such as high salinity or elevated antibiotic concentration. After optimizing the reaction conditions, 0.2 mM sulfamerazine (SMZ) was almost completely degraded within 1.5 h. The results highlighted the catalytic role of Fe(II) on the com-FeS2 surface. Combined with quenching experiments and quantitative analysis of reactive oxygen species (ROS), the removal of SMZ was primarily attributed to the generation of •OH, ordered by 1O2 > •O2- > •OHads, a possible degradation pathway was proposed by HR-LC-MS. The biological toxicity after the reaction was detected, and the introduction of polyvinylpyrrolidone (PVP) was beneficial to reduce the biological toxicity of iron dissolution. This work provides new insights into the green and efficient resource utilization of natural pyrite and significantly expands the pH applicability range of the Fenton process, demonstrating the large-scale industrial application potential of pyrite.

Elimination of representative antibiotic-resistant bacteria, antibiotic resistance genes and ciprofloxacin from water via photoactivation of periodate using FeS2

The propagation of antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs) induced by the release of antibiotics poses great threats to ecological safety and human health. In this study, periodate (PI)/FeS2/simulated sunlight (SSL) system was employed to remove representative ARB, ARGs and antibiotics in water. 1 × 107 CFU mL-1 of gentamycin-resistant Escherichia coli was effectively disinfected below limit of detection in PI/FeS2/SSL system under different water matrix and in real water samples. Sulfadiazine-resistant Pseudomonas and Gram-positive Bacillus subtilis could also be efficiently sterilized. Theoretical calculation showed that (110) facet was the most reactive facet on FeS2 to activate PI for the generation of reactive species (·OH, ·O2-, h+ and Fe(IV)=O) to damage cell membrane and intracellular enzyme defense system. Both intracellular and extracellular ARGs could be degraded and the expression levels of multidrug resistance-related genes were downregulated during the disinfection process. Thus, horizontal gene transfer (HGT) of ARB was inhibited. Moreover, PI/FeS2/SSL system could disinfect ARB in a continuous flow reactor and in an enlarged reactor under natural sunlight irradiation. PI/FeS2/SSL system could also effectively degrade the HGT-promoting antibiotic (ciprofloxacin) via hydroxylation and ring cleavage process. Overall, PI/FeS2/SSL exhibited great promise for the elimination of antibiotic resistance from water.

Enhanced mineralization of pharmaceutical residues at circumneutral pH by heterogeneous electro-Fenton-like process with Cu/C catalyst

Conventional electro-Fenton (EF) process at acidic pH ∼3 is recognized as a highly effective strategy to degrade organic pollutants; however, homogeneous metal catalysts cannot be employed in more alkaline media. To overcome this limitation, pyrolytic derivatives from metal-organic frameworks (MOFs) have emerged as promising heterogeneous catalysts. Cu-based MOFs were prepared using trimesic acid as the organic ligand and different pyrolysis conditions, yielding a set of nano-Cu/C catalysts that were analyzed by conventional methods. Among them, XPS revealed the surface of the Cu/C-A2-Ar/H2 catalyst was slightly oxidized to Cu(I) and, combined with XRD and HRTEM data, it can be concluded that the catalyst presents a core-shell structure where metallic copper is embedded in a carbon layer. The antihistamine diphenhydramine (DPH), spiked into either synthetic Na2SO4 solutions or actual urban wastewater, was treated in an undivided electrolytic cell equipped with a DSA-Cl2 anode and a commercial air-diffusion cathode able to electrogenerate H2O2. Using Cu/C as suspended catalyst, DPH was completely degraded in both media at pH 6-8, outperforming the EF process with Fe2+ catalyst at pH 3 in terms of degradation rate and mineralization degree thanks to the absence of refractory Fe(III)-carboxylate complexes that typically decelerate the TOC abatement. From the by-products detected by GC/MS, a reaction sequence for DPH mineralization is proposed.

Electro-Fenton degradation of pefloxacin using MOFs derived Cu, N co-doped carbon as a nanocomposite catalyst

Electro-Fenton (EF) can in-situ produce H2O2 and effectively activate H2O2 to generate powerful reactive species for the destruction of contaminants under acidic conditions, however, the production of iron-containing sludge and requirement of low working pH significantly hinder its practical application. Herein, a novel Cu, N co-doped carbon (Cu-N@C) with metal organic framework (MOF) as a precursor was constructed and adopted for the elimination of pefloxacin (PEF) in the heterogeneous electro-Fenton (HEF) process. PEF could be almost completely removed within 1 h and total organic carbon (TOC) removal efficiency was 48.57% within 6 h. Meanwhile, Cu-N@C had good repeatability and environmental adaptability, it can still maintain excellent catalytic performance after 10 cycles, and it exhibited satisfactory remediation performance in simulated water matrix. In addition, the HEF process catalyzed by Cu-N@C also showed satisfactory degradation effect on other organic pollutants including atrazine, methylene blue, and chlorotetracycline. Under the action of impressed current, the HEF system could generate H2O2 in-situ, and the active species could be generated in the redox cycle of Cu0/Cu1+/Cu2+. Electron paramagnetic resonance and quenching experiments confirmed that •OH was the dominant active species in the degradation of organic compounds. The degradation process of PEF was studied by mass spectrometry analysis of intermediate products. This study provided a simple method to prepare MOF-based electrocatalyst, which exhibits promising application potential for treatment wastewater.

Heterogeneous activation of self-generated H2O2 by Pd@UiO-66(Zr) for trimethoprim degradation: Efficiency and mechanism

The Fenton reaction is recognized as an effective technique for degrading persistent organic pollutants, such as the emerging pollutant trimethoprim (TMP). Recently, due to the excellent reducibility of active hydrogen ([H]), Pd-H2 has been preferred for Fenton-like reactions and the specific H2 activation of Pd-based catalysts. Herein, a heterogeneous Fenton catalyst named the hydrogen-accelerated oxygen reduction Fenton (MHORF@UiO-66(Zr)) system was prepared through the strategy of building ships in the bottle. The [H] has been used for the acceleration of the reduction of Fe(III) and self-generate H2O2. The systematic characterization demonstrated that the nano Pd0 particle was highly dispersed into the UiO-66(Zr). The results found that 20 mg L-1 of TMP was thoroughly degraded within 90 min in the MHORF@UiO-66(Zr) system under conditions of initial pH 3, 30 mL min-1 H2, 2 g L-1 Pd@UiO-66(Zr) and 25 μM Fe2+. The hydroxyl radical as well as the singlet oxygen were evidenced to be the main reactive oxygen species by scavenging experiments and electron spin resonance. In addition, both reducing Fe(III) and self-generating H2O2 could be achieved due to the strong metal-support interaction (SMSI) between the nano Pd0 particles and UiO-66(Zr) confirmed by the correlation results of XPS and calculation of density functional theory. Finally, the working mechanism of the MHORF@UiO-66(Zr) system and the possible degradation pathway of the TMP have been proposed. The novel system exhibited excellent reusability and stability after six cyclic reaction processes.

Size-Dependent Catalysis in Fenton-like Chemistry: From Nanoparticles to Single Atoms

State-of-the-art Fenton-like reactions are crucial in advanced oxidation processes (AOPs) for water purification. This review explores the latest advancements in heterogeneous metal-based catalysts within AOPs, covering nanoparticles (NPs), single-atom catalysts (SACs), and ultra-small atom clusters. A distinct connection between the physical properties of these catalysts, such as size, degree of unsaturation, electronic structure, and oxidation state, and their impacts on catalytic behavior and efficacy in Fenton-like reactions. In-depth comparative analysis of metal NPs and SACs is conducted focusing on how particle size variations and metal-support interactions affect oxidation species and pathways. The review highlights the cutting-edge characterization techniques and theoretical calculations, indispensable for deciphering the complex electronic and structural characteristics of active sites in downsized metal particles. Additionally, the review underscores innovative strategies for immobilizing these catalysts onto membrane surfaces, offering a solution to the inherent challenges of powdered catalysts. Recent advances in pilot-scale or engineering applications of Fenton-like-based devices are also summarized for the first time. The paper concludes by charting new research directions, emphasizing advanced catalyst design, precise identification of reactive oxygen species, and in-depth mechanistic studies. These efforts aim to enhance the application potential of nanotechnology-based AOPs in real-world wastewater treatment.

Synthesis of Fe-N doped porous carbon/silicate composites regulated by minerals in coal gasification fine slag for synergistic electrocatalytic treatment of phenolic wastewater

Coal gasification fine slag (CGFS), as a difficult-to-dispose solid waste in the coal chemical industry, consists of minerals and residual carbon. Due to the aggregate structure of minerals blocking pores and encapsulating active substances, the high-value utilization of CGFS still remains a challenge. Based on the intrinsic characteristics of CGFS, this study synthesized Fe-N doped porous carbon/silicate composites (Fe-NC) by alkali activation and pyrolysis for electrocatalytic degradation of phenolic wastewater. Meanwhile, minerals were utilized to regulate the surface chemical and pore structure, turning their disadvantages into advantages, which caused a sharp increase in m-cresol mineralization. The positive effect of minerals on composite properties was investigated by characterization techniques, electrochemical analyses and density functional theory (DFT) calculations. It was found that the mesoporous structure of the mineral-regulated composites was further developed, with more carbon defects and reactive substances on its surface. Most importantly, silicate mediated iron conversion through strong interaction with H2O2, high work function gradient with electroactive iron, and excellent superoxide radical (•O2-) production capacity. It effectively improved the reversibility and kinetics of the entire electrocatalytic reaction. Within the Fe-NC311 electrocatalytic system, the m-cresol removal rate reached 99.55 ± 1.24%, surpassing most reported Fe-N-doped electrocatalysts. In addition, the adsorption and electrooxidation experiment confirmed that the synergistic effect of Fe-N doped porous carbon and silicate simultaneously promoted the capture of pollutants and the transformation of electroactive molecules, and hence effectively shortened the diffusion path of short-lived radicals, which was further supported by molecular dynamics simulation. Therefore, this research provides new insights into the problem of mineral limitations and opens an innovative approach for CGFS recycling and environmental remediation.

Reduced sulfur accelerates Fe(III)/Fe(II) recycling in FeS2 surface for enhanced electro-Fenton reaction

FeS2 is well-known for its role in redox reactions. However, the mechanism within heterogeneous electron-Fenton (Hetero-EF) systems remains unclear. In this study, a novel FeS2 based three-dimensional system (GF/Cu-FeS2) with self-generation of H2O2 was investigated for Hetero-EF degradation of sulfamethazine (SMZ). The results revealed that SMZ could be completely removed in 1.5 h, accompanying with the mineralization efficiency of 96% within 4 h. This system performed excellent stability, evidenced by consistently eliminated 100% of SMZ within 2 h over 4 cycles. The generated Reactive Oxygen Species (ROS) of •OH and •O2- in every degradation cycle were quantitatively measured to confirm the stability of the GF/Cu-FeS2 system. Additionally, the redox reaction mechanism on the surface of FeS2 was thoroughly analyzed in detail. The accelerated reduction of Fe(III) to Fe(II), triggered by S22- on the surface of FeS2, promoted the iron cycling, thereby quickening the Fenton process. Density Functional Theory (DFT) results illustrated the process of S22- to be oxidized to in detail. Therefore, this work provides deeper insight into the mechanistic role of S22- in FeS2 for environmental remediation.

Sulfidated nanoscale zero-valent iron derived from iron sludge for tetracycline removal: Role of sulfur and iron in reactivity and mechanisms

Iron sludge, produced during the drinking water treatment process, can be recycled as potential iron resource to create environmental functional material. In this study, sulfur-iron composites derived from iron sludge (S-Fe composites) was synthesized through sulfidation and carbonization, and used for the tetracycline (TC) removal under aerobic and anoxic conditions. The reactivities of these as-prepared products were strongly depended on pyrolysis temperatures. In particular, sulfidated nanoscale zero-valent iron loaded on carbon (S-nFe0@CIS) carbonized at 800 °C exhibited the highest TC removal efficiency with 86.6% within 30 min at circumneutral pH compared with other S-Fe composites. The crystalline structure of α-Fe0, FeSx and S0 as main active sites in S-nFe0@CIS promoted the degradation of TC. Moreover, the Fe/S molar ratios significantly affected the TC removal rates, which reached the best value as the optimal S/Fe of 0.27. The results illustrated that the optimized extent of sulfidation could facilitate electron transfer from nFe0 towards contaminants and accelerate Fe(III)/Fe(II) cycle in reaction system compared to bared nFe0@CIS. We revealed that removal of TC by S-nFe0@CIS in the presence of dissolved oxygen (DO) is mainly attributed to oxidation, adsorption and reduction pathways. Their contribution to TC removal were 31.6%, 25.2% and 28.8%, respectively. Furthermore, this adsorption-oxygenation with the formation of S-nFe0@CIS-TC* complexes was a surface-mediated process, in which DO was transformed by the structural FeSx on complex surface to •OH with the generation of H2O2 intermediate. The intermediates of TC and toxicity analysis indicate that less toxicity products generated through degradation process. This study provides a new reclamation of iron sludge and offers a new insight into the TC removal by S-nFe0@CIS under aerobic conditions.

Deep Oxidation of Chlorinated VOCs by Efficient Catalytic Peroxide Activation over Nanoconfined Co@NCNT Catalysts

The catalytic removal of chlorinated VOCs (CVOCs) in gas-solid reactions usually suffers from chlorine-containing byproduct formation and catalyst deactivation. AOP wet scrubber has recently attracted ever-increasing interest in VOC treatment due to its advantages of high efficiency and no gaseous byproduct emission. Herein, the low-valence Co nanoparticles (NPs) confined in a N-doped carbon nanotube (Co@NCNT) were studied to activate peroxymonosulfate (PMS) for efficient CVOC removal in a wet scrubber. Co@NCNT exhibited unprecedented catalytic activity, recyclability, and low Co ion leakage (0.19 mg L-1) for chlorobenzene degradation in a very wide pH range (3-11). The chlorobenzene removal efficiency was kept stable above 90% over Co@NCNT, much higher than that of nonconfined Co@NCNS (45%). The low-valence Co NPs achieved a continuous electron redox cycling (Co0/Co2+ → Co3+ → Co0/Co2+) and greatly promoted the O-O bond dissociation of PMS with the least energy (0.83 eV) inside the channel of Co@NCNT to form abundant HO• and SO4•-. Thus, the deep oxidation of chlorobenzene was achieved without any biphenyl byproducts from the coupling reaction. This study provided a new avenue for designing novel nanoconfined catalysts with outstanding activity, paving the way for the deep oxidation of CVOC waste gas via AOP wet scrubber.

Metal-free electrified membranes for contaminants oxidation: Synergy effect between membrane rejection and nanoconfinement

Electro-Fenton processes are frequently impeded by depletion of metal catalysts, unbalance between H2O2 generation and activation, and low concentration of reactive species (e.g., •OH) in the bulk solution. A metal-free electro-Fenton membrane was fabricated with nitrogen-doped carbon nanotube (N-CNT) and reduced graphene oxide (RGO). N-CNT acted as a catalyst for both H2O2 generation and activation, while the incorporated RGO served as the second catalyst for H2O2 generation and improved the performance of membrane rejection. The electrified membrane was optimized in terms of nitrogen precursors selection and composition of N-CNT and RGO to achieve optimal coupling between H2O2 generation and activation. The membrane fabricated with 67% mass of N-CNT with urea as the precursor achieved over 95% removal of the target contaminants in a single pass through the membrane with a water flux of 63 L m-2 h-1. This membrane also exhibited efficient transformation of various concentrations of contaminants (i.e., 1-10 mg L-1) over a broad range of pH (i.e., 3-9). Due to its good durability and low energy consumption, the metal-free electro-Fenton membrane holds promise for practical water treatment application. The concentration-catalytic oxidation model elucidated that the elevated contaminant concentration near the membrane surface enhanced the transformation rate by 40%. The nanoconfinement enhanced the transformation rate constant inside the membrane by a factor of 105 because of elevated •OH concentration inside the nanopores. Based on the prediction of this model, the configuration of the membrane reactor has been optimized.

Comprehensive analysis of research trends and prospects in electrochemical advanced oxidation processes (EAOPs) for wastewater treatment

Electrochemical advanced oxidation processes (EAOPs) have emerged as a promising approach for efficient wastewater treatment. However, despite their promising potential, there is a lack of comprehensive analysis regarding the research trends, bibliometric data, and research frontiers of EAOPs. To address this gap, this study conducted a thorough and comprehensive analysis of 2347 related articles in the Web of Science Core Collection Database from 2012 to 2022. The analysis included information on countries, authors, institutions, and more, with a focus on summarizing trends and cutting-edge research hotspots in the field. The University of Barcelona in Spain is the most effective institution. Brillas E. is the most productive author in the world. Research hotspots in EAOPs have evolved from traditional anodic oxidation (AO) to novel electro-Fenton (EF) technology, which focuses on efficient generation of H2O2 and the use of metal-organic frameworks to enhance performance and efficiency. Through systematic research hotspot analysis, the importance of performance comparison of different types of EAOPs, development of new materials, optimization of device parameters, and toxicity assessment of byproducts is highlighted. Concurrently, the rise and mechanisms of emerging EAOPs are predicted and analyzed. Finally, future research on EAOPs technologies should focus on technological coupling, development of new materials, reduction of energy consumption and cost, evaluation and minimization of toxicity, and exploration of green renewable energy sources for larger-scale applications in wastewater treatment pilot plants. In this way, these technologies can contribute to the sustainability of larger industrial wastewater treatment applications and make an important contribution to environmental protection and scientific and technological progress.

Tuning the Hierarchical Pore Structure and the Metal Site in a Metal-Organic Framework Derivative to Unravel the Mechanism for the Adsorption of Different Volatile Organic Compounds

Volatile organic compounds (VOCs) are one of the main classes of air pollutants, and it is important to develop efficient adsorbents to remove them from the atmosphere. To do this most efficiently, we need to understand the mechanism of VOC adsorption. In this work, we described how the metal organic framework (MOF), ZIF-8, was used as a precursor to generate MOF derivatives (Zn-GC) through temperature-controlled calcination, which had adjustable metal sites and hierarchical pore structure. It was used as a model adsorbent to study the adsorption and desorption characteristics of different VOCs. Zn-GC-850 with developed pores exhibited higher adsorption performance for the benzene series, whereas Zn-GC-650 with more metal sites had a better adsorption capacity for oxygen-containing VOCs. By tuning the molecular structure of the VOCs, we revealed the adsorption mechanism of different VOCs at the molecular level. The more developed hierarchical pore structure obtained at the higher temperature facilitates the diffusion of the benzene series, and the noncovalent interaction between their methyl group(s) and the carbonized MOF derivatives improves the adsorption affinity; while the higher exposure of Zn sites obtained at lower temperature favors the adsorption of oxygen-containing VOCs by Zn-O bonds. The mass transfers of VOCs and the role of the adsorbent were simulated by multiple theoretical models. This study strengthens the basis for the design and optimization of the adsorbent and catalyst for VOCs treatment.

Boron Bifunctional Catalysts for Rapid Degradation of Persistent Organic Pollutants in a Metal-Free Electro-Fenton Process: O2 and H2O2 Activation Process

Metals usually served as the active sites of the heterogeneous bifunctional electro-Fenton reaction, which faced the challenge of poor stability under acidic or even neutral conditions. Exploring a metal-free heterogeneous bifunctional electro-Fenton catalyst can effectively solve the above problems. In this work, a stable metal-free heterogeneous bifunctional boron-modified porous carbon catalyst (BTA-1000) was synthesized. For the BTA-1000 catalyst, the yield of H2O2 (294 mg/L) significantly increased. The degradation rate of phenol by BTA-1000 (0.242 min-1) increased by an order of magnitude, compared with the porous carbon catalyst (0.0105 min-1). The BTA catalyst could rapidly degrade industrial dye wastewater, and its specific energy consumption was 5.52 kW h kg-1 COD-1, lower than that in previous reports (6.38-7.4 kW h kg-1 COD-1). DFT and XPS revealed that C═O and -BC2O groups jointly promoted the generation of H2O2, and the -BCO2 group played dominant roles in the generation of •OH because the oxygen atom near the electron-giving groups (-BCO2 group) facilitated the formation of hydrogen bond and H2O2 adsorption. This work gained deep insights into the reaction mechanism of the boron-modified porous carbon catalyst, which helped to guide the development of metal-free heterogeneous bifunctional electro-Fenton catalysts.

High-performance catalytic reduction of 4-nitrophenol to 4-aminophenol using M-BDC (M = Ag, Co, Cr, Mn, and Zr) metal-organic frameworks

The catalytic activity of pure metal nanoparticles is always limited by aggregation during the reaction. Therefore, promising candidates such as metal-organic frameworks possess benefits due to their 3D porous structures, high stability, and high specific surface area. In this study, effective and reusable catalysts based on M-BDC metal-organic frameworks were synthesized utilizing five different coordinating metal ions (M = Ag, Co, Cr, Mn, and Zr) as metal nodes and 1-4-benzene dicarboxylic acid (BDC) as an organic linker and used in catalytic reduction of 4-Nitrophenol (4-NP) to 4-Aminophenol (4-AP) for the first time. The as-prepared catalysts were characterized using SEM, EDX, XRD, and FTIR techniques. Based on catalytic performance, Co-BDC showed the best catalytic efficiency compared to the other M-BDC MOF catalysts with a conversion yield of about 99.25 in 2 min. All of the catalysts could catalyze the complete reduction of 4-NP to 4-AP at different reaction times (2-10); however, Mn-BDC could not finish the catalytic reduction reaction even after 20 min. The two more efficient catalysts including Co-BDC and Cr-BDC demonstrated high stability and reusability (more than 85% catalytic efficiency) even after 5 cycles.

Investigation on the reaction kinetic mechanism of polydopamine-loaded copper as dual-functional catalyst in heterogeneous electro-Fenton process

Heterogeneous electro-Fenton (HEF) process has been regarded as a promising method in environmental remediation. However, the reaction kinetic mechanism of the HEF catalyst for simultaneous production and activation of H₂O₂ remained confounded. Herein, the copper supported on polydopamine (Cu/C) was synthesized by a facile method and employed as a bifunctional HEFcatalyst, and the catalytic kinetic pathways were deeply investigated by using rotating ring-disk electrode (RRDE) voltammetry based on the Damjanovic model. Experimental results substantiated that a two-electron oxygen reduction reaction (2e^- ORR) and a sequential Fenton oxidation reaction were proceeded on 1.0-Cu/C, where metallic copper played a crucial role in the fabrication of 2e^- active sites as well as utmost H₂O₂ activation to produce highly reactive oxygen species (ROS), resulting in the high H₂O₂ productivity (52.2%) and the almost complete removal of contaminant ciprofloxacin (CIP) after 90 min. The work not only expanded the idea of reaction mechanism on Cu-based catalyst in HEF process but also provided a promising catalyst for pollutants degradation in wastewater treatment.

FeMo@porous carbon derived from MIL-53(Fe)@MoO3 as excellent heterogeneous electro-Fenton catalyst: Co-catalysis of Mo

An ultra-efficient electro-Fenton catalyst with porous carbon coated Fe-Mo metal (FeMo@PC), was prepared by calcining MIL-53(Fe)@MoO₃. This FeMo@PC-2 exhibited impressive catalytic performance for sulfamethazine (SMT) degradation with a high turnover frequency value (7.89 L/(g·min)), much better than most of reported catalysts. The mineralization current efficiency and electric energy consumption were 83.2% and 0.03 kWh/g_TOC, respectively, at low current (5 mA) and small dosage of catalyst (25.0 mg/L). The removal rate of heterogeneous electro-Fenton (Hetero-EF) process catalyzed by FeMo@PC-2 was 4.58 times that of Fe@PC/Hetero-EF process. Because the internal-micro-electrolysis occurred between PC and Fe⁰, while the co-catalysis of Mo accelerated the rate-limiting step of the Fe³⁺/Fe²⁺ cycle and greatly improved the H₂O₂ utilization efficiency. The results of radical scavenger experiments and electron paramagnetic resonance confirmed the main role of surface-bound hydroxyl radical oxidation. This process was feasible to remove diverse organic contaminants such as phenol, 2,4-dichlorophenoxyacetic acid, carbamazepine and SMT. This paper enlightened the importance of the doped Mo, which could greatly improve the activity of the iron-carbon heterogeneous catalyst derived from metal-organic frameworks in EF process for efficient removal of organic contaminants.

Electrochemical aptasensor based on the engineered core-shell MOF nanostructures for the detection of tumor antigens

It is essential to develop ultrasensitive biosensors for cancer detection and treatment monitoring. In the development of sensing platforms, metal-organic frameworks (MOFs) have received considerable attention as potential porous crystalline nanostructures. Core-shell MOF nanoparticles (NPs) have shown different diversities, complexities, and biological functionalities, as well as significant electrochemical (EC) properties and potential bio-affinity to aptamers. As a result, the developed core-shell MOF-based aptasensors serve as highly sensitive platforms for sensing cancer biomarkers with an extremely low limit of detection (LOD). This paper aimed to provide an overview of different strategies for improving selectivity, sensitivity, and signal strength of MOF nanostructures. Then, aptamers and aptamers-modified core-shell MOFs were reviewed to address their functionalization and application in biosensing platforms. Additionally, the application of core-shell MOF-assisted EC aptasensors for detection of several tumor antigens such as prostate-specific antigen (PSA), carbohydrate antigen 15-3 (CA15-3), carcinoembryonic antigen (CEA), human epidermal growth factor receptor-2 (HER2), cancer antigen 125 (CA-125), cytokeratin 19 fragment (CYFRA21-1), and other tumor markers were discussed. In conclusion, the present article reviews the advancement of potential biosensing platforms toward the detection of specific cancer biomarkers through the development of core-shell MOFs-based EC aptasensors.

Flow-through heterogeneous electro-Fenton system using a bifunctional FeOCl/carbon cloth/activated carbon fiber cathode for efficient degradation of trimethoprim at neutral pH

The synthesis of multifunctional cathode with high-efficiency and stable catalytic activity for simultaneously producing and activating H₂O₂ is an effective way for promoting the performance of heterogeneous electro-Fenton process (HEF). In addition, accelerating mass transfer by adopting a flow-through reactor is also great importance because of its better utilization of catalysts and adequate contact of the contaminant with the oxidants generated on the electrode surface. Herein, a novel flow-through HEF (FHEF) system was designed for the degradation of trimethoprim (TMP) using bifunctional cathode with a sandwich structure FeOCl nanosheets loaded onto carbon cloth (CC) and activated carbon fiber (ACF) (FeOCl/CC/ACF). The cathode exhibited excellent performance in activating H₂O₂ for the in-situ generation of hydroxyl radicals (^•OH). The electron spin resonance (ESR) measurements and radical quenching tests proved that the high production of ^•OH in the FHEF process was favorable to the high catalytic efficiency. 25 mg L^-1 TMP was entirely degraded after 60 min, with the TOC removal of 62.6% (180 min) at pH 6.8, 9.0 mA cm^-2, and flux rate 210 mL min^-1. Moreover, the degradation rate still could reach 83% (60 min) after 10 cycles without obvious valence and crystal phase changes. Simultaneously, the current utilization rate has also been greatly enhanced, with an average current efficiency of 69.9% and a low energy consumption of 0.28 kWh kg^-1. The reasonable degradation pathways for TMP were proposed based on the UPLC-QTOF-MS/MS results. Finally, the results of toxicological simulation showed a declining trend in the toxicity of the samples during TMP degradation. These results claim that the FeOCl/CC/ACF-FHEF system is an efficient and economical technology for the treatment of organic contaminants in effluents.

Challenges and Future Roadmaps in Heterogeneous Electro-Fenton Process for Wastewater Treatment

The efficiency of heterogeneous electro-Fenton technology on the degradation of recalcitrant organic pollutants in wastewater is glaringly obvious. This green technology can be effectively harnessed for addressing ever-increasing water-related challenges. Due to its outstanding performance, eco-friendliness, easy automation, and operability over a wide range of pH, it has garnered significant attention from different wastewater treatment research communities. This review paper briefly discusses the principal mechanism of the electro-Fenton process, the crucial properties of a highly efficient heterogeneous catalyst, the heterogeneous electro-Fenton system enabled with Fe-functionalized cathodic materials, and its essential operating parameters. Moreover, the authors comprehensively explored the major challenges that prevent the commercialization of the electro-Fenton process and propose future research pathways to countervail those disconcerting challenges. Synthesizing heterogeneous catalysts by application of advanced materials for maximizing their reusability and stability, the full realization of H2O2 activation mechanism, conduction of life-cycle assessment to explore environmental footprints and potential adverse effects of side-products, scale-up from lab-scale to industrial scale, and better reactor design, fabrication of electrodes with state-of-the-art technologies, using the electro-Fenton process for treatment of biological contaminants, application of different effective cells in the electro-Fenton process, hybridization of the electro-Fenton with other wastewater treatments technologies and full-scale analysis of economic costs are key recommendations which deserve considerable scholarly attention. Finally, it concludes that by implementing all the abovementioned gaps, the commercialization of electro-Fenton technology would be a realistic goal.

Graphical Abstract

Recent advances in application of heterogeneous electro-Fenton catalysts for degrading organic contaminants in water

Over the last decades, advanced oxidation processes (AOPs) have been widely used in surface and ground water pollution control. The heterogeneous electro-Fenton (EF) process has gained much attention due to its properties of high catalytic performance, no generation of iron sludge, and good recyclability of catalyst. As of October 2022, the cited papers and publications of EF are around 1.3 × 10^-5 and 3.4 × 10^-3 in web of science. Among the AOP techniques, the contaminant removal efficiencies by EF process are above 90% in most studies. Current reviews mainly focused on the mechanism of EF and few reviews comprehensively summarized heterogeneous catalysts and their applications in wastewater treatment. Thus, this review focuses on the current studies covering the period 2012-2022, and applications of heterogeneous catalysts in EF process. Two kinds of typical heterogeneous EF systems (the addition of solid catalysts and the functionalized cathode catalysts) and their applications for organic contaminants degradation in water are reviewed. In detail, solid catalysts, including iron minerals, iron oxide-based composites, and iron-free catalysts, are systematically described. Different functionalized cathode materials, containing Fe-based cathodes, carbonaceous-based cathodes, and heteroatom-doped cathodes, are also reviewed. Finally, emphasis and outlook are made on the future prospects and challenges of heterogeneous EF catalyst for wastewater treatments.

Degradation of carbamazepine over MOFs derived FeMn@C bimetallic heterogeneous electro-Fenton catalyst

A highly efficient heterogeneous electro-Fenton (Hetero-EF) catalyst with core-shell structure was successfully prepared by calcination of Mn-doped Mil-53 (Fe) precursor at high temperature. FeMn@C-800/2 prepared at pyrolysis temperature of 800 °C and Fe:Mn molar doping ratio of 2:1 showed the best catalytic performance for the degradation of carbamazepine (CBZ). The characterization, properties and stability of FeMn@C-800/2 were systematically investigated, obtaining the apparent first-order reaction rate of Hetero-EF was 8.9 and 17.8 times higher than that on Fe@C-800 and Mn@C-800 at the optimized conditions of current density 10 mA cm^-2, catalyst dosage of 50 mg L^-1 and initial pH 4.0, respectively. The incorporation of Mn promoted the generation of more Fe⁰ and Fe₃C during the pyrolysis process, and enhanced the internal micro-electrolysis between Fe⁰ and carbon shell. At the same time, the presence of Mn⁰ also promoted the regeneration of Fe²⁺, and improved the activity of iron-carbon heterogeneous catalysis in the EF process, so as to degrade organic pollutants more effectively. This work would help to gain insight into the design of MOFs derived Fe-Mn bimetal catalyst and its mechanism for enhanced heterogeneous electro-Fenton.

Iron carbon particle dosing for odor control in sewers: Laboratory tests

Treatment of malodor in the sewer system is a priority in many municipalities for human health concerns, sewer pipe corrosion prevention. In this study, the removal effects of iron-carbon (Fe-C) particles on the inhibition of sulfide in the liquid phase, as well as hydrogen sulfide (H₂S) and methyl mercaptan (MeSH) in the headspace were investigated using laboratory-scale reactors simulating gravity-flow sewer system. The results indicated that the sulfide in the liquid phase can be reduced from 15.1 to 16.5 mg S/L to 0.05 and 0.14 mg S/L after 70 g/L and 50 g/L Fe-C particles dosing. The flux of H₂S and MeSH in the headspace was also inhibited, and its flux decreased by up to 99%. Meanwhile, the microbial community structures of sulfate-reducing bacteria (SRB) and sulfur-oxidizing bacteria (SOB) in the sediment surface and water were also analyzed, and the results revealed that the relative abundance of SRB in the water and sediment surface was inhibited greatly after adding Fe-C particles, especially for Sulfurospirillum, Clostridium, and Desulfovibrio, while Fe-C particles promoted the growth of SOB. Moreover, the surface deposition was collected and analyzed through X-ray photoelectron spectroscopy (XPS), and the results indicated that sulfide can be removed by co-precipitation with ferrous ions formed through micro-electrolysis of Fe-C. This study provides a new approach to control the in-situ odor pollution for sewage systems.

Ferrate modified carbon felt as excellent heterogeneous electro-Fenton cathode for chloramphenicol degradation

In this study, a novel and efficient heterogeneous electro-Fenton (EF) process with a potassium ferrate (K₂FeO₄) modified carbon felt (Fe-CF) cathode was developed for chloramphenicol (CAP) removal. The catalytic activity was assessed by the comparison of different systems and the effects of multiple operating parameters (K₂FeO₄ dosage, initial solution pH, applied current) and co-existing constituents. Results indicated that the Fe-CF cathode exhibited excellent performance for CAP degradation (almost 100% removal efficiency within 60 min) over a wide range of pH (pH 3-9) during heterogeneous EF ascribed to the synergistic effect of embedded iron species and porous graphitic carbon structure and effective utilization of the in-situ generated H₂O₂. Moreover, the Fe-CF cathode possessed good recyclability with low metal leaching (98.2% CAP removal efficiency after reused for 5 times) and outstanding real water application performance. The ∙OH and O₂∙^- were responsible for CAP degradation, while ∙OH played a main role. Moreover, the toxicity evaluation by E. coli growth experiments demonstrated an efficient toxicity reduction in this system. Overall, a novel heterogeneous EF functional cathode with superior performance was fabricated via a green, low-cost one-step method, which shows promising application potential for actual wastewater treatment.

Electronic Control of Traditional Iron-Carbon Electrodes to Regulate the Oxygen Reduction Route to Scale Up Water Purification

Shifting four-electron (4e^-) oxygen reduction in fuel cell technology to a two-electron (2e^-) pathway with traditional iron-carbon electrodes is a critical step for hydroxyl radical (HO^•) generation. Here, we fabricated iron-carbon aerogels with desired dimensions (e.g., 40 cm × 40 cm) as working electrodes containing atomic Fe sites and Fe₃C subnanoclusters. Electron-donating Fe₃C provides electrons to FeN₄ through long-range activation for achieving the ideal electronic configuration, thereby optimizing the binding energy of the *OOH intermediate. With an iron-carbon aerogel benefiting from finely tuned electronic density, the selectivity of 2e^- oxygen reduction increased from 10 to 90%. The resultant electrode exhibited unexpectedly efficient HO^• production and fast elimination of organics. Notably, the kinetic constant k_M for sulfamethoxazole (SMX) removal is 60 times higher than that in a traditional iron-carbon electrode. A flow-through pilot device with the iron-carbon aerogel (SA-Fe_0.4NCA) was built to scale up micropolluted water decontamination. The initial total organic carbon (TOC) value of micropolluted water was 4.02 mg L^-1, and it declined and maintained at 2.14 mg L^-1, meeting the standards for drinking water quality in China. Meanwhile, the generation of emerging aromatic nitrogenous disinfection byproducts (chlorophenylacetonitriles) declined by 99.2%, satisfying the public safety of domestic water. This work provides guidance for developing electrochemical technologies to satisfy the flexible and economic demand for water purification, especially in water-scarce areas.

Amplifying anti-flooding electrode to fabricate modular electro-fenton system for degradation of antiviral drug lamivudine in wastewater

The advanced oxidation based on in-situ hydrogen peroxide production using carbon air cathode is very potential for wastewater treatment. However, catalyst flooding and complex assembly patterns are the bottleneck limiting the air cathode to the long-term and large-scale application. In this work, a novel anti-flooding air-breathing cathode (ABC) was prepared by a simple rolling-spraying method with relatively low price commercial materials. The novel method changed the morphology of gas diffusion layer as well as adjusted the hydrophobicity of air side of the catalyst layer. As a result, water-air distribution management was achieved and TPI disequilibrium was prevented. Compare with traditional ABC, the H₂O₂ yield and current efficiency (CE) of optimized anti-flooding ABC (ABC_0.9) increased by 13.5% (941 ± 10 mg·L^-1·h^-1 with CE of 84% at 30 mA·cm^-2), the material cost and fabrication time decreased by 10.1% (2.32 ¥·dm^-2, ~0.36 $·dm^-2) and 40%. Amplified ABC coupled with Ti/IrO₂ anodes were integrated into a modular electrode used for H₂O₂generation. When the current density (j) increased from 10 to 30 mA·cm^-2, the energy cost increased from 0.19 to 0.43 ¥·mol^-1 H₂O₂ (from 0.03 to 0.07 $·mol^-1 H₂O₂). The modular electrode was utilized in a 2 L pre-pilot scale reactor for antiviral drug lamivudine degradation by electro-Fenton (EF) process. 100% of lamivudine and 78.1% of total organic carbon (TOC) were removed within 60 min at 20 mA·cm^-2. The susceptible sites on the lamivudine toward hydroxyl radicals were investigated and transformation products (TP) as well as degradation pathway were studied.

Derivatives of metal-organic frameworks for heterogeneous Fenton-like processes: From preparation to performance and mechanisms in wastewater purification - A mini review

Organic pollution is an ever-growing issue in aquatic environment, Fenton-like processes have gained widespread acceptance due to their high oxidative potential and environmental compatibility. Derivatives of metal-organic frameworks (MOFs) are emerging heterogeneous Fenton-like catalysts, which have advantages of large surface area, diversity of structures, and abundant active sites. This work focuses on the recent advances in MOFs derivatives including metal compounds and metal incorporated carbons for Fenton-like processes. First, preparation strategies, structures and compositions are introduced. And then, the removal of organic pollutant in Fenton, electro-Fenton, and photo-Fenton process catalyzed by MOFs derivative is summarized, respectively. The contents particularly devote efforts to build connections among preparation, structures, compositions, and performance. Furthermore, the mechanisms of improving performance are discussed in detail. Finally, the perspectives of MOFs derivatives toward Fenton-like applications are proposed.

A novel electrokinetic remediation with in-situ generation of H2O2 for soil PAHs removal

Electrokinetic-Fenton (EK-Fenton) technology requires a high dose of H₂O₂ to produce •OH radicals, which adds a high cost to the remediation process and raises safety concerns during transportation and storage of H₂O₂. Moreover, the remediation efficiency of the conventional EK-Fenton process is low due to the meaningless consumption of H₂O₂ on the electrodes and the alkaline environment near the cathode. In this work, a modified CMK3-gas diffusion electrode (CMK3-GDE) is fabricated. This cathode can continuously generate H₂O₂, and the cumulative H₂O₂ concentration can reach 0.23 M during 10 days of the test. The utilization of cation exchange membranes (CEMs) efficiently restricts the decomposition of H₂O₂ on the electrodes and prevents the alkalization of the soil near the cathode, resulting in a 13.7-43.2% increase of the removal efficiency of polycyclic aromatic hydrocarbons (PAHs). In this new treatment process, PAHs are mainly oxidized into quinones, ketones, alcohols, and small molecule acids, and all these products have lower toxicities than PAHs. The EK-Fenton/CMK3-GDE-CEM system exhibits excellent remediation efficiency for treating PAHs polluted soil, which could be a sustainable, eco-friendly, and low-cost strategy for soil remediation.

Synergistic effects of oxidation, coagulation and adsorption in the integrated fenton-based process for wastewater treatment: A review

Fenton process is the most popular for wastewater treatment among all available advanced oxidation processes (AOPs). Numerous endeavors have been devoted to improving the oxidation efficiency of Fenton reaction in terms of promoting ·OH generation, accelerating iron redox cycle and extending applicable pH range. However, in addition to oxidation, coagulation and adsorption also simultaneously occur in the Fenton process, which play important role in the removal of pollutants. Rapid progress has revealed the synergistic effects of oxidation, coagulation and adsorption in the Fenton process, providing new ideas for the treatment of complex and refractory wastewater. Based on available studies, this review is the first to systematically summarize the research progress regarding the synergistic effects of oxidation, coagulation and adsorption in the integrated Fenton-based processes for wastewater treatment. The involved mechanism of the synergistic effects in different Fenton processes (homogeneous Fenton, heterogeneous Fenton and physical field-assistant Fenton coupling process) are critically reviewed. Furthermore, special attention has been paid to the representative applications of the synergistic effects in wastewater treatment (such as industrial organic wastewater, landfill leachate and heavy metal-organic complexes, etc.), particularly focusing on the operation parameters and removal performance. Finally, a conclusion of the review and subsequently, perspectives are given for possible research directions. We believe this review can provide useful information for researchers and end-users involved in the development and application of the Fenton process in wastewater treatment.

Tannic acid-Fe complex derivative-modified electrode with pH regulating function for environmental remediation by electro-Fenton process

A heterogeneous electro-Fenton (hetero-EF) system can effectively broaden the applicable pH range, although the decreased electrogeneration efficiency of H₂O₂ at elevated pH (especially neutral conditions) is unfavorable for the efficient removal of organic pollutants. Herein, a tannic acid-Fe complex derivative-modified carbon felt (TFD@CF) cathode was prepared for hetero-EF treatment of organic pollutants over a wide pH range. Interestingly, the as-prepared hetero-EF cathode could act as a pH regulator that acidified the solution over a wide pH range. As expected, the TFD@CF cathode exhibited excellent hetero-EF activity for the removal of diverse organic pollutants (such as methyl orange, methylene blue, sulfamerazine, bisphenol A and 2,4-dichlorophenoxyacetic acid) at neutral and even alkaline pH (removal efficiency >90 %). A total of 2.98 kWh kg^-1 COD^-1 with 83.2 % COD removal could be achieved by the TFD@CF cathode for the treatment of actual textile dyeing secondary wastewater. Electrochemical characterizations proved that the TFD@CF cathode had excellent electrochemical properties with improved electron transfer ability and a well-pronounced Fe(III) electroreductive response. Meanwhile, more acidic groups were newly generated during the electrochemical reaction (an increase of 30.1 %), thus dissociating more H⁺ into solution. The identification of reactive oxygen species suggested that OH and ¹O₂ could be responsible for the removal of organic pollutants in the TFD@CF EF system. These interesting findings may provide new insights into the design of multifunctional hetero-EF cathodes for the removal of refractory organic pollutants.

Carbon nanotube filter functionalized with MIL-101(Fe) for enhanced flow-through electro-Fenton

Herein, an electrochemical carbon nanotubes (CNT) filter modified with MIL-101(Fe) has been designed for the electro-Fenton applications by serving as a functional flow-through electrode. Under an electric field, the hybrid filter enabled the in situ generation of H₂O₂via the two-electron oxygen reduction reaction, which promoted the production of HO by the accelerated Fe²⁺/Fe³⁺ cycling of MIL-101(Fe). It was observed that 93.2 ± 1.2% tetracycline and 69.0 ± 0.8% total organic carbon (TOC) were removed in 2 h under the optimized conditions. The electron paramagnetic resonance (EPR) analysis and radical scavenging experiments revealed that HO predominated the tetracycline degradation. As compared to the batch reactor, the performance of the proposed system was improved by 5.6 times owing to the convection-enhanced mass transport. The plausible working mechanism and degradation pathway were also subsequently proposed. The findings reported in this study provide a promising insight for the environmental remediation by integrating nanotechnology and Fenton chemistry.

S-doped MIL-53 as efficient heterogeneous electro-Fenton catalyst for degradation of sulfamethazine at circumneutral pH

The reduced S-modified MIL-53(Fe) was prepared by sulfurizing MIL-53(Fe) at low temperature, which was an efficient electro-Fenton catalyst at wide pH range (3-9) for sulfamethazine (SMT) degradation. The best temperature and MIL-53(Fe)/S ratio were 350 °C and 1:2, at which the BET surface area was much enlarged. The MIL-53(Fe) surface was etched by S to many 2D nanosheets with the thickness of ~50 nm, while S_2-2 replaced OH^- to coordinate with Fe²⁺ and increased the Fe²⁺ content, which improved the catalytic performance. Even at initial pH of 7.0, the SMT removal was 95.8%, and the rate constant (k) in the Hetero-EF process was 16-folds of that in the Homo-EF process. The turnover frequency (TOF_d) value of MIL-53(Fe)/S(1:2)-350 was 0.48 L g^-1 min^-1, which was 6.8 times that of commercial FeS₂. The S_2-2in catalyst adjusted the pH superfast, and promoted the generation of Fe²⁺ and thus efficiently activating H₂O₂ to form surface ^·OH, which was verified to be the main radical by EPR and radical scavenger experiments. This catalyst showed promising prospect for environmental application and could be regenerated by sulfidation method. S-doped MIL-53(Fe) was an excellent pH regulator, thus promoting promising application in Hetero-EF processes.

Pyrite-embedded porous carbon nanocatalysts assembled in polyvinylidene difluoride membrane for organic pollutant oxidation

FeS₂-embedded in porous carbon (FeS₂/C) was prepared by simultaneous sulfidation and carbonization of an iron-based metal-organic framework precursor, and subsequently immobilized in polyvinylidene fluoride membranes (FeS₂/C@PVDF) for organics removal via peroxymonosulfate (PMS) activation. The composition, structure, and morphology of the FeS₂/C@PVDF membrane were extensively characterized. Scanning electron microscopy images manifest that the FeS₂/C nanoparticles with an average diameter of 40 nm are assembled on the external and internal membrane surface. The as-prepared FeS₂/C@PVDF membrane exhibits excellent performances over a wide pH range of 1.53-9.50, exceeding carbon-free syn-FeS₂@PVDF. The effective degradation could be improved by inner pyrite FeS₂ cores and thus enhanced the electron transfer between carbon shell and PMS. Electron paramagnetic resonance and quenching experiments elucidated that radical (HO^∙, SO₄^∙-) and nonradical (¹O₂) species were the predominant reactive oxidants. In addition, FeS₂/C@PVDF exhibited high stability with low Fe leaching (0.377 mg/L) owing to the effective protection of the outer carbon skeleton. Plentiful porosity of PVDF membranes not only affords a controlled size and confined uniform distribution of the immobilized FeS₂/C nanoparticles, but also enables a persistent exposure of active sites and enhanced mass transfer efficiency. Our findings demonstrate a promise for utilizing the novel FeS₂/C@PVDF membrane as an efficient catalyst for the environmental cleanup.

H2O2 production at gas-diffusion cathodes made from agarose-derived carbons with different textural properties for acebutolol degradation in chloride media

The excessive cost, unsustainability or complex production of new highly selective electrocatalysts for H₂O₂ production, especially noble-metal-based ones, is prohibitive in the water treatment sector. To solve this conundrum, biomass-derived carbons with adequate textural properties were synthesized via agarose double-step pyrolysis followed by steam activation. A longer steam treatment enhanced the graphitization and porosity, even surpassing commercial carbon black. Steam treatment for 20 min yielded the greatest surface area (1248 m² g^-1), enhanced the mesopore/micropore volume distribution and increased the activity (E_1/2 = 0.609 V) and yield of H₂O₂ (40%) as determined by RRDE. The upgraded textural properties had very positive impact on the ability of the corresponding gas-diffusion electrodes (GDEs) to accumulate H₂O₂, reaching Faradaic current efficiencies of ~95% at 30 min. Acidic solutions of β-blocker acebutolol were treated by photoelectro-Fenton (PEF) process in synthetic media with and without chloride. In urban wastewater, total drug disappearance was reached at 60 min with almost 50% mineralization after 360 min at only 10 mA cm^-2. Up to 14 degradation products were identified in the Cl^--containing medium.

Micro/macrostructure and multicomponent design of catalysts by MOF-derived strategy: Opportunities for the application of nanomaterials-based advanced oxidation processes in wastewater treatment

Advanced oxidation processes (AOPs) have demonstrated an effective wastewater treatment method. But the application of AOPs using nanomaterials as catalysts is challenged with a series of problems, including limited mass transfer, surface fouling, poor stability, and difficult recycling. Recently, metal-organic frameworks (MOFs) with high tunability and ultrahigh porosity are emerging as excellent precursors for the delicate design of the structure/composition of catalysts and many MOF-derived catalysts with distinct physicochemical characteristics have shown optimized performance in various AOPs. Herein, to elucidate the structure-composition-performance relationship, a review on the performance optimization of MOF-derived catalysts to overcome the existing problems in AOPs by micro/macrostructure and multicomponent design is given. Impressively, MOF-derived strategy for the design of catalyst materials from the aspects of microstructure, macrostructure, and multicomponent (polymetallic, heteroatom doping, M/C hybrids, etc.) is firstly presented. Moreover, important advances of MOF-derived catalysts in the application of various AOPs (Fenton, persulfate-based AOPs, photocatalysis, electrochemical processes, hybrid AOPs) are summarized. The relationship between the unique micro/macrostructure and/or multicomponent features and performance optimization in mass transfer, catalytic efficiency, stability, and recyclability is clarified. Furthermore, the challenges and future work directions for the practical application of MOF-derived catalysts in AOPs for wastewater treatment are provided.

A review on future wastewater treatment technologies: micro-nanobubbles, hybrid electro-Fenton processes, photocatalytic fuel cells, and microbial fuel cells

The future prospect in wastewater treatment technologies mostly emphasizes processing efficiency and the economic benefits. Undeniably, the use of advanced oxidation processes in physical and chemical treatments has played a vital role in helping the technologies to remove the organic pollutants efficiently and reduce the energy consumption or even harvesting the electrons movements in the oxidation process to produce electrical energy. In the present paper, we review several types of wastewater treatment technologies, namely micro-nanobubbles, hybrid electro-Fenton processes, photocatalytic fuel cells, and microbial fuel cells. The aims are to explore the interaction of hydroxyl radicals with pollutants using these wastewater technologies, including their removal efficiencies, optimal conditions, reactor setup, and energy generation. Despite these technologies recording high removal efficiency of organic pollutants, the selection of the technologies is dependent on the characteristics of the wastewater and the daily production volume. Hence the review paper also provides comparisons between technologies as the guidance in technology selection.

Generation of hydroxyl radicals by metal-free bifunctional electrocatalysts for enhanced organics removal

Low yields of H₂O₂ and a narrow range of appropriate pH values have been two major drawbacks for electro-Fenton (EF) process. Herein, metal-free electrochemical advanced oxidation processes (EAOPs) were developed with nitrogen and sulfur co-doped electrochemically exfoliated graphene (N, S-EEGr) electrocatalysts, which was confirmed as an outstanding bifunctional catalyst for synchronous generation and activation of H₂O₂ via (2 + 1) e^- consecutive reduction reactions. Specifically, two elements (N, S) in metal-free N, S-EEGr-CF cathode synergize to promote the formation of H₂O₂ followed by its activation. With N, S-EEGr-CF cathode, phenol of initial 50 mg L^-1 could be effectively removed within pH 3-11 and 6.25 mA cm^-2, and 100% removal efficiency could be achieved within 15-min even at neutral pH. The pseudo-first-order rate constant for phenol removal in metal-free EAOPs with N,S-EEGr-CF at neutral pH was 10 times higher than that with EF process. Detection of active species, coupled with decay kinetics with specific trapping agents, confirmed that OH was the dominant oxidizing species promoting removal efficiencies of organics (phenol, antibiotics and dyes) at pH 3 and pH 7. In the actual wastewater treatment, the synergistic effect of bifunctional catalyst would also be used for improving the degradation efficiency of organics. Thus, the metal-free EAOPs with N,S-EEGr-CF cathode may serve as an alternative in wastewater treatment with a broadened range of solution pH values and avoiding Fe²⁺ (catalyst) addition.