Recent Developments in Nanomaterials-Modified Membranes for Improved Membrane Distillation Performance

Advancements in Nanoenabled Membrane Distillation for a Sustainable Water-Energy-Environment Nexus

The emergence of nano innovations in membrane distillation (MD) has garnered increasing scientific interest. This enables the exploration of state-of-the-art nano-enabled MD membranes with desirable properties, which significantly improve the efficiency and reliability of the MD process and open up opportunities for achieving a sustainable water-energy-environment (WEE) nexus. This comprehensive review provides broad coverage and in-depth analysis of recent innovations in nano-enabled MD membranes, focusing on their role in achieving desirable properties, such as strong liquid-repellence, high resistance to scaling, fouling, and wetting, as well as efficient self-heating and self-cleaning functionalities. The recent developments in nano-enhanced photothermal-catalytic applications for water-energy co-generation within a single MD system are also discussed. Furthermore, the bottlenecks are identified that impede the scale-up of nanoenhanced MD membranes and a future roadmap is proposed for their sustainable commercialiation. This holistic overview is expected to inspire future research and development efforts to fully harness the potential of nano-enabled MD membranes to achieve sustainable integration of water, energy, and the environment.

Flexible, durable, and anti-fouling maghemite copper oxide nanocomposite-based membrane with ultra-high flux and efficiency for oil-in-water emulsions separation

In this study, we developed a novel nanocomposite-based membrane using maghemite copper oxide (MC) to enhance the separation efficiency of poly(vinyl chloride) (PVC) membranes for oil-in-water emulsions. The MC nanocomposite was synthesized using a co-precipitation method and incorporated into a PVC matrix by casting. The resulting nanocomposite-based membrane demonstrated a high degree of crystallinity and well-dispersed nanostructure, as confirmed by TEM, SEM, XRD, and FT-IR analyses. The performance of the membrane was evaluated in terms of water flux, solute rejection, and anti-fouling properties. The pinnacle of performance was unequivocally reached with a solution dosage of 50 mL, a solution concentration of 100 mg L-1, and a pump pressure of 2 bar, ensuring that every facet of the membrane's potential was fully harnessed. The new fabricated membrane exhibited superior efficiency for oil-water separation, with a rejection rate of 98% and an ultra-high flux of 0.102 L/m2 h compared to pure PVC membranes with about 90% rejection rate and an ultra-high flux of 0.085 L/m2 h. Furthermore, meticulous contact angle measurements revealed that the PMC nanocomposite membrane exhibited markedly lower contact angles (65° with water, 50° with ethanol, and 25° with hexane) compared to PVC membranes. This substantial reduction, transitioning from 85 to 65° with water, 65 to 50° with ethanol, and 45 to 25° with hexane for pure PVC membranes, underscores the profound enhancement in hydrophilicity attributed to the heightened nanoparticle content. Importantly, the rejection efficiency remained stable over five cycles, indicating excellent anti-fouling and cycling stability. The results highlight the potential of the maghemite copper oxide nanocomposite-based PVC membrane as a promising material for effective oil-in-water emulsion separation. This development opens up new possibilities for more flexible, durable, and anti-fouling membranes, making them ideal candidates for potential applications in separation technology. The presented findings provide valuable information for the advancement of membrane technology and its utilization in various industries, addressing the pressing challenge of oil-induced water pollution and promoting environmental sustainability.

Novel Hydrophobic Functionalized UiO-66 Series: Synthesis, Characterization, and Evaluation of Their Structural and Physical-Chemical Properties

A novel set of four functionalized hydrophobic UiO-66-NHR series were synthesized through postsynthetic procedures, utilizing various benzoyl chlorides and UiO-66-NH2 as starting materials. This synthesis method was carried out by employing p- (1) and o-toluoyl (2), as well as 2- (3) and 4-fluorobenzoyl (4) substituents. The analysis of the resulting compounds was performed using conventional spectroscopic methods such as FT-IR and 1H NMR to quantify the conversion rate into amide. Furthermore, SEM and XPS techniques were employed for morphological and surface analysis. Finally, the evaluation of the chemical stability and contact angle using the sessile drop method was performed to evaluate the technological potential of these compounds for application in aqueous and acidic media (such as selective separation of different metals and wastewater recovery).

Harvesting Blue Energy Based on Salinity and Temperature Gradient: Challenges, Solutions, and Opportunities

Greenhouse gas emissions associated with power generation from fossil fuel combustion account for 25% of global emissions and, thus, contribute greatly to climate change. Renewable energy sources, like wind and solar, have reached a mature stage, with costs aligning with those of fossil fuel-derived power but suffer from the challenge of intermittency due to the variability of wind and sunlight. This study aims to explore the viability of salinity gradient power, or "blue energy", as a clean, renewable source of uninterrupted, base-load power generation. Harnessing the salinity gradient energy from river estuaries worldwide could meet a substantial portion of the global electricity demand (approximately 7%). Pressure retarded osmosis (PRO) and reverse electrodialysis (RED) are more prominent technologies for blue energy harvesting, whereas thermo-osmotic energy conversion (TOEC) is emerging with new promise. This review scrutinizes the obstacles encountered in developing osmotic power generation using membrane-based methods and presents potential solutions to overcome challenges in practical applications. While certain strategies have shown promise in addressing some of these obstacles, further research is still required to enhance the energy efficiency and feasibility of membrane-based processes, enabling their large-scale implementation in osmotic energy harvesting.

Progress on Improved Fouling Resistance-Nanofibrous Membrane for Membrane Distillation: A Mini-Review

Nanofibrous membranes for membrane distillation (MD) have demonstrated promising results in treating various water and wastewater streams. Significant progress has been made in recent decades because of the development of sophisticated membrane materials, such as superhydrophobic, omniphobic and Janus membranes. However, fouling and wetting remain crucial issues for long-term operation. This mini-review summarizes ideas as well as their limitations in understanding the fouling in membrane distillation, comprising organic, inorganic and biofouling. This review also provides progress in developing antifouling nanofibrous membranes for membrane distillation and ongoing modifications on nanofiber membranes for improved membrane distillation performance. Lastly, challenges and future ways to develop antifouling nanofiber membranes for MD application have been systematically elaborated. The present mini-review will interest scientists and engineers searching for the progress in MD development and its solutions to the MD fouling issues.

Enhancing Mechanical Properties and Flux of Nanofibre Membranes for Water Filtration

Nanofibres have gained attention for their highly porous structure, narrow pore size, and high specific surface area. One of the most efficient techniques for producing nanofibres is electrospinning. These fibres are used in various fields, including water filtration. Although they possess the ability to filter various components, the fibres generally have low mechanical strength, which can mitigate their performance over time. To address this, studies have focused on enhancing nanofibre membrane strength for water filtration. Previous analyses show that the mechanical properties of nanofibre mats can be improved through solvent vapour treatment, thermal treatment, and chemical crosslinking. These treatments promote interfibre bonding, leading to the improvement of mechanical strength. However, excessive treatment alters nanofibre behaviour. Excessive heat exposure reduces interfibre bonding, while too much solvent vapour decreases pore size and mechanical strength. Thus, a comprehensive understanding of these post-treatments is crucial. This review examines post-treatments aiming to increase the mechanical strength of nanofibre mats, discussing their advantages and disadvantages. Understanding these treatments is essential for optimising nanofibre membrane performance in water filtration and other applications.

Hydrophobic modification of a PVDF hollow fiber membrane by plasma activation and silane grafting for membrane distillation

Polyvinylidene fluoride (PVDF) hollow fibers were hydrophobically modified using a simple and scalable method of plasma activation and silane grafting. The effects of plasma gas, applied voltage, activation time, silane type, and concentration were investigated according to the membrane hydrophobicity and direct contact membrane distillation (DCMD) performance. Two kinds of silane were used, including methyl trichloroalkyl silane (MTCS) and 1H,1H,2H,2H-perfluorooctane trichlorosilane silanes (PTCS). The membranes were characterized by techniques such as Fourier transform infrared (FTIR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and contact angle. The contact angle of the pristine membrane was 88°, which increased to 112°-116° after modification. Meanwhile, the pore size and porosity decreased. In DCMD, the maximum rejection reached 99.95% by the MTCS-grafted membrane, while the flux decreased by 35% and 65% for the MTCS- and PTCS-grafted membranes, respectively. Treating humic acid-contained solution, the modified membrane showed steadier water flux and higher salt rejection than the pristine membrane, and 100% flux recovery was achieved by simple water flushing. This two-step method of plasma activation and silane grafting is very simple and effective to improve the hydrophobicity and DCMD performance of PVDF hollow fibers. However, further study on improving the water flux should be carried out.

A Comparison of Unmodified and Sawdust Derived-Cellulose Nanocrystals (CNC)-Modified Polyamide Membrane Using X-ray Photoelectron Spectroscopy and Zeta Potential Analysis

Cellulose nanocrystals (CNC) obtained from waste sawdust were used to modify the polyamide membrane fabricated by interfacial polymerization of m-phenylene-diamine (MPDA) and trimesoyl chloride (TMC). The efficiency of the modification with sawdust-derived CNC was investigated using zeta potential and X-ray photoelectron spectroscopy (XPS). The effect of the modification on membrane mechanical strength and stability in acidic and alkaline solutions was also investigated. Results revealed that the negative zeta potential decreased at a high pH and the isoelectric point shifted into the acidic range for both modified and unmodified membranes. However, the negative charges obtained on the surface of the modified membrane at a pH lower than 8 were higher than the pristine membrane, which is an indication of the successful membrane modification. The XPS result shows that the degree of crosslinking was lowered due to the presence of CNC. Enhanced stability in solution in all pH ranges and the increase in mechanical strength, as indicated by higher Young's modulus, maximum load, and tensile strength, confirmed the robustness of the modified membrane.

Microplastics waste in environment: A perspective on recycling issues from PPE kits and face masks during the COVID-19 pandemic

During the COVID-19 pandemic, the extensive use of face masks and protective personal equipment (PPE) kits has led to increasing degree of microplastic pollution (MP) because they are typically discarded into the seas, rivers, streets, and other parts of the environment. Currently, microplastic (MP) pollution has a negative impact on the environment because of high-level fragmentation. Typically, MP pollution can be detected by various techniques, such as microscopic analysis, density separation, and Fourier transform infrared spectrometry. However, there are limited studies on disposable face masks and PPE kits. A wide range of marine species ingest MPs in the form of fibers and fragments, which directly affect the environment and human health; thus, more research and development are needed on the effect of MP pollution on human health. This article provides a perspective on the origin and distribution of MP pollution in waterbodies (e.g., rivers, ponds, lakes, and seas) and wastewater treatment plants, and reviews the possible remediation of MP pollution related to the excessive disposal of face masks and PPE kits to aquatic environments.

Advancements in modification of membrane materials over membrane separation for biomedical applications-Review

A comprehensive overview of various modifications carried out on polymeric membranes for biomedical applications has been presented in this review paper. In particular, different methods of carrying out these modifications have been discussed. The uniqueness of the review lies in the sense that it discusses the surface modification techniques traversing the timeline from traditionally well-established technologies to emerging new techniques, thus giving an intuitive understanding of the evolution of surface modification techniques over time. A critical comparison of the advantages and pitfalls of commonly used traditional and emerging surface modification techniques have been discussed. The paper also highlights the tuning of specific properties of polymeric membranes that are critical for their increased applications in the biomedical industry specifically in drug delivery, along with current challenges faced and where the future potential of research in the field of surface modification of membranes.

Study of MOF incorporated dual layer membrane with enhanced removal of ammonia and per-/poly-fluoroalkyl substances (PFAS) in landfill leachate treatment

Landfill leachate is a highly polluted and complex wastewater as it contains large amounts of organic matters, ammonia‑nitrogen, heavy metals, and per-/poly-fluoroalkyl substances (PFAS), which makes its treatment very challenging. In this paper, hydrophilic/hydrophobic dual layer membranes combining advantages of pervaporation and membrane distillation was employed to treat leachate in a direct contact membrane distillation (DCMD) configuration. An aluminum fumarate (AlFu) metal organic framework (MOF) incorporated poly(vinyl alcohol) (PVA) hydrophilic layer was coated on hydrophobic PTFE membrane to overcome the low separation efficiency of PFAS and ammonia and wetting issues encountered by the conventional hydrophobic PTFE membrane used for DCMD. The rejections of dual layer membranes with different MOF loading to PFAS, ammonia, TOC and TDS were assessed based on the amount of AlFu MOF incorporated into the PVA layer. Based on the conducted adsorption tests, it was found that AlFu MOF increases the rejection of PVA layer to PFAS and ammonia. The coating of the hydrophilic layer could enhance the wetting resistance with/without MOF addition. In comparison with the pristine PTFE membrane using synthetic feed containing 3 wt% NaCl, 1 wt% addition of AlFu MOF into the PVA layer showed slightly increased flux. All the tested membranes showed more than 99% rejection to TOC. The rejection to ammonia was increased as more MOF was incorporated into the PVA layer. The maximum rejection of ammonia was 99.8% when the PVA layer containing 10% MOF. All the membranes showed more than 99% rejection to PFOS and PFHxS. However, PTFE membrane did not show any rejection to PFOA. As more MOF was added into the hydrophilic layer, the rejection to PFOA increased, but plateaued at 65.6% with 5% MOF incorporation into the hydrophilic layer.

A Review on the Use of Membrane Technology Systems in Developing Countries

Fulfilling the demand of clean potable water to the general public has long been a challenging task in most developing countries due to various reasons. Large-scale membrane water treatment systems have proven to be successful in many advanced countries in the past two decades. This paves the way for developing countries to study the feasibility and adopt the utilization of membrane technology in water treatment. There are still many challenges to overcome, particularly on the much higher capital and operational cost of membrane technology compared to the conventional water treatment system. This review aims to delve into the progress of membrane technology for water treatment systems, particularly in developing countries. It first concentrates on membrane classification and its application in water treatment, including membrane technology progress for large-scale water treatment systems. Then, the fouling issue and ways to mitigate the fouling will be discussed. The feasibility of membrane technologies in developing countries was then evaluated, followed by a discussion on the challenges and opportunities of the membrane technology implementation. Finally, the current trend of membrane research was highlighted to address future perspectives of the membrane technologies for clean water production.

A Single Step Preparation of Photothermally Active Polyvinylidene Fluoride Membranes Using Triethyl Phosphate as a Green Solvent for Distillation Applications

Membrane distillation is a growing technology that can address the growing problem of water shortage. The implementation of renewable energy and a reduction in the environmental impact of membrane production could improve the sustainability of this process. With this perspective, porous hydrophobic polyvinylidene fluoride (PVDF) membranes were prepared using triethyl phosphate (TEP) as a green solvent, using the non-solvent induced phase separation technique. Different amounts of carbon black were added to dope solutions to improve the photothermal properties of the membranes and to enable direct heating by solar energy. By optimizing the preparation conditions, membranes with porosity values as high as 87% were manufactured. Vacuum membrane distillation tests carried out using a concentrated NaCl solution at 50 °C showed distillate fluxes of up to 36 L/m² h and a complete salt rejection. Some preliminary studies on the photothermal performance were also conducted and highlighted the possibility of using such membranes in a direct solar membrane distillation configuration.

Recent Improvement Strategies on Metal-Organic Frameworks as Adsorbent, Catalyst, and Membrane for Wastewater Treatment

The accumulation of pollutants in water is dangerous for the environment and human lives. Some of them are considered as persistent organic pollutants (POPs) that cannot be eliminated from wastewater effluent. Thus, many researchers have devoted their efforts to improving the existing technology or providing an alternative strategy to solve this environmental problem. One of the attractive materials for this purpose are metal-organic frameworks (MOFs) due to their superior high surface area, high porosity, and the tunable features of their structures and function. This review provides an up-to-date and comprehensive description of MOFs and their crucial role as adsorbent, catalyst, and membrane in wastewater treatment. This study also highlighted several strategies to improve their capability to remove pollutants from water effluent.

Recent Progress in the Membrane Distillation and Impact of Track-Etched Membranes

Membrane distillation (MD) is a rapidly developing field of research and finds applications in desalination of water, purification from nonvolatile substances, and concentration of various solutions. This review presents data from recent studies on the MD process, MD configuration, the type of membranes and membrane hydrophobization. Particular importance has been placed on the methods of hydrophobization and the use of track-etched membranes (TeMs) in the MD process. Hydrophobic TeMs based on poly(ethylene terephthalate) (PET), poly(vinylidene fluoride) (PVDF) and polycarbonate (PC) have been applied in the purification of water from salts and pesticides, as well as in the concentration of low-level liquid radioactive waste (LLLRW). Such membranes are characterized by a narrow pore size distribution, precise values of the number of pores per unit area and narrow thickness. These properties of membranes allow them to be used for more accurate water purification and as model membranes used to test theoretical models (for instance LEP prediction).

Stability of Ar/O2 Plasma-Treated Polypropylene Membranes Applied for Membrane Distillation

In the present work, Ar/O₂ plasma treatment was used as a surface modification tool for polypropylene (PP) membranes. The effect of the plasma conditions on the properties of the modified PP surface has been investigated. For this purpose, the influence of gas composition and its flow rate, plasma power excitation as well as treatment time on the contact angle of PP membranes has been investigated. The properties of used membranes were determined after various periods of time: immediately after the modification process as well as after one, four and five years of storage. Moreover, the used membranes were evaluated in terms of their performance in long-term MD process. Through detailed studies, we demonstrated that the performed plasma treatment process effectively enhanced the performance of the modified membranes. In addition, it was shown that the surface modification did not affect the degradation of the membrane matrix. Indeed, the used membranes maintained stable process properties throughout the studied period.

An Improved Modelling Approach for the Comprehensive Study of Direct Contact Membrane Distillation

Direct Contact Membrane Distillation (DCMD) is a promising and feasible technology for water desalination. Most of the models used to simulate DCMD are one-dimensional and/or use a linear function of vapour pressure which relies on experimentally determined parameters. In this study, the model of DCMD using Nusselt correlations was improved by coupling the continuity, momentum, and energy equations to better capture the downstream alteration of flow field properties. A logarithmic function of vapour pressure, which is independent from experiments, was used. This allowed us to analyse DCMD with different membrane properties. The results of our developed model were in good agreement with the DCMD experimental results, with less than 7% deviation. System performance metrics, including water flux, temperature, and concentration polarisation coefficient and thermal efficiency, were analysed by varying inlet feed and permeate temperature, inlet velocity, inlet feed concentration, channel length. In addition, twenty-two commercial membranes were analysed to obtain a real vision on the influence of membrane characteristics on system performance metrics. The results showed that the feed temperature had the most significant effect on water flux and thermal efficiency. The increased feed temperature enhanced the water flux and thermal efficiency; however, it caused more concentration and temperature polarisation. On the other hand, the increased inlet velocity was found to provide increased water flux and reduced temperature and concertation polarisation as well. It was also found that the membrane properties, especially thickness and porosity, can affect the DCMD performance significantly. A two-fold increase of feed temperature increased the water flux and thermal efficiency, 10-fold and 27%, respectively; however, it caused an increase in temperature and concertation polarisation, at 48% and 34%, respectively. By increasing Reynolds number from 80 to 1600, the water flux, CPC, and TPC enhanced by 2.3-fold, 2%, and 21%, respectively. The increased feed concentration from 0 to 250 [g/L] caused a 26% reduction in water flux. To capture the downstream alteration of flow properties, it was shown that the ratio of inlet value to outlet value of system performance metrics decreased significantly throughout the module. Therefore, improvement over the conventional model is undeniable, as the new model can assist in achieving optimal operation conditions.

Performance of PVDF Based Membranes with 2D Materials for Membrane Assisted-Crystallization Process

Membrane crystallization (MCr) is a promising and innovative process for the recovery of freshwater from seawater and for the production of salt crystals from the brine streams of desalination plants. In the present work, composite polymeric membranes for membrane crystallization were fabricated using graphene and bismuth telluride inks prepared according to the wet-jet milling (WJM) technology. A comparison between PVDF-based membranes containing a few layers of graphene or bismuth telluride and PVDF-pristine membranes was carried out. Among the 2D composite membranes, PVDF with bismuth telluride at higher concentration (7%) exhibited the highest flux (about 3.9 L∙m-2h-1, in MCr experiments performed with 5 M NaCl solution as feed, and at a temperature of 34 ± 0.2 °C at the feed side and 11 ± 0.2 °C at the permeate side). The confinement of graphene and bismuth telluride in PVDF membranes produced more uniform NaCl crystals with respect to the pristine PVDF membrane, especially in the case of few-layer graphene. All the membranes showed rejection equal to or higher than 99.9% (up to 99.99% in the case of the membrane with graphene). The high rejection together with the good trans-membrane flux confirmed the interesting performance of the process, without any wetting phenomena, at least during the performed crystallization tests.

Mass Transfer Analysis of Air-Cooled Membrane Distillation Configuration for Desalination

It has been proposed that the air-cooled configuration for air gap membrane distillation is an effective way to simplify the system design and energy source requirement. This offers potential for the practical applications of membrane distillation on an industrial scale. In this work, membrane distillation tests were performed using a typical water-cooled membrane distillation (WCMD) configuration and an air-cooled membrane distillation (ACMD) configuration with various condensing plates and operating conditions. To increase the permeate flux of an ACMD system, the condensing plate in the permeate side should transfer heat to the atmosphere more effectively, such as using a more thermally conductive plate, adding fins, or introducing forced convection air flow. Importantly, a practical mass transfer model was proposed to describe the ACMD performance in terms of permeate flux. This model can be simplified by introducing specific correction values to the mass transfer coefficient of a WCMD process under the same conditions. The two factors relate to the capacity (B) and the efficiency (σ), which can be considered as the characteristic factors of a membrane distillation (MD) system. The experimental results are consistent with the theoretical estimations based on this model, which can be used to describe the performance of an MD process.

Boron Nitride Nanotube (BNNT) Membranes for Energy and Environmental Applications

Owing to their extraordinary thermal, mechanical, optical, and electrical properties, boron nitride nanotubes (BNNTs) have been attracting considerable attention in various scientific fields, making it more promising as a nanomaterial compared to other nanotubes. Recent studies reported that BNNTs exhibit better properties than carbon nanotubes, which have been extensively investigated for most environment-energy applications. Irrespective of its chirality, BNNT is a constant wide-bandgap insulator, exhibiting thermal oxidation resistance, piezoelectric properties, high hydrogen adsorption, ultraviolet luminescence, cytocompatibility, and stability. These unique properties of BNNT render it an exceptional material for separation applications, e.g., membranes. Recent studies reported that water filtration, gas separation, sensing, and battery separator membranes can considerably benefit from these properties. That is, flux, rejection, anti-fouling, sensing, structural, thermal, electrical, and optical properties of membranes can be enhanced by the contribution of BNNTs. Thus far, a majority of studies have focused on molecular simulation. Hence, the requirement of an extensive review has emerged. In this perspective article, advanced properties of BNNTs are analyzed, followed by a discussion on the advantages of these properties for membrane science with an overview of the current literature. We hope to provide insights into BNNT materials and accelerate research for environment-energy applications.

Vertically Aligned Carbon Nanotube Membranes: Water Purification and Beyond

Vertically aligned carbon nanotube (VACNT) membranes have attracted significant attention for water purification owing to their ultra-high water permeability and antibacterial properties. In this paper, we critically review the recent progresses in the synthesis of VACNT arrays and fabrication of VACNT membrane methods, with particular emphasis on improving water permeability and anti-biofouling properties. Furthermore, potential applications of VACNT membranes other than water purification (e.g., conductive membranes, electrodes in proton exchange membrane fuel cells, and solar electricity–water generators) have been introduced. Finally, future outlooks are provided to overcome the limitations of commercialization and desalination currently faced by VACNT membranes. This review will be useful to researchers in the broader scientific community as it discusses current and new trends regarding the development of VACNT membranes as well as their potential applications.