Polyelectrolyte Multilayer Nanofiltration Membranes Research Articles

Tailoring Polyelectrolyte Multilayer Nanofiltration Membranes by Aerosol-Assisted Printing: Insights into Membrane Formation Mechanisms.

Polyelectrolyte multilayer (PEM) membranes, with advantageous features of versatile chemistry and structures, are driving the development of advanced nanofiltration (NF) membranes with exceptional performance. While developing a printing method holds great promise for the eventual mass production of these membranes, reports on the printing method and the underlying mechanisms of membrane formation are currently scarce. Herein, we develop an aerosol-assisted printing (AAP) system for fabricating PEM NF membranes with highly tunable separation characteristics. Our study unveils the three stages of membrane formation from assembly of polyethylenimine (PEI) and poly(sodium 4-styrenesulfonate) (PSS): aerosol deposition, single PE layer formation, and PEM assembly. The droplet deposition is governed by inertial impaction, and the deposited PEs migrate/entangle to form a single PE layer. The thicknesses of the PE layer and PEM exhibit linear growth as the number of printing scan increases. Furthermore, PE interdigitation forms an effective polymeric network barrier, which increases the resistance to solute and water transport. By manipulating the PE deposition mass and layering, PEM membranes with tunable pore radii (0.40-0.56 nm) and water permeability (5-60 L·m-2·h-1·bar-1) were obtained for various water treatment applications, ranging from micropollutant removal to humic acid filtration. Our study offers valuable mechanistic insights into the PEM formation and precise structural adjustment via printing, thus facilitating scalable manufacturing and widespread applications of the PEM NF membranes.

Cation-modulated permselectivity regulation of polyelectrolyte nanofiltration membranes for water purification

Polyelectrolyte multilayer (PEM) membranes fabricated by layer-by-layer (LBL) assembly have been widely adopted for nanofiltration (NF) due to their unique merits of highly tunable surface charge and pore size. However, current PEM membranes generally suffer from cumbersome preparation procedures and unsatisfying water permeance and/or solute retention rates, which stymie their practical applications. This study discloses a facile and effective method to deliberately regulate the interactions between polyelectrolytes and their LBL assembly behaviors by using cations with various valences and sizes as the modulators. Compared with the acquiescent background electrolyte NaCl, the PEMs modulated by divalent cations (i.e., Ca2+) exhibit intensified surface electronegativity with dense packing structures, and the resulting PEM membranes thus show high rejections for both inorganic salts (i.e., 94.0 % for Na2SO4) and negatively charged emerging micropollutants (EMPs) (i.e., 94.9 % and 99.9 % for tetracycline hydrochloride and perfluorododecanoic acid, respectively). In contrast, monovalent cations (i.e., Li+) lead to the formation of PEMs with loose structures and strong surface electronegativity, which endow the PEM membranes with a high water permeance of 15.0 LMH/bar and slightly reduced solute rejections. Nevertheless, the retention rates of Na2SO4 and perfluorooctanoic acid still surpass those obtained by the NaCl-modulated PEM membrane. Interestingly, trivalent cations such as Fe3+ and Al3+ are unable to facilitate the formation of an intact PEM. Characterization data and theoretical models imply that cations play a pivotal role in the stacking structure of PEMs through synergistically modulating polyelectrolyte-counterion interactions and the overcompensation and subsequent site diffusion of polyanions. This work demonstrates an effective approach to finely tune the nanostructure and permselectivity of the PEMs, which provides a valuable platform for the rational design of PEM NF membranes for effective separations of salts and EMPs.

Enhanced Mg2+/Li+ separation performance of polyelectrolyte multilayers nanofiltration membranes modified by trimethylamine N-oxide zwitterions

Polyelectrolyte multilayer nanofiltration (NF) membranes have garnered attention for their potential in Mg2⁺/Li⁺ separation applications, particularly due to their positively charged nature and the versatility offered by the layer-by-layer (LbL) self-assembly technique. However, these membranes often face significant challenges in balancing high ion selectivity with adequate water permeability. Addressing these limitations, this study introduces a novel approach by grafting a zwitterionic material, trimethylamine N-oxide (TMAO), onto polyacrylamide hydrochloride (PAH), creating a new cationic polyelectrolyte (TPAH). By cyclically depositing sodium polystyrene sulfonate (PSS) and TPAH onto a polyethersulfone (PES) substrate, we have successfully fabricated a high-performance NF membrane (i.e., (PSS/TPAH)n) that demonstrates enhanced Mg2⁺/Li⁺ selectivity with high water permeability. TMAO features shorter distances between its positive and negative charge groups and smaller dipoles compared to other zwitterions. These characteristics enhance its ability to resistance to salt effects and transfer charge from amine oxide to water molecule. Furthermore, the grafting of TMAO enhanced the hydrophilicity of TPAH and regulated the charge distribution of the polyelectrolyte. Compared to the control membrane (PSS/PAH)n, the optimized membrane (PSS/TPAH)n showed reduced rejection of LiCl from 60.9 ± 2.7 % to 35.9 ± 1.5 %, while that of MgCl2 increased from 94.3 ± 1.6 % to 96.1 ± 0.7 %. Additionally, water permeability improved from 9.2 ± 0.9 LMH/bar to 16.1 ± 0.5 LMH/bar. Notably, the membranes exhibited an improved selectivity for Mg2+/Li+ ions in synthetic saline, reaching a maximum of 30.5 ± 1.5, highlighting its potential for practical separation applications. Density functional theory simulations indicate that TMAO not only enhances Donnan effect and intensify ion dehydration of polyelectrolyte multilayers NF membranes but also increases their hydrophilicity. In general, these polyelectrolyte multilayers NF membranes exhibit considerable promise for multiple applications, such as seawater pretreatment, groundwater softening and lithium extraction.

Retention of micropollutants by polyelectrolyte multilayer based hollow fiber nanofiltration membranes under fouled conditions

To meet the increasing global water demand, local solutions, such as the reuse of greywater, are becoming of greater importance. While greywater reclamation has been implemented, full-scale adaptation has been limited by the occurrence of micropollutants in treated greywater effluent. To reduce the risks posed by micropollutants, advanced post-treatment is required. One promising technology is the implementation of hollow fiber nanofiltration, which has proven to effectively remove micropollutants. However, the effect of fouling on the overall membrane performance has received limited attention. In this research, the effect of fouling on the performance of polyelectrolyte multilayer nanofiltration membranes was evaluated. Filtration experiments were performed using hollow fiber nanofiltration membranes using model solutions and greywater effluent. Synthetic greywater effluent with different model foulants was implemented to simulate bio- and colloidal fouling. Afterwards, the fouling propensity of greywater effluent from a source-separated treatment plant was evaluated. The performed experiments demonstrated that the studied hollow fiber nanofiltration membranes are mainly sensitive to biofouling. Permeability loss was observed in all foulant experiments, and overall, sodium alginate most severely reduced the flux. Regardless of the loss in permeability, micropollutant retention remained relatively constant, demonstrating the potential suitability of hollow fiber nanofiltration membranes in greywater reclamation schemes.

Progress towards Stable and High-Performance Polyelectrolyte Multilayer Nanofiltration Membranes for Future Wastewater Treatment Applications

The increasing demand for nanofiltration processes in drinking water treatment, industrial separation and wastewater treatment processes has highlighted several shortcomings of current state-of-the-art thin film composite (TFC NF) membranes, including limitations in chemical resistance, fouling resistance and selectivity. Polyelectrolyte multilayer (PEM) membranes provide a viable, industrially applicable alternative, providing significant improvements in these limitations. Laboratory experiments using artificial feedwaters have demonstrated selectivity an order of magnitude higher than polyamide NF, significantly higher fouling resistance and excellent chemical resistance (e.g., 200,000 ppmh chlorine resistance and stability over the 0-14 pH range). This review provides a brief overview of the various parameters that can be modified during the layer-by-layer procedure to determine and fine-tune the properties of the resulting NF membrane. The different parameters that can be adjusted during the layer-by-layer process are presented, which are used to optimize the properties of the resulting nanofiltration membrane. Substantial progress in PEM membrane development is presented, particularly selectivity improvements, of which the most promising route seems to be asymmetric PEM NF membranes, offering a breakthrough in active layer thickness and organic/salt selectivity: an average of 98% micropollutant rejection coupled with a NaCl rejection below 15%. Advantages for wastewater treatment are highlighted, including high selectivity, fouling resistance, chemical stability and a wide range of cleaning methods. Additionally, disadvantages of the current PEM NF membranes are also outlined; while these may impede their use in some industrial wastewater applications, they are largely not restrictive. The effect of realistic feeds (wastewaters and challenging surface waters) on PEM NF membrane performance is also presented: pilot studies conducted for up to 12 months show stable rejection values and no significant irreversible fouling. We close our review by identifying research areas where further studies are needed to facilitate the adoption of this notable technology.

Dealcoholization of Unfiltered and Filtered Lager Beer by Hollow Fiber Polyelectrolyte Multilayer Nanofiltration Membranes-The Effect of Ion Rejection.

Membrane-based beverage dealcoholization is a successful process for producing low- and non-alcoholic beer and represents a fast-growing industry. Polyamide NF and RO membranes are commonly applied for this process. Polyelectrolyte multilayer (PEM) NF membranes are emerging as industrially relevant species, and their unique properties (usually hollow fiber geometry, high and tunable selectivity, low fouling) underlines the importance of testing them in the food industry as well. To test PEM NF membranes for beer dealcoholization at a small pilot scale, we dealcoholized filtered and unfiltered lager beer with the tightest available commercial polyelectrolyte multilayer NF membrane (NX Filtration dNF40), which has a MWCO = 400 Da, which is quite high for these purposes. Dealcoholization is possible with a reasonable flux (10 L/m2h) at low pressures (5-8.6 bar) with a real extract loss of 15-18% and an alcohol passage of ~100%. Inorganic salt passage is high (which is typical for PEM NF membranes), which greatly affected beer flavor. During the dealcoholization process, the membrane underwent changes which substantially increased its salt rejection values (MgSO4 passage decreased fourfold) while permeance loss was minimal (less than 10%). According to our sensory evaluation, the process yielded an acceptable tasting beer which could be greatly enhanced by the addition of the lost salts and glycerol.

Relating uncharged solute retention of polyelectrolyte multilayer nanofiltration membranes to effective structural properties

A novel way of making Nanofiltration (NF) membranes is to apply the Layer-by-Layer method, where polyelectrolytes are alternatingly coated on top of a porous Ultrafiltration membrane to form a separation layer with a controllable thickness in the nanometer range, also known as Polyelectrolyte Multilayer (PEM). An important precondition to make use of the variety this fabrication method offers for membrane optimization is knowledge of design rules. Therefore, the structural properties of PEMs and their relation to both coating conditions and membrane performance is an ongoing field of research. In this work, the separation performance of NF PEM membranes, based on PDADMAC/PSS and PAH/PAA, towards uncharged molecules is related to PEM structure. The structure of the membrane is represented by a nanoporous film with distribution in pore size. As, up to date, no experimental technique is available to directly measure pore sizes in the nanometer range of a wet film, a representative pore size distribution is estimated via the use of a theoretical transport description fitted to experimental data. Here, the studied PEM systems showed distinctive differences in both film thickness (PDADMAC/PSS: 44nm/BL, PAH/PAA: 689nm/BL) and mean pore size (PDADMAC/PSS: 0.44 nm, PAH/PAA: 0.27 nm). Within the range of layer numbers, the pore size of both PEMs in the layer dominated regime was independent of layer number. This indicates that there is an optimum layer number for PEM NF membranes regarding uncharged solute retention. Surprisingly, despite forming much thicker layers the PAH/PAA system closes off the support membrane pores at a higher bilayer number.

A high stability GO nanofiltration membrane preparation by co-deposition and crosslinking polydopamine for rejecting dyes.

In order to improve the stability of nanofiltration membrane in separation and purification, a novel polyelectrolyte multilayer nanofiltration membrane was facilely prepared by co-deposition of polydopamine (PDA) and polyethyleneimine (PEI) on the polyethersulfone (PES) ultrafiltration membrane substrate, followed by immersing graphene oxide (GO) solution, and crosslinking PDA. The modified surfaces were characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM), water contact angle, their saline flux and ability to reject salt and dye were determined. The results also exhibited salt rejection ability as Na2SO4 > K2SO4 > MgSO4 > NaCl > KCl > MgCl2, suggesting the higher rejection of divalent anion. Also, the retention order of the dye by the GO modified membrane is DY86 > DB19 > AG27 > DY142 > DB56 > AR151 > VB5, indicating that the GO modified membrane has better rejection of negatively charged dyes as well as higher molecular weight dyes. Ethanol and hypochlorite resistance tests under different pH conditions showed the membranes coated GO enhanced stability in regard to salt rejection properties. Significantly, the anti-biological test confirmed the growth rate of microalgae on the GO introduced membrane was decreased greatly due to enhanced stability and lower roughness.

Relating Uncharged Solute Retention of Polyelectrolyte Multilayer Nanofiltration Membranes to Effective Structural Properties

Relating Uncharged Solute Retention of Polyelectrolyte Multilayer Nanofiltration Membranes to Effective Structural Properties

Influence of Molecular Weight on the Performance of Polyelectrolyte Multilayer Nanofiltration Membranes

Polyelectrolyte multilayers (PEMs) are highly promising selective layers for membrane applications, especially because of their versatility. By careful choice of the types of polyelectrolyte and the coating conditions, the PEM material properties can be controlled to achieve desired separations. Less understood, however, is how the molecular weight (Mw) of the chosen polyelectrolytes (PEs) will impact layer build-up and thus separation properties. In this work, we investigate the influence of Mw on the performance of two types of PEM-based membranes. PEM membranes have been fabricated from low (15–20 kDa) and high (150–250 kDa) Mw poly(allylamine hydrochloride) (PAH), poly(sodium-4-styrenesulfonate)(PSS), and poly(acrylic acid) (PAA) to obtain PAH/PSS- and PAH/PAA-based nanofiltration membranes. For the linear growing PSS/PAH system, with low PE mobility, the Mw is found to influence the pore closure of the support membrane during coating but not its subsequent performance. In contrast, for the exponentially growing PAH/PAA system with a high PE mobility, much stronger effects of Mw are observed. For low-Mw PAH/PAA PEM membranes, separation properties are found that would be expected of a negatively charged separation layer, while for high-Mw PAH/PAA PEMs a positive separation layer is found. Moreover, molecular weight cutoff (MWCO) measurements show that the low-Mw PAH/PAA multilayers are much denser than their high-Mw counterparts. Here the higher mobility of the small PE chains is expected to lead to more optimal binding between the oppositely charged PEs, explaining the denser structure. Lastly, we find that PEM pH stability is lowest for low-Mw PAH/PAA multilayers which can again be attributed to their higher mobility. Clearly, the Mw can significantly influence the separation performance of PEM-based membranes, especially for more mobile PEM systems such as PAA/PAH.

Charge distribution in polyelectrolyte multilayer nanofiltration membranes affects ion separation and scaling propensity

Polyelectrolyte multilayer nanofiltration membranes (PEMMs) achieve tailor-made rejection and selectivity of ions for water treatment applications through a layer-by-layer coating procedure, in which a charged support membrane surface is sequentially contacted with positively and negatively charged polyelectrolytes. This results in the adsorption and formation of such selective multilayer membrane skins with defined molecular compositions. The selective properties of the PEMM depend on the intrinsic properties of the respective layers. Today’s research efforts aim to correlate the membrane’s selective characteristics, its structural parameters, and the operating conditions to a model representation of the membrane’s properties. We use our previously published pEnPEn model, which solves the pressure (p) driven transport of ions through n electrolyte layers (En) and n polyelectrolyte layers (PEn). Here, we expand the model to predict the multi-ionic pressure-induced transport through PEMMs solving one-dimensional Nernst–Planck–Poisson equations. The simulations quantify the influence of asymmetric charge distributions and individual PE layers on the ion selectivity for multi-ion solutions. These asymmetric layer properties represent the nanometer-scale membrane properties emerging from the ionic crosslinking, fixed charge compensation, and overcompensation. The model gives insight into each ion’s concentration profile for n layers of electrolyte and n layers of polyelectrolytes. Now, multi-ion compositions inside and outside of the membrane are simulated, and it is shown that the membrane charge distribution even influences the onset of scaling at the fluid membrane interface. As pEnPEn provides a detailed understanding of the rejection and selectivity characteristics as a function of membrane flux and feed concentration including feed side concentration polarization, it can now predict flux-scaling boundaries for the different membrane charge distributions, making it a powerful tool for choosing process parameters and even for designing tailored PEMMs for specific separation tasks.

Fast and Versatile Pathway in Fabrication of Polyelectrolyte Multilayer Nanofiltration Membrane with Tunable Properties

Thin film composite nanofiltration (NF) membranes are relatively new membranes compared to other types of pressure-driven membranes. However, they attract interest from researchers due to their versatility to be used in various applications. In this work, a new class of NF membrane was successfully fabricated through spin-assisted layer-by-layer assembly by depositing alternate layers of branched polyethylenimine (PEI) and poly(sodium 4-styrenesulfonate) (PSS) on ultrafiltration polysulfone (PSF) membrane. The suitability of the fabricated membranes for removal of divalent ions was investigated. It was found that the membrane consisting of (PEI/PSS)10–0.05 M NaCl showed MgCl2 rejection rate of 93.95% and permeation flux of 0.9 L/m2·h bar during tests performed using a crossflow permeation cell at a crossflow velocity of 0.65 m/s, MgCl2 feed concentration of 6530 ppm, pressure of 10 bar, temperature of 32.5°C, and pH of 6.5. This result suggests that this new fabrication method is suitable for producing polyelectrolyte multilayered (PEM) NF membranes that exhibit comparable membrane performance to commercial ones.

Removal of Emerging Wastewater Organic Contaminants by Polyelectrolyte Multilayer Nanofiltration Membranes with Tailored Selectivity

Emerging organic contaminants (EOCs) discharged from wastewater effluents into drinking water resources are of growing concern for human health and the environment. In this study, we demonstrate th...

Mechanism of Permselectivity Enhancement in Polyelectrolyte-Dense Nanofiltration Membranes via Surfactant-Assembly Intercalation.

Enhancing the water permeance while maintaining the solute rejection of a nanofiltration (NF) membrane can potentially result in significant cost-reduction for NF-a membrane process that excels in several unique environmental applications of growing interests. In this work, we demonstrate for the first time that intercalation of surfactant self-assemblies in the polyelectrolyte multilayer (PEM) can lead to significant performance enhancement of salt-rejecting dense NF membranes fabricated using layer-by-layer assembly of polyelectrolytes. Specifically, the intercalation of sodium dodecyl sulfate (SDS) bilayers in a PEM comprising poly(diallyldimethylammonium chloride) (PDADMAC) and poly (sodium 4-styrenesulfonate) (PSS) resulted in a decrease in PEM thickness, increase in pore size, and a smoother and more hydrophilic surface. The water permeance of the resulting PEM NF membrane increased by 100% without compromising the rejection of Na2SO4. Experiments with a quartz crystal microbalance also provide direct evidence that the intercalation of the surfactants substantially reduces the subsequent adsorption of the polyelectrolytes of a similar charge. Based on its mechanism of performance enhancement, surfactant intercalation may become a universally applicable and highly cost-effective approach for dramatically enhancing the performance of PEM NF membranes.

In-situ monitoring of polyelectrolytes adsorption kinetics by electrochemical impedance spectroscopy: Application in fabricating nanofiltration membranes via layer-by-layer deposition

We herein report a novel approach based on electrochemical impedance spectroscopy (EIS) for in-situ monitoring of the adsorption kinetics in preparing polyelectrolyte multilayer nanofiltration membranes using layer-by-layer (LbL) deposition. Unlike existing methods for monitoring adsorption kinetics, this new approach is non-destructive and applicable to various substrates (as it does not use the substrate as a sensor). The model nanofiltration membrane used in this study is prepared by alternate depositions of Poly(diallyldimethylammonium chloride) and Poly(sodium 4-styrenesulfonate) on a polyacrylonitrile ultrafiltration membrane as the support. In each deposition step, the EIS measurements yield two important parameters, including the interfacial layer solution resistance and the film resistance, for probing the extent of polyelectrolyte deposition and membrane performance. The extent of polyelectrolyte deposition as probed by the EIS measurements is well corroborated by independent measurements of the membrane surface potential and the nanofiltration performance including water permeability and Na2SO4 rejection. This EIS-based approach enables the optimization of membrane fabrication using LbL deposition by conveniently identifying the minimum deposition time required to attain surface saturation.

On the long-term pH stability of polyelectrolyte multilayer nanofiltration membranes

Long-term pH stability is critical for nanofiltration membranes in many applications, e.g. dairy and mining industry. We present a systematic study on the long-term pH stability of four different polyelectrolyte multilayer (PEM) nanofiltration membranes. The stability was assessed by comparing their performance before and after exposure to up to 1 M HNO3 (~pH 0) and 1 M NaOH (~pH 14), in terms of pure water permeance (PWP), salt retention, and molecular weight cut-off (MWCO).Poly(diallyldimethylammonium chloride) (PDADMAC)/poly(styrenesulfonate) (PSS) nanofiltration membranes show excellent stability under extreme acidic and basic conditions for more than 2 months (10.7 L m−2h−1bar−1 PWP, 95.5% MgSO4 retention, 279 g mol−1 MWCO), attributed to the use of strong polyelectrolytes, of which the charge is unaffected by pH. Poly(allylamine hydrochloride) (PAH)/PSS membranes show stable performance when exposed to extreme acidic conditions (9.7 L m−2h−1bar−1 PWP, 97.5% MgSO4 retention, 249 g mol−1 MWCO). Under these conditions, PAH remains charged and therefore a stable multilayer is maintained. PDADMAC/poly(acrylic acid) (PAA) and PAH/PAA membranes are not stable at extreme pH conditions.These results highlight that PEM nanofiltration membranes, especially PDADMAC/PSS membranes, have tremendous potential for use at extreme pH conditions. Compared to most commercially available membranes they have superior long-term stability and very relevant performance.

Unraveling the effect of charge distribution in a polyelectrolyte multilayer nanofiltration membrane on its ion transport properties

Polyelectrolyte (PE) multilayer nanofiltration membranes are composite membranes obtained by layer-by-layer (LbL) adsorption/advection of oppositely charged polyelectrolytes. The mass transport properties of such polyelectrolyte multilayer membranes (PEMMs) strongly depend on membrane structural parameters related to the synthetic preparation conditions as well as on the operating conditions. Understanding the relationship between such structural features and transport properties remains a difficult question to answer. We propose a one-dimensional numerical simulation framework solving the Nernst-Planck-Poisson equations for the transport of ions through n electrolyte layers En and n polyelectrolyte layers PEn - coining this pressure (p) driven transport model as pEnPEn. We utilize different EnPEn-architectures of this model to interpret experimental data of new LbL-membranes. The proposed model framework systematically evaluates the impact of the effect of polycation/polyanion architecture with respect to the rejection of symmetric and asymmetric salts. pEnPEn can be tuned to reveal and explain the influence of the ionic crosslinking, charge compensation, and overcompensation. The model enables to formulate an effective charge distribution for different polyelectrolyte multilayer (PEM) structures. As such, the model framework gives insightful details on ion rejection phenomena, yet it will be a prerequisite and will become even more valuable when combined with neural network modeling based on a hybrid data set combining pEnPEn simulations with experimental input parameter.

Energy barriers to anion transport in polyelectrolyte multilayer nanofiltration membranes: Role of intra-pore diffusion

We investigated the relative contributions of intra-pore diffusion (via membrane thickness) and partitioning into nanofiltration (NF) membrane pores (via membrane pore size and ion hydration energy) to the apparent energy barriers for ion transport in NF membranes. Using polyelectrolyte layer-by-layer assembly, we independently altered NF membrane thickness as well as membrane pore size and then determined the apparent energy barriers to bromide and fluoride transport through the fabricated membranes. Membrane thickness and pore size were estimated using an AFM scratch technique and the hydrodynamic pore transport model, respectively. By increasing the number of polyelectrolyte bilayers from four to ten, the polyelectrolyte film thickness increased from 28 to 77 nm, while the apparent energy barriers to bromide transport through the membranes with four, seven, and ten bilayers were negligibly affected (4.4, 3.4, and 3.9 kcal mol−1, respectively, at 1.7 bar). Instead, we found that solute flux and the apparent energy barriers to ion transport were significantly affected by both membrane pore size and ion hydration energy. Overall, our results support the traditional energy barrier-based models for ion transport in membranes and the recently proposed notion that ion dehydration at the solution-membrane interface is the rate-limiting step during transport through NF membranes.

Controlling pore structure of polyelectrolyte multilayer nanofiltration membranes by tuning polyelectrolyte-salt interactions

Nanofiltration membranes have limited ion-ion selectivity in water treatment applications, especially when separating ions with similar size and charge. To achieve greater size-based selectivity in nanofiltration, more control of pore structure is required during membrane fabrication. We demonstrate how to tailor membrane pore size and thickness using polyelectrolyte layer-by-layer assembly by alternately applying two strong polyelectrolytes, PDADMAC and PSS, to a polysulfone substrate while systematically controlling the polyelectrolyte and salt concentrations in the deposition solution. Results suggest that increasing polyelectrolyte concentration or salt concentration in the deposition solution increases polyelectrolyte multilayer thickness, but the effects on pore size may be categorized into two distinct regimes. In the first growth regime, increasing polyelectrolyte concentration in the deposition solution led to larger polymer deposition rates and smaller pore sizes. In the second growth regime, increasing polyelectrolyte concentration produced larger pore sizes. We attribute the second regime to less adsorbed polyelectrolyte on the membrane and/or less coiled polymer chains as a result of changing polyelectrolyte-salt interactions. Overall, results show that pore size modification is achievable using layer-by-layer assembly by tuning polyelectrolyte-salt interactions and can be used to study and improve size-based selectivity in membrane separation processes.

Selective removal of divalent cations by polyelectrolyte multilayer nanofiltration membrane: Role of polyelectrolyte charge, ion size, and ionic strength

We fabricated polyelectrolyte multilayer (PEM) nanofiltration (NF) membranes using a layer-by-layer (LbL) method for effective removal of scale-forming divalent cations (Mg2+, Ca2+, Sr2+, and Ba2+) from feedwaters with different salinities. Two polymers with opposite charges, polycation (poly(diallyldimethylammonium chloride), PDADMAC) and polyanion (poly(sodium 4-styrenesulfonate), PSS), were sequentially deposited on a commercial polyamide NF membrane to form a PEM. Compared to pristine and PSS-terminated membranes, PDADMAC-terminated membranes demonstrated much higher rejection of divalent cations and selectivity for sodium transport over divalent cations (Na+/X2+) due to a combination of both Donnan- and size-exclusion effects. A PDADMAC-terminated membrane with 5.5 bilayers exhibited 97% rejection of Mg2+ with selectivity (Na+/Mg2+) greater than 30. We attribute the order of cation rejection (Mg2+ > Ca2+ > Sr2+ > Ba2+) to the ionic size, which governs both the hydration radius and hydration energy of the cations. The ionic strength (salinity) of the feed solution had a significant influence on both water flux and cation rejection of PEM membranes. In feed solutions with high ionic strength, abundant NaCl salt screened the charge of the polyelectrolytes and led to swelling of the multilayers, resulting in decreased selectivity (Na+/X2+) and increased water permeability. The fabricated PEM membranes can be potentially applied to the pretreatment of mild-salinity brackish waters to reduce membrane scaling in the main desalination stage.