Preparation of high performance TFC RO membranes by surface grafting of small-molecule zwitterions

A review on antifouling polyamide reverse osmosis membrane for seawater desalination

Reverse osmosis (RO) membrane technology is well-established in desalination. Aromatic polyamide (PA) thin-film composite (TFC) membrane dominates the commercial RO membrane market due to its high-salt rejection, water flux, and excellent chemical, thermal, and mechanical stabilization. However, membrane fouling is a common problem that has seriously hindered the wide application of RO membrane technology. This paper reviewed the PA RO membrane fouling types, and membrane fouling factors. Antifouling measures for RO membranes were summarized, including pretreatment, periodic cleaning, and modification of the support layer and PA layer. The future development of antifouling RO membranes was clarified.

Biomimetic redox-responsive smart coatings with resistance-release functions for reverse osmosis membranes

Membrane fouling induces catastrophic loss of separation performance and seriously restricts the applications of reverse osmosis (RO) membranes. Inspired by the mussel structure, polydopamine (PDA) and cystamine molecules (CA) with excellent anti-fouling properties were used to prepare accessible, biocompatible, and redox-responsive coatings for RO membranes. The PDA/CA-coated RO membranes exhibit a superior water flux of 65 L m-2 h-1 with a favourable NaCl rejection exceeding 99%. The water permeability through the PDA/CA-coated membrane is much higher than that of most membranes with similar rejection rates. Due to the formed protective hydration layers by PDA/CA coatings, anti-fouling properties against proteins, polysaccharides and surfactants were evaluated separately, and ultralow fouling properties were demonstrated. Moreover, the disulfide linkages in CA molecules can cleave in a reducing environment, yielding the degradation of PDA/CA coatings, thereby removing the foulants deposited on the coatings. The degradation endows the coated membranes with satisfying longtime anti-fouling properties, where the flux recovery reaches up to 90%. The construction of redox-responsive smart coatings not only provided a promising route to alleviate membrane fouling but can also be upscaled for use in numerous practical applications like sensors, medical devices, and drug delivery.

When self-assembly meets interfacial polymerization

Interfacial polymerization (IP) and self-assembly are two thermodynamically different processes involving an interface in their systems. When the two systems are incorporated, the interface will exhibit extraordinary characteristics and generate structural and morphological transformation. In this work, an ultrapermeable polyamide (PA) reverse osmosis (RO) membrane with crumpled surface morphology and enlarged free volume was fabricated via IP reaction with the introduction of self-assembled surfactant micellar system. The mechanisms of the formation of crumpled nanostructures were elucidated via multiscale simulations. The electrostatic interactions among m-phenylenediamine (MPD) molecules, surfactant monolayer and micelles, lead to disruption of the monolayer at the interface, which in turn shapes the initial pattern formation of the PA layer. The interfacial instability brought about by these molecular interactions promotes the formation of crumpled PA layer with larger effective surface area, facilitating the enhanced water transport. This work provides valuable insights into the mechanisms of the IP process and is fundamental for exploring high-performance desalination membranes.

Distinct Antifouling Mechanisms on Different Chain Densities of Zwitterionic Polymers

Antifouling polymer coating surfaces are used in widespread industries applications. Zwitterionic polymers have been identified as promising materials in developing polymer coating surfaces. Importantly, the density of the polymer chains is crucial for acquiring superior antifouling performance. This study introduces two different zwitterionic polymer density surfaces by applying molecular modeling tools. To assess the antifouling performance, we mimic static adsorption test, by placing the foulant model bovine serum albumin (BSA) on the surfaces. Our findings show that not only the density of the polymer chain affect antifouling performance, but also the initial orientation of the BSA on the surface. Moreover, at a high-density surface, the foulant either detaches from the surface or anchor on the surface. At low-density surface, the foulant does not detach from the surface, but either penetrates or anchors on the surface. The anchoring and the penetrating mechanisms are elucidated by the electrostatic interactions between the foulant and the surface. While the positively charged ammonium groups of the polymer play major role in the interactions with the negatively charged amino acids of the BSA, in the penetrating mechanism the ammonium groups play minor role in the interactions with the contact with the foulant. The sulfonate groups of the polymer pull the foulant in the penetrating mechanism. Our work supports the design of a high-density polymer chain surface coating to prevent fouling phenomenon. Our study provides for the first-time insights into the molecular mechanism by probing the interactions between BSA and the zwitterion surface, while testing high- and low-densities polymer chains.

Surface modification of commercial reverse osmosis membranes using both hydrophilic polymer and graphene oxide to improve desalination efficiency

Various methods have been applied to modify the surface of reverse osmosis (RO) membranes to modify the membrane performance to enhance the flux, rejection, and resistance to various factors of fouling. Hence, the main objective of the current study is to modify the surface of commercial RO membranes using the synergistic effect of the hydrophilic polymer and graphene oxide (GO). GO nanosheets were firstly synthesized by the modified hummer method, then characterized by FTIR, XRD, and SEM analyses. Then, the polyacrylic acid (PAA) was grafted on the membrane surface for membrane fabrication. Furthermore, effective factors of grafting such as monomer concentration, time, and temperature of polymerization were optimized. After that, different amounts of GO nanosheets were loaded in PAA optimized layer. Then, the effect of GO loading on the RO membrane structure and performance was investigated. The outcomes of membrane characterization demonstrated that modified RO membranes had a smoother surface, more negative surface charge, a little better hydrophilicity, and more thickness. Moreover, the results of PAA and GO optimization were shown that grafting 1.5 mM of PAA and loading 0.1 wt% of GO nanosheets give the best membrane performance. This membrane (GO 0.1@1.5M PAA/RO) between all modified membranes has the most water flux (37.1 L/m²h), the highest NaCl rejection (98%), and the best antifouling efficiency. Ultimately, it was concluded that the grafting of GO@PAA on the surface of a commercial RO membrane is an efficient approach for the enhancement of desalination and antifouling performance of this kind of membrane.

In Situ Chemical Modification with Zwitterionic Copolymers of Nanofiltration Membranes: Cure for the Trade-Off between Filtration and Antifouling Performance

Breaking the trade-off between filtration performance and antifouling property is critical to enabling a thin-film nanocomposite (TFC) nanofiltration (NF) membrane for a wide range of feed streams. We proposed a novel design route for TFC NF membranes by grafting well-defined zwitterionic copolymers of [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (SBMA) and 2-aminoethyl methacrylate hydrochloride (AEMA) on the polyamide surfaces via an in situ surface chemical modification process. The successful grafting of a zwitterionic copolymer imparted the modified NF membranes with better surface hydrophilicity, a larger actual surface area (i.e., nodular structures), and a thinner polyamide layer. As a result, the water permeability of the modified membrane (i.e., TFC-10) was triple that of the pristine TFC membrane while maintaining high Na₂SO₄ rejection. We further demonstrated that the TFC-10 membrane possessed exceptional antifouling properties in both static adsorption tests and three cycles of dynamic protein and humic acid fouling tests. To recap, this work provides valuable insights and strategies for the fabrication of TFC NF membranes with simultaneously enhanced filtration performance and antifouling property.

3D Printed and Conventional Membranes—A Review

Polymer membranes are central to the proper operation of several processes used in a wide range of applications. The production of these membranes relies on processes such as phase inversion, stretching, track etching, sintering, or electrospinning. A novel and competitive strategy in membrane production is the use of additive manufacturing that enables the easier manufacture of tailored membranes. To achieve the future development of better membranes, it is necessary to compare this novel production process to that of more conventional techniques, and clarify the advantages and disadvantages. This review article compares a conventional method of manufacturing polymer membranes to additive manufacturing. A review of 3D printed membranes is also done to give researchers a reference guide. Membranes from these two approaches were compared in terms of cost, materials, structures, properties, performance. and environmental impact. Results show that very few membrane materials are used as 3D-printed membranes. Such membranes showed acceptable performance, better structures, and less environmental impact compared with those of conventional membranes.

Eco-friendly surface modification approach to develop thin film nanocomposite membrane with improved desalination and antifouling properties

Introduction

Nanomaterials aggregation within polyamide (PA) layer of thin film nanocomposite (TFN) membrane is found to be a common issue and can negatively affect membrane filtration performance. Thus, post-treatment on the surface of TFN membrane is one of the strategies to address the problem.

Objective

In this study, an eco-friendly surface modification technique based on plasma enhanced chemical vapour deposition (PECVD) was used to deposit hydrophilic acrylic acid (AA) onto the PA surface of TFN membrane with the aims of simultaneously minimizing the PA surface defects caused by nanomaterials incorporation and improving the membrane surface hydrophilicity for reverse osmosis (RO) application.

Methods

The TFN membrane was first synthesized by incorporating 0.05 wt% of functionalized titania nanotubes (TNTs) into its PA layer. It was then subjected to 15-s plasma deposition of AA monomer to establish extremely thin hydrophilic layer atop PA nanocomposite layer. PECVD is a promising surface modification method as it offers rapid and solvent-free functionalization for the membranes.

Results

The findings clearly showed that the sodium chloride rejection of the plasma-modified TFN membrane was improved with salt passage reduced from 2.43% to 1.50% without significantly altering pure water flux. The AA-modified TFN membrane also exhibited a remarkable antifouling property with higher flux recovery rate (>95%, 5-h filtration using 1000 mg/L sodium alginate solution) compared to the unmodified TFN membrane (85.8%), which is mainly attributed to its enhanced hydrophilicity and smoother surface. Furthermore, the AA-modified TFN membrane also showed higher performance stability throughout 12-h filtration period.

Conclusion

The deposition of hydrophilic material on the TFN membrane surface via eco-friendly method is potential to develop a defect-free TFN membrane with enhanced fouling resistance for improved desalination process.

Surface modification of polyamide reverse osmosis membranes with small-molecule zwitterions for enhanced fouling resistance: a molecular simulation study

Surface modification with small-molecule zwitterions is experimentally proved to be an effective solution to improve the antifouling performance of polyamide membranes. However, there is no comprehensive understanding of their microscopic mechanism. In order to address this issue, in this work we constructed two atomistic models, PA (a pure polyamide membrane) and QDAP-PA (a polyamide membrane surface-modified with QDAP), where QDAP was a zwitterion that was prepared by 2,6-daaminopyridine quaternized with 3-bromopropionic acid experimentally. Density functional theory was adopted to elucidate the variations in the electrostatic potential before and after modification. Then, equilibrium molecular dynamics (EMD) simulations were conducted to investigate the structure and hydrophobic/hydrophilic nature of the membrane surface in the two models. Finally, we introduced two typical organic foulants, sodium dodecyl sulfonate (SDS) and dodecyl trimethyl ammonium chloride (DTAC), to evaluate the antifouling performance of the membranes with the umbrella sampling method. The analyses of the membrane structure and properties show that surface modification with small-molecule zwitterions can densify the membrane surface as well as enlarge the distribution of electrostatic potential on the membrane surface. Water molecules tend to have more interactions with the membrane and more hydrogen bonds near the membrane surface are observed in QDAP-PA. The antifouling test supports that QDAP-PA shows a better antifouling performance, as the surface-modified membrane exhibits a stronger resistance to SDS and DTAC. Even if the foulant is adsorbed to the membrane surface, the denser interface region can prevent a further pollution of the foulant. Also, the free energy needed during the process for QDAP-PA to desorb a foulant is relatively small, indicating that this kind of membrane is easy to clean. The current work might provide a comprehensive understanding of the enhanced fouling resistance of polyamide membranes after surface modification with small-molecule zwitterions.