Pulsed hydraulic-pressure-responsive self-cleaning membrane.

Pressure-driven membranes is a widely used separation technology in a range of industries, such as water purification, bioprocessing, food processing and chemical production1,2. Despite their numerous advantages, such as modular design and minimal footprint, inevitable membrane fouling is the key challenge in most practical applications3. Fouling limits membrane performance by reducing permeate flux or increasing pressure requirements, which results in higher energetic operation and maintenance costs4-7. Here we report a hydraulic-pressure-responsive membrane (PiezoMem) to transform pressure pulses into electroactive responses for in situ self-cleaning. A transient hydraulic pressure fluctuation across the membrane results in generation of current pulses and rapid voltage oscillations (peak, +5.0/-3.2 V) capable of foulant degradation and repulsion without the need for supplementary chemical cleaning agents, secondary waste disposal or further external stimuli3,8-13. PiezoMem showed broad-spectrum antifouling action towards a range of membrane foulants, including organic molecules, oil droplets, proteins, bacteria and inorganic colloids, through reactive oxygen species (ROS) production and dielectrophoretic repulsion.

Solar-enhanced lithium extraction with self-sustaining water recycling from salt-lake brines.

Lithium is an emerging strategic resource for modern energy transformation toward electrification and decarbonization. However, current mainstream direct lithium extraction technology via adsorption suffers from sluggish kinetics and intensive water usage, especially in arid/semiarid and cold salt-lake regions (natural land brines). Herein, an efficient proof-of-concept integrated solar microevaporator system is developed to realize synergetic solar-enhanced lithium recovery and water footprint management from hypersaline salt-lake brines. The 98% solar energy harvesting efficiency of the solar microevaporator system, elevating its local temperature, greatly promotes the endothermic Li+ extraction process and solar steam generation. Benefiting from the photothermal effect, enhanced water flux, and enriched local Li+ supply in nanoconfined space, a double-enhanced Li+ recovery capacity was delivered (increase from 12.4 to 28.7 mg g-1) under one sun, and adsorption kinetics rate (saturated within 6 h) also reached twice of that at 280 K (salt-lake temperature). Additionally, the self-assembly rotation feature endows the microevaporator system with distinct self-cleaning desalination ability, achieving near 100% water recovery from hypersaline brines for further self-sufficient Li+ elution. Outdoor comprehensive solar-powered experiment verified the feasibility of basically stable lithium recovery ability (>8 mg g-1) directly from natural hypersaline salt-lake brines with self-sustaining water recycling for Li+ elution (440 m3 water recovery per ton Li2CO3). This work offers an integrated solution for sustainable lithium recovery with near zero water/carbon consumption toward carbon neutrality.

Electrostatic-induced green and precise growth of model catalysts.

Crystallographic control of crystals as catalysts with precise geometrical and chemical features is significantly important to develop sustainable chemistry, yet highly challenging. Encouraged by first principles calculations, precise structure control of ionic crystals could be realized by introducing an interfacial electrostatic field. Herein, we report an efficient in situ dipole-sourced electrostatic field modulation strategy using polarized ferroelectret, for crystal facet engineering toward challenging catalysis reactions, which avoids undesired faradic reactions or insufficient field strength by conventional external electric field. Resultantly, a distinct structure evolution from tetrahedron to polyhedron with different dominated facets of Ag3PO4 model catalyst was obtained by tuning the polarization level, and similar oriented growth was also realized by ZnO system. Theoretical calculations and simulation reveal that the generated electrostatic field can effectively guide the migration and anchoring of Ag+ precursors and free Ag3PO4 nuclei, achieving oriented crystal growth by thermodynamic and kinetic balance. The faceted Ag3PO4 catalyst exhibits high performance in photocatalytic water oxidation and nitrogen fixation for valuable chemicals production, validating the effectiveness and potential of this crystal regulation strategy. Such an electrically tunable growth concept by electrostatic field provides new synthetic insights and great opportunity to effectively tailor the crystal structures for facet-dependent catalysis.

Engineering the Co(II)/Co(III) Redox Cycle and Coδ+ Species Shuttle for Nitrate-to-Ammonia Conversion.

Electroreduction of waste nitrate to valuable ammonia offers a green solution for environmental restoration and energy storage. However, the electrochemical self-reconstruction of catalysts remains a huge challenge in terms of maintaining their stability, achieving the desired active sites, and managing metal leaching. Herein, we present an electrical pulse-driven Co surface reconstruction-coupled Coδ+ shuttle strategy for the precise in situ regulation of the Co(II)/Co(III) redox cycle on the Co-based working electrode and guiding the dissolution and redeposition of Co-based particles on the counter electrode. As result, the ammonia synthesis performance and stability are significantly promoted while cathodic hydrogen evolution and anodic ammonia oxidation in a membrane-free configuration are effectively blocked. A high rate of ammonia production of 1.4 ± 0.03 mmol cm-2 h-1 is achieved at -0.8 V in a pulsed system, and the corresponding nitrate-to-ammonia Faraday efficiency is 91.7 ± 1.0%. This work holds promise for the regulation of catalyst reactivity and selectivity by engineering in situ controllable structural and chemical transformations.

Membraneless Electrochemical Synthesis Strategy toward Nitrate-to-Ammonia Conversion.

Electroreduction of nitrate (NO3RR) to ammonia in membraneless electrolyzers is of great significance for reducing the cost and saving energy consumption. However, severe chemical crossover with side reactions makes it challenging to achieve ideal electrolysis. Herein, we propose a general strategy for efficient membraneless ammonia synthesis by screening NO3RR catalysts with inferior oxygen reduction activity and matching the counter electrode (CE) with good oxygen evolution activity while blocking anodic ammonia oxidation. Consequently, screening the available Co-Co system, the membraneless NO3--to-NH3 conversion performance was significantly higher than H-type cells using costly proton-exchange membranes. At 200 mA cm-2, the full-cell voltage of the membraneless system (∼2.5 V) is 4 V lower than that of the membrane system (∼6.5 V), and the savings are 61.4 kW h (or 56.9%) per 1 kg NH3 produced. A well-designed pulse process, inducing reversible surface reconstruction that in situ generates and restores the active Co(III) species at the working electrode and forms favorable Co3O4/CoOOH at the CE, further significantly improves NO3--to-NH3 conversion and blocks side reactions. A maximum NH3 yield rate of 1500.9 µmol cm-2 h-1 was achieved at -0.9 V (Faraday efficiency 92.6%). This pulse-coupled membraneless strategy provides new insights into design complex electrochemical synthesis.

O2-Independent H2O2 Production via Water-Polymer Contact Electrification.

Hydrogen peroxide (H2O2), as a critical green chemical, has received immense attention in energy and environmental fields. The ability to produce H2O2 in earth-abundant water without relying on low solubility oxygen would be a sustainable and potentially economic process, applicable even to anaerobic microenvironments, such as groundwater treatment. However, the direct water to H2O2 process is currently hindered by low selectivity and low production rates. Herein, we report that poly(tetrafluoroethylene) (PTFE), a commonly used inert polymer, can act as an efficient triboelectric catalyst for H2O2 generation. For example, a high H2O2 production rate of 24.8 mmol gcat-1 h-1 at a dosage of 0.01 g/L PTFE was achieved under the condition of pure water, ambient atmosphere, and no sacrificial agents, which exceeds the performance of state-of-the-art aqueous H2O2 powder catalysts. Electron spin resonance and isotope experiments provide strong evidence that water-PTFE tribocatalysis can directly oxidize water to produce H2O2 under both anaerobic and aerobic conditions, albeit with different synthetic pathways. This study demonstrates a potential strategy for green and effective tribocatalytic H2O2 production that may be particularly useful toward environmental applications.

Electrostatic Field in Contact-Electro-Catalysis Driven C-F Bond Cleavage of Perfluoroalkyl Substances.

Perfluoroalkyl substances (PFASs) are persistent and toxic to human health. It is demanding for high-efficient and green technologies to remove PFASs from water. In this study, a novel PFAS treatment technology was developed, utilizing polytetrafluoroethylene (PTFE) particles (1-5 µm) as the catalyst and a low frequency ultrasound (US, 40 kHz, 0.3 W/cm2) for activation. Remarkably, this system can induce near-complete defluorination for different structured PFASs. The underlying mechanism relies on contact electrification between PTFE and water, which induces cumulative electrons on PTFE surface, and creates a high surface voltage (tens of volts). Such high surface voltage can generate abundant reactive oxygen species (ROS, i.e., O2⋅-, HO⋅, etc.) and a strong interfacial electrostatic field (IEF of 109~1010 V/m). Consequently, the strong IEF significantly activates PFAS molecules and reduces the energy barrier of O2⋅- nucleophilic reaction. Simultaneously, the co-existence of surface electrons (PTFE*(e-)) and HO⋅ enables synergetic reduction and oxidation of PFAS and its intermediates, leading to enhanced and thorough defluorination. The US/PTFE method shows compelling advantages of low energy consumption, zero chemical input, and few harmful intermediates. It offers a new and promising solution for effectively treating the PFAS-contaminated drinking water.

New Indole Derivative Heterogeneous System for the Synergistic Reduction and Oxidation of Various Per-/Polyfluoroalkyl Substances: Insights into the Degradation/Defluorination Mechanism.

The hydrated electron (eaq-) system is typically suitable for degrading perfluoroalkyl substances (PFASs). To enhance eaq- utilization, we synthesized a new indole compound (DIHA) that forms stable nanospheres (100-200 nm) in water via a supramolecular assembly. Herein, the DIHA nanoemulsion system exhibits high degradation efficiencies toward a broad category of PFASs, regardless of the headgroup, chain length, and branching structure, under UV (254 nm) irradiation. The strong adsorption of PFAS on the DIHA surface ensures its effective degradation/defluorination. Quenching experiments further demonstrated that the reaction took place on the surface of DIHA nanospheres. This specific heterogeneous surface reaction unveiled novel PFAS degradation and defluorination mechanisms that differ from previously reported eaq- systems. First, the photogenerated surface electrons nonselectively attacked multiple C-F bonds of the -CF2- chain. This plays a dominant degrading/defluorinating role in the DIHA system. Second, abundant hydroxyl radicals (•OH) were also produced, leading to synergistic reduction (by surface electron) and oxidation (by surface •OH) in a single system. This facilitates faster and deeper defluorination of different structured PFASs through multiple pathways. The new mechanism inspires the design of innovative organo-heterogeneous eaq- systems possessing synergistic reduction and oxidation functions, thereby making them potentially effective for treating PFAS-contaminated water.

Electrical Pulse-Driven Periodic Self-Repair of Cu-Ni Tandem Catalyst for Efficient Ammonia Synthesis from Nitrate.

Electrocatalytic nitrate reduction sustainably produces ammonia and alleviates water pollution, yet is still challenging due to the kinetic mismatch and hydrogen evolution competition. Cu/Cu2 O heterojunction is proven effective to break the rate-determining NO3 - -to-NO2 - step for efficient NH3 conversion, while it is unstable due to electrochemical reconstruction. Here we report a programmable pulsed electrolysis strategy to achieve reliable Cu/Cu2 O structure, where Cu is oxidized to CuO during oxidation pulse, then regenerating Cu/Cu2 O upon reduction. Alloying with Ni further modulates hydrogen adsorption, which transfers from Ni/Ni(OH)2 to N-containing intermediates on Cu/Cu2 O, promoting NH3 formation with a high NO3 - -to-NH3 Faraday efficiency (88.0±1.6 %, pH 12) and NH3 yield rate (583.6±2.4 µmol cm-2 h-1 ) under optimal pulsed conditions. This work provides new insights to in situ electrochemically regulate catalysts for NO3 - -to-NH3 conversion.

Dielectrophoresis-Based Universal Membrane Antifouling Strategy toward Colloidal Foulants.

Membrane fouling compromises the benefits of membrane technology, leading to its performance deterioration and incremental cost. Coupling with an electric field has been attractive but is limited by the electrical dependence of the electrophoresis (EP) mechanism and undesired faradic reactions. This study reports a universal dielectrophoresis-based (DEP) membrane antifouling strategy for electronegative, electropositive, and neutral colloidal foulants, which depends on the particle polarizability rather than its charge. The porous Ni@PVDF model electroconductive membrane was fabricated to construct a nonuniform electric field inducing DEP, while applying a low voltage avoided side electrochemical reactions. For electronegative SiO2(-) and electropositive Al2O3(+) particles with a lower relative permittivity than the medium water (78), the membrane permeability all remarkably increased by 90.1% under AC/DC (±1.0 V) fields. By contrast, serious membrane fouling occurred for the BaTiO3 colloids with a higher relative permittivity (∼2000). Notably, the permittivity of nearly all colloids in wastewater treatment is much less than that of water, which makes the dielectrophoresis-based antifouling strategy universal. The theoretical simulation systematically analyzed the forces on particles including DEP, EP, and others, indicating that the formed protected area on the membrane pore wall by DEP forces prevented the irreversible membrane blockage of colloids and facilitated loose cake layer formation for alleviating membrane fouling. In brief, this work reported a hopeful concept for dielectrophoresis-based membrane antifouling and verified its antifouling mechanism.

Prospects of an Electroactive Carbon Nanotube Membrane toward Environmental Applications.

Rapid population growth and industrialization have driven the emergence of advanced electrochemical and membrane technologies for environmental and energy applications. Electrochemical processes have potential for chemical transformations, chloralkali disinfection, and energy storage. Membrane separations have potential for gas, fluid, and chemical purification. Electrochemical and membrane technologies are often used additively in the same unit process, e.g., the chloroalkali process where a membrane is used to separate cathodic and anodic products from scavenging each other. However, to access the maximal potential requires intimate hybridization of the two technologies into an electroactive membrane. The combination of the two discrete technologies results in a range of synergisms such as reduced footprint, increased processing kinetics, reduced fouling, and increased energy efficiency.Due to their high specific surface area, excellent electric conductivity, and desirable robustness, 1D carbon nanotubes (CNTs) hold promise for many applications over a range of industry sectors such as a base material for electrodes and membranes. Importantly, CNT morphology and surface chemistry can be rationally modified and fine-tuning of these CNT physicochemical properties can enhance their functionality toward practical applications. The CNT 1D form allows assembly of a stable thin-film fibrous network by a variety of facile techniques. These CNT networks have pore sizes in the range of 10-500 nm (dpore ∼ 6-8dCNT) and thicknesses of 10-200 µm, both similar to those of classical polymer membranes, thus allowing for straightforward incorporation into commercial membrane devices modified for electroactivity inclusion.In this Account, CNTs and their composites are used as model electroactive porous materials to exemplify the design strategies and environmental applications of emerging electroactive membrane technology. The Account begins with a brief summary of the electroactive membrane design principles and flow processes developed by our groups. After the methodology section, a detailed discussion is provided on the underlying physical-chemical mechanisms that govern the electroactive membrane technology. Then we summarize our findings on the rational design of several flow-through electrochemical CNT filtration systems focused on either anodic oxidation reactions or cathodic reduction reactions. Subsequently, we discuss a recently discovered electrochemical valence-state-regulation strategy that is capable to detoxify and sequester heavy metal ions. Finally, we conclude the Account with our perspectives toward future development of the electroactive membrane technology.

Peroxydisulfate Activation and Singlet Oxygen Generation by Oxygen Vacancy for Degradation of Contaminants.

Oxygen vacancies (OVs) play a crucial role in the catalytic activity of metal-based catalysts; however, their activation mechanism toward peroxydisulfate (PDS) still lacks reasonable explanation. In this study, by taking bismuth bromide (BiOBr) as an example, we report an OV-mediated PDS activation process for degradation of bisphenol A (BPA) employing singlet oxygen (1O2) as the main reactive species under alkaline conditions. The experimental results show that the removal efficiency of BPA is proportional to the number of OVs and is highly related to the dosage of PDS and the catalyst. The surface OVs of BiOBr provide ideal sites for the inclusion of hydroxyl ions (HO-) to form BiIII-OH species, which are regarded as the major active sites for the adsorption and activation of PDS. Unexpectedly, the activation of PDS occurs through a nonradical mechanism mediated by 1O2, which is generated via multistep reactions, involving the formation of an intermediate superoxide radical (O2•-) and the redox cycle of Bi(III)/Bi(IV). This work is dedicated to the in-depth mechanism study into PDS activation over OV-rich BiOBr samples and provides a novel perspective for the activation of peroxides by defective materials in the absence of additional energy supply or aqueous transition metal ions.

Multifunctional Piezoelectric Heterostructure of BaTiO3@Graphene: Decomplexation of Cu-EDTA and Recovery of Cu.

About 3.93 billion tons of wastewater containing heavy metal complexes are discharged (e.g., from the electroplating industry) every year in China alone. It is challenging to appropriately treat such wastewaters. Here, a multifunctional composite nanowires BaTiO3@graphene was designed based on Comsol simulations and made into 3D millimeter-sphere in order to facilitate practical application. Results indicate 100% of Cu-EDTA was decomplexed in situ via piezoelectric potential by BaTiO3@graphene. Notably, the addition of graphene sharply increased the surface potential (from 19.8 ± 0.97 to 96.8 ± 1.48 mV) of BaTiO3@graphene by its flexoelectric effect then effectively promoted piezoelectric electrons to be separated and transferred, which favors the piezoelectric catalysis. Moreover, the released Cu(II) from Cu-EDTA decomplexation were recovered simultaneously via the interaction on graphene groups. This method efficiently recovered Cu(II) to avoid the consumption of massive chemical reagents and the generation of secondary hazardous solid waste containing heavy metal ions, compared with the conventional oxidative decomplexation/precipitation strategy for heavy metal complexes removal. Piezoelectric catalysis paves a new possibility for advanced oxidation in wastewater treatment.

Environmentally Friendly in Situ Regeneration of Graphene Aerogel as a Model Conductive Adsorbent.

Adsorption is a classical process widely used in industry and environmental protection, and the regeneration of exhausted adsorbents, as the reverse process of adsorption, is vital to achieving a sustainable adsorption process. Chemical and thermal regeneration, which feature high costs and environmental side effects, are classical but not environmentally friendly methods. Herein, a new regeneration method based on an electrochemical process using graphene aerogel (GA) as a model conductive adsorbent was proposed. First, 3D GA was prepared to adsorb organic and inorganic pollutants, avoiding the inconvenience of using powdered graphene. Then, the exhausted GA was cleaned by the electrochemical desorption and degradation of adsorbed organic pollutants if undesired and the electrorepulsion of adsorbed metal ions in the absence of any additional chemicals, showing a high processing capability of 1.21 L g-1 GA h-1 and low energy consumption (∼0.2 kWh m-3 solution). The mechanisms involved in the electrochemistry-induced desorption process cover a decline in the GA adsorption performance depended on the electrochemically adjustable surface charge conditions, and the further repulsion and migration of adsorbates is subject to the strong in situ electric field. This work has important implications for the development of environmentally friendly regeneration processes and qualified adsorbents as well as the application of a green and efficient regeneration concept for traditional adsorption processes.

Adsorption and Reduction of Cr(VI) Together with Cr(III) Sequestration by Polyaniline Confined in Pores of Polystyrene Beads.

The simultaneous reduction and sequestration of Cr(VI) from wastewater is desirable as a cost-effective and environmentally friendly approach. In this study, we execute a one-step facile synthesis strategy on polyaniline (PANI) composites based on aniline adsorption and polymerization on pores of millimeter-scale polystyrene balls (PANI@PS). The well-defined PANI@PS increased the removal capacity of Cr(VI) by 5.4 times, going from 43.6 (bulky PANI) to 233.7 mg g-1 near neutral pH (6.0) instead of the pH 1-3 documented in other reports, which was higher than that of many reported adsorbents due to its porous structure, numerous interaction sites, and confinement effects in the polymer. Most importantly, PANI@PS could efficiently sequester positive Cr(III) after reducing Cr(VI) to Cr(III) due to its negative surface created by confinement effects confined to the nanopores of PS. Conversely, positively charged bulky PANI repelled electrostatically positive Cr(III); thus, additional precipitation or adsorption treatments were needed in practical applications. Moreover, a coating of PANI can protect PS, as a substrate and a composite, from irreversible damage due to the strong oxidation capacity of Cr(VI), which is another major concern in adsorbing strong oxidants using polymers. A novel strategy to regenerate the exhausted PANI@PS was efficiently executed based on the electrochemical redox reversibility of PANI. Finally, the comprehensive adsorption/reduction/sequestration of Cr on PANI@PS was elucidated in detail.

Flat Graphene-Enhanced Electron Transfer Involved in Redox Reactions.

Graphene is easily warped in the out-of-plane direction because of its high in-plane Young's modulus, and exploring the influence of wrinkled graphene on its properties is essential for the design of graphene-based materials for environmental applications. Herein, we prepared wrinkled graphene (WGN-1 and WGN-2) by thermal treatment and compared their electrochemical properties with those of flat graphene nanosheets (FGN). FGN exhibit activities that are much better than those of wrinkled graphene nanosheets (WGN), not only in the electrochemical oxidation of methylene blue (MB) but also in the electrochemical reduction of nitrobenzene (NB). Transformation ratios of MB and NB in FGN, WGN-1, and WGN-2 were 97.5, 80.1, and 57.9% and 94.6, 92.1, and 81.2%, respectively. Electrochemical impedance spectroscopy and the surface resistance of the graphene samples increased in the following order: FGN < WGN-1 < WGN-2. This suggests that the reaction charges transfer faster across the reaction interfaces and along the surface of FGN than that of WGN, and wrinkles restrict reaction charge transfer and reduce the reaction rates. This study reveals that the morphology of the graphene (flat or wrinkle) greatly affects redox reaction activities and may have important implications for the design of novel graphene-based nanostructures and for our understanding of graphene wrinkle-dependent redox reactions in environmental processes.

Facet Energy and Reactivity versus Cytotoxicity: The Surprising Behavior of CdS Nanorods.

Responsible development of nanotechnology calls for improved understanding of how nanomaterial surface energy and reactivity affect potential toxicity. Here, we challenge the paradigm that cytotoxicity increases with nanoparticle reactivity. Higher-surface-energy {001}-faceted CdS nanorods (CdS-H) were less toxic to Saccharomyces cerevisiae than lower-energy ({101}-faceted) nanorods (CdS-L) of similar morphology, aggregate size, and charge. CdS-H adsorbed to the yeast's cell wall to a greater extent than CdS-L, which decreased endocytosis and cytotoxicity. Higher uptake of CdS-L decreased cell viability and increased endoplasmatic reticulum stress despite lower release of toxic Cd(2+) ions. Higher toxicity of CdS-L was confirmed with five different unicellular microorganisms. Overall, higher-energy nanocrystals may exhibit greater propensity to adsorb to or react with biological protective barriers and/or background constituents, which passivates their reactivity and reduces their bioavailability and cytotoxicity.

Synthesis of mesoporous wall-structured TiO2 on reduced graphene oxide nanosheets with high rate performance for lithium-ion batteries.

Mesoporous wall-structured TiO2 on reduced graphene oxide (RGO) nanosheets were successfully fabricated through a simple hydrothermal process without any surfactants and annealed at 400 °C for 2 h under argon. The obtained mesoporous structured TiO2 -RGO composites had a high surface area (99 0307 m(2) g(-1)) and exhibited excellent electrochemical cycling (a reversible capacity of 260 mAh g(-1) at 1.2 C and 180 mAh g(-1) at 5 C after 400 cycles), demonstrating it to be a promising method for the development of high-performance Li-ion batteries.

Facet-dependent catalytic activity of nanosheet-assembled bismuth oxyiodide microspheres in degradation of bisphenol A.

Photocatalysts with different exposed facets often exhibit different photochemical performances, but the underlying mechanisms are not fully understood. In this study, we synthesized two nanosheet-assembled bismuth oxyiodide (BiOI) microspheres with exposed (110) and (001) facets, respectively, to further investigate facet-dependent photocatalytic activity. Our experimental results showed that the BiOI microspheres with exposed (110) facets exhibited much greater catalytic activity than the BiOI microspheres with exposed (001) facets in the degradation of bisphenol A under visible light irradiation. Density functional theory calculation revealed that the (110) facets can adsorb a greater amount of O2 and, thus, form more O2(•-) and (•)OH radicals than the (001) facets. The electron spin resonance spectroscopy and radical scavenging experiments verified that the BiOI microspheres with exposed (110) facets could produce a greater amount of O2(•-) radicals than the BiOI microspheres with exposed (001) facets, and more importantly, between the two BiOI products, only the BiOI microspheres with exposed (110) facets could generate (•)OH radicals directly. The facet-dependent radical formation mechanisms were previously unidentified. The findings of this study may have important implications for the understanding of the facet-dependent photochemical performance of photocatalysts and the design of novel catalytic materials with inorganic nanostructures.

Carbon nanotube membrane stack for flow-through sequential regenerative electro-Fenton.

Electro-Fenton is a promising advanced oxidation process for water treatment consisting a series redox reactions. Here, we design and examine an electrochemical filter for sequential electro-Fenton reactions to optimize the treatment process. The carbon nanotube (CNT) membrane stack (thickness ∼ 200 µm) used here consisted of 1) a CNT network cathode for O2 reduction to H2O2, 2) a CNT-COOFe(2+) cathode to chemical reduction H2O2 to (•)OH and HO(-) and to regenerate Fe(2+) in situ, 3) a porous PVDF or PTFE insulating separator, and 4) a CNT filter anode for remaining intermediate oxidation intermediates. The sequential electro-Fenton was compared to individual electrochemical and Fenton process using oxalate, a persistent organic, as a target molecule. Synergism is observed during the sequential electro-Fenton process. For example, when [DO]in = 38 ± 1 mg L(-1), J = 1.6 mL min(-1), neutral pH, and Ecell = 2.89 V, the sequential electro-Fenton oxidation rate was 206.8 ± 6.3 mgC m(-2) h(-1), which is 4-fold greater than the sum of the individual electrochemistry (16.4 ± 3.2 mgC m(-2) h(-1)) and Fenton (33.3 ± 1.3 mgC m(-2) h(-1)) reaction fluxes, and the energy consumption was 45.8 kWh kgTOC(-1). The sequential electro-Fenton was also challenged with the refractory trifluoroacetic acid (TFA) and trichloroacetic acid (TCA), and they can be transferred at a removal rate of 11.3 ± 1.2 and 21.8 ± 1.9 mmol m(-2) h(-1), respectively, with different transformation mechanisms.