Salt Permeability Research Articles

Novel technique for measuring salt concentrations in food using silver dichromate

Curing with salt reduces water content in food and extends its shelf life; therefore, understanding salt permeability in food is critical to food quality and safety. Although there are many new techniques to measure salt concentrations in food, they cannot be applied in practice owing to drawbacks such as the need for expensive equipment or the inability to image quickly; therefore, development of a fast and simple measurement method is crucial. Silver dichromate is converted to silver chloride when it encounters chloride ions. Based on this principle, we soaked radishes in NaCl solutions of different concentrations and added a layer of silver dichromate to it. Salt concentrations was observed in 5 min, as areas with NaCl concentration of ≥1 % appeared white, whereas the other regions appeared red. A strong correlation (r = 0.94/0.96) was observed between the area of salt concentrations measured using this method and that measured using the conventional salt measurement method, demonstrating the reliability of the method. This method will help advance research related to salt concentrations in food and its application in industrial scenarios.

Comparison of 2D and 3D materials on membrane modification for improved pressure retarded osmosis (PRO) process

Pressure retarded osmosis (PRO) is a sustainable process that convert Gibbs free energy to osmotic energy by mixing two solutions of different salinities. The main challenges in the design of PRO membranes are obtaining a membrane with high water permeability and low salt permeability but also very high mechanical strength because the PRO process involves high pressure on the draw solution. Commercially available RO membranes with potential utility in a PRO system exhibit a high salt rejection rate but low water permeability and mechanical stability. Surface modification is a promising strategy for tuning the fundamental properties of the membranes (e.g. hydrophilicity, surface charge and thickness) that can improve the filtration performance of the membranes. The coating layer can also improve the mechanical stability of the membranes. Therefore, in this work, various types of modification materials were applied to the commercial available RO membranes to enhance their performance.With the assistance of hydrophilic materials (e.g. polydopamine – PDA), filtration performance of the membranes can be increased through membrane modification by 2D materials with high charge intensities (e.g. polyelectrolytes and graphene oxides) and by 3D mesoporous materials (e.g. zeolites), which increases the thickness of the membrane that can be beneficial in mechanically reinforcing the membrane. In this work, we modified commercial RO membrane with PDA, polyelectrolytes, graphene oxide and zeolites (ZSM-5). Improved filtration performance (increased water permeability and maintained salt permeability) of the modified membrane was observed. Tensile tests showed enhanced mechanical strength of the modified membranes, especially following 3D zeolites modification (up to 35 % of higher tensile strain was reported). Interestingly, a lower concentration of PDA (2 mg/mL) and zeolites resulted in higher mechanical strength of the modified membranes. Such results were likely due to a more homogenous coating layer when a low modifier concentration was applied. The thin and uniform layer can better absorb energy when membranes are under high pressure.

Small and wide angle X-ray scattering to reveal nanoscale structure evolution of the polyamide RO membranes triggered by ethyl acetate

The intrinsic structure of the polyamide separation layer determines the transport of water and salt through a thin film composite membrane. In this work, we probed nanoscale structure variation of the polyamide layers generated by ethyl acetate through small/wide angle X-ray scattering. The gyration radius and physical radius of the polyamide colloidal particles were calculated from the small angle X-ray scattering (SAXS) curves, and the results are confirmed by AFM and SEM. The form factor derived from SAXS curves and horizontal distance between the centers of adjacent benzene rings calculated from the wide angle X-ray scattering (WAXS) curves showed quantitative relationships with membrane performance including water permeability coefficient and salt permeability coefficient. The results derived from SAXS and WAXS curves provided evidence for the formation mechanism of polyamide layers.

THE ROLE OF THE MOLECULAR WEIGHT OF POLYVINYLPYRROLIDONE IN THE FORMATION OF TWO-LAYER POLYAMIDE/HYDROGEL MEMBRANES OF INCREASED STRENGTH

The article presents the results of a study of the influence of the molecular weight of polyvinylpyrrolidone on the properties of two-layer hydrogel/polycaproamide membranes. Composite membranes are obtained by the developed method, which consists in the formation of hydrogel membranes with subsequent deposition from a solution into their outer surface of a reinforcing layer based on polyamide modified with polyvinylpyrrolidone. It was established that the molecular weight of polyvinylpyrrolidone, both in the composition of the hydrogel membrane and in the modifying solution, has a significant effect on the interaction between the layers of composite membranes and their characteristics, such as water content, bursting strength , and osmotic salt permeability.

Polysulfone/sepiolite nanocomposites and disulfonated polysulfone as desalination membranes

This research targeted the preparation of new sepiolite-based nanocomposites and sulfonated polysulfone and compare them as desalination dense membranes. At the first step, a polysulfone with appropriate molecular weight was synthesized by condensation polymerization. The hydrophilic nature of the synthesized polymer was improved using two different methods: addition of various percentage (0.5–5% by weight) of sepiolite nanorods into the polymeric matrix, and introduction of sulfonated groups with 35–45 wt% sulfonation content to the polysulfone main chain. Precipitation by solvent evaporation was used to produce dense membranes after casting polymer solutions. FTIR, 1HNMR, and EDAX were used to identify the chemical structures of polysulfone, related nanocomposites, and sulfonated polysulfone. Also, the prepared samples were evaluated by equilibrium volume fraction of water, contact angle, water absorption, density, SEM, TGA, DSC, AFM and tensile test. Finally, the operation tests of the membrane including water flux, sodium chloride rejection, hydraulic water permeability, and salt permeability were investigated.

Overcoming the trade-off between water flux and salt rejection of an RO membrane via simultaneous optimization using highly porous microstructured support and sulfonated porous organic polymer-based polyamide active layer

Many researchers have devoted much effort to addressing the trade-off between water flux and salt rejection of reverse osmosis (RO) membranes till now, but it still needs to be solved. In this work, as a strategy to overcome the trade-off, simultaneous optimization was carried out with porous organic polymers (POPs) and highly porous microstructure support (HPμS) layers featuring regularly arranged columnar macrovoids formed by carefully adjusting the thermodynamic instability of a polymer solution. POPs were introduced into the active layer of an RO membrane to impart the water channel and hydrophilicity. POP fillers were functionalized with amine and sulfonic groups, and the synthesized POPs were introduced into different phases depending on sulfonation. Sulfonated POPs with high hydrophilicity provided additional water pathways and hydrophilic properties while maintaining the active layer's thickness, notably improving water permeability. Surprisingly, the simultaneous optimization not only enhanced the permselectivity further but also made thin-film nanocomposite (TFN) membranes overcome the trade-off because the shortened diffusion pathway by the HPμS was more beneficial for improving water permeability than salt permeability. As a result, the simultaneously optimized TFN exhibited 2.2 to 2.7 times higher water permeability and 2.6%p to 3.4%p higher salt rejection than a control TFC membrane.

Water and salt transport properties of zwitterionic poly (arylene ether ketone) for desalination membrane applications

A series of novel poly (arylene ether ketone) with pendant zwitterion groups were designed. Three molecules, 1,4-butane sultone, sodium 2-bromoethanesulphonate and sodium bromoacetate were selected to quaternize with tertiary amine-containing poly (arylene ether ketone) to prepare sulfobetaine poly (arylene ether ketone) (PAEK-SB4 and PAEK-SB2) and carboxylbetaine poly (arylene ether ketone) (PAEK-CB) with controllable content of zwitterion group. The fundamental water and salt transport properties in zwitterionic PAEKs were characterized. Except PAEK-SB2, the water and salt diffusivity and permeability of PAEK-SB4 and PAEK-CB increase with the increase of water sorption, which follows the trend of Yasuda's free volume theory. Salt diffusivity and permeability in PAEK-SB2 were suppressed relative to PAEK-SB4 and PAEK-CB with comparable water sorption, which is due to shorter distance between positively charged group (-N+(CH3)2-) and negatively one (SO3−) in SB2 group leading to stronger self-association interaction between zwitterions. The water/salt permeability selectivity of PAEK-SB2 and PAEK-CB change little with the increase of diffusive water permeability, which break through the trade-off between them. The permeability selectivity of zwitterionic PAEKs is more than one order of magnitude higher than sulfonated polysulfone (BPS), and its orders is PAEK-SB2>PAEK-CB > PAEK-SB4>BPS at the comparable diffusive water permeability. This study suggests that shorter distance between positively charged group and negatively one in zwitterion group benefits water/salt selectivity of zwitterionic PAEKs.

Fabrication of antifouling UiO-66 nanofiltration membranes via surface fluorination engineering

Membrane fouling is a primary hindrance in the efficient utilization of nanofiltration technology in wastewater treatment, which increases energy requirements and reduces membrane life. In this work, a highly antifouling UiO-66 nanofiltration (NF) membrane was fabricated via a facile surface engineering for dye rejection, where fluoromonomer with a long fluoroalkyl chain was fast UV-polymerized on the surface of UiO-66. Fluoroalkyl chains have low surface energy and provide the surface with non-stick and fouling-release properties, and fluoroalkyl is highly incompatible with dyes. Meanwhile, the bridged rigid fluoroalkyl produces an effective barrier to repel the foulants because of the narrowed pore structure and smooth surface. Results showed that the resulting membrane shows an excellent NF performance including high rejection of Methyl Blue, Cango Red and Chrome Black T (all over 99.7 %) with a high water permeance of 249.6 L m2 h−1 bar−1, and salt permeation of MgSO4 (89.26 %) and NaCl (99.02 %). The long-term stability testing exhibits basically stable performance for separating the CT/NaCl mixture. The outcome of the work offers an effective strategy to solve the membrane fouling and maintain the outstanding separation performance by fluorination modification. This newly UiO-66-based NF membrane showed excellent application potential in the dye wastewater treatment.

Standardizing practices and flux predictions in membrane science via simplified equations and membrane characterization

The development of membranes and membrane-based separation processes should be accompanied by a standardization of the protocols applied for membrane characterization and for data analysis. Here, streamlined equations for the estimation of the water flux and of the observed salt permeability coefficient in pressure-driven processes deploying dense membranes are presented. Also, a protocol for the experimental characterization of the transport properties of dense membranes is presented and the results are validated against the proposed equations. The proposed water flux equation is algebraic, whereas the ordinary equation needs to be solved iteratively. Moreover, in contrast to the traditional expression for the solute transport coefficient, which requires estimation of the concentration polarization, the respective equation proposed in this study only requires bulk parameters. Dimensionless variables for water flux, driving pressure, and mass transfer are introduced, and a filtration efficiency is defined, a useful parameter in terms of process design.

Salt transport of the carboxylated aromatic polyamide: Efforts to elucidate the structure-function relation of polyamide TFC desalination membranes

Five carboxylated polyamides with the number average molecular weight in the range of 15000–17000 Da, are synthesized via low-temperature solution polycondensation to study the effect of carboxyl group (-COOH) content on the carboxylated polyamide salt transport property. The glass transition temperature (Tg) of the carboxylated polyamide increases with the –COOH content due to the formation of more hydrogen bonds, which also leads to a decrease in free volume fraction (FFV). Interestingly, we observe that polymer having the –COOH content of 80%–100% possesses similar water uptake and KW, which provides a special perspective that how the –COOH group influences salt transport by excluding the effect of water content in polymer membranes. Water sorption can disrupt the hydrogen bonding within the –COOH groups, so setting out –COOH groups to bind water, leading to lower free water content and enhanced water/salt sorption selectivity. As the content of –COOH increases, the more negatively charged surface and stronger hydrogen bond interactions result in significant decrease in salt permeability (PS) and salt diffusivity (DS). These results implicate that the introduction of more –COOH groups to the polyamide layer can enhance the water/salt selectivity of TFC membrane without by changing the water sorption.

A machine learning approach for prediction of reverse solute flux in forward osmosis

This study introduces a novel approach using a supervised machine learning model to accurately predict reverse solute flux (RSF) in the forward osmosis process. This study employed feature engineering techniques to identify significant process parameters that influence RSF. Notably, the results demonstrate the high effectiveness of the Categorical boosting (CatBoost) machine learning algorithm in RSF prediction, achieving an R-square value of 0.94 and a root mean square error of 0.44 when comparing the actual and predicted data. Furthermore, the model underwent simulation using real experimental data, revealing a minimal percentage error ranging from 0 to 2 % compared to the experimental reverse solute flux. The result showcases the potential of machine learning to save valuable time typically spent on experimental data while offering accurate predictions of reverse solute flux. The implications are particularly valuable in various applications involving reverse salt flux, where precise predictions can be achieved solely based on input parameters related to the forward osmosis process, membrane water permeability, and salt permeability.

Covalent organic frameworks modified with TA embedded in the membrane to improve the separation of heavy metals in the FO

The Schiff base network-1 (SNW-1) was synthesized by the Schiff-base reaction method and then modified using tannic acid (TA). To make a thin film nanocomposite (TFC) membrane by using the nonsolvent-induced phase inversion (NIPs) method, it was embedded in the matrix of the forward osmosis (FO) polyethersulfone (PES) membrane and used to check the performance of the desalination and removal of heavy metals. SNW-1 and covalent organic framework modified tannic acid (STA) nanoparticles were characterized using FTIR, XRD, FESEM, TEM, BET, and TGA analyses. The results indicated the successful synthesis and modification nanoparticles. The effect of SNW-1 and STA nanoparticles on the surface properties of membranes were investigated using contact angle, porosity, pore size, ATR-FTIR, SEM, and AFM analyses. The results showed an extraordinary increase in hydrophilicity, softening of the membrane surface, and an enhancement in finger-like pores for water passage. Due to the presence of the hyperbranched polymer like TA next to SNW-1, in addition to creating a synergistic effect in increasing hydrophilicity and porosity, caused the removal of impurities. The results showed an increase in the permeability and rejection of the membranes modified with STA compared to the pure membrane and the modified membrane with SNW-1. Also, the TFC-STA membrane showed lower coefficient of salt permeability (B) to coefficient of water permeability (A) of RO setup (B/A), and lower salt permeation (Js) to water permeation (Jw) of FO setup (Js/Jw) coefficients than the modified TFC@SNW-1 membranes.

Date Palm Tree Leaf-Derived Cellulose Nanocrystal Incorporated Thin-Film Composite forward Osmosis Membranes for Produced Water Treatment.

Worldwide water shortage and significant issues related to treatment of wastewater streams, mainly the water obtained during the recovery of oil and gas operations called produced water (PW), has enabled forward osmosis (FO) to progress and become advanced enough to effectively treat as well as retrieve water in order to be productively reused. Because of their exceptional permeability qualities, thin-film composite (TFC) membranes have gained increasing interest for use in FO separation processes. This research focused on developing a high water flux and less oil flux TFC membrane by incorporating sustainably developed cellulose nanocrystal (CNC) onto the polyamide (PA) layer of the TFC membrane. CNCs are prepared from date palm leaves and different characterization studies verified the definite formations of CNCs and the effective integration of CNCs in the PA layer. From the FO experiments, it was confirmed that that the membrane with 0.05 wt% of CNCs in the TFC membrane (TFN-5) showed better FO performance in PW treatment. Pristine TFC and TFN-5 membrane exhibited 96.2% and 99.0% of salt rejection and 90.5% and 97.45% of oil rejection. Further, TFC and TFN-5 demonstrated 0.46 and 1.61 LMHB pure water permeability and 0.41 and 1.42 LHM salt permeability, respectively. Thus, the developed membrane can help in overcoming the current challenges associated with TFC FO membranes for PW treatment processes.

Reverse Osmosis Membrane Scaling Inhibition using Kinetic Degradation Fluxion Media

Membrane fouling can lead to reduced efficiency, increased operating costs, and shorter membrane life, which can ultimate[1]ly impact the quality of potable water produced from seawater. The use of Kinetic Degradation Fluxion (KDF) media as a pretreatment method is promising because it can potentially reduce fouling caused by CaCO3 scaling and improve the per[1]formance of RO membranes. The study's methodology, which involves comparing two identical reverse osmosis systems with and without KDF pretreatment, is appropriate and well-designed. By comparing the salt rejection and permeability flow of the two membrane systems, the study can provide valuable insights into the effectiveness of KDF media in reducing foul[1]ing caused by CaCO3 scaling. The explanation of how KDF media works to prevent the formation of mineral hardness scale by altering the morphology of the insoluble Ca and Mg carbonate and sulfate crystals and creating a redox environment is helpful in understanding the potential mechanism behind the effectiveness of KDF media as a pretreatment method. By con[1]trolling the formation of mineral scale and other contaminants, KDF media can potentially lead to improved efficiency of the RO membrane. Results suggest that KDF media can be an effective pretreatment method for reducing fouling caused by mineral scaling in RO systems. The potential mechanism behind the effectiveness of KDF media is explained, and the obser[1]vation that only a small number of KDF discs were degraded over the 40-hour period indicates that KDF media can be used for multiple cycles of RO operation, reducing the frequency of media replacement and associated operating costs. The finding that the RO-KDF membrane remained healthy and efficient for 40 hours while fouling was observed in the RO system with[1]in 20 hours is also promising, as it indicates that the KDF media can effectively reduce membrane fouling caused by scaling, leading to improved efficiency and extended membrane life. However, further studies can explore the optimal conditions for using KDF media in RO systems to maximize its effectiveness in reducing fouling and extending the lifetime of RO mem[1]branes. Investigating the potential for KDF media to reduce fouling caused by other contaminants, such as organic matter or silica, would also be interesting

Solution-diffusion-electromigration approximation model (SDE-A) for strongly charged, weakly charged and effectively uncharged reverse osmosis membranes

The solution-diffusion-electromigration approximation model (SDE-A) was formulated to describe the salt permeability of effectively uncharged, weakly charged and strongly charged reverse osmosis (RO) membranes. The model uses three of fewer parameters, depending on the type of charge behavior that a particular membrane exhibits under the allowable experimental conditions regarding feedwater salinity and acidity. The resulting algebraic equation is an approximation of an analytical solution of the Nernst-Planck equations for a 1:1 salt where the membrane charge depends on the feedwater conditions. The SDE-A model was compared to the more complex solution-friction (SF) model, by converting SF parameters to SDE-A parameters, resulting in a typical difference in computed salt permeability of less than 20%. Furthermore, the SDE-A model was applied to datasets from literature, corresponding to strongly charged, weakly charged and uncharged membranes. The SDE-A model can describe the effect of salinity and pH in these datasets comparably to the SF model. Because the model parameters can be easily determined experimentally and the algebraic model equation does not require elaborate solvers, the model is suitable for process design and optimization.

Novel method for accurately estimating membrane transport properties and mass transfer coefficients in reverse osmosis

We present a simple and robust method to simultaneously characterize the water and salt permeability (A, B) of reverse osmosis (RO) membranes and mass transfer coefficient (k) in membrane modules. The proposed methodology comprises a set of RO experiments performed at different operating pressures or stages. The measured water and salt fluxes in each stage are simultaneously fitted to the RO transport equations by performing a non-linear regression, using A, B, and k as regression parameters. We first perform a systematic accuracy analysis of the proposed method across the full operational range of RO. The assessment shows that the method accuracy is substantially higher than current methods and increases with number of experimental stages and driving forces. This assessment is used to inform the design of an experimental protocol that minimizes errors in estimated A, B, and k. We then evaluate two commercial RO membranes following the new protocol. For both membranes, A and B parameters decrease by 17% and 15% from the dilute solution to seawater concentrations, whereas the k parameter remains constant. Our study demonstrates that the proposed method, informed by data-driven experimental designs, provides a new approach for accurately characterizing transport phenomenon in membrane processes with feeds of less than 100 g/L total dissolved solids.

Effect of pore size distribution on the desalination performance of the selective layer of nanoporous atomically-thin membranes

Over the last decade, molecular simulations and experiments have shown that nanoporous atomically-thin membranes (NATMs) have the potential for high-permeance, high-selectivity separations, including seawater desalination. Realistic NATMs contain polydisperse pore sizes that could impede their performance, as large pores lead to salt leakage. This paper computationally examines the effect of pore size distributions (PSDs) on the desalination performance of the selective layer of NATMs by reverse osmosis (RO), considering size exclusion as the dominant selection mechanism. Analogous to thin-film composite RO membranes, the finite width of PSDs leads to a trade-off between water permeability and water/salt selectivity in NATMs. Tight PSDs with average pore sizes slightly below the size of the salt are needed to ensure high selectivity comparable to TFC membranes with high water permeance. Sealing of large, salt-permeable pores (e.g. by interfacial polymerization) limits salt leakage but substantially reduces water permeance. Introducing energy barriers that impede salt permeation by tuning nanopore structure and chemistry can make NATMs more robust to wider PSDs. In summary, some combination of control of PSDs, leakage mitigation, and pore functionalization is essential for NATMs to surpass the permeability/selectivity trade-off of polymeric RO membranes and achieve high water permeance with good salt rejection.

Selective separation of dye/salt mixture using diatomite-based sandwich-like membrane

A novel nanofiltration membrane was developed by entrapping a layer of modified diatomaceous earth between two layers of electrospun polysulfone (E-PSf) nanofibers. The diatomaceous earth particles and the fabricated membrane were characterized using FTIR, SEM, EDS, zeta potential, and water contact angle techniques. The static adsorption and dynamic separation of pristine E-PSF and sandwich-like membranes for methylene blue (MB) with/without salt were investigated under different operating conditions. The Langmuir model suited the MB adsorption isotherm data with a linear regression correlation coefficient (R2) >0.9955. As pH increased, both flux and MB rejection of the sandwich-like membrane improved by up to 183.8 LMH and 99.7%, respectively, when operated under gravity. The water flux of the sandwich-like membrane was sharply increased by increasing the pressure up to 19,518.2 LMH at 4.0 bar. However, this came at the expense of MB rejection (10.93%) and reduced its practical impact. At a high salt concentration, the sandwich-like membrane also indicated remarkable dye/salt separation with a higher permeation of salt (<0.2% NaCl rejection) and MB rejection (>99%). The performance of the regenerated diatomaceous material and membrane was maintained during five cycles of operation compared to that of the original ones.

A mechanistic model for salt and water transport in leaky membranes: Implications for low-salt-rejection reverse osmosis membranes

The transport of salt and water through “leaky” membranes is of fundamental importance in many disciplines of science and engineering. One important application is in low-salt-rejection reverse osmosis (LSRRO), an emerging membrane-based brine concentration technology with the potential to reduce the energy consumption and cost of brine management. For further development and optimization of the LSRRO technology, models that accurately describe salt transport through LSRRO membranes are critically needed. In this work, we use the solution-friction model to describe salt transport in LSRRO and demonstrate that the model can be simplified to the classic Spiegler-Kedem-Katchalsky model, where the phenomenological parameters of salt permeability and reflection coefficient are expressed in terms of friction and partitioning coefficients. We obtained membranes with different salt permeabilities by treating commercial polyamide RO membranes with an alkaline chlorine solution. Our results reveal the dependence of the phenomenological membrane parameters on the feed salt concentration, with salt permeability increasing and reflection coefficient decreasing as salt concentration increases. We further quantify the frictions among salt, water, and membrane for the chlorine-treated membranes with different salt permeabilities. We find that the friction between water and membrane is inversely proportional to water permeability, while friction between the salt ions and water is relatively insensitive to the feed concentration and chlorine treatment. Further, the salt-membrane friction coefficient is shown to be controlled by the friction between co-ions and membrane. Overall, our study presents a framework to quantify the frictions between water and salt ions as they move through LSRRO membranes, providing mechanistic insights and accurate model for salt transport in LSRRO.

Theory for salt transport in charged reverse osmosis membranes: Novel analytical equations for desalination performance and experimental validation

Reverse osmosis (RO) is one of the most successful membrane technologies for desalination and contaminant removal from water. RO is applied globally, and can be used for both small- and large-scale applications. To characterize membrane performance, standard testing uses membrane coupons and a NaCl solution in a labscale setup under controlled conditions. Ideally, experiments are done for a range of applied hydrostatic pressures and salt concentrations, with water flux and salt rejection measured in each experiment. This full dataset can then be checked for internal consistency, and all these data must then be described by a comprehensive theoretical framework, i.e., we need an appropriate set of equations to parametrize these data. Parameters derived from this procedure, such as water and salt permeability, can then be compared to those obtained in other studies, for other membranes, salts, or temperatures. If this theory indeed correctly describes data for water flux and salt flux, it can also be applied in larger scale models for RO modules and combinations of modules, which are the basis of engineering design and economic optimization. Herein, we present a novel equation for salt flux that we derive from the full solution-friction (SF) theory. This equation interpolates between an equation for neutral membranes on the one hand, and an equation for highly-charged membranes on the other hand, and thus it is more generally applicable. We apply this new equation to several datasets of seawater RO membranes, and we propose an accurate method to compare the salt permeability of different membranes.