A facile zwitterionization in the interfacial modification of low bio-fouling nanofiltration membranes

Tailoring the Surface Chemistry of Anion Exchange Membranes with Zwitterions: Toward Antifouling RED Membranes

Fouling is a pressing issue for harvesting salinity gradient energy with reverse electrodialysis (RED). In this work, antifouling membranes were fabricated by surface modification of a commercial anion exchange membrane with zwitterionic layers. Either zwitterionic monomers or zwitterionic brushes were applied on the surface. Zwitterionic monomers were grafted to the surface by deposition of a polydopamine layer followed by an aza-Michael reaction with sulfobetaine. Zwitterionic brushes were grafted on the surface by deposition of polydopamine modified with a surface initiator for subsequent atom transfer radical polymerization to obtain polysulfobetaine. As expected, the zwitterionic layers did increase the membrane hydrophilicity. The antifouling behavior of the membranes in RED was evaluated using artificial river and seawater and sodium dodecylbenzenesulfonate as the model foulant. The zwitterionic monomers are effective in delaying the fouling onset, but the further build-up of the fouling layer is hardly affected, resulting in similar power density losses as for the unmodified membranes. Membranes modified with zwitterionic brushes show a high potential for application in RED as they not only delay the onset of fouling but they also slow down the growth of the fouling layer, thus retaining higher power density outputs.

Carbodiimide-assisted zwitterionic modification of poly(piperazine amide) thin-film composite membrane for enhanced separation and anti-depositing performances to cationic/anionic dye aqueous solutions

In this work, a novel method of carbodiimide-assisted zwitterionic modification was proposed and implemented to incorporate zwitterionic moieties onto poly(piperazine amide) membrane for improved water permeability and anti-depositing property, which are crucial for highly efficient nanofiltration of dye-contained effluents. Carboxyl groups of polyamide layer were firstly transferred into N-acylurea using excess l-ethyl-3-(3-(dimethylamino)propyl)-carbodiimide. Zwitterions were then incorporated through ring-opening reaction between tertiary amine groups of N-acylurea and 1, 4-butanesultone. Carbodiimide-assisted zwitterionic modification was verified by ATR-IR and XPS analyses and was found to not affect membrane pore size but significantly enhance membrane's permeation and anti-dye-deposition performances. Compared with those of virgin membrane, water permeabilities of the desired zwitterionic membrane to pure water, Congo red aqueous solution and Victoria blue B aqueous solution were higher by 42.9, 62.3 and 95.2 %, respectively, hydraulic resistances from irreversible deposition of Congo red and Victoria blue B molecules were dramatically lowered by 68.4 and 91.8 %, respectively. Furthermore, the perm-selectivity performance of the desired zwitterionic membrane in terms of molecular weight cut-off and pure water permeability was better than most of the reported zwitterionic membranes, and the separation and anti-depositing performances to both anionic and cationic dye aqueous solutions were better than commercial membrane NF270.

Nanofiltration for Arsenic Removal: Challenges, Recent Developments, and Perspectives

Arsenic (As) removal is of major significance because inorganic arsenic is highly toxic to all life forms, is a confirmed carcinogen, and is of significant environmental concern. As contamination in drinking water alone threatens more than 150 million people all over the world. Therefore, several conventional methods such as oxidation, coagulation, adsorption, etc., have been implemented for As removal, but due to their cost-maintenance limitations; there is a drive for advanced, low cost nanofiltration membrane-based technology. Thus, in order to address the increasing demand of fresh and drinking water, this review focuses on advanced nanofiltration (NF) strategy for As removal to safeguard water security. The review concentrates on different types of NF membranes, membrane fabrication processes, and their mechanism and efficiency of performance for removing As from contaminated water. The article provides an overview of the current status of polymer-, polymer composite-, and polymer nanocomposite-based NF membranes, to assess the status of nanomaterial-facilitated NF membranes and to incite progress in this area. Finally, future perspectives and future trends are highlighted.

Constructing zwitterionic polymer brush layer to enhance gravity-driven membrane performance by governing biofilm formation

In this study, zwitterionic polymer brushes with controlled architecture were grafted on the surface of gravity-driven membrane (GDM) via surface-initiated reaction to impart antifouling property. A variety of membrane characterization techniques were conducted to demonstrate the successful functionalization of zwitterionic polymers on PVDF hollow fiber membrane. The membrane underwent 90 min of reaction time possessing strong hydrophilicity and high permeability was determined as the optimal modified membrane. Long-term GDM dynamic fouling experiments operated for 30 days using sewage wastewater as feed solution indicated zwitterionic polymer modified membrane exhibit excellent membrane fouling resistance thus enhanced stable flux. Confocal laser scanning microscopy (CLSM) imaging implied that zwitterionic polymer modification significantly inhibit the adsorption of extracellular polymeric substances (EPS) which dominates fouling propensity, resulting in the formation of a thin biofilm with high porosity under synthetic functions of foulants deposition and microbial activities. Interfacial free energy prediction affirmed the presence of zwitterionic functional layer on membrane surface could substantially decrease the interactions (e.g., electrostatic attractions and hydrophobic effects) between membrane and foulants, thereby reduced flux decline and high stable flux. Our study suggests surface hydrophilic functionalization shows promising potential for improving the performance of ultra-low pressure filtration.

Designs of Zwitterionic Interfaces and Membranes

Zwitterionic materials are the latest generation of materials for nonfouling interfaces and membranes. They outperform poly(ethylene glycol) derivatives because they form tighter bonds with water molecules and can trap more water molecules. This feature article summarizes our laboratory's fundamental developments related to the functionalization of interfaces and membranes using zwitterionic materials. Our molecular designs of zwitterionic polymers and copolymers, sulfobetaine-based, carboxybetaine-based, or phosphobetaine-based, are first reviewed. Then, the strategies used to functionalize surfaces/membranes by coating, grafting onto, grafting from, or in situ modification are examined and discussed, and the third part of this article shifts the focus to key applications of zwitterionic materials. Finally, some potential future directions for molecular designs, functionalization processes, and applications are presented.

Antifouling enhancement of polyimide membrane by grafting DEDA-PS zwitterions

In order to improve the water flux and antifouling property of polyimide (PI) membrane, zwitterions are grafted on PI membrane surface via a two-step modification route by reactions with N,N-diethylethylenediamine (DEDA) and 1,3-propane sultone (PS) sequentially. The reaction mechanism and physicochemical properties of membranes are confirmed via various characterization techniques. The anti-biofouling performance of the zwitterion-grafted PI membranes is evaluated by bacterial suspension immersion tests in Escherichia coli (E. coli) and staphylococcus aureus (S. aureus) solutions. The antifouling property is assessed via the filtration test using the bovine serum albumin (BSA) and dodecyl trimethyl ammonium bromide (DTAB) aqueous feed solutions. The effect of the reaction time with DEDA in the zwitterion-grafted process on the antifouling property is further investigated systematically. The results show that both the anti-biofouling and antifouling performances of zwitterion-grafted PI membranes are significantly improved.

Recent Advances in the Fabrication of Membranes Containing "Ion Pairs" for Nanofiltration Processes

In the face of serious environmental pollution and water scarcity problems, the membrane separation technique, especially high efficiency, low energy consumption, and environmental friendly nanofiltration, has been quickly developed. Separation membranes with high permeability, good selectivity, and strong antifouling properties are critical for water treatment and green chemical processing. In recent years, researchers have paid more and more attention to the development of high performance nanofiltration membranes containing “ion pairs”. In this review, the effects of “ion pairs” characteristics, such as the super-hydrophilicity, controllable charge character, and antifouling property, on nanofiltration performances are discussed. A systematic survey was carried out on the various approaches and multiple regulation factors in the fabrication of polyelectrolyte complex membranes, zwitterionic membranes, and charged mosaic membranes, respectively. The mass transport behavior and antifouling mechanism of the membranes with “ion pairs” are also discussed. Finally, we present a brief perspective on the future development of advanced nanofiltration membranes with “ion pairs”.

Antifouling Thin-Film Composite Membranes by Controlled Architecture of Zwitterionic Polymer Brush Layer

In this study, we demonstrate a highly antifouling thin-film composite (TFC) membrane by grafting a zwitterionic polymer brush via atom-transfer radical-polymerization (ATRP), a controlled, environmentally benign chemical process. Initiator molecules for polymerization were immobilized on the membrane surface by bioinspired catechol chemistry, leading to the grafting of a dense zwitterionic polymer brush layer. Surface characterization revealed that the modified membrane exhibits reduced surface roughness, enhanced hydrophilicity, and lower surface charge. Chemical force microscopy demonstrated that the modified membrane displayed foulant-membrane interaction forces that were 1 order of magnitude smaller than those of the pristine TFC membrane. The excellent fouling resistance imparted by the zwitterionic brush layer was further demonstrated by significantly reduced adsorption of proteins and bacteria. In addition, forward osmosis fouling experiments with a feed solution containing a mixture of organic foulants (bovine-serum albumin, alginate, and natural organic matter) indicated that the modified membrane exhibited significantly lower water flux decline compared to the pristine TFC membrane. The controlled architecture of the zwitterionic polymer brush via ATRP has the potential for a facile antifouling modification of a wide range of water treatment membranes without compromising intrinsic transport properties.