Internal Concentration Polarization Research Articles

Enhancing water permeability with super-hydrophilic metal–organic frameworks and hydrophobic straight pores

Hydrophobic straight pores and super-hydrophilic metal–organic framework (MOF) in the supporting and active layers reduced flow resistance and improved wettability, raising water permeability with low internal concentration polarization.

Smart TFC membrane for simulated textile wastewater concentration at elevated temperature enabled by thermal-responsive microgels

Substrate modification is advantageous to ameliorate the internal concentration polarization (ICP) phenomenon encountered by most thin-film composite (TFC) membrane in forward osmosis (FO) process, therefore promoting FO implementation to a large extent. Optimization of the intrinsic parameters of substrate (e.g. porosity and tortuosity) can generally be achieved by incorporating hydrophilic or pore-containing materials into the dope solution before the substrate fabrication. In this work, polyvinylidene fluoride (PVDF) substrate with embedded thermal-response poly (N-isopropyl acrylamide) (PNIPAM) microgels is employed for the construction of TFC-FO membrane, where the reversible shrinkage of PNIPAM microgels brings about a lower tortuosity and a higher porosity of the substrate at higher operation temperature, thereby mitigating the ICP impact of the TFC-FO membranes significantly. Effects of PNIPAM content on the properties of the substrates and the resultant polyamide (PA) layer, as well as the corresponding performance of the TFC-FO membranes were comprehensively investigated. The benchmarking shows that, TFC membrane with PNIPAM-modified substrate exhibits remarkably improved water flux of 40.2 L·m−2·h−1 and a reduced reverse salt flux of 10.1 g·m−2·h−1 , with DI water as feed (50 °C) and 1 M NaCl as draw solution (ambient temperature), which suggests its great potential for treating feed solution with relatively high temperature in the practical applications, for instance, the dyestuff wastewater enrichment.

Understanding Multi-Ion Transport Mechanisms in Bipolar Membranes.

Bipolar membranes (BPMs) have the potential to become critical components in electrochemical devices for a variety of electrolysis and electrosynthesis applications. Because they can operate under large pH gradients, BPMs enable favorable environments for electrocatalysis at the individual electrodes. Critical to the implementation of BPMs in these devices is understanding the kinetics of water dissociation that occurs within the BPM as well as the co- and counter-ion crossover through the BPM, which both present significant obstacles to developing efficient and stable BPM-electrolyzers. In this study, a continuum model of multi-ion transport in a BPM is developed and fit to experimental data. Specifically, concentration profiles are determined for all ionic species, and the importance of a water-dissociation catalyst is demonstrated. The model describes internal concentration polarization and co- and counter-ion crossover in BPMs, determining the mode of transport for ions within the BPM and revealing the significance of salt-ion crossover when operated with pH gradients relevant to electrolysis and electrosynthesis. Finally, a sensitivity analysis reveals that the performance and lifetime of BPMs can be improved substantially by using of thinner dissociation catalysts, managing water transport, modulating the thickness of the individual layers in the BPM to control salt-ion crossover, and increasing the ion-exchange capacity of the ion-exchange layers in order to amplify the water-dissociation kinetics at the interface.

Zwitterionic Hydrogel-Impregnated Membranes with Polyamide Skin Achieving Superior Water/Salt Separation Properties.

Support-free nonporous membranes have emerged as a new material platform for osmotic pressure-driven processes due to its insusceptibility to internal concentration polarization (ICP). Herein, we demonstrate high-performance membranes of zwitterionic hydrogels impregnated in porous membranes with a skin layer of highly cross-linked polyamides on both sides prepared by gel-liquid interfacial polymerization (GLIP). Such a configuration eliminates the pores and thus ICP, while the thin polyamide layer provides high salt rejection but negligible resistance to the water transport compared with the hydrogels. The polyamide skin layers are characterized using scanning electron microscopy and atomic force microscopy. The effect of the hydrogel compositions and polyamide formation conditions on the water/salt separation properties is thoroughly investigated. Example membranes show water permeance and salt rejection comparable to state-of-the-art commercial forward osmosis membranes and essentially no ICP.

Synthesis of novel thin film composite (TFC) forward osmosis (FO) membranes incorporated with carboxylated carbon nanofibers (CNFs)

In this study, thin film composite (TFC) membranes were modified using carbon nanofibers (CNFs) for forward osmosis (FO) applications. Carboxyl functionalization of CNFs was performed to increase their surface hydrophilicity and dispersion in a polysulfone (PSF) matrix as the membrane substrate. Different concentrations of both carboxyl functionalized CNFs (f-CNFs) (0.1, 0.3, and 0.5 wt%,) and raw CNFs (0.3 wt%) were incorporated in the polymer matrix and hydrophilicity, porosity, surface roughness, tensile strength and morphology of the nanocomposite substrates were characterized. The optimal membrane was achieved based on the best performance (higher water flux and lower reverse solute flux). Thereby, the TFC membrane containing 0.3 wt% of the f-CNFs was selected as the optimal membrane. The FO water flux of this membrane was almost 2 times higher than the neat TFC membrane in both FO (active layer facing FS) and PRO (active layer facing DS) modes of operation. Water flux of 13.08 L m− 2 h− 1 and specific reverse solute flux of 0.24 g L− 1 were achieved when 1 M NaCl was used as the draw solution (DS) and Deionized (DI) water was used as the feed solution (FS) under the FO mode. Furthermore, incorporation of the f-CNFs decreased the structural parameter (S) of the TFC membranes leading to lower internal concentration polarization (ICP). From the results, it was concluded that the f-CNFs is a suitable candidate for TFC-FO membranes modification to achieve high membrane performance.

Extended performance study of forward osmosis during wastewater reclamation: Quantification of fouling-based concentration polarization effects on the flux decline

The long-term performance of forward osmosis during simulated wastewater reclamation was investigated for 120 h operation with a focus upon the influence of flux on flux decline and the synergistic effect of fouling on concentration polarization. Our comprehensive investigation focused on different fluxes (25; 30; 34 LMH) for simulated wastewater containing either a high protein or a low protein fraction. Compared to an initial flux of 25 LMH, operation at an initial 34 LMH favored the formation of a thicker and more compact cake layer which resulted in significant increase in both cake structural parameter (four-fold) and cake layer enhanced concentration polarization (ten-fold). After 40 h operation without physical cleaning the additional effect of cake layer enhanced concentration polarization and fouling resistance consumed 25% of the total driving force; the significant internal concentration polarization still had the greatest impact. In contrast operation at the lower flux of 25 LMH generated less fouling with a lower cake structural parameter (119 μm). The resultant flux decline was only 3% in contrast to the 15–18% found for the higher flux of 34 LMH. For operation above an initial 30 LMH it was found that FO fouling became irreversible if the wastewater contained a high protein fraction. Overall for a thin film composite membrane and a wastewater with a foulant concentration of 160 mg/L an initial flux of 25 LMH is the recommended threshold; this is 25% less that the critical value determined in earlier short-term studies.

Concentration of lithium by forward osmosis

As an applying potential for a part of production of lithium (Li) from salt lake brine, forward osmosis was investigated by using semi-permeable membranes, sodium chloride (NaCl) and magnesium chloride (MgCl2) as draw solutions under several operational conditions. Cellulose triacetate and thin-film composite membranes and their orientations were compared. The results indicated that concentration of Li was higher using the thin-film composite membrane with MgCl2 as the draw solution. The effect of pH was insignificant. The results also showed that water flux decreased with increasing Li concentration on the feed side. Internal concentration polarization played an important role when rejection of Li was higher in the active layer facing feed solution system.

Electrospun nanofiber supports with bio-inspired modification enabled high-performance forward osmosis composite membranes

Electrospun nanofiber mats (ENMs) are desired candidates as supports for composite forward osmosis (FO) membranes owning to the high porosity and low tortuosity of pores. However, it remains challenge to construct defect-free thin selective layer on ENMs controllably to achieve simultaneous high permeability and selectivity. In order to meet the demands, a novel thin film composite (TFC) FO membrane was fabricated via interfacial polymerization on a polydopamine (PDA) modified ENM substrate (PDA-HPENM), which was hot-pressed (HPENMs) prior to the PDA deposition to improve the mechanical strength and reduce the mat thickness. The internal concentration polarization (ICP) effects and the transmembrane resistance in FO membranes are reduced significantly attributed to the unique pore structure and good hydrophilicity of the PDA-HPENMs, resulting in doubled water flux compared with that of the TFC membranes using HPENM substrates. Moreover, the optimized TFC FO membranes exhibit relatively high rejection (~100%) to heavy metal ions and antibiotics, holding great potential for wastewater treatment.

Porous nano-hydroxyapatites doped into substrate for thin film composite forward osmosis membrane to show high performance

The incorporation of inorganic nanoparticles into thin film composite forward osmosis (TFC FO) membranes is an effective method to alleviate internal concentration polarization (ICP) and enhance the flux performance of the FO membrane. In this paper, synthetic hydrophilic rod-like porous nano-hydroxyapatites (PNHAs) were doped into polysulfone (PSf) casting solution to form support layer by phase inversion; further interfacial polymerization was carried out to prepare a high performance TFC FO membrane. The results showed that the incorporation of PNHAs not only improved the thickness, porosity, hydrophilicity, and connectivity of the support layer, but also enhanced the roughness of the active layer. The measured mass transfer parameters prove that these improvements were beneficial. Further FO experiments showed that when using deionized water as the feed solution and 1mol/L NaCl as the draw solution, TFN 0.75 showed higher water flux than TFC FO membrane in both AL-FS (18.5 vs 7.16 L/m2·h) and AL-DS (33.26 vs 9.93 L/m2·h) modes. Reverse salt flux had not increased significantly. At the same time, TFN 0.75 (697 μm vs 1,960 μm) showed the smallest structural parameter. This study shows that PNHA is a suitable nanomaterial for mitigating the ICP effect of FO membranes.

Forward osmosis for industrial effluents treatment – sustainability considerations

Access to clean water resources has become a global challenge in recent times, especially in developing countries, where huge amounts of highly polluted industrial and municipal effluents are produced and discharged into the receiving environments. Applications of membrane technologies to deal with effluents from various origins have recently received a great deal of attention due to their inherent advantages compared to other physico-chemical methods developed so far. Forward osmosis (FO) is among one of the efficient membrane-based processes adopted by wastewater treatment facilities, with various configurations currently being transferred from laboratory and pilot-scales to large-scale applications. Still, FO technologies are plagued with drawbacks such as fouling, internal concentration polarization (ICP), reverse solute flux and draw solution recovery, which invariably increases the cost of operation and restricts the feasibility for large-scale and long-term use because economic considerations are the most important sustainability criteria when selecting a wastewater treatment technique among the various alternatives. Several modifications have been introduced in recent years to overcome the existing limitations, such as incorporation of engineered nanomaterials onto the membrane surface to mitigate membrane fouling and to enhance their life-time, thereby minimizing the cleaning and (when necessary) replacement costs. Several FO based pre-treatment technologies have also been introduced for complex effluents treatment to minimize the operational costs arising from cleaning and replacement activities. Therefore, assessing the performance of such technologies according to sustainability indicators is the key to ensure long-term benefits from the application of FO technologies for the treatment of highly polluted effluents. In this review, treatment and water reclamation from industrial effluents using FO process and the current states of the standalone and hybrid technologies considering sustainability criteria are discussed. Scaling-up opportunities for the existing lab-scale modules have been evaluated and environmental footprint of the under-developed technologies are discussed. This review will therefore aid in selecting the most suitable configurations of FO technologies (standalone/hybrid) to deal with the highly polluted effluents for real world applications and to direct future studies in this emerging area. Perspectives and recommendations for future studies are also included.

A novel empirical method for predicting concentration polarization in forward osmosis for single and multicomponent draw solutions

Concentration polarization is one of the inherent problems in forward osmosis membrane process. A quantitative evaluation of concentration polarization is therefore vital to understand its impact on the performance of the forward osmosis. Limited data in the literature exists for the diffusion coefficient of mixed electrolyte or multicomponent solutions, which makes the calculation of mass transfer coefficient and solute resistance to diffusion in forward osmosis complicated. Therefore, an empirical method based on a limited set of well-defined experiments for evaluating and predicting the concentration polarization, water flux, and reverse solute flux is presented for single and mixed, or multi-ions draw solutions. The proposed method does not rely on the hydrodynamic conditions and flow regime in the system and provides an approach to measure and predict concentration polarization, water flux, and reverse salt flux when the diffusion coefficient of a feed solution (FS) or draw solution (DS) is challenging to determine. The developed numerical method is two steps method to measure internal and external concentration polarization using different concentrations of the draw and feed solutions. Experimental work was carried out with a single, and highly soluble sodium chloride (NaCl) DS and a mixture of NaCl and magnesium sulphate (MgSO4) were used as a selected multicomponent DS. The results showed a 95% to 99% agreement with the experimental data.

Improving water flux and salt rejection by a tradeoff between hydrophilicity and hydrophobicity of sublayer in TFC FO membrane

This paper presents a novel approach using sulfonated polyethersulfone/zeolitic imidazolate framework (SPES/ZIF-8) porous nanocomposite substrate to enhance the salt rejection and water flux of forward osmosis (FO) membrane. The effect of SPES/ZIF-8 substrate on FO membrane properties containing morphologies, hydrophilicity, water permeability and salt rejection were investigated and completely discussed. Based on the obtained results, the selectivity and permeability of thin-film nanocomposite (TFN) FO membranes can be improved simultaneously through adjusting the both hydrophilic and hydrophobic characteristics. But these two results, is achieved in two different processes and with reverse impacts. After the sulfonation process and using the more hydrophilic polymer (SPES) instead of the neat PES, the porosity and consequently the water flux was increased. Because of more hydrophilicity of SPES which reduces the cross-linking degree of the corresponding polyamide (PA) layer, the salt rejection was reduced. In contrast, after the incorporation of ZIF-8 into the SPES substrate, the salt rejection was increased due to the hydrophobic nature of these nanoparticles (NPs). In terms of both salt rejection and water flux, the sulfonated TFN membrane with 0.5% ZIF-8 NPs (TFN-S0.5 membrane) showed the improved results comparing with the bare Thin-film composite (TFC) membrane. Furthermore, the best result for the ratio of reverse salt flux to water flux (Js/Jw) as the selectivity parameter, was about 0.16 g/L using 10 mM and 2 M NaCl solutions as the feed solution (FS) and the draw solution (DS), respectively, in the FO experiment. Also, the structural parameter (S) which is believed to have a strong relationship with the internal concentration polarization (ICP), was decreased to 0.39 nm as the best result, after modification of FO membrane.

The synergy between osmotically assisted reverse osmosis (OARO) and the use of thermo-responsive draw solutions for energy efficient, zero-liquid discharge desalination

Conventional zero-liquid discharge (ZLD) processes alleviate environmental concerns associated with brine discharge, but are not widely applied, due to their high cost and significant energy consumption. However, new and promising ZLD technologies, such as those integrated with osmotically assisted reverse osmosis (OARO), may be technically and economically more favourable than existing technologies.In this study, a numerical comparison between an FO and an OARO integrated ZLD process is performed, in which a thermo-responsive draw solution is utilised for both. The findings from the parametric and techno-economic analysis indicate that OARO mitigates several shortcomings of FO, making it more energy efficient (energy savings of ≈10.8%), less susceptible to internal concentration polarisation and more economical (cost savings of ≈15.5%). Furthermore, a key advantage of OARO is that it operates with lower draw solute concentrations allowing for the draw solute to be regenerated at temperatures 12 °C lower than with FO operation. This means that low-grade waste heat (LGWH), which is essentially ‘free energy’, is more easily utilised as an energy source.In comparison to other hydraulic pressure-driven ZLD technologies, such as low-salt-rejection RO (LSRRO), the electrical energy consumption of OARO can be significantly offset when powered by LGWH and using a thermo-responsive draw solute. The specific electrical energy consumption can be as low as 3.47 kWh/m3 when concentrating a saline stream from 0.6 M to 4 M using 4 membrane stages. At the same time, a 4-stage LSRRO system would consume approximately 48% more electricity to achieve the same concentration factor. Furthermore, OARO processes operating with a thermo-responsive draw solution can outperform those using non-responsive draw solutes (i.e. NaCl). Brine concentrations of up to 245 g/kg can be achieved using technically feasible operating pressures and draw solute concentrations, while the number of required membrane stages is almost halved from 6 to 4.

Dissolved methane rejection by forward osmosis membrane to achieve in-situ energy recovery from anaerobic effluent

Dissolved CH4 (D-CH4) rejection by forward osmosis (FO) membranes requires systematically investigate to mitigate anaerobic effluents pollution and energy loss. In this study, effects of membrane materials, orientations and alginate fouling on dissolved CH4, H2 and CO2 rejections were evaluated. FO was further connected with UASB effluent (FO-UASB) to recover D-CH4 for the first time. Results showed that D-CH4 rejections by CTA-ES and CTA-NW membranes were above 99.99% in the active layer facing feed solution (AL-FS) and active layer facing draw solution (AL-DS) orientations. Dissolved H2 rejections (87.5–92.3%) achieved higher than dissolved CO2 rejections (52.5–73.8%) in AL-FS orientation which was attributed to dissolved gas permeability through FO. Alginate fouling layer improved dissolved H2 rejection while concentrative internal concentration polarization aggravated dissolved CO2 rejection by fouled CTA-ES membrane in AL-DS orientation. D-CH4 was successfully collected by FO-UASB in the synergy effect of FO rejection and liquid-to-gas mass transfer. The maximum total CH4 collection rate was 599.0 mg COD/(L d) and 603.4 mg COD/(L d) at 35 °C and 25 °C in the FO-UASB.

Multistage osmotically assisted reverse osmosis process for concentrating solutions using hollow fiber membrane modules

Reverse osmosis (RO) is commonly used to concentrate solutions in food preparation and environmental applications among others. Limitations on the applied pressure, which must be higher than the osmotic pressure of the feed solution, determine the concentration that can be achieved by RO. Recently, an osmotically assisted reverse osmosis (OARO) system that can be operated even with low applied pressure has been suggested as a new concentration process. In this study, we performed concentration tests on a multistage OARO system that used two types of hollow fiber (HF) membrane modules made from cellulose triacetate with different numbers of HFs. Water permeated through the membrane and the concentration increased as it progressed through the modules, even when using concentrated NaCl solutions (1.0 and 2.0 M) with low applied pressures (10 and 15 bar). The concentration increased in each of ten stages in a multistage concentration test. Water flux and concentration ratio were strongly influenced by the internal concentration polarization in the modules.

Membrane structure-dependent limiting flux behavior and membrane selectivity loss during gypsum scaling: Implications for pressure-retarded osmosis operation and membrane design

Herein, we systematically investigated the influence of membrane structural properties on limiting flux behavior and selectivity loss during gypsum scaling in osmotically driven membrane processes. We selected two typical osmotic membranes, thin-film composite (TFC) polyamide (PA) membrane and integrally asymmetric cellulose triacetate (CTA) membrane with different structures, for gypsum scaling tests in active-layer-facing-draw-solution orientation (an operating mode preferred for pressure-retarded osmosis). Compared to the CTA membrane, the TFC membrane suffered severer internal scaling and achieved a lower limiting flux primarily due to its greater structural parameter that induced severer internal concentration polarization (ICP)-enhanced scaling. The limiting flux is inversely proportional to the membrane structural parameter. For the first time we observed that the TFC membrane suffered a drastic loss of integrity and selectivity after gypsum scaling in PRO. We confirmed that the thin PA layer of TFC membrane is more prone to being damaged by the growth of gypsum crystals inside the confined and unstirred support layer, whereas the integrally asymmetric membrane with a thicker active layer could better maintain its integrity. While TFC membrane is the mainstream for PRO in osmotic power harvesting, our study suggests that the integrally asymmetric membrane may be more suitable under severe scaling conditions.

Immobilization of sulfonated polysulfone via 2D LDH nanosheets during phase-inversion: A novel strategy towards greener membrane synthesis and enhanced desalination performance

Sulfonated polysulfone (sPSf) is a commonly used hydrophilic additive to polysulfone (PSf) substrates for preparing polyamide membranes with enhancement desalination performance. However, severe leaching of water-soluble sPSf into the coagulation water bath during substrate formation can lead to weakened mechanical strength of the substrate, loss of the expensive sPSf polymer, and potential environmental pollution. In this study, we report a novel and efficient strategy to “anchor” sPSf in the PSf matrix by using 2D layered double hydroxide (LDH) nanosheets. LDH nanosheets effectively immobilized sPSf due to their electrostatic interaction, resulting in greener membrane synthesis. Substrates modified with LDH anchored sPSf (PSf/sPSf5-LDHx) exhibited enhanced mechanical strength and water permeability compared to the pristine PSf substrate as well as the sPSf-blended substrates (sPSf/sPSf5). Interfacial polymerization on the PSf/sPSf5-LDHx substrate resulted in a polyamide rejection film containing more extensive nanovoids and thus greater effective filtration area, which enhanced water permeability without major loss of salt rejection. In forward osmosis tests, this novel membrane enjoyed an additional advantage of less severe internal concentration polarization, as reflected by its significantly reduced structural parameter.

Improved performance of thin-film composite membrane supported by aligned nanofibers substrate with slit-shape pores for forward osmosis

Substrate construction is considered as the most effective approach to alleviate the internal concentration polarization (ICP) effect, and therefore achieves the excellent performance of resultant thin-film composite (TFC) membrane in forward osmosis (FO) process. In this work, a well-aligned nanofiber substrate was prepared and employed for constructing the high-performance TFC-FO membrane for the first time. The physicochemical properties of the substrate, the micro-structure of polyamide layer and the separation performance of resultant TFC-FO membranes were systematically investigated. Taking advantages of the larger pore size and the better pore interconnectivity of the well-aligned nanofiber substrate, the resultant TFC-FO membrane achieves the best water fluxes of 50.7 and 62.9 LMH, accompanied with the ultralow reverse salt flux in FO and PRO modes, using deionized water feed solution and 1 M NaCl aqueous draw solution. This work may provide a new orientation in developing high-performance TFC membranes.

Advancement of forward osmosis (FO) membrane for fruit juice concentration

Simultaneous fruit juice concentration and preservation of the heat-sensitive sensorial and nutritional components is a challenging topic in food processing. Forward-osmosis (FO) membrane is one of the promising alternatives to concentrate juice at low temperature and pressure. There are a number of negative propensities in FO operation, inter alia, internal and external concentration polarization, organic fouling, and product contamination due to back salt diffusion from the draw solution. To address these issues, preparation methods have been developed to obtain highly hydrophilic membrane with low fouling tendency, concentration polarization, and solute diffusion. The desired membrane has been obtained by modifying active and support layers of the membrane. Draw solutions are also optimized recently with desirable characteristics such as low reverse salt permeation, easy to re-concentrate, favorable price, stable and minimal toxicity. This paper reviews mechanism and principle of FO membrane technology, recent advancement of the membrane (including membrane preparation and modification, and important parameters), as well as challenges and strategies for the future development of FO membranes in fruit juice concentration.

Water flux increase by inverting the membrane from its normal position – Is it occurring in FO and PRO?

Flux increase occurs when a membrane is inverted from normal position of active layer (AL)-facing-feed solution (FS). It was noticed at the very early stage of reverse osmosis (RO) membrane development but much attention has not been paid since then, possibly because of severe internal concentration polarization that occurs in the porous sublayer. In forward osmosis (FO) and pressure retarded osmosis (PRO), when they are run in the preferred mode of AL-facing-draw solution (DS), the water flow direction is from the porous sublayer to the AL, which is the same as the inverted position of RO. Hence, it is interesting to know if a similar flux increase, from the normal to the inverted position, is also occurring in FO and PRO. The objective of this work is to confirm the effect of membrane inversion on the flux and separation of two RO membranes, as well as to know if a similar increase in water flux is occurring in FO/PRO. Results showed that significant flux increase, up to an order of magnitude, takes place when the membrane is in the inverted position, while exhibiting almost no selectivity. As for the possibility of such a flux increase in FO/PRO, it was denied by the multivariable regression analysis of the experimental FO/PRO data found in the literature.