Separation Performance Of Nanofiltration Membranes Research Articles

Tailoring separation performance of nanofiltration membranes by construction of Tin(II)-α-diimine coordination bonds in polyimides

Polyimide nanofiltration (NF) membranes with customizable chemical structures and excellent overall performance have attracted great interest in desalination and wastewater treatment. This work introduced novel Tin(II)-α-diimine coordination interactions to tailor the separation performance of polyimide NF membranes. Microstructural analysis confirmed the successful construction of coordination sites between Tin(II) and the diimine moieties. As expected, the resulting coordinated NF membranes exhibited uniform nanopores and reduced absolute negative charges on their surfaces, facilitating outstanding NF separation. The optimal coordinated polyimide NF membranes exhibited a high rejection rate of over 98 % for methylene blue (MB) and a low rejection rate of less than 2 % for NaCl and Na2SO4, demonstrating significant potential for application in dye desalination. Additionally, the obtained NF membranes demonstrate better long-term separation stability. As a preliminary study, this work provides a new strategy to modulate the microstructure and surface electric charge of NF membranes, potentially leading to greater application potential in dye desalination, seawater desalination, etc.

Extreme Li-Mg selectivity via precise ion size differentiation of polyamide membrane

Achieving high selectivity of Li+ and Mg2+ is of paramount importance for effective lithium extraction from brines, and nanofiltration (NF) membrane plays a critical role in this process. The key to achieving high selectivity lies in the on-demand design of NF membrane pores in accordance with the size difference between Li+ and Mg2+ ions, but this poses a huge challenge for traditional NF membranes and difficult to be realized. In this work, we report the fabrication of polyamide (PA) NF membranes with ultra-high Li+/Mg2+ selectivity by modifying the interfacial polymerization (IP) process between piperazine (PIP) and trimesoyl chloride (TMC) with an oil-soluble surfactant that forms a monolayer at oil/water interface, referred to as OSARIP. The OSARIP benefits to regulate the membrane pores so that all of them are smaller than Mg2+ ions. Under the solely size sieving effect, an exceptional Mg2+ rejection rate of over 99.9% is achieved. This results in an exceptionally high Li+/Mg2+ selectivity, which is one to two orders of magnitude higher than all the currently reported pressure-driven membranes, and even higher than the microporous framework materials, including COFs, MOFs, and POPs. The large enhancement of ion separation performance of NF membranes may innovate the current lithium extraction process and greatly improve the lithium extraction efficiency.

A homogeneous carbon nitride nanomodifier for promoting the water permeation of polyamide desalination membranes

Nanomaterial modification provides an effective way to enhance the separation performance of nanofiltration membranes, but the limited hydrophilicity of conventional nanostructures causes their poor solubility in solvents, affecting the quality of polyamide membranes for performance enhancement. Herein, we explored quasi-water soluble carbon nitride as a homogeneous modifier to improve the nanofiltration performance of polyamide membranes. With sufficient hydroxyl, amino, and cyano groups, the introduced quasi-water soluble carbon nitride significantly enhanced the surface hydrophilicity of the support membrane, resulting in the formation of an ultrathin functional layer for high-efficiency desalination. The high permeation flux of 108.64 L m-2h−1 (4 bar) was even twice and three times higher than those of the pristine polyamide membrane and commercial nanofiltration membranes, with a high Na2SO4 barrier rate of 98.84 % and a Cl-/SO42- selectivity of 76. This work provides an effective way to precisely regulate the structure of nanofiltration membranes for high-performance water purification.

Novel macrocyclic polyamines regulated nanofiltration membranes: Towards efficient micropollutants removal and molecular separation

The discharge of wastewater containing organic micropollutants not only causes severe environmental pollution, but also exacerbates the scarcity of freshwater resources and poses a serious threat to human health. The nanofiltration (NF) membrane has unique advantages and obvious energy-saving effects in the demineralization of organic micropollutants. Poly(piperazine) amide NF membrane exhibits the poor organic micropollutants/salt selectivity and low water permeance. Therefore, developing a novel NF membrane with high water permeance and superior selectivity is of significance. In this study, a loose nanofiltration (LNF) membrane is fabricated by interfacial polymerization with 1, 4, 7, 10-tetraazacyclododecane (Cyclen) as a new aqueous phase monomer to modulate the polyamide microstructure. The steric structure of Cyclen is larger than that of PIP, and the steric hindrance increases, resulting in a low stacking density of the formed polyamide polymer. Compared with the PIP-based NF membrane, the fabricated Cyclen-TMC LNF membrane has a smoother surface, a higher density of carboxyl groups, and a lower degree of cross-linking. The Cyclen-TMC membrane exhibits the considerable water permeance, with the corresponding rejection of 99.9% and 92.7% for methyl blue and tetracycline (pH = 9), respectively, while maintaining a low rejection for monovalent salts (e.g., 4.5% for NaCl). Furthermore, the LNF membrane shows a superior anti-fouling performance with the relatively smooth, highly negatively charged surface and low operation pressure. The novel LNF membrane exhibits a high potential for textile and pharmaceutical wastewater treatment. This study provides a new candidate to regulate the separation performance of NF membranes using new aqueous monomers.

Triethanolamine-modulated interfacial polymerization toward microcrumpled nanofiltration membranes: Performances and mechanisms

Highly permeable thin-film composite (TFC) nanofiltration (NF) membranes featuring high separation efficiencies are of great importance for purifying surface water and ground water. Increasing the filtration area of NF membranes via accurate control of the interfacial polymerization (IP) is proposed to be an efficient way to achieve this goal. In this work, a triethanolamine (TEOA)-modulated IP was utilized to tailor the IP reaction rate and thus regulate the separation performance of NF membranes. The resultant NF membranes were characterized in terms of chemical structures, morphologies, and separation behaviors in detail. The reaction dynamic of the TEOA-doped IP was evaluated by stopped-flow spectroscopy and molecular dynamics. The results showed that the incorporation of TEOA could efficiently regulate the IP reaction rate through hydrogen bonds, increased viscosity, and acid-neutralization effects. As result, a high-performance NF membrane possessing a microcrumpled structure and excellent separation efficiency for ground water was achieved. Hence, the TEOA-modulated IP method is expected to be a maneuverable approach for tailoring excellent NF membranes for ground water treatments.

Study on Spacing Regulation and Separation Performance of Nanofiltration Membranes of GO.

Graphene oxide (GO) membranes have attracted significant attention in the field of water processing in recent years due to their unique characteristics. However, few reports focus on both membrane stability and the “trade-off” effect. In this study, a series of aliphatic diamines (1, 2-ethylenediamine, 1, 4-butanediamine, and 1, 6-hexamethylenediamine) of covalent crosslinked GO were used to prepare diamine-modified nanofiltration membranes, BPPO/AX-GO, with adjustable layer spacing using the vacuum extraction–filtration method. Moreover, Ax-GO-modified nanofiltration membranes modified with adipose diamine had higher layer spacing, lower mass-transfer resistance, and better stability. When the number of carbon atoms was 5, the best layer spacing was reached, and when the number of carbon atoms was greater than 4, the modified membrane nanosheets more easily accumulated. With the increase in layer spacing, the water flux of the composite film increased to 26.27 L/m2·h·bar. Meanwhile, adipose diamine crosslinking significantly improved the stability of GO films. The interception sequence of different valence salts in the composite membrane was NaCl > Na2SO4 > MgSO4, and the rejection rate of bivalent salts was higher than that of monovalent salts. The results can provide some experimental basis and research ideas for overcoming the “trade-off” effect of a lamellar GO membrane.

Fabrication of halloysite nanotubes embedded thin film nanocomposite membranes for dye removal

AbstractEnvironmental friendly Halloysite nanotubes (HNTs) are used to fabricate novel nanofiltration membranes by in situ interfacial polymerization of piperazine and trimesoyl chloride. The removal of excess amine solution from the porous support membrane surface is a critical step to obtain defect free active layer. Hereby, two main removal tools for the excess aqueous amine solution; a rubber roll or air knife are compared to fabricate a defect free thin film nanocomposite (TFN) nanofiltration (NF) membrane. Removal by the rubber roll is eventuated more favorable than air knife in terms of the reproducibility of NF membranes by comparing salt rejections. By determining the removal step of excess amines, various HNTs concentrations are used to fabricate NF membranes and, these membranes are tested with salt and dye solutions at various pH and temperature ranges. R2 membrane (containing 0.02% [w/v] HNTs) performs the best flux results beside higher rejections of MgSO4 (93.0%) and dye (99.5%). To evaluate the extreme conditionals, further performance tests such as pH and temperature resistance are also performed for R2 membrane. Considering the performances of R2 membrane, HNTs can be demonstrated for tailoring the balance between flux and separation performance of NF membranes.

Simulating the separation performance of nanofiltration membranes for salt lake brine

Simulating the separation performance of nanofiltration membranes for salt lake brine

Manipulating the separation performance of nanofiltration membranes by coating thickness of organic phase during interfacial polymerization

ABSTRACTNanofiltration (NF) membranes have excellent separation capabilities and selective performance, and are widely applied in the field of concentration and separation of salt and inorganic salt mixtures. In this article, NF membranes with a higher comprehensive performance were fabricated using a thinner coated organic phase during interfacial polymerization, which greatly reduced the rejection of sodium chloride and significantly improved the water flux without reducing the rejection of magnesium sulfate. Structure comparison found that thinner polyamide separation layer resulted in the higher crosslinking degree, smaller particle, and relatively rough surface. Based on these analyses, we hypothesized that thermodynamically efficiency in the system varies with the coating thickness. Finally, the mechanism of the influence of the coating thickness of organic phase during heat treatment on the interfacial polymerization process and membrane structure was discussed. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48284.

Improvement of MWCO determination by using branched PEGs and MALDI method

For the first time, matrix-assisted laser desorption/ionization (MALDI) method was successfully applied for identification of the concentration of individual polyethylene glycol (PEG) oligomer used in the complex mixture solutions used for evaluation of separation performance of nanofiltration membranes. As a result, it was possible to plot MWCO curve based on the filtration complex solution contained PEG oligomers with a molecular weight of up to 2000 g/mol. To eliminate the influence of porous support, the dense membranes made of two polymers of intrinsic microporosity, poly(1‐trimethylsilyl‐1‐propyne) (PTMSP) and poly(4-methyl-1-pentene) (PMP), were selected in this study. It was found that the molecular weight cut-off (MWCO) parameter for PTMSP and PMP determined by using of linear PEG were as 1550 ± 30 and 910 ± 30 g/mol, respectively. Such noticeable difference in MWCO was explained by the fact that PMP has about 2 times lower fraction accessible volume comparing with PTMSP (FAVPTMSP = 30%, FAVPMP = 16%). It was shown that the branched PEGs, trimethylolpropane ethoxylates (TMPE), can overcome the problem of high flexbility of linear PEGs. For example, the corresponded MWCO values for PTMSP and PMP determined with TMPE were 910 ± 30 and 820 ± 30 g/mol, respectively.

Fabrication of New Photografted Charged Thin Film Composite (TFC) Nanofiltration Membrane Applied to Waste Water Treatment: Effect of Filtration Parameters on the Rejection of Salts and Dyes

Composite nanofiltration (NF) membrane was developed polyacrylic acid (PAA) in situ UV graft polymerization process using ultrafiltration polysulfone (PSf) membrane as porous support. Through the observation using SEM, the PSf UF membranes fabricated by phase inversion method, demonstrated numerous finger-like pores. AFM microscopy showed the roughness of surface was reduced by increase of UV irradiation times. The feasibility of membrane processes for treating simulated mixture by varying the feed pressures (1–4 bars), feed pH and UV irradiation times studied to assess the separation performance of NF membranes. The rejections of sodium chloride and sodium sulfate were moderate and declined with the increase of concentration. Color removal by NF with a high rejection of 99.90% was achieved. It was found that the efficiency of NF membranes used in the treatment of colored water effluents was greatly affected by the presence of salts and dyes in the mixture and the retention of dyes decreased with the salt concentration due to a decrease of the Donnan effect.

Modeling of the separation performance of nanofiltration membranes and its role in the applications of nanofiltration technology in product separation processes

Although there is a voluminous literature on the determination of structural parameters (the pore radius, the ratio of membrane porosity to membrane thickness) of a nanofiltration (NF) membrane and its separation performance (such as the rejection and the permeation flux) by the simplified Teorell-Meyer-Sievers (TMS) model, little of this research comments on other theories and the consequences of linking modeling evaluation to technological application. Theories used to predict the separation performance of an NF membrane usually include: the non-equilibrium thermodynamic model, the pore model, the space charge model, the TMS model, the electrostatic and steric-hindrance model, and the semiempirical model. In the article, we briefly trace the origins or the general ideas of the above-mentioned theories. From there, recent researches on the characterization of membrane structural parameters and electrical properties (such as the surface charge density qw) are reviewed. We then turn to research on the separation performance of an NF membrane for single-component solutions of inorganic electrolytes, neutral organic solutions, and a mixture solution of electrolytes or that of an electrolyte and neutral organic solute. Afterwards, we outline the applications of NF technology in the processes of product separation and conclude with a discussion on the role of models in such applications.

Modeling the separation performance of nanofiltration membranes for the mixed salts solution with Mg 2+ and Ca 2+

Many investigations showed that the dependence of the separation performance of nanofiltration (NF) membrane for the single salt with Mg 2+ or Ca 2+ on the salt concentration was distinguished from that for the salt with Na + or K +. In this paper, the results of the permeation experiments of some single salt solutions with Mg 2+ or Ca 2+ through ESNA 1 membrane showed that their observed transmission decreased with the growth of the concentration. This phenomenon is explained by the counter-ion adsorption of Mg 2+ or Ca 2+ on the surface of NF membrane. In order to evaluate the separation performance of NF membranes for the mixed salts solution with Mg 2+ and Ca 2+, the regulation coefficient was added in the model proposed in our preceding paper. A simple empirical equation was built to calculate the regulation coefficient at the different mixed salts solutions with Mg 2+ and Ca 2+. The empirical parameters were obtained on the basis of the permeation experiments of two type of binary salts solutions ((1) Na +, Mg 2+ and Cl −; (2) Na +, Ca 2+ and Cl −) though three kinds of NF membranes (ESNA 1-LF, ESNA 1 and LES 90). The regulation coefficient decreased with the growth of the total concentration of mixed salts and increased with the growth of the equivalent fraction of Mg 2+ and Ca 2+. The effect of the counter-ion adsorption of Ca 2+ on the effective fixed charge density of NF membrane was more remarkable than that of Mg 2+. Finally some complex mixed salts solutions with Mg 2+ and Ca 2+ were chosen to verify the model with the regulation coefficient. The model evaluation results of the observed transmission of each ion in the mixed salts solution with Mg 2+ and Ca 2+ through ESNA 1-LF, ESNA 1 and LES 90 membranes agreed quite well with the experimental data. The deviation between the model evaluation results and the experimental data of most ions was less than 20% for the case of ESNA 1-LF and LES 90, which of ESNA 1 membrane was less than 10%.

Modeling the separation performance of nanofiltration membranes for the mixed salts solution

A new model is proposed to evaluate the separation performance of nanofiltration (NF) membranes for the mixed salts solution. In the model, the observed transmission of an ion through a NF membrane is applied to express the separation performance of the membrane for the ion in the mixed salts solution, which has a relationship with the total concentration, the equivalent fraction and the species of each ion in the mixed salts solution. The verification of the model was carried out in the permeation experiments of some mixed salts solutions ((1) Na +, Cl − and F −; (2) Na +, K + and Cl −; (3) Na +, F −, Cl − and NO 3 −; (4) Na +, Cl −, NO 3 − and SO 4 2−) through three commercial NF membranes (ESNA 1-LF, ESNA 1 and LES 90). According to the permeation experiments of three NF membranes for some binary salts solutions, the competition coefficients of ions were obtained. The model evaluation results agreed quite well with the experimental data. Finally, the model was applied to evaluate the observed transmission of each ion in the mixed salts solution (Na +, F −, Cl −, NO 3 − and SO 4 2−) through three NF membranes. The agreement between the model evaluation results and the experimental data indicated that the model is suitable for evaluating the separation performance of three NF membranes for the mixed salts solution.

Influence of dyes, salts and auxiliary chemicals on the nanofiltration of reactive dye baths: experimental observations and model verification

The aim of this study was to investigate the effects of dyes, salts and auxiliary chemicals in reactive dye baths on the separation performance of nanofiltration membranes. A reactive dye bath was simulated for this purpose with auxiliary chemicals. A DS5-type nanofiltration membrane was used in the experimental runs. Performance of the nanofiltration membrane was evaluated by measuring permeate flux, salt and color rejections in five steps. Reactive black 5, reactive orange 16, NaCl, NaOH, Na 2SO 4, acidic acid, mollan and slipper were used to prepare synthetic dye baths. Pressures in the range of 8 to 24 bars were applied, and flow velocity was kept constant at 0.74 m/s. NaCl rejection of 20% and color rejection of more than 95% were achieved throughout the experiments. Permeate quality was satisfactory enough to recycle these effluents in reactive dyeing. Acidification ofthe original synthetic dye bath solution with HCl and H 2S0 4 decreased the membrane fouling and also increased the NaCl recovery and color rejection. Besides, using HCI instead of H 2SO 4 increased these positive effects. The effects of auxiliary chemicals were determined by using salt rejection model parameters of α and k D in the presence of an organic ion. There was a correlation among the results of experiments and the model. The model parameters (α and k D were also calculated for all steps.

Experimental investigation on separation performance of nanofiltration membranes for inorganic electrolyte solutions

Permeation experiments of KCl, NaCl, LiCl, MgCl 2, MgSO 4 and K 2SO 4 solutions were carried out using two commercial nanofiltration membranes, which were NF45 (Dow Chemical Corporation) and SU200 (Toray Corporation) membranes. The effects of the type and the concentration of electrolytes as well as the pH value of the electrolyte solution on the separation performance (such as rejections) were investigated. The experimental results showed that NF45 and SU200 membranes possess similar separation potential to the inorganic electrolytes of 1-1 type (LiCl, NaCl and KCl) and much different ones to the other electrolytes containing bivalent ions (MgCl 2, K 2SO 4 and MgSO 4) with different concentrations. The rejections to inorganic electrolytes by the two NF membranes declined with the growth of the electrolyte concentrations and approached some certain values when on much high concentrations (more than 400 mol·m −3), which implied that the size of electrolyte-ions could not be ignored by comparison to the pore radii of NF membranes. The two NF membranes behave almost the same rejections to the electrolytes such as LiCl under various pH values of the feed solution from pH=4 to 10 and behaved the similar minimum values near the isoelectric point of pH=6.5.

Characterisation and prediction of separation performance of nanofiltration membranes

The rejection of single electrolytes at six nanofiltration membranes has been experimentally studied. The membranes were chosen to cover the range from ultrafiltration to reverse osmosis and to represent a diversity of polymers used for membrane fabrication. Such experimental data has been interpreted using a model based on the extended Nernst-Planck equation. The model accounts for the hindered nature of transport in the membranes. Such an interpretation allows a characterisation of the membranes in terms of two parameters, an effective membrane thickness and an effective membrane charge density. The latter may be related to the solution ionic content by means of an isotherm. The method presented also allows an estimation of the effective pore size of the membrane. A knowledge of the effective membrane thickness, effective charge density and effective pore size allows use of the model to predict the separation of mixtures of electrolytes at a membrane. Very good agreement between such a prediction and experimental data has been obtained.