High performance reverse osmosis membrane with carbon nanotube support layer

Double-Walled Carbon Nanotubes Enable Breakdown of the Trade-off between Ion Selectivity and Water Permeability

Although evidence has been presented for desalination potentials in single-walled carbon nanotubes (SWCNTs), it is still very challenging to overcome the trade-off between ion selectivity and water permeability by simply tuning the carbon nanotube (CNT) size. In this work, we prove that double-walled carbon nanotubes (DWCNTs) can make it. Employing a series of molecular dynamics simulations, we find a striking phenomenon that tuning the combination architecture of DWCNTs can significantly improve the desalination performance, with the salt rejection rate even reaching 100% in some cases while maintaining high levels of water flux. Specifically, under a certain outer CNT (20,20), with the increase in inner CNT radius, the salt rejection rate reaches a maximum for the CNT (9,9), attributed to the small size of the inner CNT and the space between the two CNT walls that significantly impedes the ion passage; however, it still allows the passage of massive water. Furthermore, as the pressure difference increases, the water flux greatly increases, while the salt rejection rate only slightly decreases for the CNTs (8,8) and (9,9), effectively addressing the trade-off between ion selectivity and water permeability. As a result, optimizing the architecture of DWCNTs should be an effective strategy for designing an efficient desalination membrane, which is still a challenge for SWCNTs.

Study on the migration performance of Cs(I) in the treatment of simulated radioactive wastewater by electrodialysis

As a competitive radioactive wastewater treatment technology, electrodialysis (ED) has the advantages of low operating pressure and high cycles of concentration. In order to analyze the migration performance of radionuclides during the treatment of radioactive wastewater by ED, a radionuclide migration model was constructed based on the mass conservation law and Faraday's law with the typical radionuclide cesium as the research object. Experiments were carried out for the treatment of simulated radioactive wastewater in a small-scale ED system, and the average migration rate of radionuclides under different operating conditions was predicted by the model. The results showed that the experimental values of concentration and average migration rate of Cs(I) were significantly correlated with the calculated values of the model, in which the relative error of the average migration rate was 4.54%. The variation characteristics of Cs(I) concentration in diluted solution under different current and volume ratio conditions can be predicted by the model. The average variation rate of Cs(I) concentration decreases significantly with the increase of current and volume ratio.

Applications of Polymeric Membranes with Carbon Nanotubes: A Review

Nanomaterials have been commonly employed to enhance the performance of polymeric membrane materials that are used in several industrial applications. Carbon nanotubes (CNTs) have gained notable attention over the years for use in membrane technology due to their anti-biofouling properties, salt rejection capability, exceptional electrical conductivity, and mechanical properties. This paper aims to discuss some of the recent applications of CNTs in membrane technology and their effect on a larger scale. The paper reviews successful case studies of incorporation of CNTs in membranes and their impact on water purification, desalination, gas separations, and energy storage, in an effort to provide a better understanding of their capabilities. Regarding the future trends of this technology, this review emphasizes improving the large-scale production processes and addressing environmental and health-related hazards of CNTs during production and usage.

Modification of regenerated cellulose ultrafiltration membranes with multi-walled carbon nanotubes for enhanced antifouling ability: Field test and mechanism study

Although ultrafiltration (UF) has been extensively employed for drinking water purification, it is crucial to further develop novel membrane materials to improve the antifouling capacity and satisfy the practical usage. Multi-walled carbon nanotubes (MWCNTs) have characteristics that could potentially improve the membrane antifouling performance. Therefore, in this study, modified cellulose UF membranes were prepared using MWCNTs of various outer diameters ranging from 10 to 20 nm to 40-60 nm. The antifouling properties of the modified membrane and natural organic matter (NOM) removal mechanism were investigated while treating water from a local drinking water source river. Overall, the antifouling ability increased by more than one-fold when the nascent cellulose membrane was coated with MWCNTs (outer diameter of 40-60 nm) at a loading of 17.4 g/m². The molecular weight distribution profiles of the NOM in the raw water and permeates suggest the superior performance of the modified membranes in removing two major NOM fractions with molecular weights ranging from approximately 5 k-30 k and 500 k-1000 k. Based on its hydrophobicity, the NOM of the raw water was fractionated into the strong hydrophobic (SHPO), the weak hydrophobic, the strong hydrophilic and the moderately hydrophilic (MHPI) fractions. The WHPO fraction caused the highest fouling compared with the other fractions under consistent experimental conditions. Meanwhile, the modified membranes showed a preference for removing the MHPI and SHPO fractions. These results imply that MWCNTs can be employed to improve the antifouling property of cellulose UF membranes and have the potential to selectively remove moderately hydrophilic contaminants from water.

Emerging graphitic carbon nitride-based membranes for water purification

Membrane separation is a promising technology that can effectively remove various existing contaminants from water with low energy consumption and small carbon footprint. The critical issue of membrane technology development is to obtain a low-cost, stable, tunable and multifunctional material for membrane fabrication. Graphitic carbon nitride (g-C₃N₄) has emerged as a promising membrane material, owing to the unique structure characteristics and outstanding catalytic activity. This review paper outlined the advanced material strategies used to regulate the molecule structure of g-C₃N₄ for membrane separation. The presentative progresses on the applications of g-C₃N₄-based membranes for water purification have been elaborated. Essentially, we highlighted the innovation integration of physical separation, catalysis and energy conversion during water purification, which was of great importance for the sustainability of water treatment techniques. Finally, the continuing challenges of g-C₃N₄-based membranes and the possible breakthrough directions in the future research was prospected.

A Comprehensive Review on Membrane Fouling: Mathematical Modelling, Prediction, Diagnosis, and Mitigation

Membrane-based separation has gained increased popularity over the past few decades, particularly reverse osmosis (RO). A major impediment to the improved performance of membrane separation processes, in general, is membrane fouling. Fouling has detrimental effects on the membrane’s performance and integrity, as the deposition and accumulation of foulants on its surface and/or within its pores leads to a decline in the permeate flux, deterioration of selectivity, and permeability, as well as a significantly reduced lifespan. Several factors influence the fouling-propensity of a membrane, such as surface morphology, roughness, hydrophobicity, and material of fabrication. Generally, fouling can be categorized into particulate, organic, inorganic, and biofouling. Efficient prediction techniques and diagnostics are integral for strategizing control, management, and mitigation interventions to minimize the damage of fouling occurrences in the membranes. To improve the antifouling characteristics of RO membranes, surface enhancements by different chemical and physical means have been extensively sought after. Moreover, research efforts have been directed towards synthesizing membranes using novel materials that would improve their antifouling performance. This paper presents a review of the different membrane fouling types, fouling-inducing factors, predictive methods, diagnostic techniques, and mitigation strategies, with a special focus on RO membrane fouling.

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.

Applications of MXene-based membranes in water purification: A review

Since MXenes (a new family of two-dimensional materials) were first produced in 2011, they have become very attractive nanomaterials due to their unique properties and the range of potential industrial applications. Numerous recent studies have discussed the environmental applications of different MXenes in adsorption, catalysis, and membranes. Only a limited number of MXene-based membrane studies have been published to date, and most have discussed only specific MXenes (i.e., Ti₃C₂T_x), a small number of solutes (e.g., dyes and inorganic salts), and laboratory-scale short-term experiments under limited water-quality and operational conditions. In addition, to our knowledge, there has been no review of MXene-membrane studies. It is therefore essential to assess the current status of understanding of the performance of these membranes in liquid separation and water purification. Here, a comprehensive literature review is conducted to summarize the current preparation techniques for MXene-based membranes and their applications, particularly in terms of environmental and industrial applications (e.g., water treatment and organic solvent filtration), and to direct future research by identifying gaps in our present understanding. In particular, this review focuses on several key factors, including the effects of preparation techniques on membrane properties, operational conditions, and compound properties that influence liquid separation during MXene-based membrane filtration.

Recent Advances in Applications of Carbon Nanotubes for Desalination: A Review

As a sustainable, cost-effective and energy-efficient method, membranes are becoming a progressively vital technique to solve the problem of the scarcity of freshwater resources. With these critical advantages, carbon nanotubes (CNTs) have great potential for membrane desalination given their high aspect ratio, large surface area, high mechanical strength and chemical robustness. In recent years, the CNT membrane field has progressed enormously with applications in water desalination. The latest theoretical and experimental developments on the desalination of CNT membranes, including vertically aligned CNT (VACNT) membranes, composited CNT membranes, and their applications are timely and comprehensively reviewed in this manuscript. The mechanisms and effects of CNT membranes used in water desalination where they offer the advantages are also examined. Finally, a summary and outlook are further put forward on the scientific opportunities and major technological challenges in this field.