Dual-Charged Hollow Fiber Membranes for Low-Pressure

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Dual-Charged Hollow Fiber Membranes for Low-Pressure Nanofiltration Based on Polyelectrolyte Complexes: One-Step Fabrication with Tailored Functionalities Cristina Veronica Gherasim,*,†,‡ Tobias Luelf,†,‡ Hannah Roth,†,‡ and Matthias Wessling*,†,‡ †

DWI Leibniz Institute for Interactive Materials Research at RWTH Aachen, Forckenbeckstrasse 50, 52074 Aachen, Germany Department of Chemical Process Engineering, RWTH Aachen University, Turmstrasse 46, 52064 Aachen, Germany

‡

S Supporting Information *

ABSTRACT: A new nanofiltration (NF) hollow fiber membrane is developed by using two oppositely charged polyelectrolytes coagulating into a polyelectrolyte complex (PEC) onto polyether sulfone base polymer. The particular membrane architecture emerges during a single-step procedure, allowing setting both the porous negatively charged support of the hollow fiber and the separation layer containing also the positive polyelectrolyte (PEI/PDADMAC) through a single layer dry-jet wet spinning process. The novelty is two-pronged: the composition of the hollow fiber membrane itself and its fabrication procedure (one-step fabrication of membranes employing polyelectrolytes). These result in highly permeable hollow fiber membranes with a stable separation layer and performance at par with the membranes reported in literature obtained by multistep processes. More importantly, the membranes are obtained through a simple, very fast (one-step), and less expensive procedure. The best performance among these newly obtained hollow-fiber membranes is achieved by PD5% hollow fiber (MWCO of 300 Da), which showed 7.6 L/m2·h· bar permeability and ∼90% rejection of MgCl2, MgSO4, and Na2SO4 at 2 bar pressure. Thus, the resulting membranes not only have the advantages of the hollow-fiber configuration, but perform very well at extremely low pressures (the lowest reported in the literature). The broad impact of the results presented in this Article lies in the potential to dramatically reduce both the fabrication (duration and complexity) and the price and desalination costs of highly performing NF hollow fiber membranes. These might result in interesting potential applications and open new directions toward designing efficient functional NF hollow fibers for water desalination. KEYWORDS: hollow fiber membrane, nanofiltration, polyelectrolyte, polyelectrolyte complex, polyether sulfone, PEI, PSS

1. INTRODUCTION Seawater, brackish water, surface water, and even wastewater are increasingly used as water resources in the world with a growing population.1 The occurrence of pollutants in these water sources has become a serious concern during the last few decades.2,3 Membrane processes such as reverse osmosis (RO), nanofiltration (NF), ultralow-pressure reverse osmosis (ULPRO), and electrodialysis (ED) are nonpolluting water treatment techniques applied in water production and reclamation plants to achieve high removals of inorganic ions and organic compounds to provide high-quality pure water.4 NF has the unique feature of assuring high rejection of divalent ions together with only low rejection of the monovalent ions, thus constituting a promising technology for applications as water softening, drinking water production, and treatment of industrial effluents.5−7 Moreover, NF has gained importance over the past decade because it has paramount advantages such as significantly lower energy consumption and higher flux achieved.4,5 Therefore, the development of novel and efficient NF membranes that can be produced in a time and costeffective way is of high importance.8 © XXXX American Chemical Society

So far, the commercially available NF membranes are mainly flat sheet and tubular (diameter 0.5−2 cm), negatively charged, as they are mostly based on polyamide and polyether sulfone (PES). The tubular modules have a low packing density, and the spiral wound modules are subject to problems like floating matters, the impossibility of backwashing, and loss of energy due to the friction in the spacers.9,10 In comparison with these, the hollow fiber membranes (thin fibers with a diameter of 0.5−3 mm) have significant advantages such as the most favorable surface area to volume ratio, good mechanical stability, require less pretreatment and easy maintenance, and they do not need spacers and can be backwashed during the operation.10 On the other hand, there is a need of positively charged NF membranes with small pore size for relevant applications like: heavy metals removal from wastewaters, treatment of wastewaters in pulp and paper industry (by removal of Mg, Ca, and Fe ions), and removal of small Received: May 12, 2016 Accepted: July 6, 2016

A

DOI: 10.1021/acsami.6b05706 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

irregular modification are the most important disadvantages to be mentioned. All of these together with the multistep production process lead to an increase in the production time and costs.11−13 Aiming toward the one-step preparation methods, asymmetric hollow fiber membranes for ultrafiltration (UF) and gas separation were recently obtained in our group by the method called “chemistry in a spinneret”, which integrates into a single step both membrane formation and the cross-linking reaction between the membrane polymer (polyimide P84) and the cross-linking agent poly(ethylene imine) (PEI) dissolved in the bore liquid.35 For NF applications, a recent technique named dual-layer spinning was employed to prepare in a single-step NF hollow fibers containing a separation layer on a porous support by coextrusion of two polymer dope solution through a triple spinneret.36−38 These membranes surpass the drawbacks of time and tedious multistep processes, but exhibit bad adhesion between the layers, overcoming the delamination problem being the subject of further research.38 To summarize, the current state-of-the art highlights the advantages and the demand of hollow fiber membranes, and also the challenging task to produce (charged) NF hollow fibers in a time- and cost-efficient manner. Thus, while 20 m of hollow fiber membrane can be spun in around 1 min, bringing polyelectrolytes onto a support hollow fiber membrane by LbL (alternatively coating and rinsing between layers) or performing surface modification will need much more additional time and add new costs. So, besides the disadvantages of the multistep preparation processes mentioned above, the great advantage of a one-step production process is the speed of the hollow fiber membrane production. In this work, we integrate the advantages of dual-layer spinning, polymer blending, and LbL approaches for the development of a novel strategy to prepare charged NF hollow fiber membranes in a single step process. More specifically, we report for the first time one-step preparation of novel dualcharged hollow fiber membranes based on polyelectrolyte complexes (designated as PEC hollow fiber membranes) with applicability in low-pressure NF, and we show the proof of principle of this new concept. The newly developed membranes consist of a negatively charged support containing polyether sulfone and the polyanion poly(styrenesulfonate) (PSS) and a separation layer consisting of the polyelectrolyte complexes of PSS with the polycations polyethylenimine (PEI) or poly(diallyl dimethylammonium chloride) (PDADMAC). Our fabrication process involves a normal single layer spinning procedure based on the phase inversion process, the membrane separating layer (inside the hollow fiber geometry) not being applied as a separate layer in a further step of the synthesis, but deliberately set in the formation process of the membranes. The NF hollow fibers prepared are tested regarding the flux and separation performance, and comprehensively characterized in terms of charge properties, morphology, and chemical composition of the active separation layer. The insights revealed are used to explain and highlight the advantages and the innovative character of both the hollow fibers and the preparation process developed. Furthermore, this study draws the directions for setting of a general one-step methodology to prepare different types of such charged NF hollow fiber membranes based on polyelectrolyte complexes, which can achieve performances comparable with NF hollow fibers prepared by multistep processes. This one-step preparation procedure allows a lot of flexibility and, together with the

positively charged micropollutants such as pharmaceuticals.2,3,5,11 Hence, there is a growing interest to develop NF hollow fiber membranes as it was reported that a hollow fiber NF module can ensure a 100% higher performance as compared to an optimized spiral wound module.12−14 However, the fabrication of highly charged NF hollow fiber membranes is still a challenge, which might open new directions toward more effective and economically advantageous separations. To date, the only commercially available NF hollow fiber membrane is Pentair X-Flow HFW 1000, which is a PES membrane with a relatively high MWCO of ∼1000 Da. PES has a wide pH tolerance from 2 to 13 and high chlorine resistance, and it is cheap and easily fabricated in various configurations. However, it has limitations as well; they have low-pressure limits and are hydrophobic, which results in reduced flux, and it is more prone to fouling in comparison to the more hydrophilic polymers.8,15−17 During the last years, intensive research was done on the NF hollow fiber preparation, the investigations falling into several main methods. Interfacial polymerization was applied in a similar manner with the preparation of conventional thin film composite (TFC) flat sheet membranes to obtain a polyamide layer on a commercially available ultrafiltration (UF) hollow fiber membrane.13,14,18−20 Blending of charged polymers such as sulfonated poly(ether ether ketone) (SPEEK) with PES,21 or coating a layer of SPEEK onto a commercial UF hollow fibers22,23 were leading to negatively charged NF hollow fiber membranes; the preparation of positively charged NF hollow fiber membranes by chemical modification was also reported.14,24 Another modification method intensively explored recently is the layer-by-layer (LbL) technique. It allows building of dense polyelectrolyte multilayer films (PEMs) onto UF membranes by electrostatic adsorption when alternately filtering solutions of polyanions and polycations through the porous flat sheet25−27 or even recently through hollow fiber UF membranes.28−31 This is a versatile method, which allows one to obtain both positively and negatively charged membrane surfaces depending on the terminating polyelectrolyte layer, and also to tune the membrane properties by selecting desired polyelectrolytes.30 While LbL films result from self-assembling of polyelectrolytes on a surface, by mixing solutions of oppositely charged polyelectrolytes, the electrostatic interactions lead to the formation of bulk polyelectrolyte complexes (PECs).32 On the basis of this phenomenon, a recent multistep approach was proposed for preparing flat sheet NF membranes based on polyelectrolyte complexes (PECs). In this method, the PECs once prepared are filtered and dried, then redispersed in NaOH at room temperature for 48 h followed by addition of curing agents and cross-linkers; this solution is subsequently casted onto a UF membrane, which is finally cured for undergoing the cross-linking.33,34 The resulting NF membranes were showing good separation performance, mass transfer properties, and stability; however, this multistep method is restricted to flat sheet configuration. All of these techniques have, however, obvious drawbacks: time and energy consumption due to the multistep fabrication, requires large amounts of water, washing baths and drying ovens, and, in most cases, resulting membranes have also poor stability.8,9 Additionally, the support membranes need to be assembled into modules first, and then the modification is operated; defects that might occur during module preparation, pressure losses, and differences in concentration of the reagents along the module that result in B


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ACS Applied Materials & Interfaces Table 1. Preparation Parameters of the PEC Hollow Fiber Membranes R dope composition (wt %) bore fluid composition (wt %) external coagulant dope flow rate (mL/min) bore flow rate (mL/min) air gap (cm) take-up speed (m/min) spinneret dimension (mm)

PEI5%

PES/PSS/NMP/glycerol/PEG 400 = 17/4/68/9/2 NMP/H2O/glycerol 50/25/25 NMP/H2O/PEI 55/40/5 water water 2.5 2.5 2 2 6 6 6 6 i.d. 0.8; o.d. 1.5

PEI10%

PD5%

NMP/H2O/PEI 50/40/10 water 2.5 2 6 6

NMP/H2O/PDADMAC 50/45/5 water 2.5 2 6 6

detailed spinning conditions are listed in Table 1, and a schematic representation of the one-step preparation process is shown in Figure S2. After the spinning, the hollow fibers prepared were rinsed with water for 48 h and then post treated by dipping into a 20 wt % glycerol aqueous solution to prevent the pores from collapsing during drying. They were dried in air at room temperature for 2 days afterward. Three groups of PEC hollow fibers were prepared by varying the polycation type and concentration in the bore solution as shown in Table 1. Fiber R was prepared as reference without positive polyelectrolyte in the bore, with the aim to evaluate the influence of the formation of the polyelectrolyte complex layer on the mass transfer and rejection properties. 2.3. Hollow Fiber Characterization. The morphologies of the reference and of the PEC hollow fibers were observed by field emission scanning electron microscopy (FESEM Hitachi S4800). The samples were prepared by cutting the fibers on the length (for the inner surface observation) and by immersing the fibers in liquid nitrogen and subsequently fracturing them (for investigating the crosssection morphology). The samples were dried under vacuum at 30 °C for 2 days and coated with wolfram prior to the observation. The nature and stability of the active inner separation layer were investigated by observation of the phenomena taking place when placing the hollow fiber membranes in NMP and by analyzing the chemical composition of the inner surface (bore side) of the hollow fibers by X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). The hollow fiber membranes were sectioned to expose the inner (bore) side, repeatedly washed with water, and then vacuum-dried for 1 week prior to the measurements. The XPS measurements were carried out in an Ultra AxisTM spectrometer (Kratos Analytical, Manchester, UK). The samples were irradiated with mono energetic Al K*1,2 radiation (1486.6 eV), and the spectra were taken at a power of 144 W (12 kV × 12 mA). The aliphatic carbon at a binding energy of 285 eV (C 1s photo line) was used to determine the charging. XPS spectra were processed with dedicated software, and atomic concentrations of the elements were quantified by integration of the relevant photoelectron peaks, the information depth being about 10 nm for polymers. ATR-FTIR spectra were collected at room temperature by using a Nexus 470 FT-IR spectrometer (Thermo Electron Corp.) combined with a Smart SplitPea ATR-unit (Thermo Nicolet) with Silicon crystal. The spectra were acquired by scanning the range of 600−4000 cm−1 with a spectral resolution of 4 cm−1. The spectra were collected by using FT-IR software Omnic 6.1a (Thermo Nicolet). The charge properties of the inner surface of the hollow fibers were estimated by performing zeta potential measurements (Sur PASS electrokinetic analyzer, Anton Paar, Austria). One 7 cm hollow fiber was mounted in a module, and the space outside the fiber was completely filled with glue to avoid the electrolyte permeation through the membrane pores during the measurements. The modules were equilibrated with the measuring solution 10−3 M KCl for 24 h prior to the measurements. The electrolyte solution was flowing through the bore of the hollow fiber in the module mounted in the measuring cell, automatic titration with 0.1 M HCl and 0.1 M NaOH being performed for assessing the pH dependence of zeta potential in the pH range 3− 11. The principle of the tangential streaming potential created by the flow of the solutions through the lumen of the fiber was employed

applicability for low-pressure NF range, makes these membranes economically attractive in terms of both manufacturing and desalination costs.

2. EXPERIMENTAL SECTION 2.1. Materials. Polyether sulfone (PES, Ultrason E6020 P) was purchased from BASF (Germany), and the solvent N-methyl-2pyrrolidone (NMP) (Across Organics, Germany) was used for the preparation of dope and bore solutions. The polyelectrolytes used, that is, poly(sodium 4-styrenesulfonate) (PSSNa, Mw = 70 000), branched poly(ethylenimine) (PEI, Mw ≈ 750 000) solution 50 wt % in H2O, and high molecular weight poly(diallyldimethylammonium chloride) (PDADMAC, Mw ≈ 400 000−500 000) solution 20 wt % in H2O, were supplied by Sigma-Aldrich (Germany). Their chemical structures are presented in Figure S1. Neutral organic solute polyethylene glycols (PEG) with different molecular weights 200, 300, 400, 600, 1000, 1500, 2000 Da (Merck, Germany) were used for membrane characterization. Glycerol (Roth, Germany) was used as additive and for post-treatment of the membranes prepared. Inorganic salts KCl, NaCl, MgCl2, Na2SO4, and MgSO4(Merck, Germany) were used to evaluate the NF performance and the membranes surface charge by zeta potential measurements. All chemicals used for the preparation of the synthetic solutions were of analytical reagent grade or the highest purity available, and were used as received. The aqueous solutions were prepared by dissolving the reagents in highly demineralized water (conductivity < 1 μS/cm). 2.2. Fabrication of PEC Hollow Fiber Membranes. The polymer (PES) was dried in a vacuum oven at 30 °C for 24 h to remove the moisture. Poly(sodium 4-styrenesulfonate) (PSSNa) was converted into H form (PSS) by ion exchange. After solvent evaporation, the PSS was dried under vacuum at 30 °C for 48 h and further used for the preparation of dope solution. The PEC hollow fibers were prepared by dry jet-wet immersion precipitation spinning, the dope solution consisting of a blend of PES and the negatively charged polyelectrolyte (PSS), the positively charged polyelectrolyte (PEI or PDADMAC) being incorporated in the bore solution. PSS addition into the dope is aimed to both increase the PES hydrophilicity (and thus to increase the permeate flux) and also provide the negative charge to the dope material for building up of resistant polyelectrolyte complex layers. To this aim, we tried to maximize the PSS content of the dope solution while not compromising the mechanical resistance of the hollow fibers fibers (provided by the base polymer PES) and the solution spinability. A dope composition containing 4% PSS was found to answer these needs, and therefore was subsequently used in this study. Table 1 lists the compositions of dope and bore solutions prepared. The dope solution was stirred at room temperature for 24 h to form a homogeneous polymer solution, which was subsequently filtered through 25 μm mesh metal filter and then left for degassing for 24 h. The bore solutions were also left for degassing overnight prior to being used for the spinning. The dope and bore solutions were simultaneously pumped through a double orifice spinneret at certain flow rates, met at the exit of it, and passed together through the air gap before entering the water coagulation bath where the phase separation occurs. The as-spun hollow fibers are then collected by a pulling wheel at a certain take-up speed, which enables also the fibers to stretch. The C


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Figure 1. FESEM images of the hollow fibers R, PEI5%, PEI10%, and PD5%: (a) cross-section; (b) enlarged cross-section; (c) cross-sectional image of the separation layer; and (d) inner surface. here, and the Fairbrother−Masting equation was applied to obtain the zeta potential at the inner surface of the hollow fiber membranes. 2.4. Evaluation of NF Performance. The NF experiments were performed by using membrane modules containing 5 hollow fibers potted together with the effective length of about 20 cm each. The feed solution was circulated in the lumen side of the hollow fibers, the permeate being collected on the outer side of the fibers. Before the first measurement, each module was flushed with water to remove the glycerol used for conditioning the membranes after fabrication, and subsequently compacted for at least 6 h at the maximum pressure used in this study (6 bar). The permeation experiments were carried out at room temperature at applied pressures up to 6 bar. To ensure the reproducibility of the results, all experiments were performed in triplicate by using three individual modules. The pure water flux Jw (L/m2·h) was measured in dead-end mode by using compressed nitrogen for applying the desired pressure. The pure water permeability PWP (L/m2·h·bar) was subsequently calculated from the plot of pure water flux versus applied pressures. The molecular weight cutoff (MWCO), that is, the molecular weight of the solute with 90% rejection, was determined by filtration measurements performed in a lab-bench cross-flow separation unit operated at cross-flow velocity of 1.2 kg/h. The experiments were conducted at 5 bar by filtering an aqueous solution containing a

mixture of PEGs (MW = 200, 300, 400, 600, 1000, 1500, 2000 Da, 500 mg/L each fraction). The compositions of feed and permeate were analyzed with the aid of size-exclusion chromatography (SEC) by using an Agilent 1200 system equipped with a refractive index detector and a multiple-angle laser light scattering (MALLS) detector (DawnEOS, Wyatt Technology). The results were evaluated using the PSS Win GPC UniChrom software (Version 8.1.1), with the PEGs analysis and the calculation of the MWCO from the ratio of the sieve curves of permeate and feed being done following the procedure detailed described elsewhere.39 The ability of the hollow fibers to retain various salts (NaCl, Na2SO4, MgCl2, and MgSO4) was assessed by performing permeation experiments in total recycled mode at pressure up to 6 bar by using a fully automated filtration system OSMO Inspector (Convergence Industry B.V., Netherlands). The feed solutions were circulated at high cross-flow velocity (5 kg/h) and, together with the low pressures used, allow one to ignore the concentration polarization effect. The salt rejection measurements were carried on for 10−3 M single salt solutions, the salt concentration in feed and permeate at various transmembrane pressure being determined by conductivity measurements (Thermo Fischer) after the system reached the steady-state (constant permeate flux and composition, respectively). The NF experiments lasted for 24−48 h to allow the system to reach the D


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ACS Applied Materials & Interfaces steady-state for each pressure step. After each experiment, the membrane modules were cleaned by flushing with demineralized water for 3 h.

membranes prepared with polycations in the bore show clearly defined separation layers. This suggests that the positive polyelectrolytes are retained on the inner surface of the PES/ PSS nascent support after exiting the spinneret due to electrostatic interactions acting at the interface between the negatively charged dope and the positively charged PEI/ PDADMAC in the bore, thus giving rise to a separation layer consisting most likely of a polyelectrolyte complex (see Figure S2). Such unique architecture might be achieved in these innovative PEC hollow fiber membranes due to the fact that our one-step preparation technique allows the oppositely charged polyelectrolytes to coexist in the dissolved state at the interface during the spinning; the results of the subsequent characterization experiments presented in sections 3.3 and 3.4 strongly support this assumption. Moreover, this led us to assume that the dimension of the pores of the negative support (R) is reduced by incorporating the polycation during the spinning, this assumption being also confirmed by the results of the permeation experiments presented in sections 3.2 and 3.5. By analyzing the images (d) in Figure 1, it can be observed that the separation layers (inner surfaces) of the PEI5%, PEI10%, and PD5% hollow fibers are defect-free and nonporous, the PEIcontaining ones showing more dense and wrinkled appearances as compared to the PD5% one, which looks rough and more loose. Surface topology similar to that of PD5% presented in Figure 1d was observed for PSS/PDADMAC layers coated by LbL.29 The morphology of the inner layers of our PEC membranes depicted in Figure 1c,d can be explained by taking into account that PDADMAC is a polyelectrolyte with a low charge density26 and also that the conformation of polyelectrolytes in solutions strongly depends on the solvent quality.43 The polymer chains will adopt a folded (compact) conformation in a bad solvent (to reduce polymer/solvent interactions) and an extended one in a good solvent as they will try to maximize the polymer/solvent interactions.43 PEI is soluble in both water and NMP, and PDADMAC is soluble in water and insoluble in NMP, and our bore solutions contain these polyelectrolytes and more NMP than water (see Table 1). Thus, the low charge density determines smaller intrachain repulsion forces and, together with the insolubility in NMP, lead to a more loopy (folded) structure of the PDADMAC chains in the bore solution as compared to PEI ones. Therefore, while flowing the bore solution containing PDADMAC along the dope containing PSS into the nascent hollow fiber during the spinning process, the assembly of the PDADMAC chains takes place quickly and results in an irregular arrangement of the loopy polyelectrolyte chains, thus making the surface rough as observed in Figure 1d, PD5%. By contrast, the assembly of the branched PEI chains with high charge density and more extended conformation leads to a smoother peak−valley structure as shown in Figure 1d, PEI5%, and Figure 1d, PEI10%. It can be also noticed that the inner layer of the PEI10% is smoother than that of the PEI5% hollow fiber. This shows that using more PEI in the bore solution allows the deposition into the valley parts of the structure and thus results in a smoother surface for PEI10% as compared to PEI5% hollow fibers. Other researchers also observed a similar behavior of smoothing the surface when increasing the number of layers in LbL deposition of PSS and poly(allylamine hydrochloride) (PAH).29 The membranes spun with PEI in the bore have separation layers of thicknesses varying between 0.5 and 1.0 μm (PEI5%) and between 1 and 2 μm (PEI10%), while PD5% fibers prepared with PDADMAC have a separation layer with thickness varying

3. RESULTS AND DISCUSSION 3.1. Morphology of the PEC Hollow Fibers. Figure 1 shows the morphology of the PEC hollow fibers prepared by varying the positive polyelectrolyte type and concentration in the bore solution as well as the reference membrane R. The hollow fiber membranes prepared have internal diameters in the range of 670 μm (R) and 560−600 μm (PEI5%, PEI10%, and PD5%), and outer diameters of 900−970 μm. The cross-sectional images in Figure 1a show that the reference fiber (R) has a round inner channel with only imperceptible corrugations, while the fibers spun with positive polyelectrolyte in the bore exhibit a more irregular inner channel with wavy inner walls. The obtaining of such an irregular fiber cross section is known to be one of the major instabilities encountered in the fiber spinning, and it was comprehensively investigated by Bonyadi et al.40 It was shown that various parameters like dope concentration, bore fluid composition, the air gap distance, the external coagulant, and the take-up speed determine instabilities leading to the deformed cross section of the hollow fibers fabricated by dry jet-wet spinning. In our case, it seems that a new factor, that is, the electrostatic interactions between the oppositely charged dope and bore solutions during the spinning and phase inversion process, play the major role in the corrugation phenomena occurring in the inner contour of the PEC hollow fibers. However, other factors might also interfere with it, as it is known that the mechanism of formation of irregular internal contours is complicated and challenging13 even without considering the electrostatic interactions, which add as a new factor in the case of the PEC hollow fibers. It was suggested that hollow fibers with wavy inner wall could be preferable as they have a high mass transfer area per unit volume, and also the wavy shape might increase the flow turbulence and decrease the concentration polarization;40 it is also known that the rough surfaces can increase the fouling propensity.41,42 On the other hand, one strategy to obtain antifouling membranes is the introduction of hydrophilic groups in the membrane,17,41,42 PEG and PEI (both contained in our PEC membranes) being reported to be very effective.41 The fouling in NF is very complex, that is, organic solutes fouling, colloidal fouling, scaling, biofouling, and additionally occurring at nanoscale and by different mechanisms for various foulants and membrane materials.42 One can assume that the behavior of our membranes in long term fouling experiments would be an interplay between these complex factors with opposite influences, which might be an interesting subject for an extensive investigation in a future work. Analyzing the enlarged cross-sectional images of the fibers presented in Figure 1b, it can be observed that all fibers have a wall structure consisting of two layers of parallel finger-like voids situated at the inner and outer surfaces, the layers being separated by a sponge-like structure. Therefore, the instantaneous phase inversion occurs during the precipitation step as the solvent NMP and water have a strong mutual affinity.8 Figure 1b,c highlights that all hollow fibers have a good internal porosity with interconnected open-cell pores, which is favorable for water transfer through the membranes. Figure 1c,d also shows that the reference membrane (R) has no distinct separation layer in the bore side, while the E


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ACS Applied Materials & Interfaces from 0.5 to 1.5 μm. Therefore, the thickness of the separation layer is proportional to the PEI concentration in the bore solution for the membranes PEI5% and PEI10%. Additionally, it can be observed that the PEI-containing inner separation layers penetrate more into the porous PES/PSS support as compared to the PDADMAC-containing layer. These observations suggest that chains (branches) of the hyperbranched PEI may interdiffuse more or even might enter the nascent pores during their formation process by phase inversion. Menne et al. reported a thickness of the separation layer of about 140 nm for LbL deposition of 8 bilayers of PDADMAC/PSS onto a UF hollow fiber membrane support.29 Thus, our preparation procedure allows us to obtain in only one step a significantly thicker separation layer, and thus a better coverage of the pores of the “support”, which can expect to result in retention performance suitable for NF applications. On the basis of the findings above, one can suggest that our single-step preparation of an innovative PEC hollow fiber is a faster, simpler, and cheaper process, which shows significant advantages and might have promising perspectives toward important practical applications. 3.2. Permeability and MWCO. The hollow fibers prepared were characterized regarding the pure water permeability (PWP) and subsequently for uncharged solutes rejection by using a mixture of PEGs with molecular weights in the range of 200−2000 Da. Table 2 summarizes these transport characteristics.

membranes show a denser inner separation layer and have lower permeability. These results correlate with our previous observations and hypotheses in section 3.1 to explain this MWCO order. Thus, the previous morphological observations allow one to assume that intrusion of PEI branches/molecules into the nascent pores of the PES/PSS support might cap some small pores or decrease the diameter of bigger ones, while the linear and loopy chains of PDADMAC cannot enter the nascent pores and align and assemble at the interface; the entanglement of the buckled chains might create a loose network on the surface (as depicted in Figure 1), a network that hinders the solute transport and allows the water passage, so explaining the low MWCO (about 300 Da) and the high permeability of the PD5% membrane. However, other factors like increasing in the transport resistance due to the differences in the active layer or even pores’ connectivity might also contribute to the permeability and MWCO behavior presented above.44 We highlight that the permeability decreases by a factor of around 4 in the series of the hollow fibers prepared, and, by considering both permeability and MWCO results, it is obvious that the PD5% membrane has the best performance among the PEC hollow fibers prepared. 3.3. Surface Charge of the Hollow Fibers. The separation mechanism in NF is a combination of size exclusion and electrical interactions between the solutes in the feed solution and the charged NF membranes. Therefore, the assessment of the electrical properties of NF membranes is an important step in understanding, applying, and predicting the NF processes. The tangential streaming potential is used for evaluating the electrical properties of membranes without pores or with nanometer-size pores like RO and NF membranes, and it was employed in this study to evaluate the charge properties of the separation layer of the hollow fibers prepared. Figure 2 shows the pH dependence of the surface zeta potential measured in an indifferent electrolyte (10−3 M KCl)

Table 2. Water Permeability (PWP) and Molecular Weight Cut Off (MWCO) of the Hollow Fiber Membranes PWP (L/m2·h·bar) MWCO (Da)a

R

PEI5%

PEI10%

PD5%

28 n.d.

2 1040

0.4 700

7.6 300

a

The feed solution is a mixture of PEG 200, 300, 400, 600, 1000, 1500, 2000, each one 100 mg/L; pressure: 5 bar.

The data in Table 2 show that the hollow fibers’ permeability dramatically decreases for the membranes with PEI-containing inner layers as compared to the PDADMAC-containing ones, both being much less permeable than the reference fiber R. The permeability order R > PD5% > PEI5% > PEI10% is in good agreement with our hypotheses in section 3.1 regarding the decrease in the pore size of the membranes due to the assembly of the positively charged polyelectrolyte during the spinning onto the negatively charged dope; this order is also consistent with the previous observations about the thickness and compactness of the inner active layers of the PEC hollow fibers, as it is known that the separation layer has a paramount contribution to the overall mass transfer resistance. Moreover, the hollow fibers R and PD5% show a high flux, thus confirming that the addition of PSS to the PES dope solution increases the hydrophilicity and subsequently the permeability of the membranes. The MWCO of the membrane R could not be determined as the pores are too big to reach the 90% rejection for the PEGs used, while the membranes spun with positive polyelectrolyte in the bore have MWCOs, which place them in the NF range (see Table 2). The MWCO order for the hollow fibers prepared, R > PEI5% > PEI10% > PD5%, surprisingly differs from the permeability order discussed above, R > PD5% > PEI5% > PEI10%; thus, the PD5% membrane has the lowest MWCO and is coupled with a high permeability, although the PEI

Figure 2. Zeta potential as a function of pH for the hollow fibers prepared (lines connect the points to guide the eye).

for the inner (bore) side of the reference (R) and of the PEC hollow fiber membranes PEI5%, PEI10%, and PD5%. It can be seen that the reference hollow fiber (R) spun without polycation in the bore has a strong negative charge in the range of −30 to −43 eV in the pH range investigated. Also, the PD5% membrane is slightly negatively charged in the whole pH range, and the hollow fibers PEI5% and PEI10% are positively F


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sheet24−27 or hollow fibers having IEP in the 5−8 range and a relatively strong negative charge of about −20 to −40 eV at pH 9.14,28 The results above show that the formation of a positively charged separation layer on a negatively charged support and obtaining charged NF hollow fibers are possible to achieve in only one step by the fabrication process presented here. The hollow fibers have high permeability, especially when PDADMAC is used in the bore; by using PEI the flux reduces, but the hollow fibers are positively charged in a wide pH range (pH < 9). Thus, the formation of a highly positively charged NF hollow fiber membrane or that of a membrane with high flux and low MWCO can be also tuned in this one-step process. 3.4. Chemistry of the Hollow Fiber Membranes. To further investigate and obtain more insight into the properties of the separation layer of the PEC membranes, the stability of the prepared hollow fibers was investigated by immersing them in NMP. It has been observed that the reference fiber R completely dissolves immediately, while the PEC hollow fibers spun with positive polyelectrolyte in the bore exhibited all the same behavior, that is, dissolution within seconds of the exterior layer and persistence of a thin inner layer (see Figure S3). These layers maintained the same appearance even after being kept in NMP for 1 month, therefore showing a high stability. This is a surprising result as PES, PSS, and PEI are soluble in NMP and only PDADMAC is insoluble. Additionally, it can be observed that the separation layers are slightly transparent, the PEI-containing layers looking denser than the PDADMACcontaining one; the layer of PEI10% is denser than that of PEI5%, which is consistent with the amount of PEI in the bore solutions used for spinning. The visual aspect of the inner layers is in perfect agreement with the morphological characterization in section 3.2 and confirms our assumptions regarding the assembling of the PDADMAC chains onto the PES/PSS dope in a more open structure as compared to the PEI ones; at the same time, the more loose and thin layer that remained after dissolution of PD5% can explain the slightly negative surface charge, while the denser layers might lead to positive surfaces of PEI5% and PEI10% as revealed by the zeta potential measurements. To elucidate both the phenomena leading to such stable separation layers and also their intimate structure, we further conducted XPS analyses on the inner surface of the hollow fibers. The XPS survey spectra acquired are shown in Figure S4, and the quantitative elemental compositions obtained by integration of the elements peaks are presented in Table 3.

charged in a wide pH range with an isoelectric point (IEP) at about pH 9. A pure PES hollow fiber membrane has a negative charge around −5 to −10 eV.28 Therefore, the addition of PSS into the dope solution allows one to obtain a strongly negatively charged hollow fiber R, thus a support that strongly interacts with the polycation in the bore during the spinning of the PEC hollow fibers PEI5%, PEI10%, and PD5%. Indeed, Figure 3 shows that the zeta potential of the hollow fibers PEI5%,

Figure 3. Effect of transmembrane pressure on Na2SO4 rejection and permeate flux of the hollow fiber membranes (feed solutions with initial concentration of 10−3 M; lines connect the points to guide the eye).

PEI10%, and PD5% increases to around −2 to −12 eV for the hollow fiber PD5% and rises to positive values until around +20 eV for the membranes spun with PEI-containing bore solutions (PEI5%, PEI10%). The surface charge usually alternates between positive and negative after deposition of polycations and polyanions by LbL technique.25−30 However, in contrast to this typical LbL behavior, in this study the surface charge of the PD5% hollow fiber remains slightly negative in the whole pH range despite the positively charged quaternary ammonium groups contained by the PDADMAC. Su et al.27 observed a similar behavior of persistent negative surface charge after LbL deposition of PDADMAC layers alternating with PSS on a polysulfone UF membrane support; the behavior was attributed to the fact that the strong negative charge of the PSS shields off the positive charge of the low charge density polyelectrolyte PDADMAC, and thus the zeta potential of the surface remains negative even after the PDADMAC layer.27 By contrast, the surface charge of the hollow fibers PEI5% and PEI10% is only slightly negative at pH > 9 and gets positive around +20 eV at pH < pH 9.0, the PEI10% membrane being slightly more positively charged than PEI5%. Therefore, the high charge density polyelectrolyte PEI can reverse the charge of the support to get the inner layer positively charged, and the charge properties of these membranes are characteristic of a typical weak polyelectrolyte whose protonation degree varies with the solution pH.45 The hollow fibers PEI5% and PEI10% are positively charged in a wide pH range, which is applied in most of the separation processes (pH < 9), and they can be promising candidates for the separation of positively charges solutes by NF. Few other studies also reported about positively charged NF membranes obtained by multistep processes, flat

Table 3. Elemental Surface Composition of the Hollow Fiber Membranes %C %O %S %N N/S ratio O/S ratio

R

PEI5%

PEI10%

PD5%

66.95 20.31 12.74

60.45 18.86 8.49 12.20 1.44 2.22

61.77 17.04 7.49 13.71 1.83 2.28

67.41 18.56 9.05 4.98 0.55 2.05

1.60

The XPS spectra in Figure S4 reveal the constitutive elements of the hollow fibers’ separation layers, the values of the binding energies of the elemental peaks being in good agreement with those previously reported in the literature.46 As it was expected, the surface of the reference membrane R contains only C, O, and S, while N was also found on the G


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hollow fibers containing PEI. To investigate this assumption, ATR-FTIR analyses were conducted on the inner surface of the hollow fibers, the results being depicted in Figure S5. The FTIR spectra show that new adsorption peaks can be found in the spectra of the hollow fibers PEI5%, PEI10%, and PD5% spun with positive polyelectrolyte in the bore solution as compared to the reference hollow fiber R. These peaks were assigned as follows: at 1033 cm−1 (PSS characteristic peak determined by the sulfonated moieties in the PSS), at 2840 and 2930 cm−1 (stretching vibrations of the N−H in amine, and C−H in methyl and methylene groups), at 3430 cm−1 (−OH stretching in alcohol and in hydrogen bonds), and at 1125 cm−1 (C−N bond stretching). Additionally, the new peak of CN at 1640 cm−1 can be ascribed to the formation of imine bonds CN between the amine groups from PEI and the aldehyde impurities in glycerol, and assess for a covalent cross-linking of the separation layer of PEI-containing hollow fibers. As the PD5% membranes do not have primary amine groups, they cannot form imine bonds. For these hollow fibers, we assume a cross-linking between the quaternary ammonium groups of PDADMAC (positively charged and possessing a deficit of electrons) and the primary OH groups of glycerol, which can act as an electron donor. Such interactions leading to complexes formation are reported in the literature for obtaining deep eutectic solvents by mixing quaternary ammonium salts acting as a hydrogen-bond acceptor (HBA) with a hydrogenbond donor (HBD) molecule (nitrogen-based compounds, carboxylic acids, or various alcohols, including polyols such as glycerol or ethylene glycol).49 Therefore, the ATR-FTIR analysis provides supplementary evidence for the presence of both positively and negatively charged polyelectrolytes in the separation layer of the PEC hollow fibers, and also completes the picture about the structure of it. Thus, the active layers of the PEI and PDADMAC-containing hollow fibers have an interwoven and stable structure, which results as a combination of aggregation between the polycation and polyanion into polyelectrolyte complexes (PECs) at the interface during the spinning by electrostatic interactions (ionic cross-linking), and a chemical cross-linking between the positive polyelectrolyte and glycerol. These account for both the stable layer formed without using a dedicated step/cross-linking agent during fabrication, and also for the fact that the reference R membrane is not cross-linked, even containing and being post treated with glycerol. Thus, besides the obvious advantages of the one-step preparation process developed for the preparation of these novel PEC hollow fibers, the increase in the stability of the separation layer by chemical cross-linking during the normal post treatment procedure is a supplementary advantage, which, to the best of our knowledge, was not reported until now. 3.5. Hollow Fibers’ Separation Performance. The ion separation in NF is achieved by a combination of size exclusion and electrical interactions with the charged NF membranes.7 The PEC hollow fibers have unique charge and structural characteristics, and therefore assessing the contribution of these mechanisms is an important step in understanding their structure−properties dependence and for further tuning the membrane properties. The rejections for single salt solutions NaCl, Na2SO4, MgCl2, and MgSO4 of the hollow fibers prepared were measured at low pressures of 2, 3, 4, 5, 6 bar for R, PD5%, and PEI5% and 5 and 6 bar for the hollow fiber with lower flux PEI10%. The effect of operating pressure on the flux and rejection performances for

surface of the membranes spun with PEI and PDADMAC in the bore. The amount of N in PEI10% is higher than that in PEI5% but not proportional to the PEI concentrations in the bore solution used, and the surface of PD5% contains less than one-half of N in PEI5%, which was spun with the same amount of positive polyelectrolyte in the bore solution (5%). The order of the N content PD5% < PEI5% < PEI10% is the same as those of the membrane surface charge depicted in Figure 2, and is in agreement with the charge density characteristics and concentrations of the polyelectolytes used. Table 3 shows that the surfaces of the PEC hollow fibers PEI5%, PEI10%, and PD5% contain also S in considerable amounts, which originate from PSS in the dope, thus adding new information to the investigations in the previous sections and also confirming our assumptions above regarding the polyelectrolyte complex nature of the separation layer of these novel hollow fibers. The values of the N/S ratio in Table 3 show values more than unity for the PEI-containing hollow fibers, and of only 0.55 for the PDADMAC-containing ones, which is consistent with the differences in charge densities of the positive polyelectrolytes used. This indicates that branched PEI exposes more N on the surface as compared to the linear PDADMAC (thus resulting in a positive surface charge), while more PSS is exposed at the surface in the PSS/PDADMAC separation layer, which results in a slightly negative surface as determined by zeta potential measurements. Such structural organization of interwoven oppositely charged polyelectrolytes and ionic cross-linking between them was previously reported in the LbL assemblies on UF membranes as the oppositely charged polyelectrolytes can easily interdifuse during the LbL coating procedure.25−28,47 In our case, the presence of both positively and negatively charged polyelectrolytes in the separation layer proves that the separation layer of these novel hollow fiber membranes arises by interdiffusion and aggregation of the oppositely charged polyelectrolytes at the interface between the dope and the bore solutions during the spinning. In this way, the separation layer of these PEC NF hollow fibers prepared by this one-step process is much thicker than that reported for coating of 16 layers by LbL method in ref 29, and has also unique structural and transport features as presented in the previous sections. By analyzing the O/S ratio in Table 3, it can be noticed that the value for the reference membrane R is 1.59 and therefore close to the O/S ratios in PES and PSS, which are 1.50 and 1.54, respectively. The ratio O/S is much bigger for the PEC hollow fibers PEI5%, PEI10%, and PD5%, and as both PEI and PDADMAC do not contain O, this indicates the presence of another O-containing substance in their surface layer. This also correlates with their insolubility in NMP, and led us to assume that this O-containing substance might have an additional cross-linking effect determined by the presence of the positive polyelectrolytes. Using PEI for cross-linking polymers with carbonil functionalities such as P84 or PAI is widely applied in preparing thin film composite membranes by interfacial polymerization,13−15,19,20,25 and also cross-linking PEI with glutaraldehyde in LbL assembly was reported.28 Bello et al. reported the chemical modification and cross-linking of proteins by reducible impurities as aldehydes or peroxides existing in small amounts in glycerol originating from the fabrication process or formed by mild oxidation in air during the glycerol storage.48 In our work, glycerol was used in the dope solution and subsequently for post-treatment of the hollow fibers for preserving the pores. Therefore, a similar cross-linking as reported by Bello et al. might undergo in our H


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Figure 4. Rejection versus flux performance of the hollow fibers for various salts (feed salt solutions with initial concentration of 10−3 M; pressure 2, 3, 4, 5, 6 bar for (a) R, (b) PD5%, (c) PEI5%, and 5 and 6 bar for (d) PEI10%,; lines connect the points to guide the eye).

fiber is strongly negative charged. Thus, the rejection behavior of the hollow fiber R is based on charge effects and not on steric hindrance effects as Figure 4a shows that the large magnesium ions are enriched in the permeate.50 Data in Figure 4b highlight very good separation performance of the membrane PD5% spun with PDADMAC in the bore, the rejection reaching 90% for all salts except NaCl whose retention reaches 50%. The 90% rejections stay almost constant in the whole pressure range of 2−6 bar, thus indicating that the rejection properties are due to the steric effects as this hollow fiber is almost uncharged (see Figure 2). The relatively high rejection of NaCl also supports this assumption, as it can be observed from Figure 4b that sodium chloride rejection is the only one increasing with the transmembrane pressure. This suggests that the increasing NaCl rejection is determined by the dilution effect (increasing water flux with the applied pressure, while the salt is retained in feed by steric effects).51 In light of these experimental data and by considering also the low MWCO of 300 Da and the slightly negative (almost neutral) surface of the hollow fiber PD5%, one can assume that the steric effects play a major role and are decisive for reaching very good retention of both ions and molecules by the PD5% hollow fiber. By analyzing the retention behavior of the PEC hollow fibers spun with PEI in the bore depicted in Figure 4c,d, one can observe that the salt retention order is the same MgCl2 > MgSO4 > NaCl > Na2SO4 and indicates a positive membrane

Na2SO4 is depicted in Figure 3, and the salt rejection sequences for each type of hollow fibers are presented in Figure 4. Figure 3 shows that the rejections do not change significantly in the pressure range investigated, and the permeate fluxes through the hollow fibers differ significantly in magnitude but all increase linearly with the transmembrane pressure. These indicate that the hollow fibers can be effective even at very low pressures, thus performing energy efficient separations. The flux order is the same as the water permeability sequence (R > PD5% > PEI5% > PEI10%) as a consequence of the individual characteristics of the separation layers discussed in sections 3.1 and 3.2. The linear dependence of fluxes with transmembrane pressure also indicates that the influence of the osmotic pressure and concentration polarization phenomena can be neglected due to the low-pressure range and low feed concentrations used in this study. The flux versus pressure behaviors are similar for the other salts investigated (data not presented here). Figure 4a−d presents the salt rejection sequences of the hollow fibers investigated. It can be observed that the reference hollow fiber R shows high flux and low rejections for all salts, with negative rejection even being observed for MgCl2. The rejection order Na2SO4 > NaCl > MgSO4 > MgCl2 is consistent with the Donnan exclusion theory, and it is typical for a negatively charged membrane with large pores. The zeta potential measurements revealed that the surface of this hollow I


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ACS Applied Materials & Interfaces surface charge.50,52 Indeed, it was found in section 3.3 that both hollow fibers PEI5% and PEI10% have a positive separation layer, only slightly more positively charged for the hollow fiber PEI10% spun with more PEI in the bore. Figure 4c,d shows a not obvious increase in the membrane performance by doubling PEI concentration in the bore solution, but with a drastic 4-fold reduction in the flux instead (see Figure 5). These results are

consistent with the observation of a thicker layer on the inner surface of PEI10% as compared to PEI5% (section 3.2), which however seems to not be enough to significantly enhance the MWCO and salt retention. Similar results were obtained in other studies where a 20-fold flux reduction was registered for a minimal increase in rejection when using 25 times higher PEI concentration for chemical modification of a copolyimide P84 membrane.24 An overview of rejection and permeance performances of the hollow fibers developed in this work is shown in Figure 5. The flux of all PEC hollow fibers is lower than of the reference fiber R, which is attributed to the formation of inner separation layers (see section 3.1) that leads to an increase in salt rejection and a decrease in the membrane flux. It can be noticed that the permeate flux of the hollow fiber PD5% is still high when concomitantly achieving outstanding rejection performance (∼90%) even at very low applied pressure of 2 bar. Similar performances were reported for multilayer membranes containing PDADMAC obtained by LbL technique,25,26,28,29 and it is superior to other LbL membranes,27,30 the high flux of the PDADMAC-containing LbL membranes being attributed to the loose structure determined by PDADMAC in the polyelectrolyte multilayers.25−27 At the same time, the rejection of the reference membrane R is not suitable for NF applications, and the PEI-containing hollow fibers have acceptable NF properties and modest permeate flux. The flux and rejection behaviors of the PEC hollow fibers confirm our assumptions in sections 3.1 and 3.2 regarding the differences in the assembling of the polyelectrolytes at the inner surface, and together with the results of the surface charge measurements led us to propose a model structure of our PEC hollow fibers. Thus, all PEC hollow fibers have a negatively charged PES/PSS

Figure 5. Effect of variation of type and concentration of polycation in the bore solution on the hollow fibers’ rejection performance (full symbols) and permeate flux (open blue symbols) for retention of Na2SO4 (squares), MgSO4 (circles), NaCl (triangles), and MgCl2 (diamonds) at 6 bar (feed solutions with initial concentration of 10−3 M).

Table 4. Comparison between PEC Hollow Fibers Prepared in This Work and Literature Data NF membranes NTR7450 (Nitto Denko)

preparation procedure

a

CA30 (Hoechst)

PEI10% PEI5% a

PWP (L/m2·h·bar)

600−800

NF270 (Dow) UTC20 (Toray Industry)

Desal 5DK (GE) PEI_2K PEI_20K PEI_60K PES/SPEEK 2 LbL*1.5C LbL (30 LMH, 4−10 bilayers) (PDADMAC/PSBMA/PSS)2/ PDADMAC (PDADMAC/PSBMA/PSS)2/ PDADMAC/PSBMA (PDADMAC/PSBMA/PSS)3 (PAH/PAA)4PAH DL6 PD5%

MWCO (Da)

chemical modification chemical modification chemical modification polymer blend LbL LbL LbL

6.3−23

200−400 350

11−13.5 25

1000

23.2

200 1278 1278 912 ∼1000 205

5.4 0.637 0.485 0.358 10.6−14.4 ∼15 3.9

salt rejection (%)

ref

41−50 (NaCl), 88 (Na2SO4), 15 (MgCl2), 56 (MgSO4)a 97−100 (MgSO4) 94−98 (MgCl2, MgSO4, Na2SO4), 47% (NaCl)a 8 (NaCl), 29 (Na2SO4), 20 (MgCl2), 53 (MgSO4)a 98 (MgSO4) ∼90 (MgCl2) ∼95 (MgCl2) ∼97 (MgCl2) 50 (NaCl), 90 (Na2SO4) 95 (MgCl2, MgSO4, Na2SO4) 85−90 (MgSO4) ∼52 (Na2SO4), 62 (CaCl2), ∼42 (NaCl)

53, 54

54 14 14 14 21 28 29 30

54, 56 53−55 53, 54

LbL

3.7

∼53 (Na2SO4), ∼72 (CaCl2), ∼57 (NaCl)

30

LbL LbL dual-layer spinning one-step single layer spinning one-step single layer spinning one-step single layer spinning

4.5

98 (Na2SO4), ∼20 (CaCl2), ∼40 (NaCl) 24 (NaCl), ∼60 (Na2SO4) ∼15 (MgCl2), ∼97 (Na2SO4) 90 (MgCl2, MgSO4, Na2SO4), ∼50 (NaCl)

30 31 36 this work this work this work

1910 300

11.93 7.6

700

0.4

∼75 (MgCl2), ∼43 (NaCl)

1040

2.0

∼70 (MgCl2), ∼35 (NaCl)

Measured at 10 bar. J


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ACS Applied Materials & Interfaces support. On this support, low charge density loopy chains of PDADMAC aggregate with PSS in a loose, rough, and almost uncharged network ionically cross-linked, which preserves the high porosity of the support and determines high water permeability and high solute rejections due to steric hindrance effects. By contrast, high charge density hyperbranched PEI is not folded (due to strong intrachain repulsions) and forms more compact and smoother PEC layers, which cover small pores of the substrate but not completely the big ones; it also exposes free branches of PEI chains at the interface, which provides the positive surface charge. These result in a less permeable structure (low permeate flux), but the ions (and small molecules too) can still pass through the uncapped pores, thus explaining the not very high rejections of NaCl and Na2SO4 (as it can be observed from Figure 4c and d). Moreover, the PEC separation layers are further supplementary chemically cross-linked by the aid of the glycerol used in the fabrication process. A comparison of the performances of the PEC hollow fibers prepared in this work with those of commercially available flat sheet NF membranes and with other hollow fibers prepared by multistep processes reported in the literature is given in Table 4. Without being exhaustive, data in Table 4 highlight that the novel PEC hollow fibers developed in this work have NF properties situated in the top range of those reported for hollow fibers prepared by other techniques involving multistep procedures, and also as compared to well-known commercial NF flat sheet membranes. However, these membranes result from multistep processes, which are difficult to handle and are also of long time, of high chemicals, and are water consuming. Additionally, the commercial flat sheet membranes are missing the advantages of the hollow fiber configuration. By contrast, the new technique presented here only involves one step, the support and the separation layer of the hollow fiber membranes being set in the same time during the spinning. On the other hand, for applications in drinking water production, it is not always required to have rejection close to 100% because an amount of salts (hardness) is desired to remain in water. Therefore, the retention and permeability of the PEC hollow fibers reported here might meet the needs of some relevant separation task, which therefore makes them very attractive. Ongoing work is currently carried out on the optimization of these hollow fibers by varying the parameters that influence the separation and mass transport properties (i.e., the polyelectrolytes type and concentrations), and on the investigation of their possible applications on micro pollutants removal.

with characteristic structures containing oppositely charged polyelectrolytes are present on the inner surface of the PEC hollow fibers. The oppositely charged polymers are assembled in an interwoven and ionic cross-linked structure, which also undergo a supplementary chemical cross-linking by the aid of the glycerol used for the membrane post-treatment for pore stabilization. The zeta potential measurements revealed that the PEI-containing hollow fibers have a positively charged separation layer onto a negatively charged support, while use of PDADMAC leads to a neutrally slightly negatively charged separation layer. The NF experiments highlight that the type and the concentration of the polycation play a decisive role in determining the properties of the NF hollow fibers. The hollow fiber with the best NF performances is PD5% spun with 5% PDADMAC in the bore (MWCO ≈ 300 Da and PWP = 7.6 L/ m2·h·bar), and PEI-containing hollow fibers have good retention properties but modest flux. The PD5% hollow fiber performs effective removal of divalent ions (∼90% rejection of MgCl2, MgSO4, Na2SO4) together with ∼50% NaCl rejection. The high rejection is accompanied by a high permeate flux attained in a range of very low pressures of 2−6 bar, which make the NF process both highly efficient and economically attractive. The results indicate that the PEC hollow fibers prepared are suitable for low-pressure NF and might be successfully used for efficient cleaning of waters containing pollutants of great concern nowadays such as heavy metals, dyes, medicines, etc. In view of the availability of a wide variety of polyelectrolytes with various chemical functionalities, this new fabrication technology can open new directions toward economically advantageous and feasible design of highly charged hollow fibers with properties tuned toward interesting applications.

4. CONCLUSIONS In this work, a novel one-step preparation procedure is developed for the fabrication of dual-charged NF hollow fiber membranes. The newly developed procedure allows one to build up the NF separation layer during the hollow fibers spinning as a result of aggregation between oppositely charged polyelectrolytes dissolved in the dope and bore solutions into polyelectrolyte complexes (PECs), thus giving rise to a new type of membrane with unique features. The characteristics of the newly developed PEC hollow fibers were thoroughly investigated, and their NF separation performances were compared to other types of hollow fibers prepared by multistep processes and with commercially available flat-sheet NF membranes. FESEM, XPS, and FTIR analyses show that a highly permeable structure is achieved and separation layers

*E-mail: [email protected]. *E-mail: [email protected].

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b05706. Chemical structures of the polyelectrolytes used in this study, schematic representation of the spinning process, photo of hollow fibers in NMP, and XPS and ATR-FTIR spectra of the hollow fiber membranes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

C.V.G. gratefully acknowledges financial support for the research provided by the Alexander von Humboldt Foundation. We thank Karin Faensen and Lucius Esser for their help with the FESEM images and zeta potential measurements, respectively. This work was partially performed at the Center for Chemical Polymer Technology CPT, which is supported by the EU and the federal state of North Rhine-Westphalia (grant no. EFRE 30 00 883 02). K


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Dual-Charged Hollow Fiber Membranes for Low-Pressure

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