Carbon nanotubes intercalated RGO electro-Fenton membrane for coenhanced permeability, rejection and catalytic oxidation of organic micropollutants

Electrified activation of peroxymonosulfate using carbonaceous composite membranes for sulfamethoxazole removal: Treatment efficiency, mechanistic insights, and intermediate toxicity evaluation

Metal-free carbonaceous materials can effectively activate peroxymonosulfate (PMS) for organic pollutant degradation by utilizing their surface active sites. In this study, an electrified membrane was fabricated using nitrogen-doped graphene (NG) sheets and carbon nanotubes (N-CNTs) to investigate the impact of different electrified modes on membrane performance for sulfamethoxazole (SMX) removal from water matrices. Characterization results indicated that NG/N-CNT mats exhibited superior electron transfer ability for PMS activation due to their abundant defects and nitrogen-doped species. When used as the cathode (Mode III), the carbon mats achieved a pseudo-first-order kinetic constant (kobs) of 5.305 s-1 (318.2 min-1) for SMX removal, which was 51.63 % and 24.95 % higher than those in Mode I (no applied potentials) and Mode II (carbon anode), respectively. Reactive oxygen species identification revealed that non-radical pathways govern in-situ catalytic oxidation, with the relative contributions of surface-confined oxidation and singlet oxygenation significantly varying across different electrified modes. In contrast, Mode III significantly enhanced PMS activation, minimized the depletion of active sites (such as defects and pyridinic nitrogen), and reduced the accumulation of oxidation intermediates within the carbon mats. After continuous filtration of 12,000 bed volumes of carbon mats, Mode III still achieved over 85 % SMX removal from real river water while maintaining a high water flux of 116 L m-2.h-1.bar-1. Intermediate composition analysis demonstrated that the filtrate from Mode III posed lower risks of acute and developmental toxicity compared to those from the other electrified modes. These findings provide a reliable and enhanced approach for efficient in-situ catalytic oxidation.

Electrochemical filtration for drinking water purification: A review on membrane materials, mechanisms and roles

Electrochemical filtration can not only enrich low concentrations of pollutants but also produce reactive oxygen species to interact with toxic pollutants with the assistance of a power supply, making it an effective strategy for drinking water purification. In addition, the application of electrochemical filtration facilitates the reduction of pretreatment procedures and the use of chemicals, which has outstanding potential for maximizing process simplicity and reducing operating costs, enabling the production of safe drinking water in smaller installations. In recent years, the research on electrochemical filtration has gradually increased, but there has been a lack of attention on its application in the removal of low concentrations of pollutants from low conductivity water. In this review, membrane substrates and electrocatalysts used to improve the performance of electrochemical membranes are briefly summarized. Meanwhile, the application prospects of emerging single-atom catalysts in electrochemical filtration are also presented. Thereafter, several electrochemical advanced oxidation processes coupled with membrane filtration are described, and the related working mechanisms and their advantages and shortcomings used in drinking water purification are illustrated. Finally, the roles of electrochemical filtration in drinking water purification are presented, and the main problems and future perspectives of electrochemical filtration in the removal of low concentration pollutants are discussed.

Rational design of a hydrophilic nanoarray-structured electro-Fenton membrane for antibiotics removal and fouling mitigation: An intensified catalysis process in an oxygen vacancy-mediated cathodic microreactor

Electro-Fenton membranes (EFMs) can synchronously realize organic micropollutants destruction and fouling mitigation in a single filtration process with the assistance of hydroxyl radicals (•OH). Herein, a nanoarray-structured EFM (NS-EFM) was designed by assembling Fenton reactive CoFe-LDH nanowires using a low-temperature hydrothermal method. Combined with a defect-engineering strategy, the oxygen vacancies (OVac) in the CoFe-LDH nanoarrays were tailored by manipulating the stoichiometry of cations to optimize the Fenton reactivity of NS-EFMs. The optimized NS-EFM demonstrated exceptional sulfamethoxazole (SMX) removal (99.4%) and fast degradation kinetics (0.0846 min-1), but lower energy consumption (0.22 kWh m-3 per log removal of SMX). In-depth mechanism analysis revealed that the intrinsic electronic properties of OVac endowed NS-EFM with enhanced reactivity and charge transferability at metallic active sites of CoFe-LDH, thereby intensifying •OH generation. Besides, the nanoarray-structured NS-EFM built a confined microreactor space, leading to expedited •OH microflow to SMX. Meanwhile, the hydrophilic nature of CoFe-LDH nanoarrays synergistically contributed to the high flux recovery (95.0%) and minimal irreversible membrane fouling (5.0%), effectively alleviating membrane fouling within pores and on surfaces. This study offers insights into the potential of defect engineering as a foundational strategy in the design of EFMs, significantly advancing the treatment of organic pollutants and control of membrane fouling.

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.

Electrochemical degradation of favipiravir (anti-viral) drug from aqueous solution: optimization of operating parameters using the response surface method

The aim of the current study is to investigate the efficacy of the electro-Fenton process in the degradation of favipiravir drugs from aqueous solutions, which has increased in use as a result of the COVID-19 pandemic. The Response Surface Methodology (RSM) was developed using a Central Composite Design (CCD) in which five independent variables, including Fe2+ concentration, current density, initial FVP concentration, pH, and reaction time, were coded with high and low levels, and the maximum removal percentage of FVP (97.8%) and COD (91.65%) were determined as responses. In the EF process, 530 mg/L H2O2 was produced in-situ by cathodic reduction of O2 in aqueous solution and thus FVP has been successfully oxidized through hydroxyl radicals. The H2O2/Fe2+ ratio was determined to be 0.51 under optimum conditions. At the end of the experiment, the maximum energy consumption was found to be 2.12 kWh per g COD. The FVP was completely mineralized in a very short time by the EF process, according to the LC-MS/MS examination. The EF process followed the pseudo first-order kinetic model with the rate constants of 0.023, 0.016 and 0.006 1/min for pH 2, 3 and 4, respectively. According to the findings of this study, the electro-Fenton process is an effective method for removing FVP from aqueous solutions. To the authors' knowledge, this is the first study to show the degradation and optimum conditions of FVP in aqueous solution using the electro-Fenton (EF) process.

A critical review on graphene oxide membrane for industrial wastewater treatment

An important way to promote the environmental industry's goal of carbon reduction is to promote the recycling of resources. Membrane separation technology has unique advantages in resource recovery and advanced treatment of industrial wastewater. However, the great promise of traditional organic membrane is hampered by challenges associated with organic solvent tolerance, lack of oxidation resistance, and serious membrane fouling control. Moreover, the high concentrations of organic matter and inorganic salts in the membrane filtration concentrate also hinder the wider application of the membrane separation technology. The emerging cost-effective graphene oxide (GO)-based membrane with excellent resistance to organic solvents and oxidants, more hydrophilicity, lower membrane fouling, better separation performance has been expected to contribute more in industrial wastewater treatment. Herein, we provide comprehensive insights into the preparation and characteristic of GO membranes, as well as current research status and problems related to its future application in industrial wastewater treatment. Finally, concluding remarks and future perspectives have been deduced and recommended for the GO membrane separation technology application for industrial wastewater treatment, which leads to realizing sustainable wastewater recycling and a nearly "zero discharge" water treatment process.

Source prevention of halogenated antibiotic resistance genes proliferation: UV/sulfite advanced reduction process achieved accurate and efficient elimination of florfenicol antibacterial activity

The production and consumption of halogenated antibiotics, such as florfenicol (FLO), remain high, accompanied by a large amount of antibiotic-containing wastewater, which would induce the potential proliferation and transmission of antibiotic resistance genes (ARGs) in conventional biological systems. This study revealed that the introduction of reductive species (mainly H) by adding sulfite during UV irradiation process accelerated the decomposition rate of FLO, increasing from 0.1379 min^-1 in the single UV photolytic system to 0.3375 min^-1 in the UV/sulfite system. The enhanced photodecomposition in UV/sulfite system was attributed to the improved dehalogenation performance and additional removal of sulfomethyl group at the site of the benzene ring, which were the representative structures consisting of FLO antibacterial activity. Compared with single UV photolysis, UV/sulfite advanced reduction process saved the light energy requirement by 40 % for the evolutionary suppression of floR, and its corresponding class of ARGs in subsequent biotreatment system was controlled at the level of the negative group. Compared with UV/H₂O₂ and UV/persulfate systems, the decomposition rate of FLO in the UV/S system was the highest and preserved the corresponding carbon source of the coexisting organic compounds for the potential utilization of microbial metabolism in subsequent biotreatment process. These results demonstrated that UV/sulfite advanced reduction process could be adopted as a promising pretreatment option for the source prevention of representative ARGs proliferation of halogenated antibiotics in subsequent biotreatment process.

Ganzoury M, Zhang N, de Lannoy CF. Nanocomposite Polymeric Membranes for Organic Micropollutant Removal: A Critical Review

The prevalence of organic micropollutants (OMPs) and their persistence in water supplies have raised serious concerns for drinking water safety and public health. Conventional water treatment technologies, including adsorption and biological treatment, are known to be insufficient in treating OMPs and have demonstrated poor selectivity toward a wide range of OMPs. Pressure-driven membrane filtration has the potential to remove many OMPs detected in water with high selectivity as a membrane's molecular weight cutoff (MWCO), surface charge, and hydrophilicity can be easily tailored to a targeted OMP's size, charge and octanol-water partition coefficient (Kow). Over the past 10 years, polymeric (nano)composite microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF) membranes have been extensively synthesized and studied for their ability to remove OMPs. This review discusses the fate and transport of emerging OMPs in water, an assessment of conventional membrane-based technologies (NF, reverse osmosis (RO), forward osmosis (FO), membrane distillation (MD) and UF membrane-based hybrid processes) for their removal, and a comparison to the state-of-the-art nanoenabled membranes with enhanced selectivity toward specific OMPs in water. Nanoenabled membranes for OMP treatment are further discussed with respect to their permeabilities, enhanced properties, limitations, and future improvements.

Progress of homogeneous and heterogeneous electro-Fenton treatments of antibiotics in synthetic and real wastewaters. A critical review on the period 2017-2021

Antibiotics are widely supplied over all the world to animals and humans to fight and heal bacteriological diseases. The uptake of antibiotics has largely increased the average-life expectancy of living beings. However, these recalcitrant products have been detected at low concentrations in natural waters, with potential health risks due to alterations in food chains and an increase in the resistance to bacterial infection, control of infectious diseases, and damage of the beneficial bacteria. The high stability of antibiotics at mild conditions prevents their effective removal in conventional wastewater treatment plants. A powerful advanced oxidation processes such as the electro-Fenton (EF) process is being developed as a guarantee for their destruction by ^•OH generated as strong oxidant. This review presents a critical, exhaustive, and detailed analysis on the application of EF to remediate synthetic and real wastewaters contaminated with common antibiotics, covering the period 2017-2021. Homogeneous EF and heterogeneous EF involving iron solid catalysts or iron functionalized cathodes, as well as their hybrid and sequential treatments, are exhaustively examined. Their fundamentals and characteristics are detailed, and the main results obtained for the removal of the most used antibiotic families are carefully described and discussed. The role of generated oxidizing agents is explained, and the by-products generated, and reaction sequences proposed are detailed.

Confining peroxymonosulfate activation in carbon nanotube intercalated nitrogen doped reduced graphene oxide membrane for enhanced water treatment: The role of nanoconfinement effect

Coupling membrane filtration with peroxymonosulfate (PMS) activation is promising to overcome the selectivity-permeability trade-off in membrane-based water treatment. However, the PMS catalytic efficiency of membrane still needs improvement to offset the insufficient reaction time during filtration process. Herein, an oxidized carbon nanotube intercalated nitrogen doped reduced graphene oxide (NRGO-OCNT) membrane with PMS activation function was firstly designed and prepared, which confined PMS activation in membrane interlayer for enhanced water treatment. The influence of confinement scale on membrane performance was studied through changing the OCNT intercalation ratio. Under the optimal confinement condition, the NRGO-OCNT membrane filtration integrated with PMS activation (MFPA) could realize 100% 4-chlorophenol removal at a high permeate flux of 290.2 L m^-2 h^-1 bar^-1 (retention time of only 0.36 s), whose performance was 2.8, 1.7 and 5.0 times higher than that of filtration alone, NRGO MFPA (excessive confinement) and NRGO-OCNT powder-based batch reaction (no confinement), respectively. Moreover, NRGO-OCNT MFPA preferentially removed smaller-sized organics which easily entered and diffused in confined interlayer. The outstanding performance of NRGO-OCNT MFPA was owing to the nanoconfinement effect in appropriate confined interspacing, where the mass transfer rate of reactants was greatly boosted for enhanced generation of SO₄^- and OH towards pollutant.

Toward Selective Oxidation of Contaminants in Aqueous Systems

The presence of diverse pollutants in water has been threating human health and aquatic ecosystems on a global scale. For more than a century, chemical oxidation using strongly oxidizing species was one of the most effective technologies to destruct pollutants and to ensure a safe and clean water supply. However, the removal of increasing amount of pollutants with higher structural complexity, especially the emerging micropollutants with trace concentrations in the complicated water matrix, requires excessive dosage of oxidant and/or energy input, resulting in a low cost-effectiveness and possible secondary pollution. Consequently, it is of practical significance but scientifically challenging to achieve selective oxidation of pollutants of interest for water decontamination. Currently, there are a variety of examples concerning selective oxidation of pollutants in aqueous systems. However, a systematic understanding of the relationship between the origin of selectivity and its applicable water treatment scenarios, as well as the rational design of catalyst for selective catalytic oxidation, is still lacking. In this critical review, we summarize the state-of-the-art selective oxidation strategies in water decontamination and probe the origins of selectivity, that is, the selectivity resulting from the reactivity of either oxidants or target pollutants, the selectivity arising from the accessibility of pollutants to oxidants via adsorption and size exclusion, as well as the selectivity due to the interfacial electron transfer process and enzymatic oxidation. Finally, the challenges and perspectives are briefly outlined to stimulate future discussion and interest on selective oxidation for water decontamination, particularly toward application in real scenarios.

FeS2/carbon felt as an efficient electro-Fenton cathode for carbamazepine degradation and detoxification: In-depth discussion of reaction contribution and empirical kinetic model

Carbamazepine (CBZ) decay by electro-Fenton (EF) oxidation using a novel FeS₂/carbon felt (CF) cathode, instead of a soluble iron salt, was studied with the aim to accelerate the reaction between H₂O₂ and ferrous ions, which helps to produce more hydroxyl radicals (^•OH) and eliminate iron sludge. First, fabricated FeS₂ and its derived cathode were characterized by scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. Anodes were then screened, with DSA (Ti/IrO₂-RuO₂) showing the best performance under EF oxidation regarding CBZ degradation and electrochemical characterization. Several operating parameters of this EF process, such as FeS₂ loading, current density, gap between electrodes (GBE), initial [CBZ], and electrolyte type, were also investigated. Accordingly, a nonconsecutive empirical kinetic model was established to predict changes in CBZ concentration under the given operational parameters. The contribution of different oxidation types to the EF process was calculated using kinetic analysis and quenching experiments to verify the role of the FeS₂-modified cathode. The reaction contributions of anodic oxidation (AO), H₂O₂ electrolysis (EP), and EF oxidation to CBZ removal were 12.81%, 7.41%, and 79.77%, respectively. The ^•OH exposure of EP and EF oxidation was calculated, confirming that ^•OH exposure was approximately 22.45-fold higher using FeS₂-modified CF. Finally, the 19 intermediates formed by CBZ degradation were identified by ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry. Accordingly, four CBZ degradation pathways were proposed. ECOSAR software was used to assess the ecotoxicity of intermediates toward fish, daphnia, and green algae, showing that this novel EF oxidation process showed good toxicity reduction performance. A prolonged EF retention time was proposed to be necessary to obtain clean and safe water, even if the targeted compound was removed at an earlier time.