New directions on membranes for removal and degradation of emerging pollutants in aqueous systems

Emerging pollutants (EPs) refer to a group of non-regulated chemical or biological substances that have been recently introduced or detected in the environment. These pollutants tend to exhibit resistance to conventional treatment methods and can persist in the environment for prolonged periods, posing potential adverse effects on ecosystems and human health. As we enter a new era of managing these pollutants, membrane-based technologies hold significant promise in mitigating impact of EPs on the environment and safeguarding human health due to their high selectivity, efficiency, cost-effectiveness and capability for simultaneous separation and degradation. Moreover, these technologies continue to evolve rapidly with the development of new membrane materials and functionalities, advanced treatment strategies, and analyses for effectively treating EPs of more recent concerns. The objective of this review is to present the latest directions and advancements in membrane-based technologies for addressing EPs. By highlighting the progress in this field, we aim to share valuable perspectives with researchers and contribute to the development of future directions in sustainable treatments for EPs.

Development of multifunctional membranes via plasma-assisted nonsolvent induced phase separation

Demands on superhydrophobic, self-cleaning and piezoelectric membranes have gained significantly due to their potential to overcome global shortages in clean water and energy. In this study, we have discovered a novel plasma-assisted nonsolvent induced phase separation (PANIPS) method to prepare superhydrophobic, self-cleaning and piezoelectric poly(vinylidene difluoride) (PVDF) membranes without additional chemical modifications or post-treatments. The PANIPS membranes exhibit water contact angles ranging from 151.2° to 166.4° and sliding angles between 6.7° and 29.7°. They also show a high piezoelectric coefficient (d33) of 10.5 pC N-1 and can generate a high output voltage of 10 Vpp. The PANIPS membranes can effectively recover pure water from various waste solutions containing Rose Bengal dye, humic acid, or sodium dodecyl sulfate via direct contact membrane distillation (DCMD). This study may provide valuable insights to fabricate PANIPS membranes and open up new avenues to molecularly design advanced superhydrophobic, self-cleaning, and piezoelectric membranes in the fields of clean water production, motion sensor, and piezoelectric nanogenerator.

Mxenes for membrane separation: from fabrication strategies to advanced applications

Transition metal carbides/nitrides/carbonitrides, commonly referred to as MXenes, have gained widespread attention since their discovery in 2011 as a promising family of two-dimensional (2D) materials. Their impressive chemical, electrical, thermal, mechanical, and biological properties have fueled a surge in research focused on the synthesis and application of MXenes in various fields, including membrane-based separation. By engineering the materials and membrane structures, MXene-based membranes have demonstrated remarkable separation performance and added functionalities, such as antifouling and photocatalytic properties. In this review, we aim to have a timely and critical review of research on their fabrication strategy and performance in advanced molecular separation and ion exchange, beginning with a brief introduction of the preparation and physicochemical properties of MXenes. Finally, outlooks and future works are outlined with the aims to provide valuable insights and guidance for advancing membranes' applications in different separation domains.

Closed-loop recyclable membranes enabled by covalent adaptable networks for water purification

In the state-of-the-art membrane industry, membranes have linear life cycles and are commonly disposed of by landfill or incineration, sacrificing their sustainability. To date, little or no thought is given in the design phase to the end-of-life management of membranes. For the first time, we have innovated high-performance sustainable membranes, which can be closed-loop recycled after long-term usage for water purification. By synergizing membrane technology and dynamic covalent chemistry, covalent adaptable networks (CANs) with thermally reversible Diels-Alder (DA) adducts were synthesized and employed to fabricate integrally skinned asymmetric membranes via the nonsolvent-induced phase separation technique. Due to the stable and reversible features of CAN, the closed-loop recyclable membranes exhibit excellent mechanical properties and thermal and chemical stabilities as well as separation performance, which are comparable to or even higher than the state-of-the-art nonrecyclable membranes. Moreover, the used membranes can be closed-loop recycled with consistent properties and separation performance by depolymerization to remove contaminants, followed by refabrication into new membranes through the dissociation and reformation of DA adducts. This study may fill in the gaps in closed-loop recycling of membranes and inspire the advancement of sustainable membranes for a green membrane industry.

Angiography alone versus angiography plus intracoronary imaging to guide emergency percutaneous coronary intervention: outcomes from a single-centre retrospective analysis

Funding Acknowledgements

Type of funding sources: None.

Background

There is emerging evidence showing improvement in clinical outcomes with optical coherence tomography (OCT) and intravascular ultrasound (IVUS)-guided elective percutaneous coronary intervention (PCI). Yet data supporting their use in emergency setting are still conflicting.

Purpose

We aim to compare the immediate angiographic outcomes, short and long-term clinical outcomes in patients undergoing emergency PCI with intracoronary imaging to those with angiography guidance only.

Methods

We included 426 patients from May 2012 to December 2020 who presented with ST-elevation myocardial infarction (STEMI) and underwent emergency PCI within 24 hours of hospital admission. Intracoronary imaging was used in 196 of them to guide PCI. Immediate angiographic outcomes in terms of TIMI flow grade (TFG), myocardial blush grade (MBG) and corrected TIMI frame count (CTFC) are compared. Clinical outcomes including major adverse cardiac events (MACE), target vessel revascularization (TVR), hospitalization for heart failure and all-cause mortality were also compared.

Results

196 patients (46%) underwent intracoronary imaging-guided PCI. Use of imaging was associated with a higher post procedural CTFC (27.0 vs. 25.8, p=0.11), yet it failed to reach statistical significance. Comparing with angiography-guided PCI, imaging-guided PCI was associated with significantly larger proportion of high CTFC post procedure (slow coronary flow) (OR, 0.62; 95% CI: 0.38-1.0, P=0.05), and this result was consistent after adjustment of variables (adjusted OR, 0.57; 95% CI 0.34-0.98, P=0.04). Subjective measures of TIMI flow grade and MBG, however, were not different between the 2 groups. In the subgroup of patients with high Syntax score and American Heart Association (AHA)/American College of Cardiology (ACC) type C culprit lesion morphology, imaging guided PCI was associated with significantly worse post procedural MBG (OR, 0.36; 95% CI 0.17-0.78, P=0.01 vs. OR, 0.40; 95% CI 0.18-0.94, P=0.04), and the results were consistent after adjusting for variables. In the subgroup of patients with high Syntax score, imaging guided PCI was associated with significantly worse post procedural TIMI flow grade (adjusted OR, 0.35; 95% CI 0.12-0.95, P=0.05) and a trend towards higher CTFC (adjusted OR, 0.26; 95% CI: 0.06-0.94, P=0.05) after adjusting for variables. The cumulative incidences of all clinical outcome measures were not significantly different between the 2 groups before and after adjusting for confounders.

Conclusions

Imaging-guided PCI was not associated with improved angiographic or clinical outcomes in patients with STEMI who underwent emergency PCI. Use of intracoronary imaging in emergency PCI may associate with worse immediate angiographic outcomes.

Novel Sandwich-Structured Hollow Fiber Membrane for High-Efficiency Membrane Distillation and Scale-Up for Pilot Validation

Hollow fiber membranes were produced from a commercial polyvinylidene fluoride (PVDF) polymer, Kynar HSV 900, with a unique sandwich structure consisting of two sponge-like layers connected to the outer and inner skin layers while the middle layer comprises macrovoids. The sponge-like layer allows the membrane to have good mechanical strength even at low skin thickness and favors water vapor transportation during vacuum membrane distillation (VMD). The middle layer with macrovoids helps to significantly reduce the trans-membrane resistance during water vapor transportation from the feed side to the permeate side. Together, these novel structural characteristics are expected to render the PVDF hollow fiber membranes more efficient in terms of vapor flux as well as mechanical integrity. Using the chemistry and process conditions adopted from previous work, we were able to scale up the membrane fabrication from a laboratory scale of 1.5 kg to a manufacturing scale of 50 kg with consistent membrane performance. The produced PVDF membrane, with a liquid entry pressure (LEPw) of >3 bar and a pure water flux of >30 L/m2·hr (LMH) under VMD conditions at 70−80 °C, is perfectly suitable for next-generation high-efficiency membranes for desalination and industrial wastewater applications. The technology translation efforts, including membrane and module scale-up as well as the preliminary pilot-scale validation study, are discussed in detail in this paper.

Supramolecular Polymer Network Membranes with Molecular-Sieving Nanocavities for Efficient Pre-Combustion CO2 Capture

Pre-combustion membrane CO₂ capture from syngas before utilizing the clean hydrogen fuel, demands very challenging membrane materials with simultaneous high thermal resistance, precise subnanometer size-selectivity, and robust processability. Here, an unconventional yet ultra-facile nanocomposite membrane design using 4-sulfocalix[4]arene (SCA4) molecules, a highly interactive member of soluble organic macrocyclic cavitands (OMCs) with a precise ≈3.0Å open cavity, is reported, to effectively sieve CO₂ (3.3Å) from H₂ (2.89Å). By simply infiltrating dissolved SCA4 molecules into prefabricated polymer membranes, they form extensive 3D supramolecular polymer networks (SPNs) with the polymer backbones through multi-site ionic interactions. Bearing distinctly molecular-sieving nanocavities, these otherwise amorphous SPN membranes deliver drastically enhanced mixed-gas H₂ /CO₂ separation under an industrial high-temperature-and-pressure environment with 4.35 times higher selectivity being achieved, allowing them to well outperform most existing polymer-based materials and even rival many state-of-the-art but delicate inorganic and framework-based membranes. They also demonstrate enhanced mechanical properties and long-term operation stability. Most attractively, the SPN membranes obtain a molecularly homogeneous, single-phase composite structure that can significantly surpass conventional phase-segregated mixed-matrix membranes in processability. Accompanied by the widely tunable OMC structures, this work can provide a versatile toolbox for designing advanced molecular-sieving membranes with an optimal balance of performance, robust properties, and scalability.

Tunable Supramolecular Cavities Molecularly Homogenized in Polymer Membranes for Ultraefficient Precombustion CO2 Capture

Processable molecular-sieving membranes are important materials for realizing energy-efficient precombustion CO₂ capture during industrial-scale hydrogen production. However, the promising design of mixed matrix membranes (MMMs) that aims to integrate the molecular-sieving properties of nanoporous architectures with industrial processable polymers still faces performance and fabrication issues due to the formation of segregated nanofiller domains in their polymer matrices. Here, an unconventional nanocomposite membrane design is proposed using soluble organic macrocyclic cavitands (OMCs) with tunable open cavity sizes that not only mitigate the formation the discrete nanofiller phases but also deliver distinct molecular-sieving separations. The versatile organic-solvent solubility coupled with highly interactive functionalities of OMCs allows them to obtain molecularly homogeneous mixing with matrix polymers and form only one integral continuous phase crucial to the robust processability of polymers. A series of polybenzimidazole-based molecularly mixed composite membranes (MMCMs) are fabricated via the incorporation of a soluble and thermally stable OMC choice, sulfocalixarenes, with various cavity sizes. These membranes achieve outstanding high-temperature mixed-gas H₂ /CO₂ separation performances comparable with several state-of-the-art molecular-sieving membranes owing to effective size-sieving gas passages through the open or partially-intruded supramolecular cavities. The broadly tunable structures and functionalities of OMCs would make their MMCMs attractive for other energy-intensive molecular separations.

Polyphenylsulfone (PPSU)-Based Copolymeric Membranes: Effects of Chemical Structure and Content on Gas Permeation and Separation

Although various polymer membrane materials have been applied to gas separation, there is a trade-off relationship between permeability and selectivity, limiting their wider applications. In this paper, the relationship between the gas permeation behavior of polyphenylsulfone(PPSU)-based materials and their chemical structure for gas separation has been systematically investigated. A PPSU homopolymer and three kinds of 3,3',5,5'-tetramethyl-4,4'-biphenol (TMBP)-based polyphenylsulfone (TMPPSf) copolymers were synthesized by controlling the TMBP content. As the TMPPSf content increases, the inter-molecular chain distance (or d-spacing value) increases. Data from positron annihilation life-time spectroscopy (PALS) indicate the copolymer with a higher TMPPSf content has a larger fractional free volume (FFV). The logarithm of their O₂, N₂, CO₂, and CH₄ permeability was found to increase linearly with an increase in TMPPSf content but decrease linearly with increasing 1/FFV. The enhanced permeability results from the increases in both sorption coefficient and gas diffusivity of copolymers. Interestingly, the gas permeability increases while the selectivity stays stable due to the presence of methyl groups in TMPPSf, which not only increases the free volume but also rigidifies the polymer chains. This study may provide a new strategy to break the trade-off law and increase the permeability of polymer materials largely.

Nanovoid-Enhanced Thin-Film Composite Reverse Osmosis Membranes Using ZIF-67 Nanoparticles as a Sacrificial Template

In this work, nanovoid-enhanced thin-film composite (TFC) membranes have been successfully fabricated using ZIF-67 nanoparticles as the sacrificial template. By incorporating different amounts of ZIF-67 during interfacial polymerization, the resultant TFC membranes can have different degrees of nanovoids after self-degradation of ZIF-67 in water, consequently influencing their physiochemical properties and separation performance. Nanovoid structures endow the membranes with additional passages for water molecules. Thus, all the newly developed TFC membranes exhibit better separation performance for brackish water reverse osmosis (BWRO) desalination than the pristine TFC membrane. The membrane made from 0.1 wt % ZIF-67 shows a water permeance of 2.94 LMH bar^-1 and a salt rejection of 99.28% when being tested under BWRO at 20 bar. This water permeance is 53% higher than that of the pristine TFC membrane with the salt rejection well maintained.

Nanofiltration-Inspired Janus Membranes with Simultaneous Wetting and Fouling Resistance for Membrane Distillation

Membranes with robust antiwetting and antifouling properties are highly desirable for membrane distillation (MD) of wastewater. Herein, we have proposed and demonstrated a highly effective method to mitigate wetting and fouling by designing nanofiltration (NF)-inspired Janus membranes for MD applications. The NF-inspired Janus membrane (referred to as PVDF-P-CQD) consists of a hydrophobic polyvinylidene fluoride (PVDF) membrane and a thin polydopamine/polyethylenimine (PDA/PEI) layer grafted by sodium-functionalized carbon quantum dots (Na⁺-CQDs) to improve its hydrophilicity. The vapor flux data have confirmed that the hydrophilic layer does not add extra resistance to water vapor transport. The PVDF-P-CQD membrane exhibits excellent resistance toward both surfactant-induced wetting and oil-induced fouling in direct contact MD (DCMD) experiments. The impressive performance arises from the fact that the nanoscale pore sizes of the PDA/PEI layer would reject surfactant molecules by size exclusion and lower the propensity of surfactant-induced wetting, while the high surface hydrophilicity resulted from Na⁺-CQDs would induce a robust hydration layer to prevent oil from attachment. Therefore, this study may provide useful insights and strategies to design novel membranes for next-generation MD desalination with minimal wetting and fouling propensity.

Pulmonary pressure-to-longitudinal strain ratio by echocardiography: a rapid surrogate to magnetic resonance for right ventricular failure assessment

Funding Acknowledgements

Type of funding sources: Public hospital(s). Main funding source(s): United Christian Hospital Ruttonjee and Tang Siu Kin Hospitals

Background

Better risk stratification in pulmonary hypertension (PH) by echocardiography (echo) to detect ventricular vascular uncoupling may act as gate-keeper for downstream management, such as MRI and expensive therapies. Studies showed modest correlation found between RV peak global longitudinal strain (RVGLS), which is afterload dependent, and right ventricular ejection fraction (RVEF)

Purpose

To test the accuracy and optimal cut-off of echo derived mean PA pressure-to-RVGLS ratio against MRI detected severe RV dysfunction (defined as RVEF< 35%), RV dilatation (defined as RVEDVi >87ml), and correlate native T1-values (nT1)

Method

Strain analyses by echo and volumetric assessment by 1.5 tesla MRI were performed in all patients. Contoured MRI short axis images provided RVEF. In a subgroup of pulmonary arterial hypertension (PAH), right heart catheterization and MRI non-contrast native T1 mapping were performed (Figure 1). Using previous study data, to identify a difference of 1.8 pressure-to-strain ratio between mild and severe PH with a variance  of 2.2 , power of 80% and a significance level of 0.05, a total of 11 participants per group were needed

Result

Thirty-one PH patients (13 female, age 60 ± 14y, 13 had PAH) were recruited prospectively. Strong correlation was demonstrated between the mean PA pressure-to-RVGLS ratio to MRI derived RVEF (r = 0.80, p < 0.01), and to catheterization derived pulmonary vascular resistance and indexed cardiac output (r = -0.80, p= 0.001; r= -0.75, p = 0.003 respectively). The cut-off value of -2.5 had best accuracy in ROC analyses (Table 1)

In PAH patients, this ratio correlated with global nT1 at basal short-axis level (r= -0.91, p = 0.004), but not at the mid short-axis level. Their basal posterior interventricular insertion regions had significantly higher nT1 than those of age-matched normal controls at the same region on the same scanner (1256 ± 217 ms vs. 932 ± 25 ms, p = 0.04)

Conclusion

In terms of detection of severe right ventricular dysfunction by echocardiography, mean PA pressure-to-RVGLS ratio performed better than RVGLS alone, and a ratio cutoff of -2.5 predicts MRI determined ventricular vascular uncoupling in pulmonary hypertension

Table 1 Echo detect MRI AUC standard error 95% CI sens (%) spec (%) p Mean PA pressure -to-RVGLS ratio RVEF < 35% 0.86 0.073 0.71-1.00 72 83 0.007 RVEDVi > 87ml 0.81 0.081 0.65-0.97 83 70 0.004 RVGLS RVEF < 35% 0.76 0.100 0.57-0.96 60 83 0.048 RVEDVi > 87ml 0.73 0.090 0.55-0.91 67 70 0.032 PA pulmonary artery; RVGLS: RV global longitudinal strain; RVEDVi: indexed RV end-diastolic volume Abstract Figure 1

Can Composite Janus Membranes with an Ultrathin Dense Hydrophilic Layer Resist Wetting in Membrane Distillation?

Tackling membrane wetting is an ongoing challenge for large-scale applications of membrane distillation (MD). Herein, composite Janus MD membranes comprising an ultrathin dense hydrophilic layer are developed by layer-by-layer assembling cationic polyethyleneimine and anionic poly(sodium 4-styrenesulfonate) polyelectrolytes on a hydrophobic polyvinylidene fluoride substrate. Using surfactant-containing saline water as the feed with low surface tension, experiments reveal that the number of polyelectrolyte layers, rather than surface wettability or surface charge, determines the anti-wetting performance of the composite Janus membranes. More deposited layers yield higher wetting resistance. With the aid of positron annihilation spectroscopy, this study, for the first time, demonstrates the origin of the excellent wetting resistance of the composite Janus membranes. The effective pore size of the polyelectrolyte multilayer decreases with an increase in the number of the deposited layer. The membrane with an ultrathin hydrophilic multilayer of 48 nm has a sufficiently small pore size to sieve out surfactant molecules from the feed solution via a size exclusion mechanism, thus protecting the hydrophobic substrate from being wetted by the low-surface-tension feed water. This study may pave the way for developing next-generation anti-wetting Janus membranes for robust membrane distillation.

Self-standing and flexible covalent organic framework (COF) membranes for molecular separation

Almost all covalent organic framework (COF) materials conventionally fabricated by solvothermal method at high temperatures and pressures are insoluble and unprocessable powders, which severely hinder their widespread applications. This work develops an effective and facile strategy to construct flexible and free-standing pure COF membranes via the liquid-liquid interface-confined reaction at room temperature and atmospheric pressure. The aperture size and channel chemistry of COF membranes can be rationally designed by bridging various molecular building blocks via strong covalent bonds. Benefiting from the highly-ordered honeycomb lattice, high solvent permeances are successfully obtained and follow the trend of acetonitrile > acetone > methanol > ethanol > isopropanol. Interestingly, the imine-linked COF membrane shows higher nonpolar solvent permeances than b-ketoenamine-linked COF due to their difference in pore polarity. Both kinds of COF membranes exhibit high solvent permeances, precise molecular sieving, excellent shape selectivity, and sufficient flexibility for membrane-based separation science and technology.

Emerging thin-film nanocomposite (TFN) membranes for reverse osmosis: A review

Thin-film composite (TFC) membranes are the heart of reverse osmosis (RO) processes for desalination and water reuse. In recent years, nanomaterials with high permeability, selectivity and chemical resistance, and low fouling tendency have begun to emerge and be applied in many other fields. This has stimulated the research on novel RO membranes consisting of nanomaterials (non-porous and porous) in their selective layers. Encouraging results have been demonstrated. Herein, the state-of-the-art developments of polyamide thin-film nanocomposite (TFN) membranes for RO processes are summarized since the concept of TFN was introduced in 2007. While it is obvious that nanomaterials could impart exclusive properties, it should also be noted that significant challenges still exist for research and commercialization of TFN membranes, such as selection of proper nanomaterials, prevention of leaching of nanoparticles, and performance and cost analysis before large-scale RO membrane manufacturing. Future research directions are outlined to offer insights for the fabrication of much advanced TFN membranes with optimal interface morphology and separation performance.

Design of omniphobic interfaces for membrane distillation - A review

Membrane distillation (MD) has a great potential in treating high salinity industrial wastewater due to its unique characteristics. Nevertheless, the implementation of MD for industrial wastewater reclamation must be conducted with precaution because low-surface-tension contaminates in feed solutions may easily wet the membranes. In recent years, omniphobic membranes that exhibit strong repellence towards liquids with a wide range of surface tensions have been proposed as a promising solution to deal with the wetting problem. In this paper, we aim to provide a comprehensive review of omniphobic interfaces and illustrate their fundamental working principles, innovative design approaches and novel applications on membrane distillation. The review may provide insights in designing stable solid-liquid-vapor interfaces and serve as a guidance for the development of robust anti-wetting membranes for industrial wastewater reclamation via membrane distillation.

Thin film nanocomposite hollow fiber membranes comprising Na+-functionalized carbon quantum dots for brackish water desalination

We have incorporated Na⁺-functionalized carbon quantum dots (Na-CQDs) into the polyamide layer via interfacial polymerization reaction and developed novel thin film nanocomposite (TFN) hollow fiber membranes for brackish water desalination. Comparing with the conventional thin film composite (TFC) membranes, the TFN membranes comprising Na-CQDs have a larger effective surface area, thinner polyamide layer and more hydrophilic oxygen-containing groups in the polyamide layer. Besides, the interstitial space among the polyamide chains becomes larger due to the presence of Na-CQDs. As a result, the incorporation of 1 wt% Na-CQDs into the polyamide layer could improve the pure water permeability (PWP) of the membranes from 1.74 LMH/bar to 2.56 LMH/bar by 47.1% without compromising their NaCl rejection of 97.7%. Interestingly, stabilization of the TFN hollow fiber membranes containing 1 wt% Na-CQDs at 23 bar could further promote the PWP to 4.27 LMH/bar and the salt rejection to 98.6% under the same testing conditions due to the deformation of the membranes under a high hydraulic pressure. When using a 2000 ppm NaCl aqueous solution as the feed, the optimal water flux and rejection of the newly developed TFN membranes at 15 bar are 57.65 ± 3.26 LMH and 98.6% ± 0.35% respectively. The Na-CQDs incorporated TFN hollow fiber membranes show promising applications in the field of brackish water desalination.

Applications of carbon quantum dots (CQDs) in membrane technologies: A review

Carbon quantum dots (CQDs), which are a fascinating class of nanostructured carbons, have recently attracted extensive attention in the field of membrane technologies for their applications in separation processes. This is because they possess two unique advantages. Their productions are facile and inexpensive, while their physicochemical properties such as ultra-small sizes, good biocompatibility, high chemical inertness, tunable hydrophilicity, rich surface functional groups and antifouling characteristics are highly desirable. Leveraging on these, researchers have explored their utilizations in various membrane designs for reverse osmosis (RO), ultrafiltration (UF), nanofiltration (NF), forward osmosis (FO), pressure retarded osmosis (PRO), membrane distillation (MD), and organic solvent nanofiltration (OSN) processes. In particular, CQDs have especially stimulated exploration in the field of water treatment by membrane technologies since biocompatibility of membrane materials is of utmost importance to ensure safety of drinking water. In addition, CQDs are in a favorable position for achieving unprecedented performance of membrane separation processes in water treatment, in the light of substantial efficiency enhancement and antifouling propensity as discovered in recent studies. In this article, we will review the progress in the development of CQD incorporated membranes with discussions on their challenges and perspectives.