Conspectus 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 (d pore ∼ 6–8d CNT) 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.
Conventional water resources in many regions are insufficient to meet the water needs of growing populations, thus reuse is gaining acceptance as a method of water supply augmentation. Recent ...advancements in membrane technology have allowed for the reclamation of municipal wastewater for the production of drinking water, i.e., potable reuse. Although public perception can be a challenge, potable reuse is often the least energy-intensive method of providing additional drinking water to water stressed regions. A variety of membranes have been developed that can remove water contaminants ranging from particles and pathogens to dissolved organic compounds and salts. Typically, potable reuse treatment plants use polymeric membranes for microfiltration or ultrafiltration in conjunction with reverse osmosis and, in some cases, nanofiltration. Membrane properties, including pore size, wettability, surface charge, roughness, thermal resistance, chemical stability, permeability, thickness and mechanical strength, vary between membranes and applications. Advancements in membrane technology including new membrane materials, coatings, and manufacturing methods, as well as emerging membrane processes such as membrane bioreactors, electrodialysis, and forward osmosis have been developed to improve selectivity, energy consumption, fouling resistance, and/or capital cost. The purpose of this review is to provide a comprehensive summary of the role of polymeric membranes and process components in the treatment of wastewater to potable water quality and to highlight recent advancements and needs in separation processes. Beyond membranes themselves, this review covers the background and history of potable reuse, and commonly used potable reuse process chains, pretreatment steps, and advanced oxidation processes. Key trends in membrane technology include novel configurations, materials, and fouling prevention techniques. Challenges still facing membrane-based potable reuse applications, including chemical and biological contaminant removal, membrane fouling, and public perception, are highlighted as areas in need of further research and development.
Fine-tuning the geometric and electronic structure of catalytic metal centers via N-coordination engineering offers an effective design for the electrocatalytic transformation of O2 to singlet oxygen ...(1O2). Herein, we develop a general coordination modulation strategy to synthesize fluidic single-atom electrodes for selective electrocatalytic activation of O2 to 1O2. Using a single Cr atom system as an example, >98% 1O2 selectivity can be achieved from electrocatalytic O2 activation due to the subtle engineering of Cr–N4 sites. Both theoretical simulations and experimental results determined that “end-on” adsorption of O2 onto the Cr–N4 sites lowers the overall activation energy barrier of O2 and promotes the breakage of Cr–OOH bonds to form •OOH intermediates. In addition, the flow-through configuration (k = 0.097 min–1) endowed convection-enhanced mass transport and improved charge transfer imparted by spatial confinement within the lamellar electrode structure compared to that of batch reactor (k = 0.019 min–1). In a practical demonstration, the Cr–N4/MXene electrocatalytic system exhibits a high selectivity toward electron-rich micropollutants (e.g., sulfamethoxazole, bisphenol A, and sulfadimidine). The flow-through design of the fluidic electrode achieves a synergy with the molecular microenvironment that enables selective electrocatalytic 1O2 generation, which could be used in numerous ways, including the treatment of environmental pollution.
Abstract
Controlled generation of reactive oxygen species (ROS) is essential in biological, chemical, and environmental fields, and piezoelectric catalysis is an emerging method to generate ROS, ...especially in sonodynamic therapy due to its high tissue penetrability, directed orientation, and ability to trigger in situ ROS generation. However, due to the low piezoelectric coefficient, and environmental safety and chemical stability concerns of current piezoelectric ROS catalysts, novel piezoelectric materials are urgently needed. Here, we demonstrate a method to induce polarization of inert poly(tetrafluoroethylene) (PTFE) particles (<d > ~ 1–5 μm) into piezoelectric electrets with a mild and convenient ultrasound process. Continued ultrasonic irradiation of the PTFE electrets generates ROS including hydroxyl radicals (•OH), superoxide (•O
2
−
) and singlet oxygen
(1
O
2
) at rates significantly faster than previously reported piezoelectric catalysts. In summary, ultrasonic activation of inert PTFE particles is a simple method to induce permanent PTFE polarization and to piezocatalytically generate aqueous ROS that is desirable in a wide-range of applications from environmental pollution control to biomedical therapy.
We propose an innovative approach to impart nanomaterial-specific properties to the surface of thin-film composite membranes. Specifically, biocidal properties were obtained by covalently binding ...single-walled carbon nanotubes (SWNTs) to the membrane surface. The SWNTs were first modified by purification and ozonolysis to increase their sidewall functionalities, maximize cytotoxic properties, and achieve dispersion in aqueous solution. A tailored reaction protocol was developed to exploit the inherent moieties of hand-cast polyamide membrane surfaces and create covalent amide bonds with the functionalized SWNTs. The reaction is entirely aqueous-based and entails activation of the carboxylate groups of both the membrane and the nanomaterials to maximize reaction with ethylenediamine. The presence of SWNTs was verified after sonication of the membranes, confirming the strength of the bond between the SWNTs and the membrane surface. Characterization of the SWNT-functionalized surfaces demonstrated the attainment of membranes with novel properties that continued to exhibit high performance in water separation processes. The presence of surface-bound antimicrobial SWNTs was confirmed by experiments using E. coli cells that demonstrated an enhanced bacterial cytotoxicity for the SWNT-coated membranes. The SWNT membranes were observed to achieve up to 60% inactivation of bacteria attached to the membrane within 1 h of contact time. Our results suggest the potential of covalently bonded SWNTs to delay the onset of membrane biofouling during operation.
Organic fouling of ultrafiltration (UF) membranes results in decreased water flux and increased energy requirements. Modification of UF membrane surfaces is one possible method to mitigate natural ...organic matter (NOM) fouling, yet to date; most modifications have been passive. In this study, we investigate the use of a carbon nanotube-polyvinylidene fluoride (CNT-PVDF) porous non-Faradaic cathode on top of a UF membrane to actively produce negative surface charges via capacitive charging. The study is divided into three elements: (1) modification of the UF system with the capacitive CNT-PVDF electrodes and determination of the optimal electrode-membrane configuration, (2) analysis of the fouling mitigation mechanism, and (3) evaluation of the practical potential of capacitive fouling reduction. All experiments were completed in the cross-flow configuration. The optimal electrode-membrane configuration for organic fouling reduction was when the permeate first flowed through the porous anode, then the CNT-PVDF cathode, and finally the polyethersulfone (PES) UF membrane. The extent of capacitive fouling reduction was determined to be a function of anode material, ionic strength, and cathode potential. The primary fouling reduction mechanism is the potential-induced cathodic negative surface charges that increase the Derjaguin–Landau–Verwey–Overbeek (DLVO) energy barrier and decrease the collision efficiency of negatively-charged organic matter with the membrane surface. The capacitive system has potential to reduce energy requirements by up to 2-fold as compared to the unmodified UF system when challenged with 10ppm NOM solutions at low ionic strength.
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•Porous CNT-PVDF non-Faradaic cathode capacitively reduced UF organic fouling.•Electrode-membrane order affected hydrodynamic and electrokinetic fouling reduction.•Capacitive organic fouling reduction is related to increase in DLVO barrier energy.•At low ionic strength, O&M savings greater than material cost increase.
In this study, a TiO2-coated carbon nanotube (CNT) network filter was prepared via a simple filtration–steam hydrolysis method and evaluated with respect to aqueous arsenic removal. The TiO2 coating ...was 5.5 ± 2.7 nm thick, completely covered the CNT network surface, and had a specific surface area of 196 m2 g–1, which was ∼2-fold greater than that of the CNT network. The TiO2–CNT As sorption kinetics increased with both increasing flow rate and cell potential, with increasing flow rate having a significantly stronger effect. At 6 mL min–1 in the absence of potential and in recirculation mode, the first-order As sorption rate constants were 4.3 and 4.4 s–1 for As(III) and As(V), respectively. The TiO2–CNT electro-assisted equilibrium sorption capacities at a cell potential of 2 V for effluent As = 10 ppb in single-pass mode were 1.8 and 1.3 mg g–1 for As(III) and As(V), respectively. The enhanced TiO2–CNT filter As sorption kinetics and capacity result from increased mass transport due to internal convection and pore radius range, improved sorption site accessibility due to porosity and TiO2 dispersion, and reduced TiO2 negative surface charge due to anodic capacitance. Groundwater samples containing 44 ppb As were treated by single-pass filtration, and 12500 bed volumes (residence time of 4.5 s; 127 L m–2 h–1; 5.8 mg m–2 h–1) were filtered prior to the effluent As level reaching >10 ppb. A spent TiO2 filter was successfully regenerated by 5 mM NaOH for both As(III) and As(V).
An electrochemical carbon nanotube (CNT) filter has been reported to be effective for the adsorptive removal and oxidation of aqueous organic compounds. Here, we complete a detailed investigation of ...the aqueous dye oxidation reactive transport mechanism during electrochemical filtration. Similar to batch electrolysis, mass transfer, physical adsorption, and electron transfer are found to be three primary steps in the overall electrochemical filtration organic oxidation mechanism. Mass transfer was quantitatively examined by chronoamperometry and normal pulse voltammetry and determined to be increased 6-fold during electrochemical filtration as compared to batch electrochemistry. Convection-enhanced mass transfer to the electrode surface is determined to be the primary factor for increased current density and organic oxidation during electrochemical filtration. Physical adsorption of the organics onto the CNTs was evaluated using temperature-dependent batch adsorption and electrochemical filtration experiments. The electrochemical filtration kinetics were observed to have a minor negative temperature-dependence. Electron transfer was examined by challenging the electrochemical filter with a range of increasing dye concentrations until the mass transfer and adsorption processes were saturated. Upon surface site saturation, the electron transfer rates were determined to be 8.5 × 1015, 6.3 × 1016, and 1.3 × 1017 e– s–1 m–2 at anode potentials of 0.35, 0.77, and 1.50 V, respectively. The electron transfer mechanism was also investigated and direct electron transfer was determined to be the dominant methyl orange oxidation mechanism at all evaluated anode potentials with an increasing contribution from indirect oxidation processes at potentials ≥1.0 V. The anode potential dependent maximum electron transfer rate is also observed to be affected by the polarity of the organic charge indicating electromigration is also active. In summary, electrochemical filtration is advantageous as compared to batch electrolysis due to the liquid flow through the electrode resulting in convection-enhanced transfer of the target molecule to the electrode surface.
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-COOFe2+ cathode to chemical reduction H2O2 to •OH and HO– and to regenerate Fe2+ 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 DOin = 38 ± 1 mg L–1, J = 1.6 mL min–1, neutral pH, and E cell = 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.
In this study, a binder-free, porous, and conductive 3D carbon-nanotube (CNT) network uniformly coated with bismuth-doped tin oxide (BTO) nanoparticles was prepared via a simple ...electrosorption–hydrothermal method and utilized for the electrooxidative filtration of organics. The BTO-CNT nanocomposite was characterized by scanning electron microscopy, thermogravimetric analysis, transmission electron microscopy, X-ray photoelectron spectroscopy, linear sweep voltammetry, and Tafel analysis. The submonolayer BTO coating is composed of 3.9 ± 1.5 nm diameter nanoparticles (NPs). The oxygen-evolution potential of the BTO-CNT nanocomposite was determined to be 1.71 V (vs Ag/AgCl), which is 440 mV higher than an uncoated CNT anode. Anodic stability, characterized by CNT oxidative corrosion to form dissolved species, indicated that the BTO-CNT incurred negligible corrosion up to Vanode = 2.2 V, whereas the uncoated CNT was compromised at Vanode ≥ 1.4 V. The effect of metal oxide-nanoparticle coating on anodic performance was initially studied by oxalate oxidation followed by total organic carbon (TOC) and chemical oxygen demand (COD) analysis. The BTO-CNT displayed the best performance, with ∼98% oxalate oxidation (1.2 s filter residence time) and current efficiencies in the range of 32 to >99%. The BTO-CNT anode energy consumption was 25.7 kW h kgCOD–1 at ∼93% TOC removal and 8.6 kW h kgCOD–1 at ∼50% TOC removal, comparable to state-of-the-art oxalate oxidation processes (22.5–81.7 kW h kgCOD–1). The improved reactivity, current efficiency, and energy consumption are attributed to the increased conductivity, oxygen-evolution potential, and stability of the BTO-CNT anode. The effectiveness and efficiency of the BTO-CNT anode as compared to the uncoated CNT was further investigated by the electrooxidative filtration of ethanol, methanol, formaldehyde, and formate, and it was determined to have TOC removals 2 to 8 times greater, mineralization current efficiencies 1.5 to 3.5 times greater, and energy consumption 4 to 5 times less than the uncoated CNT anode. Electrooxidation and anode passivation mechanisms are discussed.
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