Hybrid graphene-decorated metal hollow fibre membrane reactors for efficient electro-Fenton - Filtration co-processes

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.

Subtle tuning of nanodefects actuates highly efficient electrocatalytic oxidation

Achieving controllable fine-tuning of defects in catalysts at the atomic level has become a zealous pursuit in catalysis-related fields. However, the generation of defects is quite random, and their flexible manipulation lacks theoretical basis. Herein, we present a facile and highly controllable thermal tuning strategy that enables fine control of nanodefects via subtle manipulation of atomic/lattice arrangements in electrocatalysts. Such thermal tuning endows common carbon materials with record high efficiency in electrocatalytic degradation of pollutants. Systematic characterization and calculations demonstrate that an optimal thermal tuning can bring about enhanced electrocatalytic efficiency by manipulating the N-centered annulation-volatilization reactions and C-based sp³/sp² configuration alteration. Benefiting from this tuning strategy, the optimized electrocatalytic anodic membrane successfully achieves >99% pollutant (propranolol) degradation during a flow-through (~2.5 s for contact time), high-flux (424.5 L m^-2 h^-1), and long-term (>720 min) electrocatalytic filtration test at a very low energy consumption (0.029 ± 0.010 kWh m^-3 order^-1). Our findings highlight a controllable preparation approach of catalysts while also elucidating the molecular level mechanisms involved.

A comprehensive review on reactive oxygen species (ROS) in advanced oxidation processes (AOPs)

In this account, the reactive oxygen species (ROS) were comprehensively reviewed, which were based on electro-Fenton and photo-Fenton processes and correlative membrane filtration technology. Specifically, this review focuses on the fundamental principles and applications of advanced oxidation processes (AOPs) based on a series of nanomaterials, and we compare the pros and cons of each method and point out the perspective. Further, the emerging reviews regarding AOPs rarely emphasize the involved ROS and consider the convenience of radical classification and transformation mechanism, such a review is of paramount importance to be needed. Owing to the strong oxidation ability of radical (e.g., •OH, O₂^•-, and SO₄^•-) and non-radical (e.g., ¹O₂ and H₂O₂), these ROS would attack the organic contaminants of emerging concern, thus achieving the goal of environmental remediation. Hopefully, this review can offer detailed theoretical guidance for the researchers, and we believe it able to offer the frontier knowledge of AOPs for wastewater treatment plants (WWTPs).

A critical review on paracetamol removal from different aqueous matrices by Fenton and Fenton-based processes, and their combined methods

Paracetamol (PCT), also known as acetaminophen, is a drug used to treat fever and mild to moderate pain. After consumption by animals and humans, it is excreted through the urine to the sewer systems, wastewater treatment plants, and other aquatic/natural environments. It has been detected in trace amounts in effluents of wastewater plant treatments, sewage sludge, hospital wastewaters, surface waters, and drinking water. PCT can cause genetic code damage, oxidative degradation of lipids, and denaturation of protein in cells, and its toxicity has been well-proven in bacteria, algae, macrophytes, protozoan, and fishes. To avoid its harmful health problems over living beings, powerful Fenton and Fenton-based treatments as pre-eminent advanced oxidation processes (AOPs) have been developed because of the inefficient treatment by conventional treatments. This paper presents a comprehensive and critical review over the application of such Fenton technologies to remove PCT from natural waters, synthetic wastewaters, and real wastewaters. The characteristics and main results obtained using Fenton, photo-Fenton, electro-Fenton, and photoelectro-Fenton are described, making special emphasis in the oxidative action of the generated reactive oxygen species. Hybrid processes based on the coupling with ultrasounds, gamma radiation, photocatalysis, photoelectrocatalysis, zero-valent iron-activated persulfate, adsorption, and microbial fuel cells, are analyzed. Sequential treatments involving the initiation with plasma gliding arc discharge and post-biological process are detailed. Comparative results with other available AOPs are also described and discussed. Finally, 13 aromatic by-products and 9 short-linear aliphatic carboxylic acid detected during the PCT removal by Fenton and Fenton-based processes are reported, with the proposal of three parallel pathways for its initial degradation.

Recent advances in electrocatalytic membrane for the removal of micropollutants from water and wastewater

The increasing occurrence of micropollutants in water and wastewater threatens human health and ecological security. Electrocatalytic membrane (EM), a new hybrid water treatment platform that integrates membrane separation with electrochemical technologies, has attracted extensive attention in the removal of micropollutants from water and wastewater in the past decade. Here, we systematically review the recent advances of EM for micropollutant removal from water and wastewater. The mechanisms of the EM for micropollutant removal are first introduced. Afterwards, the related membrane materials and operating conditions of the EM are summarized and analyzed. Lastly, the challenges and future prospects of the EM in research and applications are also discussed, aiming at a more efficient removal of micropollutants from water and wastewater.

Modified Polyethersulfone Ultrafiltration Membrane for Enhanced Antifouling Capacity and Dye Catalytic Degradation Efficiency

Catalytic membranes, as a combination of heterogeneous advanced oxidation and membrane technology reaction systems, have important application prospects in the treatment of dyes and other organics. In practical applications, it is still challenging to construct catalytic membranes with excellent removal efficiency and fouling mitigation. Herein, molybdenum disulfide-iron oxyhydroxide (MoS2-FeOOH) was fabricated using iron oxide and MoS2 nanoflakes, which were synthesized by the hydrothermal method. Furthermore, by changing the concentration of MoS2-FeOOH, the MoS2-FeOOH/polyethersulfone (PES) composite ultrafiltration membrane was obtained with improved hydrophilicity, permeability, and antifouling capacity. The pure water flux of the composite membrane reached 385.3 L/(m2 h), which was 1.7 times that of the blank PES membrane. Compared with the blank membrane, with the increase of MoS2-FeOOH content, the MoS2-FeOOH/PES composite membranes had better adsorption capacity and catalytic performance, and the membrane with 3.0% MoS2-FeOOH content (M4) could be achieved at a 60.2% methylene blue (MB) degradation rate. In addition, the membrane flux recovery ratio (FRR) of the composite membrane also increased from 25.6% of blank PES membrane (M0) to more than 70% after two cycles of bovine serum albumin (BSA) filtration and hydraulic cleaning. The membrane with 2.25% MoS2-FeOOH content (M3) had the best antifouling performance, with the largest FRR and the smallest irreversible ratio (Rir). Catalytic self-cleaning of the composite membrane M3 recovered 95% of the initial flux with 0.1 mol/L H2O2 cleaning. The MoS2-FeOOH/PES composite membranes with the functions of excellent rejection and antifouling capacity have a good prospect in the treatment of printing and dyeing wastewater composed of soluble dyes.

A critical review on the electrosorption of organic compounds in aqueous effluent - Influencing factors and engineering considerations

Despite being an old process from the end of the 19^th century, electrosorption has attracted renewed attention in recent years because of its unique properties and advantages compared to other separation technologies and due to the concomitant development of new porous electrode materials. Electrosorption offer the advantage to separate the pollutants from wastewater with the possibility of selectively adsorbing and desorbing the targeted compounds. A comprehensive review of electrosorption is provided with particular attention given to the electrosorption of organic compounds, unlike existing capacitive deionization review papers that only focus on inorganic salts. The background and principle of electrosorption are first presented, while the influence of the main parameters (e.g., electrode materials, electrode potential, physico-chemistry of the electrolyte solutions, type of compounds, co-sorption effect, reactor design, etc.) is then detailed and the modeling and engineering aspects are discussed. Finally, the main output and future prospects about recovery studies and combination between electro-sorption/desorption and degradation processes are given. This review particularly highlights that carbon-based materials have been mostly employed (85% of studies) as porous electrode in organics electrosorption, while existing studies lack of electrode stability and durability tests in real conditions. These electrodes have been implemented in a fixed-bed reactor design most of the time (43% of studies) due to enhanced mass transport. Moreover, the electrode potential is a major criterion: it should be applied in the non-faradaic domain otherwise unwanted reactions can easily occur, especially the corrosion of carbon from 0.21 V/standard hydrogen electrode or the water oxidation/reduction. Furthermore, there is lack of studies performed with actual effluents and without addition of supporting electrolyte, which is crucial for testing the real efficiency of the process. The associated predictive model will be required by considering the matrix effect along with transport phenomena and physico-chemical characteristics of targeted organic compounds.

Removal of p-toluenesulfonic acid from wastewater using a filtration-enhanced electro-Fenton reactor

A filtration-enhanced electro-Fenton reactor was developed for the removal of p-toluenesulfonic acid from wastewater.

Rapid electrochemical recognition of trimethoprim in human urine samples using new modified electrodes (CPE/Ag/Au NPs) analysing tunable electrode properties: experimental and theoretical studies

Pharmaceutical effluents are a serious environmental issue, which require to be treated by a suitable technique; thus, the electrochemical process is actively considered as a viable method for the treatment. In this work, new carbon paste electrodes (CPEs) were fabricated by compressing gold and silver nanoparticles (NPs), namely, CPE/Ag NPs, CPE/Au NPs, and CPE/Ag/Au NPs and then completely characterized by different analytical methods. The performance of the electrodes was studied after determining their surface area (×10^-6 cm²) as 4.17, 5.05, 5.27, and 5.12, producing high anodic currents for K₄[Fe(CN)₆] compared to the commercial electrode. This agrees with the results of impedance study, where the electron transfer rate constants (k_app, ×10^-3 cm s^-1) were determined to be 28.7, 42.6, 41.0, and 101.4 for CPE, CPE/Ag NPs, CPE/Au NPs, and CPE/Ag/Au NPs, respectively, through the Bode plot-phase shifts. This is consistent with the charge transfer resistance (R_CT, Ω), resulting as 171 for CPE/Ag/Au NPs < 395 for CPE/Ag NPs < 427 for CPE/Au NPs and < 742 for CPE. Therefore, these electrodes were employed to detect trimethoprim (TMP) since metallic NPs contribute good crystallinity, stability, conduciveness, and surface plasmon resonance to the CPE, convalescing the sensitivity; comprehensively, they were applied for its detection in real water and human urine samples, and the limit of detection (LOD) was as low as 0.026, 0.032, and 0.026 μmol L^-1 for CPE/Ag NPs, CPE/Au NPs, and CPE/Ag/Au NPs, respectively. In contrast, unmodified CPE was unable to detect TMP due to the lack of efficiency. The developed technique shows excellent electrochemical recovery of 92.3 and 97.1% in the urine sample. Density functional theory (DFT) was used to explain the impact of the metallic center in graphite through density of states (DOS).

3D Self-Supported Nitrogen-Doped Carbon Nanofiber Electrodes Incorporated Co/CoOx Nanoparticles: Application to Dyes Degradation by Electro-Fenton-Based Process

We developed free-standing nitrogen-doped carbon nanofiber (CNF) electrodes incorporating Co/CoO_x nanoparticles (NPs) as a new cathode material for removing Acid Orange 7 (AO7; a dye for wool) from wastewater by the heterogeneous electro-Fenton reaction. We produced the free-standing N-doped CNF electrodes by electrospinning a polyacrylonitrile (PAN) and cobalt acetate solution followed by thermal carbonation of the cobalt acetate/PAN nanofibers under a nitrogen atmosphere. We then investigated electro-Fenton-based removal of AO7 from wastewater with the free-standing N-doped-CNFs-Co/CoO_x electrodes, in the presence or not of Fe²⁺ ions as a co-catalyst. The electrochemical analysis showed the high stability of the prepared N-doped-CNF-Co/CoO_x electrodes in electrochemical oxidation experiments with excellent degradation of AO7 (20 mM) at acidic to near neutral pH values (3 and 6). Electro-Fenton oxidation at 10 mA/cm² direct current for 40 min using the N-doped-CNF-Co/CoO_x electrodes loaded with 25 wt% of Co/CoO_x NPs led to complete AO7 solution decolorization with total organic carbon (TOC) removal values of 92.4% at pH 3 and 93.3% at pH 6. The newly developed N-doped-CNF-Co/CoO_x electrodes are an effective alternative technique for wastewater pre-treatment before the biological treatment.

Hollow fibers: from fabrication to applications

Hollow fibers have attracted more and more attention due to their broad range of applications in numerous fields. We review the latest advance and summarize the fabrication methods, types and applications of hollow fibers. We mainly introduce the fabrication methods of hollow fibers, including co-extrusion/co-axial spinning methods, template methods, 3D printing methods, electrospinning methods, self-crimping methods and gas foaming process. Meanwhile, we summarize four types of hollow fibers: one-layered hollow fibers, multi-layered hollow fibers, multi-hollow fibers and branched hollow fibers. Next, we focus on the main applications of hollow fibers, such as gas separation, cell culture, microfluidic channels, artificial tubular tissues, etc. Finally, we present the prospects of the future trend of development. The review would promote the further development of hollow fibers and benefit their advance in sensing, bioreactors, electrochemical catalysis, energy conversion, microfluidics, gas separation, air purification, drug delivery, functional materials, cell culture and tissue engineering. This review has great significance for the design of new functional materials and development of devices and systems in the related fields.

Membrane Separation Coupled with Electrochemical Advanced Oxidation Processes for Organic Wastewater Treatment: A Short Review

Research on the coupling of membrane separation (MS) and electrochemical advanced oxidation processes (EAOPs) has been a hot area in water pollution control for decades. This coupling aims to greatly improve water quality and focuses on the challenges in practical application to provide a promising solution to water shortage problems. This article provides a summary of the coupling configurations of MS and EAOPs, including two-stage and one-pot processes. The two-stage process is a combination of MS and EAOPs where one process acts as a pretreatment for the other. Membrane fouling is reduced when setting EAOPs before MS, while mass transfer is promoted when placing EAOPs after MS. A one-pot process is a kind of integration of two technologies. The anode or cathode of the EAOPs is fabricated from porous materials to function as a membrane electrode; thus, pollutants are concurrently separated and degraded. The advantages of enhanced mass transfer and the enlarged electroactive area suggest that this process has excellent performance at a low current input, leading to much lower energy consumption. The reported conclusions illustrate that the coupling of MS and EAOPs is highly applicable and may be widely employed in wastewater treatment in the future.

Graphene Oxide Membranes for Trace Hydrocarbon Contaminant Removal from Aqueous Solution

The aim of this paper is to shed light on the application of graphene oxide (GO) membranes for the selective removal of benzene, toluene, and xylene (BTX) from wastewater. These molecules are present in traces in the water produced from oil and gas plants and are treated now with complex filtration systems. GO membranes are obtained by a simple, fast, and scalable method. The focus of this work is to prove the possibility of employing GO membranes for the filtration of organic contaminants present in traces in oil and gas wastewater, which has never been reported. The stability of GO membranes is analyzed in water solutions with different pH and salinity. Details of the membrane preparation are provided, resulting in a crucial step to achieve a good filtration performance. Material characterization techniques such as electron microscopy, x-ray diffraction, and infrared spectroscopy are employed to study the physical and chemical structure of GO membranes, while gas chromatography, UV-visible spectroscopy, and gravimetric techniques allow the quantification of their filtration performance. An impressive rejection of about 90% was achieved for 1 ppm of toluene and other pollutants in water, demonstrating the excellent performance of GO membranes in the oil and gas field.

In-situ electro-generation and activation of hydrogen peroxide using a CuFeNLDH-CNTs modified graphite cathode for degradation of cefazolin

The modified multifunctional electrodes for electro-Fenton (EF) process are suggested to be promising cathodes for in situ electro-generation and activation of H₂O₂ to produce hydroxyl radicals (^•OH). However, heterogeneous EF process still faces the challenges of limited catalytic activity and releasing of massive amounts of transition metals to the solution after removal of organic pollutants. The main aim of the present investigation was to prepare a cathode containing carbon nanotubes (CNTs) and CuFe nano-layered double hydroxide (NLDH) for degradation and mineralization of cefazolin antibiotic through electro-Fenton process. Structural and electrochemical analyses demonstrated that CuFeNLDH-CNTs nanocomposite was successfully incorporated on the surface of graphite cathode. Due to the increased formation of ^•OH in the reactor, the incorporation of CNTs into NLDH matrix with a catalyst loading of 0.1 g substantially improved the degradation efficiency of cefazolin (89.9%) in comparison with CNTs-coated (28.7%) and bare graphite cathode (22.8%) within 100 min. In the presence of 15 mM of ethanol, the degradation efficiency of cefazolin was remarkably decreased to 43.7% by the process, indicating the major role of ^•OH in the destruction of target molecules. Acidic conditions favored the degradation efficiency of cefazolin by the modified EF process. Mineralization efficiency of the bio-refractory compound was obtained to be 70.1% in terms of chemical oxygen demand (COD) analysis after 300 min. The gas chromatography-mass spectroscopy (GC-MS) analysis was also implemented to identify the intermediate byproducts generated during the degradation of cefazolin in the CuFeNLDH-CNTs/EF reactor.

Graphene-Modified Composites and Electrodes and Their Potential Applications in the Electro-Fenton Process

In recent years, graphene-based materials have been identified as an emerging and promising new material in electro-Fenton, with the potential to form highly efficient metal-free catalysts that can be employed in the removal of contaminants from water, conserving precious water resources. In this review, the recent applications of graphene-based materials in electro-Fenton are described and discussed. Initially, homogenous and heterogenous electro-Fenton methods are briefly introduced, highlighting the importance of the generation of H2O2 from the two-electron reduction of dissolved oxygen and its catalysed decomposition to produce reactive and oxidising hydroxy radicals. Next, the promising applications of graphene-based electrodes in promoting this two-electron oxygen reduction reaction are considered and this is followed by an account of the various graphene-based materials that have been used successfully to give highly efficient graphene-based cathodes in electro-Fenton. In particular, graphene-based composites that have been combined with other carbonaceous materials, doped with nitrogen, formed as highly porous aerogels, three-dimensional materials and porous gas diffusion electrodes, used as supports for iron oxides and functionalised with ferrocene and employed in the more effective heterogeneous electro-Fenton, are all reviewed. It is perfectly clear that graphene-based materials have the potential to degrade and mineralise dyes, pharmaceutical compounds, antibiotics, phenolic compounds and show tremendous potential in electro-Fenton and other advanced oxidation processes.

Enhanced degradation of antibiotics by photo-fenton reactive membrane filtration

Micropollution such as pharmaceutical residuals potentially compromises water quality and jeopardizes human health. This study evaluated the photo-Fenton ceramic membrane filtration toward the removal of sulfadiazine (SDZ) as a common antibiotic chemical. The batch experiments verified that the photo-Fenton reactions with as Goethite (α-FeOOH) as the photo-Fenton catalyst achieved the degradation rates of 100% within 60 min with an initial SDZ concentration of 12 mg·L^-1. Meanwhile, a mineralization rate of over 80% was obtained. In continuous filtration, a negligible removal rate (e.g., 4%) of SDZ was obtained when only filtering the feed solution with uncoated or catalyst-coated membranes. However, under Ultraviolet (UV) irradiation, both the removal rates of SDZ were significantly increased to 70% (no H₂O₂) and 99% (with H₂O₂), respectively, confirming the active degradation by the photo-Fenton reactions. The highest apparent quantum yield (AQY) reached up to approximately 25% when the UV₂₅₄ intensity was 100 μW·cm^-2 and H₂O₂ was 10 mmol·L^-1. Moreover, the photo-Fenton reaction was shown to effectively mitigate fouling and prevent flux decline. This study demonstrated synchronization of photo-Fenton reactions and membrane filtration to enhance micropollutant degradation. The findings are also important for rationale design and operation of photo-Fenton or photocatalytic membrane filtration systems.

Highly efficient electro-generation of H2O2by a nitrogen porous carbon modified carbonaceous cathode during the oxygen reduction reaction

In this work, active carbon fibers (ACFs) were modified with nitrogen-doped porous carbon (NPC) and carbon nanotubes (CNTs) to prepare a novel modified electrode as a cathode.