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Decomposition of micropollutants using an electrocatalytic membrane reactor is a promising alternative to traditional advanced oxidation processes due to its high efficiency and environmental compa...

Graphene oxide (GO) nanosheets with atomic thickness and tunable physicochemical properties have been considered as promising nanobuilding blocks for fabrication of separation membranes with impressive performance. There are two kinds of molecular transport channels in laminar GO membranes, interlayer nanochannels formed by adjacent nanosheets and intrinsic defects/pores/edges of GO nanosheets. It has been demonstrated that precisely controlling the transport pathways at the angstrom level, through reduction, molecule/cation cross-linking, intercalation, physical confinement, electric field adjustment, pore creation, and defect sealing, can greatly improve the separation performance of GO membranes. Herein, we first briefly review the fabrication strategies of GO membranes and then comprehensively discuss the merits and mechanisms of controlling the transport pathways of GO membranes for liquid separation applications including static diffusion, pressure-driven filtration, and pervaporation.

Wetting and fouling phenomena are the main concerns for membrane distillation (MD) in treating high-salinity industrial wastewater. This work developed an omniphobic membrane by growing titanium dioxide (TiO2) nanorods on polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) nanofibers using a hydrothermal technique. The TiO2 nanorods form a uniform pine-needle-like hierarchical nanostructure on PVDF-HFP fibers. A further fluorination treatment provides the membrane with a low-surface-energy omniphobic surface, displaying contact angles of 168° and 153° for water and mineral oil, respectively. Direct contact MD experiments demonstrated that the resulting membrane shows a high and stable salt rejection of >99.9%, while the pristine PVDF-HFP nanofibrous membrane suffers a rejection decline caused by intense pore wetting and oil fouling in the desalination process in the presence of surfactant and mineral oil. The superior antiwetting and antifouling behaviors were ascribed to a nonwetting Cassie-Baxter state established by the accumulation of a great deal of air in the hydrophobized hierarchical re-entrant structures. The development of omniphobic membranes with pine-needle-like hierarchical nanostructures provides an approach to mitigate membrane wetting and fouling in the MD process for the water reclamation from industrial wastewater.

Superhydrophobic polyvinylidene fluoride (PVDF) membrane is a promising material for membrane distillation. Existing approaches for preparing superhydrophobic PVDF membrane often involve separate manipulation of surface roughness and surface chemistry. Here we report a one-step approach to simultaneously manipulate both the surface roughness and surface chemistry of PVDF nanofibrous membranes for enhanced direct-contact membrane distillation (DCMD) performances. The manipulation was realized in a unique solvent-thermal treatment process, during which a treatment solution containing alcohols was involved. We demonstrate that by using different chain-length alcohols in the treatment solvent, surface roughness can be promoted by creating nanofin structures on the PVDF nanofibers using an alcohol which has moderate affinity with PVDF. Meanwhile, surface chemistry can be tuned by adjusting the fraction distribution of crystal phases (nonpolar α phase and polar β phase) in the membrane using different alcohols. PVDF membranes with different surface wettabilities were used to evaluate the effects of surface roughness and surface energy on the DCMD performances. Combining both low surface energy and multi-scale surface roughness, pentanol-treated PVDF membrane achieved best anti-water property (water contact angle of 164.1° and sliding angle of 8.1°), and exhibited superior water flux and enhanced anti-wetting ability to low-surface-tension feed in the DCMD application.

We report a novel and facile welding strategy using Span 80 to simultaneously enhance the mechanical robustness and superhydrophilicity of an electrospun poly(vinylidene fluoride) (PVDF) nanofibrous membrane. By taking advantage of its amphiphilic property, Span 80 micelle is able to attach PVDF nanofibers due to the strong affinity between the hydrophobic segments of Span 80 and PVDF. The as-prepared PVDF nanofibrous membrane exhibited simultaneously improved stress (from 1.7 ± 0.3 MPa to 8.8 ± 0.6 MPa). Meanwhile, supherhydrophilic property was endowed through the self-assembly of Span 80 on the hierarchical surface of PVDF membrane. Water contact angle (WCA) and under-water oil contact angle (UOCA) of the modified membrane was nearly 0 and 154.9 ± 2o, respectively. This membrane had a water permeability of 18,482.7 ± 287.3 Lm−2h−1bar−1 for 1, 2-dichloroethane-in-water emulsion with a separation efficiency of 96.3%. The welded PVDF nanofibrous membrane exhibited extraordinary robustness, resisting to long-term washing of water and oil-in-water emulsion.

Salinity-gradient is emerging as one of the promising renewable energy sources but its energy conversion is severely limited by unsatisfactory performance of available semipermeable membranes. Recently, nanoconfined channels, as osmotic conduits, have shown superior energy conversion performance to conventional technologies. Here, ion selective nanochannels in lamellar Ti3C2Tx MXene membranes are reported for efficient osmotic power harvesting. These subnanometer channels in the Ti3C2Tx membranes enable cation-selective passage, assisted with tailored surface terminal groups, under salinity gradient. A record-high output power density of 21 W·m-2 at room temperature with an energy conversion efficiency of up to 40.6% is achieved by controlled surface charges at a 1000-fold salinity gradient. In addition, due to thermal regulation of surface charges and ionic mobility, the MXene membrane produces a large thermal enhancement at 331 K, yielding a power density of up to 54 W·m-2. The MXene lamellar structure, coupled with its scalability and chemical tunability, may be an important platform for high-performance osmotic power generators.

Antifouling performance of membranes is the key obstacle limiting their practical applications for oil/water separation. In this study, a sustaining antifouling membrane was fabricated by constructing polydopamine (PDA) micro-/nano-spheres on a polyacrylonitrile (PAN) nanofibrous membrane. The secondary PDA nano-spheres not only strengthened the bonding of primary micro-spheres with the substrate, but also diversified the hierarchical structure and chemistry. The composite showed enhanced superhydrophilicity and underwater superoleophobicity. Permeability of PAN-PDAc membrane was maintained as high as 11666 ± 978 Lm−2h−1bar−1 with separation efficiency of higher than 99.9% over a 2-h continuous filtration. This permeability was about 2.7 times of pristine PAN membrane (4260 ± 430 Lm−2h−1bar−1). The extrusion and cutting demulsification on the confined space of PAN-PDA surface was proposed. Antifouling mechanism of the superhydrophilic membrane was first theoretically elucidated based on hydration ability and adhesion free energy with recourse to thermal analysis and Derjaguin-Landau-Verwey-Overbeek theory respectively. It was found that PDA micro-/nano-spheres mediated membrane showed strong hydration ability (higher fraction of non-freezable water) and weak adhesion towards toluene (low free energy of adhesion) compared to pristine PAN membrane. These findings would lead to a better understanding of antifouling demulsification mechanism and improved design of sustaining antifouling membranes for oil/water separation.

The ridge-and-valley roughness structure of a polyamide reverse osmosis (RO) membrane has a paramount impact on its separation performance. We show that this surface roughness appearance was shaped by gas nanobubbles confined between the polyamide rejection layer and the substrate. Performing interfacial polymerization (IP) under alternative confinement conditions led to drastically different surface morphologies, e.g., smooth polyamide surface formed at support-free aqueous/organic interfaces whereas crater-like features formed in inversed IP. For the first time, we demonstrated the collapse of fully hydrated balloon-like nodules into dehydrated leaf-like and donut-like roughness features during membrane drying by performing an in-situ atomic force microscopic characterization. Deformation of roughness features caused by dehydration was not fully reversible, which correlates well with the dramatic reduction of membrane permeability upon drying. Our study provides a fundamental framework for the surface roughness formation in RO membranes, which is critical for advancing roughness control technologies with enhanced membrane performance.

Salinity gradient energy extracting by a reverse electrodialysis (RED) unit using for electrodialysis (ED) desalination process is a potential way to achieve energy-economic and sustainable production of freshwater. However, the parameters in RED and ED unit synergistically influence the desalination process, resulting to the hybrid process controlled by multi-parameters. Modeling of an RED/ED is a simple way to describe the desalination process and reveal the effects of these parameters on the performance of system and then to find the better adaption of RED/ED. In this study, a model of an internal-integrated RED/ED hybrid system is first established. It found that the ratio of desalination in RED/ED is higher than 90%. The brine/river is the alternative combination to realize seawater desalination with a desalination rate of 0.38 h m2/mol. The desalination capacity of RED/ED (0.43–2.6 mol/h·m2) is much higher than that of the external-integrated RED + ED system (0.10–0.15 mol/h·m2), but it is of simpler configuration and has a lower energy requirement. Moreover, the RED/ED system is preferred for using in the pre-desalination process. The outcome of this model is helpful in the design of practical RED/ED systems, and points out the development potential of RED/ED in practical applications.

Thin film composite (TFC) polyamide membranes set the golden standard for reverse osmosis technology, but tuning their permeability and selectivity remains a major challenge because of the inherent permeability-selectivity trade-off. Creating nano-sized voids within the polyamide rejection layer can tune the membrane roughness and increase its effective filtration area to improve the water permeability. Here we prepare nano-foamed polyamide rejection layers by adding sodium bicarbonate into the aqueous solution of amine monomers. We show a systematic evolution of the roughness structure of polyamide membranes, with increasingly leaf-like and belt-like features appearing under enhanced nano-foaming conditions. These nano-foamed features can result in remarkable improvements in both water permeability and salt rejection and reduce membrane fouling propensity at the same time. Our study paves a new research direction for designing future generation of desalination membranes, which holds vast potential to reduce the cost and energy consumption of desalination while achieving improved product water quality.

We report the use of highly porous Janus membranes with unparalleled forward osmosis (FO) performance in terms of water flux and reverse salt flux. A porous Janus FO membrane, composed of a hydroph...

Thin film nanocomposite (TFN) membranes, which incorporate nanomaterials in a crosslinked polyamide matrix, often show enhanced separation properties thanks to the additional pores or channels offered by these materials. In this study, we deliberately created nanovoids in the dense polyamide rejection layer by acid-etching copper nanoparticles (CuNPs) contained in a TFN membrane. Systematic membrane characterization confirmed the complete removal of CuNPs using 1% HNO3, which formed nanovoids of approximately 10 nm in size. The water flux of the etched membrane TFC-Cu50X was nearly quadrupled compared to that of the CuNPs loaded membrane TFC-Cu50. This significantly improved water flux can be ascribed to the enhanced water transport through these nano-sized voids. The nanovoids-enhanced approach provides new possibilities for synthesizing high performance membranes.

Membranes with ultrathin rejection layers can effectively achieve high water permeability. In the current study, we report a single-step fabrication of carbon nanotubes (CNTs)-loaded ultrathin film composite (uTFC) polyamide membranes using an electrospray-assisted interfacial polymerization method. In this method, amine and acyl chloride monomer solutions were electrosprayed into micro-droplets to allow a controlled growth of the polyamide rejection layer. At the same time, CNTs were uniformly dispersed under the action of the electrical field. The loading of CNTs in the polyamide rejection layer markedly enhance the water permeability of the membrane with a 2–6 folds enhancement. The simple and versatile electrospray-assisted interfacial polymerization method, which allows simultaneous control of rejection layer thickness and dispersion of nanomaterials, paves a new dimension for the preparation of high performance ultrathin nanocomposite membranes.

Loose nanofiltration (NF) membranes are an attractive avenue in effective separation of organic matters and salts for resource recovery from highly-loaded wastewater. However, membrane fouling remains an unclear and complex factor in practical applications. In this work, the flux of a loose NF membrane fouled by humic acid at various solution compositions was systematically investigated. The strong hydrophilicity of the loose NF membrane allows for slight deposition of humic acid on the membrane surface, yielding an outstanding antifouling performance. However, a moderate flux decline was observed at low pH and high ionic strength, due to reduction in charge density of membrane surface for formation of a porous foulant layer. At higher ionic strength, cake-enhanced concentration polarization was the fouling mechanism that dominates the membrane flux. The presence of calcium ions induced bridging between humic acid molecules to generate a compact foulant layer, tremendously deteriorating the membrane flux. Based on COMSOL simulation for the membrane module, the hydrodynamics near the membrane surface had a more significant effect on membrane fouling than the solution chemistry, which is consistent with scanning electronic microscopy observation. This indicates benign hydrodynamic condition can be an effective strategy to fouling control for loose NF membranes.

The microbial safety of swimming pool waters (SPWs) becomes increasingly important with the popularity of swimming activities. Disinfection aiming at killing microbes in SPWs produces disinfection by-products (DBPs), which has attracted considerable public attentions due to their high frequency of occurrence, considerable concentrations and potent toxicity. We reviewed the latest research progress within the last four decades on the regulation, formation, exposure, and treatment of DBPs in the context of SPWs. This paper specifically discussed DBP regulations in different regions, formation mechanisms related with disinfectants, precursors and other various conditions, human exposure assessment reflected by biomarkers or epidemiological evidence, and the control and treatment of DBPs. Compared to drinking water with natural organic matter as the main organic precursor of DBPs, the additional human inputs (i.e., body fluids and personal care products) to SPWs make the water matrix more complicated and lead to the formation of more types and greater concentrations of DBPs. Dermal absorption and inhalation are two main exposure pathways for trihalomethanes while ingestion for haloacetic acids, reflected by DBP occurrence in human matrices including exhaled air, urine, blood, and plasma. Studies show that membrane filtration, advanced oxidation processes, biodegradation, thermal degradation, chemical reduction, and some hybrid processes are the potential DBP treatment technologies. The removal efficiency, possible mechanisms and future challenges of these DBP treatment methods are summarized in this review, which may facilitate their full-scale applications and provide potential directions for further research extension. Keywords: Disinfectants, Disinfection by-products (DBP), Precursors, Human exposure, DBP treatment

Using hydroxypropyl-β-cyclodextrins (OH-β-CD) as aqueous phase additive, nanocavity-contained thin film composite (TFC) nanofiltration (NF) membranes were prepared by interfacial polymerization of piperazine (PIP) and trimesoyl chloride (TMC). Attenuated total reflectance Fourier transform infrared and X-ray photoelectron spectroscopy analysis confirmed the successful incorporation of OH-β-CDs into the thin polyamide rejection layers. The rejection layer morphologies and structures, surface properties and separation performances of the prepared TFC membranes were greatly influenced by the OH-β-CD mass ratio in aqueous phase during interfacial polymerization. With appropriate OH-β-CD mass ratios (20–60%), the prepared TFC membranes showed enhanced water flux, reduced RNaCl, increased RNa2SO4 and RMgSO4, which led to highly improved divalent to monovalent ions selectivity. These nanocavity-contained TFC membranes exhibited the potential to apply in desalination pretreatment.

Nanofiltration (NF) with high water flux and precise separation performance with high Li+/Mg2+ selectivity is ideal for lithium brine recovery. However, conventional polyamide-based commercial NF membranes are ineffective in lithium recovery processes due to their undesired Li+/Mg2+ selectivity. In addition, they are constrained by the water permeance selectivity trade-off, which means that a highly permeable membrane often has lower selectivity. In this study, we developed a novel nonpolyamide NF membrane based on metal-coordinated structure, which exhibits simultaneously improved water permeance and Li+/Mg2+ selectivity. Specifically, the optimized Cu-m-phenylenediamine (MPD) membrane demonstrated a high water permeance of 16.2 ± 2.7 LMH/bar and a high Li+/Mg2+ selectivity of 8.0 ± 1.0, which surpassed the trade-off of permeance selectivity. Meanwhile, the existence of copper in the Cu-MPD membrane further enhanced anti-biofouling property and the metal-coordinated nanofiltration membrane possesses a pH-responsive property. Finally, a transport model based on the Nernst-Planck equations has been developed to fit the water flux and rejection of uncharged solutes to the experiments conducted. The model had a deviation below 2% for all experiments performed and suggested an average pore radius of 1.25 nm with a porosity of 21% for the Cu-MPD membrane. Overall, our study provides an exciting approach for fabricating a nonpolyamide high-performance nanofiltration membrane in the context of lithium recovery.

Salinity gradient power (SGP) derived from sea and fresh water through reverse electrodialysis (RED) is an emerging discipline with huge potential for carbon-free energy harvesting. SGP technology is still in an infant stage and there is a need for accurate mathematical tools to study its energy harvesting process. Previous models assume a constant salinity gradient with a continuous flow of sea water with constant salinity. In the case of recycling used sea water, such assumption is no longer valid because the salinity gradient reduces with operating time. This paper presents a generalized RED model that covers both of the continuous and recycle modes. It combines an improved kinetic battery module (KiBaM) with an electrical circuit module (ECM), for capturing the behaviors of both RED stacks operating in continuous mode (C-mode) and those in recycle mode (R-mode). To intuitively describe the compound effects of salinity variation and concentration polarization on electrical performance of the R-mode RED stack, nonlinear capacity effects (i.e., recovery effect and rate capacity effect) and self-consumed effect are introduced into the proposed model. The derivation and extraction procedures of the proposed model are included. An RED stack prototype with 50 pairs of alternating membranes is constructed for model validation. Various pulsed and constant current discharge experimental tests are performed to validate the accuracy of the proposed model. Published version This work was supported by the Research Grants Council of the Hong Kong Special Administration Region, China, under Grant C7051-17G.

The recycling of end-of-life water purification membranes is of great significance for environmental sustainability. However, only techniques for downcycling end-of-life high-pressure membranes are available. Here, we propose to upcycle fouled microfiltration membranes for fabricating new high-pressure polyamide (PA) thin-film composite membranes via interfacial polymerization (IP). A cross-linked, defect-free, and ultrathin PA active layer was formed on biopolymer-fouled substrates. In contrast to the decreased pure water permeance of substrates caused by biopolymer fouling, the upcycled membranes show excellent water permeance (∼30 L m−2 h−1 bar−1) and Na2SO4 rejection (∼95%) in nanofiltration tests for water purification. The biopolymer foulants regulate the IP process and the formation of the PA layer. Furthermore, the foulants between the PA layer and substrate can create additional channels for water transport. The PA layer formation could also be achieved on real fouled microfiltration substrates. This proof-of-concept study paves the way for upcycling fouled/end-of-life low-pressure membranes to fabricate new high-pressure membranes for water purification, forming a closed eco-loop of membrane recycling.

Frequent occurrence of harmful algal blooms (HABs) and red tides in freshwater and seawater poses serious threats to water treatment and drives the application of membrane-based technologies in algal separation. Despite the high removal efficiency of algal cells and their metabolites (e.g. organic matter and toxins) by membranes, the generation of concentrate and waste streams presents a major challenge. In this paper, we review the scenarios under which membrane-based processes are integrated with algal separation, with particular attention given to (i) drinking water production and desalination at low algal concentrations and (ii) cyanobacteria-laden water treatment/desalination. The concentrate and waste streams from backwashing and membrane cleaning in each scenario are characterised with this information facilitating a better understanding of the transport of algal cells and metabolites in membrane processes. Current strategies and gaps in managing concentrate and waste streams are identified with guidance and perspectives for future studies discussed in an Eisenhower framework.

© 2019 Elsevier B.V. The rising consumption of pharmaceuticals, personal care products, and endocrine disruptive compounds for healthcare purposes and improving living standards has resulted in the widespread occurrence of organic micropollutants (MPs) in water and wastewater. Conventional water/wastewater treatment plants are faced with inherent limitations in tackling these compounds, leading to difficulties in the provision of secure and safe water supplies. In this context, membrane technology has been found to be a promising method for resolving this emerging concern. To ensure the suitability of membrane-based treatment processes in full-scale applications, we first need to develop a better understanding of the behavior of MPs and the mechanisms behind their removal using advanced membrane technologies. This review provides a thorough overview of the advanced membrane-based treatment methods available for the effective removal of MPs, including reverse osmosis, nanofiltration, ultrafiltration, forward osmosis, and membrane distillation.

Highly efficient and economic treatment of wastewater sludges and wastewaters in one way is a challenging issue in the water treatment field. Herein we present a waste-to-resource strategy for rational fabrication of low–cost ceramic membranes, which simultaneously addresses the treatment of heavy metal-laden sludges and the separation of oil-in-water (O/W) emulsions. A thermal conversion mechanism is proposed for complicated reactions between simulated nickel-laden wastewater sludge and bauxite mineral. In addition to full stabilization and recycling of heavy metal wastewater sludges, rational tailoring of ceramic membrane structures can also be realized to achieve high water flux and favorable mechanical and surface properties. With rational structure design, the tailored spinel-based ceramic membranes exhibited high rejection and high flux (7473 LMH·bar−1) simultaneously for separation of oily wastewater, outperforming other reported state-of-the-art ceramic membranes. The membrane fouling mechanism revealed the dominance of cake layer formation at low cross flow velocities, while a combined model of cake layer formation and pore blocking dominated membrane fouling at high cross-flow velocities. The proposed strategy can be potentially extended toward design of functional ceramic membranes derived from other heavy metal wastewater sludges and for other water treatment applications.

Surface roughness enhancement by fabricating multi-scale nano/microstructure is an effective strategy to prepare superhydrophobic membranes. Here we report a novel solvent-thermal induced roughening (STIR) method. The method involves the swelling of a polymer surface to create a soft shell/hard core structure under the combined action of solvent and heating, followed by a controllable surface roughening as a result of mismatched thermal expansion between the shell and the core. We show a significant increase of surface roughness for a STIR-treated polyvinylidene fluoride nanofibrous membrane, whose nanofibers were covered with densely-packed nanofins. The treated membrane had greatly enhanced hydrophobicity, resulting in improved anti-wetting performance to low-surface-tension feed water in a membrane distillation process. The STIR method was capable of treating membranes with various pore structures. The novel surface roughening strategy opens up new directions to fabricate superhydrophobic surfaces and membranes, which can greatly benefit a wide range of applications such as membrane distillation, oil/water separation.

peer-reviewed Membrane distillation (MD) is a promising process for the treatment of highly saline wastewaters. The central component of MD is a stable porous hydrophobic membrane with a large liquid–vapor interface for efficient water vapor transport. A key challenge for current polymeric or hydrophobically modified inorganic membranes is insufficient operating stability, resulting in some issues such as wetting, fouling, flux, and rejection decline. This study presents an overall conceptual design and application strategy for a superhydrophobic ceramic–based carbon nanotube (CNT) desalination membrane having specially designed membrane structures with unprecedented operating stability and MD performance. Superporous and superhydrophobic surface structures with CNT networks are created after quantitative regulation of in situ grown CNT. The fully covered CNT layers (FC–CNT) exhibit significantly improved thermally and superhydrophobically stable properties under an accelerated stability test. Due to the distinctive structure of the superporous surface network, providing a large liquid–vapor superhydrophobic interface and interior finger-like macrovoids, the FC–CNT membrane exhibits a stable high flux with a 99.9% rejection of Na+, outperforming existing inorganic membranes. Under simple and nondestructive electrochemically assisted direct contact MD (e-DCMD), enhanced antifouling performance is observed. The design strategy is broadly applicable to be extended toward fabrication of high performance membranes derived from other ceramic or inorganic substrates and additional applications in wastewater and gas treatment.

Conventional thin-film composite (TFC) membranes suffer from the trade-off relationship between permeability and selectivity, known as the “upper bound”. In this work, we report a high performance thin-film composite membrane prepared on a tannic acid (TA)-Fe nanoscaffold (TFCn) to overcome such upper bound. Specifically, a TA-Fe nanoscaffold was first coated onto a polysulfone substrate, followed by performing an interfacial polymerization reaction between trimesoyl chloride (TMC) and piperazine (PIP). The TA-Fe nanoscaffold enhanced the uptake of amine monomers and provided a platform for their controlled release. The smaller surface pore size of the TA-Fe coated substrate further eliminated the intrusion of polyamide into the substrate pores. The resulting membrane TFCn showed a water permeability of 19.6 ± 0.5 L m2– h–1 bar–1, which was an order of magnitude higher than that of control TFC membrane (2.2 ± 0.3 L m–2 h–1 bar–1). The formation of a more order polyamide rejection layer also significantly ...

We report a facile and green approach for the synthesis of novel Pd-loaded nanofibrous membrane and its catalytic performance for dechlorinating trichloroethylene. An analogue of mussel adhesive protein, polydopamine (PDA), was used as an active and green platform for the in situ Pd reduction by taking advantage of its high adhesion and reducing activity. A PDA layer was firstly coated on the surface of a nanofibrous membrane. Soaking this PDA-coated membrane into Pd solution led to spontaneous generation of Pd nanoparticles. The resulting membrane achieved a Pd loading of 4.56 wt.%. When applied for treating trichloroethylene in an aqueous solution, the Pd-loaded nanofibrous membrane showed excellent dechlorination performance (degree of TCE dechlorination can reach up to 96.5 (±0.32) %) with high recyclability.

Catalytic membrane, due to its compact reactor assembling, high catalytic performance as well as low energy consumption, has proved to be more attractive for wastewater treatment. In this work, a free-standing α-MnO2@CuO membrane with hierarchical nanostructures was prepared and evaluated as the catalytic membrane to generate radicals from peroxymonosulfate (PMS) for the oxidative degradation of organic dyes in aqueous solution. Benefiting from the high mass transport efficiency and the hierarchical nanostructures, a superior catalytic activity of the membrane was observed for organic dyes degradation. As a typical organic dye, more than 99% of methylene blue (MB) was degraded within 0.23 s using dead-end filtration cell. The effects of flow rate, PMS concentration and buffer solution on MB degradation were further investigated. Besides MB, the catalytic membrane also showed excellent performance for the removal of other dyes, such as congo red, methyl orange, rhodamine B, acid chrome blue K and malachite green. Moreover, the mechanism study indicated that OH and SO4- generated from the interaction between PMS and Mn/Cu species with different oxidation states mainly accounted for the dyes degradation. The catalytic filtration process using α-MnO2@CuO catalytic membrane could provide a novel method for wastewater purification with high efficiency and low energy consumption.

Cytostatic drugs, mainly used as chemotherapy compounds, can pose serious threats to aqueous ecosystem and human health once released into the natural environment. We investigated the use of an anaerobic osmotic membrane bioreactor (AnOMBR) for removing cytostatic drugs from wastewater. The AnOMBR utilizes a dense forward osmosis (FO) membrane in an anaerobic digester with prolonged sludge retention time (60 days). The high rejection of the FO membrane combined with the extended organic retention time in the reactor ensured high removal rates (more than 95.6%) for all the eight cytostatic drugs investigated. With regard to their removal routes in the AnOMBR, the eight cytostatic drugs can be divided into several groups. Doxorubicin, Epirubicin and Tamoxifen were nearly completely removed through the adsorption of anaerobic sludge, while Methotrexate and Cyclophosphamide were mainly removed by biodegradation and FO rejection, respectively. In addition, Mitotane, Azathioprine and Flutamide were removed by both biodegradation and adsorption. This work provides critical insights into the removal mechanisms of high-retention AnOMBRs.

In the past decades, novel materials (e.g., aquaporin proteins, carbon nanotubes, nanoporous graphene and graphene oxide) have emerged as promising candidates for synthesizing high performance desalination membranes. These materials can potentially achieve water fluxes of several orders of magnitude higher compared to the state-of-the-art thin-film composite polyamide reverse osmosis membranes. This paper provides a comprehensive summary of the current progresses and challenges in synthesizing aquaporin-based and carbon-based membranes. After a detailed review of the material properties of aquaporin proteins, carbon nanotubes, nanoporous graphene and graphene oxide, a general framework of membrane design and material incorporation is established. The fabrication methods and separation performance for each type of membrane are summarized. Future perspectives of aquaporin-based and carbon-based membranes are discussed in lieu with their ultimate separation performance and commercial scalability.

We applied a polydopamine (PDA) coating on a thin film composite (TFC) forward osmosis (FO) membrane and investigated the effects of coating on FO mass transport and antifouling behavior. The PDA coating significantly improved membrane surface hydrophilicity as well as reduced membrane surface roughness. Using a short PDA coating duration of 0.5 h, the coated membrane TFC-C0.5 achieved enhanced FO water flux and reduced reverse solute diffusion simultaneously. The reduced reverse solute diffusion can be attributed to the enhanced membrane selectivity: TFC-C0.5 had better rejection and similar water permeability compared to the original TFC membrane. This reduction in reverse solute diffusion further reduced the internal concentration polarization inside the coated membrane, leading to an enhanced FO water flux. Nevertheless, longer PDA coating duration of 1–4 h resulted in reduced FO water flux due to the significantly increased hydraulic resistance of the coated membranes. The PDA coated membrane TFC-C0.5 also presented an improved antifouling performance compared to the control membrane using alginate as a model foulant. Our results reveal the great room for the development of effective coating materials in FO: a well-designed coating with high selectivity and low hydraulic resistance can improve solute rejection, reduce reverse solute diffusion, mitigate internal concentration polarization and enhance FO water flux in addition to control fouling. Such unprecedented opportunities break the traditional trade-off between water flux and antifouling performance when coating pressure driven reverse osmosis membranes.

We performed chlorination experiments to understand the impact of hypochlorite exposure on both cation exchange membranes (CEMs) and anion exchange membranes (AEMs) regarding their properties and performance in a reverse electrodialysis (RED) stack. Changes in membrane morphology, surface elemental composition and chemical bounding suggested the chlorine incorporation in the form of C-Cl bonds and side-chain cleavage of –SO3- or –NR3+ containing molecular fractions. These observations were further supported by observed increases in hydrophobicity and decreases in fixed charged groups of the membranes, respectively. Compared to CEMs, AEMs were less chlorine resistant such that the development of more extensive cracks in the membrane structure further increased water content and dramatically decreased membrane conductivity. The performance of both chlorinated CEMs and AEMs were tested in an RED stack and the results showed the reduced RED power density was largely attributed to the deteriorated electrical properties of AEMs.

Membrane-based technologies are increasingly used for water and wastewater treatment; however, biofouling, the adhesion of microorganisms to the membrane and subsequent formation of biofilm, remains a major obstacle in real applications. In this study, we report a novel method to fabricate a highly antibiofouling membrane by grafting quaternary ammonium compounds (QAC) onto a silica-decorated membrane via alkoxysilane polycondensation reaction. A controlled architecture was created by initially coating polydopamine (PDA)/polyethylenimine (PEI) layer, followed by in situ synthesizing a hydrophilic silica nanoparticle layer through silification reaction and then immobilizing QAC on the silica-decorated membrane to form an antibacterial surface. Although the QAC modified membrane exhibited comparable surface roughness, hydrophilicity and water permeability with the pristine membrane, the former displayed clear antibacterial effects against both Gram-positive and Gram-negative bacteria compared to the pristine membrane, i.e., ~93% and ~92% inhibition of S. aureus and E. coli, respectively. The excellent biofouling resistance imparted by the QAC layer was further confirmed by filtration experiment, showing lower water flux decline compared to the control. In addition, QAC-modified membranes exhibited high stability during repeated chemical cleaning cycles. This grafting protocol for QAC provides a new dimension to modify a wide range of water and wastewater treatment membranes for mitigating biofouling.

Recent studies have documented the existence of discrete voids in the thin polyamide selective layer of composite reverse osmosis membranes. Here we present compelling evidence that these nanovoids are formed by nanosized gas bubbles generated during the interfacial polymerization process. Different strategies were used to enhance or eliminate these nanobubbles in the thin polyamide film layer to tune its morphology and separation properties. Nanobubbles can endow the membrane with a foamed structure within the polyamide rejection layer that is approximately 100 nm in thickness. Simple nanofoaming methods, such as bicarbonate addition and ultrasound application, can result in a remarkable improvement in both membrane water permeability and salt rejection, thus overcoming the long-standing permeability–selectivity trade-off of desalination membranes.

Commercial polyamide membranes for seawater desalination and water purification have low water permeability because of their relatively thick rejection layers. We report a novel interfacial polymerization method for synthesizing ultrathin polyamide layers with a precisely controllable thickness. Monomer solutions of m-phenylenediamine and trimesoyl chloride were electrosprayed into fine microdroplets. The polymerization reaction between microdroplets of different monomers leads to a fine and controllable amount of deposition. We fabricated smooth polyamide layers from 4 nm to several tens of nanometers in thickness, with a growth rate of approximately 1 nm/min. Our study provides a new dimension for the rational design and preparation of ultrathin polyamide membranes with tunable separation properties.

Reverse electrodialysis (RED) is an emerging membrane based technology that captures electricity from controlled mixing of two water streams of different salinities. To date, great advancements have been achieved on the development of RED components (e.g., membranes and spacers), optimization of operational conditions, and development of hybrid processes. This review presents an overview on the current achievements in RED membranes and spacers. Meanwhile, the critical operation conditions and their interconnected relationships are highlighted. Moreover, several innovative hybrid systems that show strong synergistic effects are highlighted. The latest development of nano-/micro-fluidic RED and pilot scale tests are also summarized.

To prepare a new type of composite membrane with high flux and good dye removal performance, hyperbranched polyester (HPE) was selected as the reactive macromonomer and a cross-linked HPE skin layer was successfully formed on the PVC-UF membrane via one-step acteal reaction between HPE and glutaraldehyde (GA) from aqueous emulsion. The resultant membranes were characterized using ATR-FTIR, FESEM, surface zeta potential and CA, while the effects of GA concentration and heat treatment time on their performance were evaluated by permeate flux and rejection rate of sunset yellow (SY). The obtained HPE-M5 membrane exhibited a SY rejection rate of 96.4% and a permeate flux of 237.6 L m − 2 h − 1 at 0.4 MPa. Due to the good stability and the feasibility of backwashing treatment, the prepared composite hollow fiber membrane has a promising perspective of application in dye removal.

Rapid cleanup of heavy oil spills is always considered a great challenge because conventional porous oil sorbents cannot efficiently remove them due to the high viscosity of heavy oil (>103 mPa s). In this work, we take advantage of the photothermal effect to heat heavy oil by using sunlight as the energy source to significantly reduce its viscosity and thus to achieve a fast heavy oil cleanup. A carbon nanotube (CNT) modified polyurethane sponge was fabricated as a photothermal sorbent that exhibited superhydrophobicity and superoleophilicity, as well as outstanding absorption capacity for heavy oil. Thanks to the excellent photothermal effect of CNTs, the modified sponge achieved nearly full sunlight absorption (99%). The resulting solar heating effectively reduced the viscosity of the heavy oil, which enabled the modified sponge to quickly absorb 20 times its own weight of heavy oil under sun illumination. This solar-assisted heavy oil sorbent design is promising for future remediation of viscous oil-spills.

Most commercial NF membranes are negatively charged at the pH range of a typical feed solution. In order to enhance the removal of cations (such as Mg2+ or Ca2+), we utilized polyethyleneimine (PEI) and trimesoyl chloride (TMC) to perform interfacial polymerization reaction on a polydopamine coated hydrolyzed polyacrylonitrile substrate to obtain a positively charged nanofiltration membrane. Effects of polydopamine coating time, PEI concentration, TMC reaction time and concentration on the membrane physicochemical properties and separation performance were systematically investigated using scanning electron microscopy, streaming potential and water contact angle measurements. The optimal NF membrane showed high rejection for divalent ions (93.6±2.6% for MgSO4, 92.4±1.3% for MgCl2, and 90.4±2.1% for Na2SO4), accompanied with NaCl rejection of 27.8±2.5% with a permeation flux of 17.2±2.8 L∙m–2∙h–1 at an applied pressure of 8 bar (salt concentrations were all 1000 mg∙L–1). The synthesized membranes showed promising potentials for the applications of water softening.

We developed a facile approach to synthesize thin-film nano-templated composite (TFNt) nanofiltration membrane with high water permeability, high NaCl/MgSO4 selectivity and strong antimicrobial properties. A polydopamine (PDA) coating on a polysulfone support was used as a nano-template to generate silver nanoparticles (AgNPs) in situ with high loading and high uniformity. A subsequent interfacial polymerization reaction of piperazine and trimesoyl chloride was performed on this nano-template substrate to form the TFNt membrane. The TFNt membrane had significantly increased both the water permeability and salt rejection compared to the control thin-film composite (TFC) membrane as well as a thin-film nanocomposite (TFN) membrane prepared by the conventional way of loading AgNPs directly during the interfacial polymerization process. Furthermore, the TFNt membrane showed better antimicrobial properties than both the TFC and the conventional TFN membranes. The current work presents an exciting approach to fabricate novel nanofiltration membranes using nano-templates, which provides important insights for high performance NF membrane synthesis.

Ceramic membranes have attracted widespread attention in industrial applications due to their excellent performance such as high flux, excellent stability and long lifetime. However, it is still a great challenge to prepare high-performance ceramic membranes with uniform pore size and high porosity using the existing manufacturing process. To this end, we have prepared hierarchical isoporous alumina ceramic membranes (IAMs) aided by the self-assembly of polystyrene-b-polyethylene oxide (PS-b-PEO) block copolymer in a sol-gel system. The packing and coalescence of spherical micelles self-assembled by PS-b-PEO and aluminum oligomers promote the formation of Allihn condenser-like channels in the isoporous separation layer (ISL). Control over the pore structure of ISL is achieved by tailoring the PS segment of PS-b-PEO, leading to the pore window rise from 8.3 to 19.7 nm. The pure water permeability of the corresponding IAMs increases from 151.6 to 223.2 L m−2 h−1 bar−1. The selective separation performance of IAMs is evaluated using gold nanoparticles (5–37 nm) and proteins (lysozyme, ovalbumin, bovine albumin serum and human immunoglobulin). The practicability, tunability and versatility of the strategy provided in this work pave the way for high-performance isoporous ceramic membranes fabrication.

Surface modification of nanofiltration (NF) membranes has great potential to improve the removal of organic micropollutants (OMs) by NF membranes. This study used polydopamine (PDA) as a model coating to comprehensively link the changes in membrane properties with the changes in transmission of 34 OMs. The membrane characterization demonstrated that a thicker, denser, and more hydrophilic PDA coating can be achieved by increasing the PDA deposition time from 0.5 to 4 hours. Overall, the transmissions of target OMs were reduced by PDA-coated NF membranes compared to unmodified NF membranes. The neutral hydrophobic compounds showed lower transmissions for longer PDA coating (PDA4), while the neutral hydrophilic compounds tended to show lower transmissions for shorter PDA coating (PDA0.5). To explain this, competing effects provided by the PDA coatings are proposed including sealing defects, inducing cake-enhanced concentration polarization in the coating layer for neutral hydrophilic compounds, and weakened hydrophobic adsorption for neutral hydrophobic compounds. For charged compounds, PDA4 with the greatest negative charge among the PDA-coated membranes showed the lowest transmission. Depending on the molecular size and hydrophilicity of the compounds, the transmission of OMs by the PDA4 coating could be reduced by 70% with only a 26.4% decline in water permeance. The correlations and mechanistic insights provided by this work are highly useful for designing membranes with specific surface properties via surface modification to improve the removal of OMs without compromising water production.

Due to the versatile structures and functionalities as well as good compatibility with polyamide (PA) matrix, organic nanomaterials are promising candidates as fascinating nanofillers in PA selective layer to improve separation efficiency of thin film nanocomposite (TFN) membranes, which consist of a nanomaterials-embedded PA layer and a porous support layer. This review specially focuses on the current development of organic nanomaterials based TFN membranes for various aqueous-/organic-based separation processes, as organic nanomaterials are effective in avoiding the non-selective defects caused by the poor interfacial compatibility between inorganic nanoadditives and PA matrix. The synthesis, modification and utilization of organic nanofillers for constructing state-of-the-art TFN membranes as well as the proposed transport mechanism are highlighted. Encouraging results have demonstrated that the embedded organic nanomaterials not only provide possibility in breaking the trade-off between membrane permeability and selectivity and improving antifouling performance, but also render membrane with attractive properties for targeted applications, such as antimicrobial performance and boron removal. Despite of the excellent properties imparted by functional organic nanomaterials, significant challenges still exist for in-depth study and scale-up of TFN membranes. Thus, this review also attempts to offer insights on future directions of advanced TFN membranes with organic-based nanomaterials.

Separation performance of nanofiltration (NF) membranes is constrained by the trade-off between permeability and selectivity. Herein, a facile and environmental friendly water activation before microwave heating was performed to improve the performance of NF membranes and overcome this trade-off. The water activation before microwave heating facilitated the hydrolysis of acyl chloride, resulting in a loose polyamide layer with enhanced hydrophilicity and surface charge. The activated NF membrane showed a permeance of 29.9 L m-2 h-1 bar-1, which is almost 2 times higher than the control. The membrane possesses a Na2SO4 rejection of 96.8% and high selectivity to antibiotics and NaCl. At the meantime, the fouling resistance to model foulants including bovine serum albumin and sodium alginate was significantly enhanced. The water activation before microwave heating is proved to be an adequate strategy for fabricating NF membranes.

Nanovoids in polyamide rejection layers of thin film composite (TFC) reverse osmosis (RO) membranes are responsible for their characteristic “ridge-and-valley” surface roughness and have profound impact on their separation performance. However, mechanisms leading to these void-containing roughness features remain poorly understood. The current work presents compelling evidence that vaporization of the organic solvent contributes to the formation of nanovoids during the exothermic interfacial polymerization (IP) process. We used a series of alkane solvents with systematically varying chain length and vapor pressure to prepare TFC membranes. Our study revealed that an organic solvent with higher vapor pressure generated more vapor during the IP reaction, which in turn resulted in larger size of the voids in the polyamide thin film and higher membrane water permeability. We further designed a strategy to suppress the vapor effect by preparing polyamide thin films at a free interface. This led to the disappearance of nanovoids and nearly identical membrane permeability regardless of the organic solvent used for the IP process, in good agreement of the weakened confinement to the organic vapor generated by interfacial heating. The current study provides new mechanistic insights to interpret the formation of the voids-containing morphology of TFC polyamide membranes, which would facilitate improved understanding of membrane transport mechanisms and better control of membrane structural features.

A robust polytetrafluoroethylene (PTFE) nanofibrous membrane was prepared by a simple electrospinning-sintering strategy for efficient oil/water separation. Specifically, a PTFE@PVA hybrid nanofibrous membrane in which PTFE particles were uniformly distributed in PVA nanofiber was first prepared by electrospinning, then a sintering treatment was applied to obtain the PTFE nanofibrous membrane. Electron microscopic characterization revealed that the membrane was formed by a sintering mechanism of a fast decomposition of PVA followed by a slower fusion of PTFE particles. Spectroscopic characterization confirmed that the PVA polymer was completely decomposed after 8 h of sintering. The resulting membrane had a ratio of fluorine to carbon atomic ratio of 2.0, indicating that a pure PTFE nanofibrous membrane was obtained. The as-prepared PTFE membrane exhibited superhydrophobic property with a water contact angle of 155.0° and a sliding angle of 5.1°. Its tensile strength was as high as 19.7 MPa, indicating excellent mechanical strength. The membrane was successfully applied for gravity-driven oil/water separation with a permeate flux of 1215 L m−2 h−1. Moreover, its excellent corrosion resistance and mechanical stability suggest that the PTFE nanofibrous membrane could stand harsh environment existing in industrial oil/water separation processes.

In this paper, we investigated the synergistic effects of co-locating reverse electrodialysis (RED) with other water and power infrastructures. The potential benefit of greater salinity (e.g., using brine from seawater desalination plants) and higher feed water temperature (e.g., through the co-location with power plants) were studied. Maximum RED power was obtained when the low salinity stream (LS) had moderate salinity (0.01–0.02 M NaCl), which can be explained by the competing effects of reduced internal resistance and decreased electrochemical potential upon increasing the LS concentration. At the same time, greater salinity of the high salinity stream (HS) and higher feed water temperature both significantly improved the power performance. Compared to the HS temperature, the LS temperature played a more important role due to the dominance of electrical resistance of the LS compartment. When RED was applied as a pre- or post-treatment to RO, it can efficiently remove salt from the HS stream (e.g., nearly 50% reduction in the HS concentration demonstrated in our bench scale evaluation). We further show that, during the RED salt removal process, the ionic efficiency (~ 76% in the current study) was closely related to the permselectivity of the ion exchange membranes.

Wastewater treatment using forward osmosis anaerobic membrane bioreactors (FO-AnMBR) has a number of advantages over traditional wastewater treatment approaches. Previous studies have investigated the overall performance of lab-scale FO-AnMBR in treating synthetic wastewater. Due to enhanced physical rejection of osmosis membrane, salinity gradually increased and emerging contaminants such as cytostatic drugs in wastewater would also be accumulated during operation. The objective of this study was to investigate the influence of increased salinity and the accumulation of cytostatic drugs on the composition and function of the microbial community in FO-AnMBR through a metagenomics approach. We found that salinity increase in FO-AnMBR was the key factor driving compositional and functional transitions in microbial community. At high salinity condition, methane-producing archaea (MPA) became less competitive than sulphate-reducing bacteria (SRB), resulting in a reduced methane yield rate. The reduced methane production could be restored in the next successive cycle and salt-tolerant MPA was continuously enriched in the entire operation. Relative abundance of genes involved in nitrogen metabolism, cell attachment and biofilm related signalling increased from low salinity to high salinity condition, while those for methanogenesis and iron metabolism were found to decrease. The presence of cytostatic drugs caused the inhibition of microbial metabolism and increased extracellular polymeric substances (EPS) concentration. Due to their differential influences on community members, the evenness of community decreased after drug treatment. After the drug treatment, biofilm-forming and siderophore-producing bacteria were found to be more dominant in the microbial community.

Customization of thickness and roughness of thin film composite reverse osmosis (TFC-RO) membranes provides opportunities to optimize the membrane permeability and fouling resistance. We propose a novel strategy to synthesize ultrathin multi-layered polyamide (ML-PA) membranes with the versatile maneuverability of the salt rejecting layer thickness and roughness. We have employed advanced quartz crystal microbalance with dissipation (QCMD) techniques to study the deposition rate of the ultrathin PA nanolayers with a resolution of approximately 8, 15 and 25 nm per deposition cycle. At brackish water desalination condition, the ML-PA membrane exhibited ~ 60% flux increase and higher salt rejection compared with the home-made TFC membrane fabricated by the conventional method. Benefit from the low roughness, the ML-PA membrane shows much better fouling resistance to Bovine serum albumin (BSA).

We designed a highly selective surface coating to achieve enhanced rejection of endocrine disrupting compounds (EDCs) by nanofiltration membranes. A commercial NF90 membrane was first coated with polydopamine (PDA) followed by in situ immobilization of silver nanoparticles (AgNPs). This PDA/AgNPs coating greatly improved EDC rejection at the expense of slight water permeability loss (4–10%). This improvement in rejection can be attributed to a combination of enhanced size exclusion and suppressed hydrophobic interaction. A resistance-in-series analysis further reveals that the coating was highly permeable to water but highly resistant to EDCs, leading to an EDC selectivity that was an order of magnitude greater than those of the bare PDA coating and the base membrane NF90. The current study provides important insights into the design of highly selective coatings for effective retention of targeted trace organic contaminants.

This study reports efficient rejection of four typical ionic divalent heavy metals of interest (i.e. Cd2+, Pb2+, Cu2+, Zn2+) using forward osmosis (FO) by our recently developed nanoporous thin-film inorganic (TFI) membrane fabricated through tetraethylorthosilicate-driven sol-gel process. Upon lab-scale FO cell with feed solution containing heavy metal electrolytes (pH 4.5 ± 0.5) and NaCl serving as draw solution, the TFI membrane yields a high water flux of 69.0 L m−2 h−1 driven by 2.0 mol L−1-NaCl draw solution. Meanwhile, effective rejection of heavy metal ions was achieved, with an average efficiency of 94% at feed concentration of 200 mg L−1. Since the membrane is able to reject heavy metals whose hydrated ion diameters are smaller than the membrane pore size, the charge-interaction rather than size exclusion should be responsible for heavy metal rejection. Based on classical Debye-Huckel theory and Gouy-Chapman model, we demonstrate a particular significance of double layer overlap within membrane pore induced by electrostatic interaction between heavy metal ions and silica-made pore walls. As such, the selectivity of TFI membrane depends essentially on the co-function of membrane pore size, surface potential of membrane pore wall as well as Debye length. This study not only confirms the feasibility of the TFI membrane in treating acidic heavy metal-containing wastewater without pH adjustment, but also suggests a simple theoretical scheme to better understand and design charged membrane with expected selectivity for FO applications.