Prospects of an Electroactive Carbon Nanotube Membrane toward Environmental Applications

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.

Highly efficient metal-free electrochemical membrane enables zero-valent sulfur recovery from thiourea wastewater

Electrochemical recovery of zero-valent sulfur (S0) from thiourea (TU) wastewater offers a promising waste-to-value strategy that expects to promote the sulfur resource cycle in water treatment but still suffer from electrode poisoning and sulfur over-oxidation. Herein, we designed a metal-free CNT electrochemical membrane for selective oxidation of thiourea and recovery of S0. We found that defect sites on the carbon nanotube surface enable direct electron transfer for thiourea oxidation and may form carbon-sulfur bridge bonds, thereby facilitating the generation of S0 and urea. When treating real industrial wastewater with a concentration of 83.1 mg L-1 thiourea, the CNT membrane system could achieve a thiourea removal efficiency of 95.4% and a S0 recovery rate of 83.8% at 150 L m-2 h-1. The corresponding treatment energy consumption and S0 recovery energy consumption are 3.3 ± 0.2 kWh kg-TU-1 and 9.9 ± 1.0 kWh kg-S-1, respectively, lower than that of existing commercial electrodes such as Ti/SnO₂-Sb, PbO₂, Ru/IrO₂, and BDD. More importantly, the membrane system exhibits a desirable durability and stability during a 48-h continuous flow operation. Our findings advance the electrochemical membrane design for efficient S0 recovery from thiourea wastewater, contributing to the realization of economical and sustainable waste management and reclaiming resource.

Confined iron-based nanomaterials for water decontamination: Fundamentals, applications, and challenges

Nanotechnology-enabled water treatment is the most attractive approach to realizing advanced purification of contaminated waters that challenge the efficacy of traditional water treatment technologies. Confining nanomaterials inside porous scaffolds or substrates is one of the most effective strategies to push nano-enabled water treatment technologies forward from laboratory to field application. As flourishingly reported, confinement effects induce significantly improved decontamination efficiency, such as enhanced adsorption capacity, reaction kinetics, stability, and selectivity. In this review, first we provide an overview of the general fundamentals of nanoconfinement effects and their implications in environmental remediation. Next, we review confined Fe-based nanomaterials, such as different polymorphs of iron-oxides, oxyhydroxides, zero-valent iron, and single-atom iron as representative materials towards their applications in nanoconfinement systems for water decontamination. Finally, we propose future studies based on the missing scientific fundamentals regarding nanoconfinement effects and challenges for translating unique and promising nanoconfinement observations to engineering applications of confined nanomaterials-driven water treatment technologies.

Hybrid Materials Based on Carbon Nanotubes and Tetra- and Octa-Halogen-Substituted Zinc Phthalocyanines: Sensor Response Toward Ammonia from the Quantum-Chemical Point of View

This paper presents the results of quantum-chemical modeling performed by the Density Functional-Based Tight Binding (DFTB) method to investigate the change in the band structure of hybrid materials based on carbon nanotubes and unsubstituted, tetra-, or octa-halogen-substituted zinc phthalocyanines upon the adsorption of ammonia molecules. The study showed that the electrical conductivity of these materials and its changes in the case of interaction with ammonia molecules depend on the position of the impurity band formed by the orbitals of macrocycle atoms relative to the forbidden energy gap of the hybrids. The sensor response of the hybrids containing halogenated phthalocyanines was lower by one or two orders of magnitude, depending on the number of substituents, compared to the hybrid with unsubstituted zinc phthalocyanine. This result was obtained by calculations performed using the nonequilibrium Green's functions (NEGF) method, which demonstrated a change in the electrical conductivity of the hybrids upon the adsorption of ammonia molecules. The analysis showed that in order to improve the sensor characteristics of CNT-based hybrid materials, preference should be given to those phthalocyanines in which substituents contribute to an increase in HOMO energy relative to the unsubstituted macrocycles.

Turning Solid Waste into Catalysts: A Path for Environmental Solutions

Waste, often overlooked, stands out as a prime source of valuable products, meeting the demand for natural resources. In the face of environmental challenges, this study explores the crucial role of waste-derived catalysts in sustainable practices, emphasizing the transformative potential of solid waste materials. Carbon-based catalysts sourced from agricultural, municipal, and industrial waste streams can be transformed into activated carbon, biochar, and hydrochar which are extensively used adsorbents. Furthermore, the paper also highlights the potential of transition metal-based catalysts derived from spent batteries, electronic waste, and industrial byproducts, showcasing their efficacy in environmental remediation processes. Calcium-based catalysts originating from food waste, including seashells, eggshells, bones, as well as industrial and construction waste also find an extensive application in biodiesel production, providing a comprehensive overview of their promising role in sustainable and eco-friendly practices. From mitigating pollutants to recovering valuable resources, waste-derived catalysts exhibit a versatile role in addressing waste management challenges and promoting resource sustainability. By transforming waste into valuable catalysts, this study champions a paradigm shift towards a more sustainable and resource-efficient future.

Differentiation of adsorption and degradation in steroid hormone micropollutants removal using electrochemical carbon nanotube membrane

The growing concern over micropollutants in aquatic ecosystems motivates the development of electrochemical membrane reactors (EMRs) as a sustainable water treatment solution. Nevertheless, the intricate interplay among adsorption/desorption, electrochemical reactions, and byproduct formation within EMR complicates the understanding of their mechanisms. Herein, the degradation of micropollutants using an EMR equipped with carbon nanotube membrane are investigated, employing isotope-labeled steroid hormone micropollutant. The integration of high-performance liquid chromatography with a flow scintillator analyzer and liquid scintillation counting techniques allows to differentiate hormone removal by concurrent adsorption and degradation. Pre-adsorption of hormone is found not to limit its subsequent degradation, attributed to the rapid adsorption kinetics and effective mass transfer of EMR. This analytical approach facilitates determining the limiting factors affecting the hormone degradation under variable conditions. Increasing the voltage from 0.6 to 1.2 V causes the degradation dynamics to transition from being controlled by electron transfer rates to an adsorption-rate-limited regime. These findings unravels some underlying mechanisms of EMR, providing valuable insights for designing electrochemical strategies for micropollutant control.

Principles, challenges and prospects for electro-oxidation treatment of oilfield produced water

The oil industry is facing substantial environmental challenges, especially in managing waste streams such as Oilfield Produced Water (OPW), which represents a significant component of the industrial ecological footprint. Conventional treatment methods often fail to effectively remove dissolved oils and grease compounds, leading to operational difficulties and incomplete remediation. Electrochemical oxidation (EO) has emerged as a promising alternative due to its operational simplicity and ability to degrade pollutants directly and indirectly, which has already been applied in treating several effluents containing organic compounds. The application of EO treatment for OPW is still in an initial stage, due to the intricate nature of this matrix and scattered information about it. This study provides a technological overview of EO technology for OPW treatment, from laboratory scale to the development of large-scale prototypes, identifying design and process parameters that can potentially permit high efficiency, applicability, and commercial deployment. Research in this domain has demonstrated notable rates of removal of recalcitrant pollutants (>90%), utilizing active and non-active electrodes. Electro-generated active species, primarily from chloride, play a pivotal role in the oxidation of organic compounds. However, the highly saline conditions in OPW hinder the complete mineralization of these organics, which can be improved by using non-active anodes and lower salinity levels. The performance of electrodes greatly influences the efficiency and effectiveness of OPW treatment. Various factors must be considered when selecting the electrode material, such as its conductivity, stability, surface area, corrosion resistance, and cost. Additionally, the specific contaminants present in the OPW, and their electrochemical reactivity must be considered to ensure optimal treatment outcomes. Balancing these considerations can be challenging, but it is crucial for achieving successful OPW treatment. Active electrode materials exhibit a high affinity for chloride molecules, generating more active species than non-active materials, which exhibit more significant degradation potential due to the production of hydroxyl radicals. Regarding scale-up, key challenges include low current efficiency, the formation of by-products, electrode deactivation, and limitations in mass transfer. To address these issues, enhanced mass transfer rates and appropriate residence times can be achieved using flow-through mesh anodes and moderate current densities, which have proven to be the optimal configuration for this process.

A Two-Phase Hydrogenation Membrane for Contaminants Reduction at High Hydrogen Reagent Utilization Efficiency

Heterogeneous hydrogenation is surging as a promising strategy for selective removal of water pollutants, yet numerous efforts rely on catalyst design to advance catalytic activity. Herein, we enhanced the mass transfer and the utilization of hydrogen reagent through construction of a two-phase flow-through membrane reaction device (Pd/SiC-MR). Pd/SiC-MR displays high efficiency and selectivity toward removal of multiple pollutants. For instance, rapid (∼0.35 s) and exclusive hydrogenation (>99%) of carbon-chlorine bond in organohalogens were realized at high water flux (220 L/m2/h). More importantly, the two-phase Pd/SiC-MR reaction system achieved 31.4% utilization of hydrogen reagent, 1-3 orders of magnitude higher than those by classical slurry or fixed-bed reactor. The high hydrogenation performance is attributed to the close proximity of the hydrogen source, reactive hydrogen atom, and pollutant under high molecular collision frequency in membrane pores. Our study opens an approach for improved hydrogen reagent utilization while reserving the high pollutant removal efficiency through altering operating conditions, beyond complex material design limitations in hydrogenation water purification.

MXene/Carbon Nanocomposites for Water Treatment

One of the most critical problems faced by modern civilization is the depletion of freshwater resources due to their continuous consumption and contamination with different organic and inorganic pollutants. This paper considers the potential of already discovered MXenes in combination with carbon nanomaterials to address this problem. MXene appears to be a highly promising candidate for water purification due to its large surface area and electrochemical activity. However, the problems of swelling, stability, high cost, and scalability need to be overcome. The synthesis methods for MXene and its composites with graphene oxide, carbon nanotubes, carbon nanofibers, and cellulose nanofibers, along with their structure, properties, and mechanisms for removing various pollutants from water, are described. This review discusses the synthesis methods, properties, and mechanisms of water purification using MXene and its composites. It also explores the fundamental aspects of MXene/carbon nanocomposites in various forms, such as membranes, aerogels, and textiles. A comparative analysis of the latest research on this topic shows the progress in this field and the limitations for the practical application of MXene/carbon nanocomposites to solve the problem of drinking water scarcity. Consequently, this review demonstrates the relevance and promise of the material and underscores the importance of further research and development of MXene/carbon nanocomposites to provide effective water treatment solutions.

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.

Piezoceramic membrane with built-in ultrasound for reactive oxygen species generation and synergistic vibration anti-fouling

Piezoceramic membranes have emerged as a prominent solution for membrane fouling control. However, the prevalent use of toxic lead and limitations of vibration-based anti-fouling mechanism impede their wider adoption in water treatment. This study introduces a Mn/BaTiO3 piezoceramic membrane, demonstrating a promising in-situ anti-fouling efficacy and mechanism insights. When applied to an Alternating Current at a resonant frequency of 20 V, 265 kHz, the membrane achieves optimal vibration, effectively mitigating various foulants such as high-concentration oil (2500 ppm, including real industrial oil wastewater), bacteria and different charged inorganic colloidal particles, showing advantages over other reported piezoceramic membranes. Importantly, our findings suggest that the built-in ultrasonic vibration of piezoceramic membranes can generate reactive oxygen species. This offers profound insights into the distinct anti-fouling processes for organic and inorganic wastewater, supplementing and unifying the traditional singular vibrational anti-fouling mechanism of piezoceramic membranes, and potentially propelling the development of piezoelectric catalytic membranes.

Oxygen-doped carbon nanotubes with dual active cites to enhance •OH formation through three electron oxygen reduction

The electro-Fenton (EF) process generates H2O2 through the 2e- oxygen reduction reaction (ORR), which is subsequently activated to •OH by iron-based catalysts. To alleviate the potential risk of external Fe-based catalysts, along with metal dissolution in acidic or neutral environments, in this study we employed oxygen-doped carbon nanotubes (OCNT) as a bifunctional, metal-free cathode to establish a metal-free EF process for organic pollutant degradation. The results demonstrate that the metal-free electrode has excellent H2O2 accumulation (12 mg L-1 cm-1) and degrades sulfathiazole (STZ) with 97.05 % efficiency in 180 min with an explanation kinetic of 0.0189 min-1. For the first time, this enhancement came from the dual active site centers in OCNT: Ⅰ) -COOH and defects active sites were responsible for H2O2 production, Ⅱ) then -CO triggered H2O2 into •OH, avoiding the introduction of metal-based catalysts. These findings suggest that the EF system with in situ oxygen-doped cathodes have great potential for treating antibiotic wastewater.

Selective electrocatalytic transformation of highly toxic phenols in wastewater to para-benzoquinone at ambient conditions

The selective transformation of organics from wastewater to value-added chemicals is considered an upcycling process beneficial for carbon neutrality. Herein, we present an innovative electrocatalytic oxidation (ECO) system aimed at achieving the selective conversion of phenols in wastewater to para-benzoquinone (p-BQ), a valuable chemical widely utilized in the manufacturing and chemical industries. Notably, 96.4% of phenol abatement and 78.9% of p-BQ yield are synchronously obtained over a preferred carbon cloth-supported ruthenium nanoparticles (Ru/C) anode. Such unprecedented results stem from the weak Ru-O bond between the Ru active sites and generated p-BQ, which facilitates the desorption of p-BQ from the anode surface. This property not only prevents the excessive oxidation of the generated p-BQ but also reinstates the Ru active sites essential for the rapid ECO of phenol. Furthermore, this ECO system operates at ambient conditions and obviates the need for potent chemical oxidants, establishing a sustainable avenue for p-BQ production. Importantly, the system efficacy can be adaptable in actual phenol-containing coking wastewater, highlighting its potential practical application prospect. As a proof of concept, we construct an electrified Ru/C membrane for ECO of phenol, attaining phenol removal of 95.8% coupled with p-BQ selectivity of 73.1%, which demonstrates the feasibility of the ECO system in a scalable flow-through operation mode. This work provides a promising ECO strategy for realizing both phenols removal and valuable organics recovery from phenolic wastewater.

Unraveling Flow Effect on Capacitive Energy Extraction from Salinity Gradients

The harvesting of salinity gradient energy through a capacitive double-layer expansion (CDLE) technique is directly associated with ion adsorption and desorption in electrodes. Herein, we show that energy extraction can be modulated by regulating ion adsorption/desorption through water flow. The flow effects on the output energy, capacitance, and energy density under practical conditions are systematically investigated from a theoretical perspective, upon which the optimal operating condition is identified for energy extraction. We demonstrate that the net charge accumulation displays a negative correlation with the water flow velocity and so does the surface charge density, and this causes a nontrivial variation in the magnitude of output energy when water flows are introduced. When high water flows are introduced in both the charging and discharging processes, the energy extraction can be significantly reduced by 47.69-49.32%. However, when a high flow is solely exerted in the discharging process, the energy extraction can be enhanced by 12.94-14.49% even at low operation voltages. This study not only offers a comprehensive understanding of the microscopic mechanisms of surface-engineered energy extraction with water flows but also provides a novel direction for energy extraction enhancement.

A facile strategy for the preparation of carbon nanotubes/polybutadiene crosslinked composite membrane and its application in osmotic energy harvesting

The osmotic energy between riverine water and seawater can be converted into electricity by reverse electrodialysis (RED). However, the facile fabrication of advanced RED membranes with high energy conversion efficiencies, large areas, and excellent mechanical properties remains a challenge. Carbon nanotubes (CNTs) exhibit excellent conductivity and provide suitable channels for ion transport but cannot form membranes independently, which limits the related applications in osmotic energy conversion. Herein, a new organic-inorganic composite membrane is prepared by combining hydroxyl-terminated polybutadiene as a matrix and carbon nanotubes as transport nanochannels. The nanotubes are pre-subjected to plasma treatment to increase the surface charge density and transport capacity of the nanochannels, improving the ion selectivity and energy conversion efficiency. Under actual seawater/river water conditions, the developed membrane delivers a power density of ∼5.1 W/m2 and shows good mechanical strength (219 MPa). Our work provides a facile solution to the problem posed by the inability of ideal nanochannels to form membranes independently and paves the way for the application of RED membranes in osmotic energy conversion.

Recent advances in surface tailoring of thin film forward osmosis membranes: A review

The recent advancements in fabricating forward osmosis (FO) membranes have shown promising results in desalination and water treatment. Different methods have been applied to improve FO performance, such as using mixed or new draw solutions, enhancing the recovery of draw solutions, membrane modification, and developing FO-hybrid systems. However, reliable methods to address the current issues, including reverse salt flux, fouling, and antibacterial activities, are still in progress. In recent decades, surface modification has been applied to different membrane processes, including FO membranes. Introducing nanochannels, bioparticles, new monomers, and hydrophilic-based materials to the surface layer of FO membranes has significantly impacted their performance and efficiency and resulted in better control over fouling and concentration polarization (CP) in these membranes. This review critically investigates the recent developments in FO membrane processes and fabrication techniques for FO surface-layer modification. In addition, this study focuses on the latest materials and structures used for the surface modification of FO membranes. Finally, the current challenges, gaps, and suggestions for future studies in this field have been discussed in detail.

Membrane-catalysis integrated system for contaminants degradation and membrane fouling mitigation: A review

The integration of catalytic degradation and membrane separation processes not only enables continuous degradation of contaminants but also effectively alleviates inevitable membrane fouling, demonstrating fascinating practical value for efficient water purification. Such membrane-catalysis integrated system (MCIS) has attracted tremendous research interest from scientists in chemical engineering and environmental science recently. In this review, the advantages of MCIS are discussed, including the membrane structure regulation, stable catalyst loading, nano-confinement effect, and efficient natural organic matter (NOM) exclusion, highlighting the synergistic effect between membrane separation and catalytic process. Subsequently, the design considerations for the fabrication of catalytic membranes, including substrate membrane, catalytic material, and fabrication method, are comprehensively summarized. Afterward, the mechanisms and performance of MCIS based on different catalytic types, including liquid-phase oxidants/reductants involved MCIS, gas involved MCIS, photocatalysis involved MCIS, and electrocatalysis involved MCIS are reviewed in detail. Finally, the research direction and future perspectives of catalytic membranes for water purification are proposed. The current review provides an in-depth understanding of the design of catalytic membranes and facilitates their further development for practical applications in efficient water purification.

Electric Field-Assisted Nanofiltration for PFOA Removal with Exceptional Flux, Selectivity, and Destruction

Per- and polyfluoroalkyl substances (PFAS) pose significant environmental and human health risks and thus require solutions for their removal and destruction. However, PFAS cannot be destroyed by widely used removal processes like nanofiltration (NF). A few scarcely implemented advanced oxidation processes can degrade PFAS. In this study, we apply an electric field to a membrane system by placing a nanofiltration membrane between reactive electrodes in a crossflow configuration. The performance of perfluorooctanoic acid (PFOA) rejection, water flux, and energy consumption were evaluated. The reactive and robust SnO2-Sb porous anode was created via a sintering and sol-gel process. The characterization and analysis techniques included field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), ion chromatography, mass spectroscopy, porosimeter, and pH meter. The PFOA rejection increased from 45% (0 V) to 97% (30 V) when the electric field and filtration were in the same direction, while rejection capabilities worsened in opposite directions. With saline solutions (1 mM Na2SO4) present, the induced electro-oxidation process could effectively mineralize PFOA, although this led to unstable removal and water fluxes. The design achieved an exceptional performance in the nonsaline feed of 97% PFOA rejection and water flux of 68.4 L/m2 hr while requiring only 7.31 × 10-5 kWh/m3/order of electrical energy. The approach's success is attributed to the proximity of the electrodes and membrane, which causes a stronger electric field, weakened concentration polarization, and reduced mass transfer distances of PFOA near the membrane. The proposed electric field-assisted nanofiltration design provides a practical membrane separation method for PFAS removal from water.

Packed OV-SnO2-Sb bead-electrodes for enhanced electrocatalytic oxidation of micropollutants in water

Electrocatalytic oxidation is an appealing treatment option for emerging micropollutants in wastewater, however, the limited reactive surface area and short service lifetime of planar electrodes hinder their industrial applications. This study introduces an innovative electrochemical wastewater treatment technology that employs packed bead-electrodes (PBE) as a dynamic electrocatalytic filter on a dimensionally stable anode (DSA) acting as a current collector. By using PBE, the electroactive volume is expanded beyond the vicinity of the common planar anode to the thick porous media of PBE with a vast electrocatalytic surface area. This greatly enhances the efficiency of electrochemical degradation of micropollutants. The OV-SnO2-Sb PBE filter achieved a nearly 100 % degradation of moxifloxacin (MOX) in under 2 min of single-pass filtration, with a degradation rate over an order of magnitude higher than the conventional electrochemical oxidation processes. The generation of abundant radical species (•OH) and non-radical species (1O2 and O3), along with the enhanced direct oxidation, led to the outstanding performance of the charged PBE system in MOX degradation. The OV-SnO2-Sb PBE was remarkably stable, and the separation between the electroactive PBE layer and the base Ti anode allows for easy renewal of the bead-electrode materials and scaling up of the system for practical applications. Overall, our study presents a dynamic electroactive PBE that advances the electrocatalytic oxidation technology for effective control of emerging pollutants in the water environment. This technology has the potential to revolutionize electrochemical wastewater treatment and contribute to a more sustainable future environment.

In situ regulation of selectivity and permeability by electrically tuning pore size in trans-membrane ion process

Regulating ion transport behavior through pore size variation is greatly attractive for membrane to meet the need for precise separation, but fabricating nanofiltration (NF) membranes with tunable pore size remains a huge challenge. Herein, a NF membrane with electrically tunable pores was fabricated by intercalating polypyrrole into reduced graphene oxide interlayers. As the potential switches from reduction to oxidation, the membrane pore size shrinks by 11%, resulting in a 16.2% increase in salt rejection. The membrane pore size expands/contracts at redox potentials due to the polypyrrole volume swelling/shrinking caused by the insertion/desertion of cations, respectively. In terms of the inserted cation, Na+ and K+ induce larger pore-size stretching range for the membrane than Ca2+ due to greater binding energy and larger doping amount. Such an electrical response characteristic remained stable after multiple cycles and enabled application in ion selective separation; e.g., the Na+/Mg2+ separation factor in the reduced state is increased by 41% compared to that in the oxide state. This work provides electrically tunable nanochannels for high-precision separation applications such as valuable substance purification and resource recovery from wastewater.

A sustainable solution for organic pollutant degradation: Novel polyethersulfone/carbon cloth/FeOCl composite membranes with electric field-assisted persulfate activation

Sulfate radical-based advanced oxidation processes (SR-AOP) and ultrafiltration (UF) membranes have demonstrated effectiveness in treating wastewater. This investigation illuminated a pioneering two-stage procedure for fabricating polyethersulfone/carbon cloth/FeOCl (PES/CC/FeOCl) composite catalytic membranes, exhibiting proficiency in persulfate activation. Evidenced by their distinctively high degradation rates and superior stability, these innovative composite membranes efficaciously obviate tetracycline (TC), showcasing a striking TC degradation rate, with an unparalleled removal ratio peaking at 93% under applied electrical fields. The process underlying persulfate activation and TC degradation was meticulously explored through electron paramagnetic resonance (EPR) and quenching trials. These evaluations unveil that hydroxyl radicals (•OH) and sulfate radicals (SO4•-) primarily drive the eradication of diminutive organic molecules. Subsequent studies emphasized the noteworthy rejection ratio of the PES/CC/FeOCl composite membranes (90%) for sodium alginate (SA), further revealing their exceptional on-line cleansing efficiency in an electrofiltration-associated in-situ oxidation system. In essence, this study proposed a novel approach for the synthesis of composite membranes adept at the catalytic degradation of organic pollutants. This paradigm-shifting research imparted a unique lens to perceive the integration of membrane separation technology, enriching the domain of advanced wastewater treatment strategies.

Activation of peroxymonosulfate by Fe-Mn-modified MWCNTs for selective decontamination: Formation of high-valent metal-oxo species and superoxide anion radicals

The extensive presence of organic micropollutants in complex water matrices requires the development of selective oxidation technologies. In this study, a novel selective oxidation process was developed via the conjunction of FeMn/CNTs with peroxymonosulfate and successfully used to remove micropollutants such as sulfamethoxazole (SMX) and bisphenol A from aqueous solutions. FeMn/CNTs were prepared using a facile co-precipitation method, characterized using a series of surface characterization techniques, and then tested for pollutant removal. The results showed that the FeMn/CNTs had much greater reactivity than CNTs, manganese oxide, and iron oxide. The pseudo-first-order rate constant with FeMn/CNTs was more than 2.9-5.7 times that of the other tested materials. The FeMn/CNTs had great reactivity in a wide range of pH values from 3.0 to 9.0, with the best reactivity found at pH values of 5.0 and 7.0. High-valent metal-oxo species such as Fe(IV)O and Mn(IV)O and superoxide anion radicals were determined to be the reactive species and were responsible for the oxidation of SMX. These reactive species were selective; therefore, the overall removal performance of SMX was not obviously influenced by high levels of water components including chloride ions, bicarbonates, and natural organic matters. The results from this study may promote the design and application of selective oxidation technologies for micropollutant abatement.

Selective activation of peroxymonosulfate to singlet oxygen by engineering oxygen vacancy defects in Ti3CNTx MXene for effective removal of micropollutants in water

Defect engineering is an effective strategy to boost the catalytic activity of MXene towards heterogeneous peroxymonosulfate (PMS) activation for water decontamination. Herein, we developed a facile approach to fine-tune the generation of oxygen vacancies (OVs) on Ti3CNTx crystals by Ce-doping (Ce-Ti3CNTx) with the aim of mediating PMS activation for the degradation of micropollutants in water. By varying the dopant content, the OV concentrations of Ti3CNTx could be varied to enable the activation of PMS to almost 100% singlet oxygen (1O2), and hence the effective degradation of sulfamethoxazole (SMX, a model micropollutant). Various advanced characterization techniques were employed to obtain detailed information on the microstructure, morphology, and defect states of the catalysts. The experimental results showed that SMX removal was proportional to the OVs level. Density functional theory (DFT) models demonstrated that, in contrast to pristine Ti3CNTx, the OVs on 10%Ce-Ti3CNTx could adsorb the terminal O of PMS, which facilitated the formation of SO5 •- as well as the generation of 1O2. We further loaded the optimized catalysts onto a polytetrafluoroethylene microfiltration membrane and also demonstrated the efficient removal of SMX from water using a convection-enhanced mass transport flow-through configuration. This study provides new insights into the effective removal of micropollutants from water by integrating state-of-the-art defect engineering, advanced oxidation, and microfiltration techniques.

Copper Nanowire Networks: An Effective Electrochemical Peroxymonosulfate Activator toward Nitrogenous Pollutant Abatement

Herein, we developed an electrochemical filtration system for effective and selective abatement of nitrogenous organic pollutants via peroxymonosulfate (PMS) activation. Highly conductive and porous copper nanowire (CuNW) networks were constructed to serve simultaneously as catalyst, electrode, and filtration media. In one demonstration of the CuNW network's capability, a single pass through a CuNW filter (τ < 2 s) degraded 94.8% of sulfamethoxazole (SMX) at an applied potential of -0.4 V vs SHE. The exposed {111} crystal plane of CuNW triggered atomic hydrogen (H*) generation on sites, which contributed to effective PMS reduction. Meanwhile, with the involvement of SMX, a Cu-N bond was formed by the interactions between the -NH₂ group of SMX and the Cu sites of CuNW, accompanied by the redox cycling of Cu²⁺/Cu⁺, which was facilitated by the applied potential. The different charges of the active Cu sites made it easier to withdraw electrons and promote PMS oxidation. Theoretical calculations and experimental results were combined to suggest a mechanism for pollution abatement with CuNW networks. The results showed that system efficacy for the degradation of a wide array of nitrogenous pollutants was robust across a broad range of solution pH and complex aqueous matrices. The flow-through operation of the CuNW filter outperformed conventional batch electrochemistry due to convection-enhanced mass transport. This study provides a new strategy for environmental remediation by integrating state-of-the-art material science, advanced oxidation processes, and microfiltration technology.

Electrochemical CNT filter functionalized with metal-organic framework for one-step antimonite decontamination

Currently, there is a lack of advanced nanotechnology designed to efficiently remove antimony (Sb) from contaminated water systems. Sb most commonly appears as antimonite (Sb(III)) or as the anion antimonate (Sb(V)). Sb(III) is approximately ten times more toxic than Sb(V), and Sb(III) is also harder to eliminate because of its motility and charge neutrality. The work presented here developed an electrochemical filtration technology for the direct elimination of Sb(III) from contaminated water. The primary components of the filtration system are an electroactive carbon nanotube (CNT) membrane that are functionalized with the Sb-specific UiO-66(Zr), an organometallic framework. In an electric field, the UiO-66(Zr)/CNT nanohybrid filter enabled in situ transformation of Sb(III) to less harmful Sb(V). The Sb(V) was then effectively adsorbed by the UiO-66(Zr). The removal efficiency (90.5%) and rate constant (k₁ = 0.0272 min^-1) toward Sb(III) removal was 1.3 and 1.4 times greater than that of CNT filter. The filter's abundance of available adsorption sites, flow-through construction, and electrochemical activity combined to rapidly remove Sb(III) from water. The underlying functioning of the nanohybrid filter was determined with a series of process experiments and structural characterizations. The filter was effective over a broad range of pH values and in a variety of complex aqueous environments. Once loaded with Sb, the UiO-66(Zr)/CNT filter could be washed with a dilute NaOH solution to efficiently refresh its activity. The results of this work offer a direct, efficient strategy that integrates nanotechnology, electrochemistry, and membrane separation to remove antimony and potentially other heavy metals from contaminated water.

Conductive MXene ultrafiltration membrane for improved antifouling ability and water quality under electrochemical assistance

Membrane fouling is a major challenge for the membrane separation technique in water treatment. Herein, an MXene ultrafiltration membrane with good electroconductivity and hydrophilicity was prepared and showed excellent fouling resistance under electrochemical assistance. The fluxes under negative potential were 3.4, 2.6 and 2.4 times higher than those without external voltage during treatment of raw water containing bacteria, natural organic matter (NOM), and coexisting bacteria and NOM, respectively. During the treatment of actual surface water with 2.0 V external voltage, the membrane flux was 1.6 times higher than that without external voltage and the TOC removal was improved from 60.7% to 71.2%. The improvement is mainly attributed to the enhanced electrostatic repulsion. The MXene membrane presents good regeneration ability after backwashing under electrochemical assistance with the TOC removal remaining stable at around 70.7%. This work demonstrates that the MXene ultrafiltration membrane under electrochemical assistance possesses excellent antifouling ability and has great potential in advanced water treatment.

Coupling anodic and cathodic reactions using an electrocatalytic dual-membrane system actuates ultra-efficient degradation with regulable mechanisms

The versatile reaction possibilities arising from the interaction between the anodic and cathodic reactions naturally contained in electrocatalytic membrane filtration (EMF) systems are of great valuable in meeting the current complex water treatment requirements. But currently, most studies only focus on half-cell reactions with a single electrocatalytic membrane, which limits the research progress of the EMF technology. Here we report a coupling strategy that utilizes the interaction between the anodic and cathodic reactions to actuate ultra-efficient degradation performance with regulable reaction mechanisms. An electrocatalytic dual-membrane filtration (EDMF) system was established. Six typical configurations of the EDMF system were set up and systematically investigated by adjusting the electrode distance and filtration sequence. Based on the obtained results of degradation performance and mechanisms, a regulation strategy which enabled flexible tuning of direct nonradical oxidation (e.g., h⁺) and indirect oxidation (e.g., ¹O₂, ·OH, HO₂·, O₂·^-, etc.) was proposed. In particular, cathodic reactions were found to adversely affect the anodic reactions at the relatively short electrode distance of 0.9 mm. Anodic reactions could inhibit the generation of ¹O₂ at short distance of 0.9 mm but promote its generation at long distances of 9 and 17 mm. The A-C_0.9 configuration achieved the highest degradation performance, while the C-A_9 configuration was revealed to be much more conducive to ¹O₂ production. Overall, our findings demonstrate the versatility and tunability of the reaction mechanism and performance of the EDMF system due to the flexible coupling of the anodic and cathodic reactions, which potentially lays a foundation for future development of ultra-efficient mechanism-adjustable electrocatalysis technologies.

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.

Three-dimensional structured electrode for electrocatalytic organic wastewater purification: Design, mechanism and role

Considering the growing need in decentralized water treatment, the application of electrocatalytic processes (EP) to achieve organic wastewater purification will be dominant in the near future due to high efficiency, small reactor assembly as well as the flexibility of operation and management. The catalytic performance of electrode materials determines the development of this technology. Among them, the unique three-dimensional (3D) structure electrode shows better performance than two-dimensional (2D) electrode in increasing mass transfer, enhancing adsorption and exposing more active sites. Hence, this review starts with the introduction of definition, classification, advantages and disadvantages of 3D electrode materials. Then a critical discussion on the design and construction of 3D electrode materials for organic wastewater purification application is provided. Next, the removal mechanism of organic pollutants on the surface of 3D electrode, the role of 3D structure, the design of reactor with 3D electrode, the conversion and toxicity of degradation products, electrode energy efficiency, stability and cost, are comprehensively reviewed. At last, current challenges and future perspectives for the development of 3D electrode materials are addressed. We deem that this review will provide a valuable insight into the design and application of 3D electrodes in environmental water purification.

Atomic H*-mediated electrochemical removal of low concentration antimonite and recovery of antimony from water

Compounds containing antimony (Sb) are broadly used as starting materials for a wide range of industrial products, leading to serious water pollution associated with Sb rock mining as well as Sb leaching. Herein, we proposed an innovative design of an electrified membrane consisted of bimetallic palladium and iron nanoparticles (Pd-Fe NPs) supported on conductive carbon nanotube (CNT) networks. The nanohybrid filter enabled effective generation and retainment of atomic hydrogen (H*) under an electric field, which further contributed to the complete electroreduction of antimonite (Sb(III)). The highest atomic H* yield and Sb(III) removal kinetics were identified once a potential of -1.0 V vs. Ag/AgCl was exerted. Compared to the pristine CNT, Pd-CNT and Fe-CNT filters, the reaction rate constant of the Pd/Fe-CNT filter was increased 5.15-, 2.39-, and 1.76-fold, respectively for electrochemical removal of Sb(III). The results denoted that the superior performance of the Pd/Fe-CNT nanohybrid filter originated from: (1) the flow-through design, which enhanced mass transport, (2) the bimetallic design, which increased the catalytic activity, and (3) the collective contribution from atomic H*-mediated indirect reduction and direct electron transfer reduction mechanisms. The robust system performance occurred over a broad range of pH values, a variety of water matrices and can withstand several cycles of experiments. Our findings highlight an effective electro-filtration strategy to induce atomic H*-mediated electrochemical removal and recovery of Sb from water.

Next-generation graphene oxide additives composite membranes for emerging organic micropollutants removal: Separation, adsorption and degradation

In the past two decades, membrane technology has attracted considerable interest as a viable and promising method for water purification. Emerging organic micropollutants (EOMPs) in wastewater have trace, persistent, highly variable quantities and types, develop hazardous intermediates and are diffusible. These primary issues affect EOMPs polluted wastewater on an industrial scale differently than in a lab, challenging membranes-based EOMP removal. Graphene oxide (GO) promises state-of-the-art membrane synthesis technologies and use in EOMPs removal systems due to its superior physicochemical, mechanical, and electrical qualities and high oxygen content. This critical review highlights the recent advancements in the synthesis of next-generation GO membranes with diverse membrane substrates such as ceramic, polyethersulfone (PES), and polyvinylidene fluoride (PVDF). The EOMPs removal efficiencies of GO membranes in filtration, adsorption (incorporated with metal, nanomaterial in biodegradable polymer and biomimetic membranes), and degradation (in catalytic, photo-Fenton, photocatalytic and electrocatalytic membranes) and corresponding removal mechanisms of different EOMPs are also depicted. GO-assisted water treatment strategies were further assessed by various influencing factors, including applied water flow mode and membrane properties (e.g., permeability, hydrophily, mechanical stability, and fouling). GO additive membranes showed better permeability, hydrophilicity, high water flux, and fouling resistance than pristine membranes. Likewise, degradation combined with filtration is two times more effective than alone, while crossflow mode improves the photocatalytic degradation performance of the system. GO integration in polymer membranes enhances their stability, facilitates photocatalytic processes, and gravity-driven GO membranes enable filtration of pollutants at low pressure, making membrane filtration more inexpensive. However, simultaneous removal of multiple contaminants with contrasting characteristics and variable efficiencies in different systems demands further optimization in GO-mediated membranes. This review concludes with identifying future critical research directions to promote research for determining the GO-assisted OMPs removal membrane technology nexus and maximizing this technique for industrial application.

Electro-Induced Carbon Nanotube Discrete Electrodes for Sustainable Persulfate Activation

In electrochemical advanced oxidation processes (EAOPs), the rate-limiting step is the mass transfer of pollutants to the electrodes due to the limited active surface areas. To this end, we established a three-dimensional (3D) EAOP system by coupling conventional graphite electrodes with dispersed carbon nanotubes (CNTs). The electrodes (particularly the anode) induced electric field spontaneously polarized CNTs into dispersed reactive particle electrodes (CNT-PEs) in the solution, which remarkably promoted electrochemical activation of peroxydisulfate (PDS) to generate surface CNT-PDS* complexes and surface-bound radicals (SBRs). Based on the excited potential (E_CNT-PEs) at different positions in the 3D electric field, CNT-PEs were activated into three states. (i) E_CNT-PEs < E_organic, CNT-PEs are chemically inert toward DCP oxidation; (ii) E_organic < E_CNT-PEs < E_water, CNT-PEs will oxidize DCP via an electron-transfer process (ETP); (iii) E_CNT-PEs > E_water, both CNT-PDS* complexes and the anode will oxidize water to produce SBRs. Thus, DCP could be oxidized by CNT-PDS* complexes via ETP to form polychlorophenols on the CNT surface, causing rapid deactivation of the micro-electrodes. In contrast, SBRs attack DCP directly into chloride ions and hydroxylated products, maintaining the surface cleanliness and activity of CNT-PEs for long-term operations.

Metal-free graphene-based catalytic membranes for persulfate activation toward organic pollutant removal: a review

Owing to their ultrathin two-dimensional structure and efficient catalytic ability for persulfate activation, graphene-based nanocarbons exhibit considerable application potential in fabricating carbonaceous composite membranes for in situ catalytic oxidation to remove organic pollutants. This approach offers significant advantages over conventional batch systems. However, the relationships between the physicochemical properties of carbon mats and performance of graphene-based catalytic membranes in water purification remain ambiguous. Herein, we summarize the main mechanisms of in situ catalytic oxidation and the facile fabrication strategies of carbonaceous composite membranes. Different factors influencing the performance of graphene-based catalytic membranes are comprehensively discussed. The defective level, heteroatom doping, and stacking morphology of carbon mats and operational conditions during filtration play critical roles in the oxidative degradation of target pollutants. Long-term operation leads to the deterioration of catalytic activity and transmembrane pressure, especially in the complex water matrix. Finally, the present challenges and future perspectives are presented to improve the anti-fouling performance and catalytic stability of membranes and develop scalable fabrication methods to promote the engineering applications of in situ catalytic oxidation in real water purification.

Electrified nanohybrid filter for enhanced phosphorus removal from water

Phosphorus (P) has been identified as a major cause of eutrophication. One feasible way to deal with P-containing wastewater is to employ advanced adsorbents with high P affinity. Towards this end, the loading of these sorbents onto a conductive scaffold would facilitate the introduction of an electric field into the reaction system thereby permitting a continuous-flow operation and improved P sorption kinetics. Here, the preparation and evaluation of an electroactive carbon nanotube (CNT) filter functionalized with cerium-based metal organic frameworks (Ce-MOF) is reported. Various advanced characterization techniques confirmed the successful fabrication of the Ce-MOF/CNT nanohybrid filter. The results suggested that the nanohybrid filter had a maximum P adsorption capacity of 22.41 mg g^-1, which compared favorably with other state-of-the-art P sorbents. Ce-MOF loading, applied voltage and flow rate each increased the rate constants for phosphate removal by factors of 1.6, 2.1 and 5.8 times relative to the absent states. The underlying P sorption mechanisms involved outer-sphere surface complexation (electrostatic attraction), inner-sphere surface complexation (Ce-O-P) and diffusion. The performance was tolerant of a wide operational pH range and different water matrices. The Ce-MOF/CNT electrochemical filter described in this study provides a viable strategy to address the challenging issues associated with aqueous P pollution.

In-situ photothermal activation of peroxydisulfate in a carbon nanotubes membrane-based flow-by reactor toward degradation of contaminants

The energy-induced peroxydisulfate (PDS) activation is a green and effective approach for pollutant degradation, while the huge energy consumption would significantly increase the cost of wastewater treatment. In this study, by taking carbon nanotubes (CNTs) membrane as the light to heat (LTH) conversion materials, we developed a photothermal PDS activation process for degradation of organic contaminants in a flow-by reactor, with hydroxyl radicals (^•OH) and sulfate radicals (SO₄^•-) as the main reactive species. This system has excellent in-situ LTH conversion performance and heat transfer ability. As a result, various pollutants are degraded with an efficiency higher than 90%. More importantly, the LTH device exhibits satisfying stability and could be used for pollutant (i.e., methyl orange (MO)) removal under solar irradiation. In addition, some important factors (i.e., irradiation distance, residence time, solution pH, and PDS dosage) that might significantly influence the removal efficiency of pollutants are optimized. This work provides a novel perspective for the activation of PDS via CNTs as photothermal materials for pollutant degradation with a flow-by reactor.

Urea-oxidation-assisted electrochemical water splitting for hydrogen production on a bifunctional heterostructure transition metal phosphides combining metal-organic frameworks

Electrocatalyzed urea-assisted wastewater splitting is a promising approach for sustainable hydrogen production. However, the lack of cost-efficient electrocatalysts hinders its practical application. Herein, bimetal phosphide (NiCoP_x) nanowire arrays decorated with ultrathin NiFeCo metal-organic framework (NiFeCo-MOF) nanosheets on porous nickel foam (NF) were designed for urea-assisted wastewater splitting. The core-shell NiCoP_x@NiFeCo-MOF hybrids were prepared via successive hydrothermal, gas-phase phosphorization and hydrothermal strategies. Encouragingly, the novel NiCoP_x@NiFeCo-MOF/NF electrode served as an excellent bifunctional electrocatalyst for both the cathodic hydrogen evolution reaction (HER) and the anodic urea oxidation reaction (UOR) in urea-assisted water splitting, which merely required an overpotential of 44 mV to deliver a current density of 10 mA cm^-2 for HER and a voltage of 1.37 V to deliver a current density of 100 mA cm^-2 for UOR in 1.0 M KOH + 0.5 M urea. Benefiting from the highly exposed electroactive sites in exquisite three-dimensional (3D) hierarchical structure, multicomponent synergistic effect, accelerated electron transfer, easy electrolyte access and diffusion of released gas bubbles, the as-fabricated NiCoP_x@NiFeCo-MOF/NF exhibited outstanding electrocatalytic performance. The mechanism of water splitting was elucidated by density functional theory calculations. Interestingly, NiFeCo-MOF possessed optimized COO* adsorption ability on Ni sites that were beneficial to UOR intermediates. More significantly, this work paves the way for the design and fabrication of bifunctional electrocatalysts for urea-containing wastewater treatment and sustainable hydrogen production.

Mo Vacancy-Mediated Activation of Peroxymonosulfate for Ultrafast Micropollutant Removal Using an Electrified MXene Filter Functionalized with Fe Single Atoms

Developing advanced heterogeneous catalysts with atomically dispersed active sites is an efficient strategy to boost the kinetics of peroxymonosulfate (PMS) activation for micropollutant removal. Here, we report a binary Mo₂TiC₂T_x MXene-based electroactive filter system with abundant surface Mo vacancies for effective activation of PMS. The Mo vacancies assumed two essential roles: (i) as anchoring sites for Fe single atoms (Fe-SA) and (ii) as cocatalytic sites for the Fenton-like reaction. Fe-SA formed strong metal-oxygen bonds with the Mo₂TiC₂T_x support, stabilizing at the sites previously occupied by Mo. The resulting Fe-SA/Mo₂TiC₂T_x nanohybrid filter achieved 100% degradation of sulfamethoxazole (SMX) in the single-pass mode (hydraulic retention time <2 s) when assisted by an electric field (2.0 V). The rate constant (k = 2.89 min^-1) for SMX removal was 24 and 67 times greater than that of Fe nanoparticles immobilized on Mo₂TiC₂T_x and the pristine Mo₂TiC₂T_x filter, respectively. Operation in the flow-through configuration outperformed the conventional batch reactor model (k = 0.17 min^-1) due to convection-enhanced mass transport. The results obtained from experimental investigations and theoretical calculations suggested that atomically dispersed Fe-SA, anchored on Mo vacancies, was responsible for the adsorption and activation of PMS to produce sulfate radicals (SO₄^•-) in the presence of an electric field. This study provides a proof-of-concept demonstration of an electroactive Fe-SA/Mo₂TiC₂T_x filter for broader application in the treatment of water contaminated by emerging micropollutants.

A critical review on classifications, characteristics, and applications of electrically conductive membranes for toxic pollutant removal from water: Comparison between composite and inorganic electrically conductive membranes

Research efforts have recently been directed at developing electrically conductive membranes (EMs) for pressure-driven membrane separation processes to remove effectively the highly toxic pollutants from water. EMs serve as both the filter and the electrode during filtration. With the assistance of a power supply, EMs can considerably improve the toxic pollutant removal efficiency and even realize chemical degradation to reduce their toxicity. Organic-inorganic composite EMs and inorganic EMs show remarkable differences in characteristics, removal mechanisms, and application situations. Understanding their differences is highly important to guide the future design of EMs for specific pollutant removal from water. However, reviews concerning the differences between composite and inorganic EMs are still lacking. In this review, we summarize the classifications, fabrication techniques, and characteristics of composite and inorganic EMs. We also elaborate on the removal mechanisms and performances of EMs toward recalcitrant organic pollutants and toxic inorganic ions in water. The comparison between composite and inorganic EMs is emphasized particularly in terms of the membrane characteristics (pore size, permeability, and electrical conductivity), application situations, and underlying removal mechanisms. Finally, the energy consumption and durability of EMs are evaluated, and future perspectives are presented.

In situ Functionalized Mesoporous Silicas for Sustainable Remediation Strategies in Removal of Inorganic Pollutants from Contaminated Environmental Water

Low-cost mesoporous silicas of the SBA-15 family were prepared, aimed for removal of a broad spectrum of both cationic and anionic forms of hazardous metal pollutants (Cr(III, VI), Mn(II, VII), Pb(II), Cd(II), and Cu(II)) from environmental water. Series of mono- and bifunctional materials with immobilized ethylenediaminetriacetic acid (EDTA), primary amine (NH₂), and quaternary ammonium (QAS) groups were prepared in a cost-efficient one-step synthesis using two silica sources, low-cost sodium metasilicate (Na₂SiO₃ 9H₂O) and the conventional source-tetraethylorthosilicate (TEOS). The functionalized SBA-15 samples obtained from both silica sources were highly ordered, as evidenced by TEM and SAXS data. All obtained materials were mesoporous with high surface area values of up to 745 m²/g, pore volumes from 0.99 to 1.44 cm³/g, and narrow pore distributions near 7 nm. The adsorption affinity of the EDTA-functionalized samples followed the common order Pb(II)> Cd(II)> Cu(II)> Cr(III)> Mn(II), which could be explained based on the Pearson theory. The highest adsorption capacities were observed for samples functionalized by EDTA groups using TEOS for synthesis (TEOS/EDTA): 195.6 mg/g for Pb(II), 111.2 mg/g for Cd(II), 58.7 mg/g for Cu(II), 57.7 mg/g for Cr(III), and 49.4 mg/g for Mn(II). Moreover, organic matter (humic acid up to 10 mg/L) and inorganic (Na(I), K(I), Mg(II), Ca(II), etc) macrocomponents present in environmental water had almost negligible effect on the removal of these cations. The NaSi/EDTA/NH₂ sample revealed a better selectivity compared to the NaSi_/NH₂ sample towards such species as Cr(III), Mn(II), Cd(II), and Cu(II). The chromate-ions uptake at pH 7.5 by the TEOS/QAS sample turned practically unaffected by the presence of doubly charged anions (CO₃ ^2-, SO₄ ^2-). The content of functional groups on the surface of MS decreased only slightly (∼1-5%) after several regeneration cycles. The complete desorption of all heavy metal ions can be achieved using 1 mol/L EDTA solution. Reusability tests demonstrated the complete stability of the adsorbent for at least five to six consecutive adsorption/desorption cycles with no decrease in its adsorption characteristics compared to those obtained by 0.05 mol/L HNO₃ treatments. The synthesized mesoporous materials were evaluated for removal of the heavy metal ions from drinking and different natural water samples, proving their potential as sustainable, effective, and cost-efficient adsorbents.

Carbon nanotube-based materials for persulfate activation to degrade organic contaminants: Properties, mechanisms and modification insights

Removal of harmful organic matters from environment has great environmental significance. Carbon nanotube (CNT) materials and their composites have been demonstrated to possess excellent catalytic activity towards persulfate (PS) activation for the degradation of organic contaminants. Herein, detailed information concerning the function, modification methods and relevant mechanisms of CNT in persulfate-based advanced oxidation processes (PS-AOPs) for organic pollutant elimination has been reviewed. The activation mechanism of PS by CNT might include radical and nonradical pathways and their synergistic effects. The common strategies to improve the stability and catalytic capability of CNT-based materials have also been put forward. Furthermore, their practical application potential compared with other catalysts has been described. Finally, the challenges faced by CNT in practical application are clearly highlighted. This review should be of value in promoting the research of PS activation by CNT-based materials for degradation of organic pollutants and the corresponding practical applications.

Integrated flow anodic oxidation and ultrafiltration system for continuous defluorination of perfluorooctanoic acid (PFOA)

While flow anodic oxidation systems can efficiently generate hydroxyl radicals (·OH) and significantly enhance direct electron transfer (DET) processes that result in the oxidation of target contaminants via the charge percolating network of flow anode particles, challenges remain in constructing a flow anodic oxidation system that can be operated continuously with stable performance. Here we incorporate an ultrafiltration (UF) membrane module into the flow anodic oxidation system and achieve the continuous defluorination of perfluorooctanoic acid (PFOA) for 12 days with high efficiency (94.1%) and reasonable energy consumption (38.1 Wh mg^-1) compared to other advanced oxidation processes by using a mixture of conducting Ti_xO_2x-1 and Pd/CNT particles as the flow anode. The results indicate that DET, ·OH mediated oxidation and adsorption processes play critical roles in the degradation of PFOA during the flow anodic oxidation processes. The synergistic effect of the Ti_xO_2x-1 and Pd/CNT particles enhances the defluorination efficiency by 3.2 times at 4.5 V vs Ag/AgCl compared to the control experiment (no flow anode particles present) and promotes the release of F^- into solution while other intermediate products remain adsorbed to the surface of the Pd/CNT particles. Although the Pd/CNT particles were oxidized after the long-term operation, no obvious Pd ion leakage into solution was observed. Results of this study support the feasibility of continuous operation of a flow anode/UF system with stable performance and pave the way for the translation of this advanced oxidation technology to practical application.

Peroxymonosulfate Activation by Photoelectroactive Nanohybrid Filter towards Effective Micropollutant Decontamination

Herein, we report and demonstrate a photoelectrochemical filtration system that enables the effective decontamination of micropollutants from water. The key to this system was a photoelectric–active nanohybrid filter consisting of a carbon nanotube (CNT) and MIL–101(Fe). Various advanced characterization techniques were employed to obtain detailed information on the microstructure, morphology, and defect states of the nanohybrid filter. The results suggest that both radical and nonradical pathways collectively contributed to the degradation of antibiotic tetracycline, a model refractory micropollutant. The underlying working mechanism was proposed based on solid experimental evidences. This study provides new insights into the effective removal of micropollutants from water by integrating state–of–the–art advanced oxidation and microfiltration techniques.