Recent advances in nanofiltration,reverse osmosis membranes and their applications in biomedical separation field

In the face of human society’s great requirements for health industry, and the much stricter safety and quality standards in the biomedical industry, the demand for advanced membrane separation technologies continues to rapidly grow in the world. Nanofiltration(NF) and reverse osmosis(RO) as the highefficient, low energy consumption, and environmental friendly membrane separation techniques, show great promise in the application of biomedical separation field. The chemical compositions, microstr...     

Enhanced forward osmosis from chemically modified polybenzimidazole (PBI) nanofiltration hollow fiber membranes with a thin wall

To develop high-flux and high-rejection forward osmosis (FO) membranes for water reuses and seawater desalination, we have fabricated polybenzimidazole (PBI) nanofiltration (NF) hollow fiber membranes with a thin wall and a desired pore size via non-solvent induced phase inversion and chemically cross-linking modification. The cross-linking by p-xylylene dichloride can finely tune the mean pore size and enhance the salt selectivity. High water permeation flux and improved salt selectivity for water reuses were achieved by using the 2-h modified PBI NF membrane which has a narrow pore size distribution. Cross-linking at a longer time produces even a lower salt permeation flux potentially suitable for desalination but at the expense of permeation flux due to tightened pore sizes. It is found that draw solution concentration and membrane orientations are main factors determining the water permeation flux. In addition, effects of membrane morphology and operation conditions on water and salt transport through membrane have been investigated.     

Polymer-based membranes for solvent-resistant nanofiltration:A review

Separation of organic mixture is an inevitable process in most modern industrial processes. In the quest for a more sustainable and efficient separation, solvent-resistant nanofiltration (SRNF) has emerged as a promising answer. This is because SRNF is a membrane-based process which offers the key advantages of high efficacy and lowenergy intensity separation. In particular, polymer-based membranes can offer compelling opportunities for SRNFwith unprecedented cost-effectiveness.As a result, intensive research efforts have been devoted into developing novel polymer-based membraneswith solvent-resistant capacities as well as exploring potential applications in different types of industries. In this review, we aim to give an overview of the recent progress in the development of the state-of-the-art polymer-based membranes for SRNF in the first section. Emerging nanomaterials for mixed matrix and thin film nanocomposite membranes are also covered in this section. This is followed by a discussion on the current status ofmembrane engineering and SRNFmembrane commercialization. In the third section,we highlight recent efforts in adopting SRNF for relevant industrial applications such as food, bio-refinery, petrochemical, fine chemical and pharmaceutical industries followed by separations of enantiomers in stereochemistry, homogeneous catalysis and ionic liquids. Finally, we offer a perspective and provide deeper insights to help shape future research direction in this very important field of SRNF.     

Thin-film composite membranes for organophilic nanofiltration based on photo-cross-linkable polyimide

This work demonstrates that it is possible to prepare new, competitive thin-film composite (TFC) membranes with a polyolefin ultrafiltration membrane as support and with a non-porous photo-cross-linked polyimide as separation layer for organic solvent nanofiltration. The commercial polyimide Lenzing P84® was modified by a polymer-analogous reaction to introduce side groups with carbon–carbon double bonds to increase its photo-reactivity with respect to cross-linking. Polymer characterization revealed that this was successfully achieved at acceptable level of main chain scission. The higher reactivity of the photo-cross-linkable polyimide had been confirmed by comparison with the original polymer; i.e., shorter gelation times upon UV irradiation, higher suppression of swelling by solvents and complete stability in strong solvents for not cross-linked polyimide such as dimethylformamide (DMF) had been obtained. For films from unmodified and modified polyimide, the degree of swelling in various solvents could be adjusted by UV irradiation time. Photo-cross-linking of the original polyimide did not lead to stability in DMF. TFC membranes had been prepared by polymer solution casting on a polyethylene ultrafiltration membrane, UV irradiation of the liquid film and subsequent solvent evaporation. Polyimide barrier film thicknesses between 10 and 1 μm were obtained by variation of cast film thickness. Performance in organic solvent nanofiltration was analyzed by using hexane, toluene, isopropanol and DMF as well as two dyes with molar masses of ∼300 and ∼1000 g/mol. Permeances of TFC membranes from unmodified polyimide were low (<0.1 L/hm² bar) while rejections of up to 100% for the dye with ∼1000 g/mol could be achieved. TFC membranes from modified and photo-cross-linked polyimide had adjustable separation performance in DMF with a trade-off between permeance and selectivity, in the same range (e.g.: 0.3 L/hm² bar and 97% rejection for the dye with ∼1000 g/mol) as a commercial conventional polyimide membrane tested in parallel. The established membrane preparation method is promising because by tuning the degree of cross-linking of the polymeric barrier layer, the membrane separation performance could be tailored within the same manufacturing process.     

Salt rejection in nanofiltration for single and binary salt mixtures in view of sulphate removal

Three commercial membranes (NF70, NF90 and TFC-SR) were firstly characterized in terms of pure water flux and the rejection of uncharged (alcohols and sugars) compounds. Subsequently, the rejection of monovalent (sodium and chloride) and divalent (calcium and sulphate) ions in single (NaCl, CaCl, and Na₂SO₄) and binary (NaCI/Na₂,SO₄ CaCl₂/CaSO₄, NaCI/CaCl₂, and Na₂SO₄/CaSO₄) salt mixtures was studied. According to the pure water permeability the TFC-SR membrane is a loosely packed NF membrane (12.3 L.m ⁻².h⁻¹.bar⁻¹), while both NF70 and NF90 are tightly packed (2.6 and 3.6 Lm⁻².h⁻¹.bar-). According to the uncharged solute rejection, the MWCO_NF70 = 60, MWCO_NF90= 200 and MWCO_TFC-SR > 500. NF70 and NF90 were equally efficient in rejecting 1-2, 1-1 and 2-1 salts (>90%), while TFC-SR showed typical negatively charged surface behaviour, i.e., R (1-2) salt > R (11) salt > R (2-1). Sulphate rejection decreased in the presence of sodium chloride more significantly than in the presence of calcium chloride due to the more efficient retention of the bivalent calcium.     

Preparation and characterization of polyvinyl chloride based nanocomposite nanofiltration-membrane modified by iron oxide nanoparticles for lead removal from water

In this research (polyvinyl chloride-blend-cellulose acetate/iron oxide nanoparticles) nanocomposite membranes were prepared by casting technique to lead removal from wastewaters. The effect of blend ratio of polymer binder (PVC to CA) and Fe₃O₄ nanoparticles concentration on physico-chemical characteristics of membranes were studied. Water permeability and ionic rejection tests, water content and mechanical properties measurements and SEM analysis were carried out in membranes characterizations. Obviously, modified membrane containing 10 wt% CA and 0.1 wt% Fe₃O₄ nanoparticles showed better performance in lead removal compared to other modified membranes and also pristine ones.     

Correlation between retention and adsorption of phenolic compounds in nanofiltration membranes

Equilibrium adsorption experiments of phenol, 3-chlorophenol, 4-chlorophenol, and 3-nitrophenol aqueous solutions on NF90 membrane were conducted to obtain the corresponding adsorption isotherms at 25 ºC. Single-compound solutions with concentration ranging from 0.1 to 8 mmol L^− 1 were used. Freundlich and Langmuir models were compared to the experimental isotherms and their characteristic parameters were obtained from linear fits. In addition, the adsorptive behaviour of twelve aqueous phenolic compounds on the NF90 membrane was studied in order to investigate the relationship between adsorption and retention of selected solutes. An inverse correlation between the adsorbed amount, at the same equilibrium concentration (1 mmol L^− 1), and retention was found. The influence of the molecular hydrophobicity and dipole moment of phenolic compounds on membrane adsorption, solute retention and water flux decline was also investigated.     

Characteristics of thin-film nanofiltration membranes at various pH-values

Salt rejection and ion selectivity of NF-255 and NF-45 nanofiltration (NF) membranes were investigated. The rejection of two cations (Na⁺, Ca²⁺) and two anions (Cl⁻, SO_4²⁻) which are common in natural and in industrial wastewater, were studied as a function of pH at permanent pressure and temperature. The ion rejection of NF membranes were investigated in single salt solutions like NaCl, CaCl₂, Na₂SO₄, CaSO₄, and in multicomponent systems that contained all the previous ions. We found that, there is a minimum rejection of the Na⁺ and Cl⁻ ions between pH 4-5 in NF-255 and between pH 7-8 in NF-45. The rejection of calcium ions were increased in each case at lower pH in both membranes. However the pH value where the ion rejection behaviour of membranes changed, were different: pH 4 in NF-255 and pH 8 in NF-45. In NF-45 the chloride ion has negative rejection which depends on the quality of ions and the pH. We found that below pH values of 4 the selectivity of mono- and multivalent cations considerable increased in NF-255. This phenomena may be used for separation of calcium ions from sodium ions from weakly acidic (hydrochloric and sulfuric acid) solution, e.g. regeneration solution of sodium form softening ion exchangers.     

Thin-film composite nanofiltration membranes prepared by electropolymerization

A novel approach to preparation of composite asymmetric nanofiltration membranes is reported based on a thin selective layer deposited by electropolymerization (EP) on top of an asymmetrically porous and electronically conductive porous support. Support films with ultrafiltration characteristics were cast from a concentrated dispersion of carbon black particles, a few tens of nanometers large, in a solution of polysulfone followed by precipitation in a non-solvent bath (phase inversion). Composite membranes with poly(phenylene oxide) and polyaniline thin top layers were prepared by EP deposition from solutions of phenol and aniline, respectively, of which polyaniline film demonstrated a dense uniform structure and water flux and rejection to sucrose and magnesium sulfate in the nanofiltration range.     

Color and COD retention by nanofiltration membranes

In the present study the application of the nanofiltration process was investigated mainly in the retention ofcolor and chemical oxygen demand (COD) present in textile industry wastewater. Nanofiltration experiments were carried out in a pilot unit, operating in crossflow. Three different types of spiral wound membranes, DK 1073, NF 45 and MPS 31 were used simultaneously in the same unit. The results of the tests showed that for color retention, the values were around 99% for the DK 1073 and NF 45 membranes and the 87% for COD retention for the DK 1073. The permeate flux for the different wastewaters varied from 30.5 to 70 L/h.m². Fouling was observed in all membranes due to the accumulation of molecular species close to the filtering surface. The process was efficient and promising for the reuse of wastewater from this type of industry.     

Graphene-based membranes for pervaporation processes

Two-dimensional graphene and its derivatives exhibiting distinct physiochemical properties are intriguing building blocks for researchers from a large variety of scientific fields. Assembling graphene-based materials into membrane layers brings great potentials for high-efficiency membrane processes. Particularly, pervaporation by graphene-based membranes has been intensively studied with respect to the membrane design and preparation. This review aims to provide an overview on the graphene-based membranes for pervaporation processes ranged from fabrication to application. Physical or chemical decoration of graphene-based materials is elaborated regarding their effects on the microstructure and performance. The mass transport of pervaporation through graphene-based membranes is introduced, and relevant mechanisms are described. Furthermore, performances of state-of-the-art graphene-based membranes for different pervaporation applications are summarized. Finally, the perspectives of current challenges and future directions are presented.     

Reduction of disinfection by-products in natural waters using nanofiltration membranes

Three commercial nanofiltration (NF) membranes, models NF 270, NF 90 (Dow Chemical Inc.) and Desal-HL-51 (GE Water), have been used to investigate trihalomethane and haloacetic acid formation potential (THMFP and HAAFP); the influence of conductivity on permeate flow-rate was also studied. Natural waters from two different sources in Alicante province (SE Spain) were analyzed.At the end of the NF experiments on water from the Amadorio reservoir, the Desal-HL-51, NF 270 and NF 90 membranes exhibited a 20, 25 and 45% drop in permeate flow-rate, respectively. The drop in flow-rate increased with conductivity for all of the membranes, as it becomes evident when comparing the experiments at conductivities of 700 and 6000 µS/cm.Over 90% of the formation potential of bromodichloro methane and dibromochloro methane was eliminated using the NF 90 membrane. The NF 270 and Desal-HL-51 membranes produced smaller decreases in THMFP. Among the haloacetic acids, dichloroacetic acid was least reduced, by 60% and 30% for waters from the Amadorio and Pedrera reservoirs, respectively. The analytes differed considerably.     

Selectivity of polyamide nanofiltration membranes for cations and phosphoric acid

The results shown in this paper are part of a study to investigate the use of nanofiltration membranes for the recycling of phosphorus from sewage sludge. It especially contains the results of an experimental study about the influencing factors pressure, pH and feed composition on the selectivity of nanofiltration membranes for phosphoric acid and multivalent cations. For the use of pretreated sewage sludge an effective pressure of 11 bar could be identified as an effective operational point. The selectivity turned out to increase with decreasing pH which was studied in a pH-interval of pH 1-4. An increasing concentration of multivalent cations in the feed solution had a negative effect on the permeability of phosphoric acid. Accordingly it could be observed that the nanofiltration worked significantly more effective with sewage sludge ash eluates of the concentration 10 g/l than eluates of the concentration 100 g/l.     

Rejection of salt mixtures from high saline by nanofiltration membranes

Nanofiltration (NF) membranes have recently been employed as pretreatment unit operations in seawater desalination processes and as partial demineralization to seawater. The present paper investigates the performance of selected commercial NF membranes to reject salts of high concentrations at salinity levels representative of brackish and sea water. Two commercial nanofiltration membranes (NF90 and NF270) have been investigated in detail to study their performance in filtering aqueous solutions containing different salt mixtures in a cross-flow NF membrane process within the pressure range from 4 to 9 bar. Spiegler-Kedem model (SKM) was used to fit the experimental data of rejection with the permeate flux. The results showed that NF90 membrane was shown to have a distinct ability to reject both monovalent and divalent ions of all investigated mixtures with very reasonable values but with relatively low flux. This will make NF90 more suitable for the application in the pretreatment of desalination processes. On the other hand, NF270 can reject monovalent ions at relatively low values and divalent ions at reasonable values, but at very high permeate flux. The SKM model only fitted well the experimental data of divalent ions in salt mixture. Based on the evaluation of the overall performance of NF90 and NF270 membranes, their distinct ability to reject salts at high salinity from seawater is considered an advantage in the field of pretreatment of seawater feed to desalination units.     

Influence of chemical speciation on the separation of metal ions from chelating agents by nanofiltration membranes

The simultaneous separation of various metal ions (nickel, copper, calcium, and iron) from chelating agents (EDTA and citric acid) in water streams using Nanofiltration membranes is analyzed. Assuming that multiply-charged species are highly rejected, chemical speciation computations reproduce the observed patterns of metal and ligand rejection at different pH values and concentrations. The separation of metal ions from citric acid is achieved in acidic conditions, where multiply-charged free metal ions and neutral or singly charged free chelating species are abundant. Overall, speciation studies help to evaluate the applicability of Nanofiltration for recycling chelating agents used for metal extraction.     

Fouling of nanofiltration and ultrafiltration membranes applied for wastewater regeneration in the textile industry

Textile effluents usually contain high concentrations of inorganics as well as organics, and the therefore difficult to treat. Membrane processes can be used for many of these wastewaters in the textile industry. Two typical examples are discussed: (1) the use of nanofiltration for the treatment of exhausted dye baths, in view of water recycling, and (2) the use of ultrafiltration for the removal of spin finish from waste water resulting from rinsing of textile fibres. Both applications are in principle feasible, but in practice the process is negatively influenced by membrane fouling. In the first application, fouling is assumed to be caused by (ad)sorption of organic compounds, which has a large influence because of the high concentrations used in textile dyeing. Furthermore, the high salt concentrations result in a decrease of the effective driving force because of the high osmotic pressures obtained for typical dye baths. Experimental results are discussed, and the applicability of nanofiltration is related to the characteristics of the dye baths for different dyeing methods. In the second application, the concentration of organic compounds is relatively low, but because of the hydrophobic nature of the spin finish compounds, a significant effect of membrane fouling is expected. An improvement is suggested by using nanofiltration membranes instead of ultrafiltration membranes.     

Dual layer composite nanofiltration hollow fiber membranes for low-pressure water softening

Nanofiltration (NF) membrane process has become increasingly attractive due to their unique characteristics to selectively remove specific compounds or ions. The most commonly NF membranes are negatively charged which is unsuitable for hardness removal. Therefore, the development of novel NF membranes with a positively charged skin has become a key issue for low pressure water softening.     

Negatively charged organic–inorganic hybrid silica nanofiltration membranes for lithium extraction

Effective extraction of lithium from high Mg~(2+)/Li+ratio brine lakes is of great challenge. In this work, organic–inorganic hybrid silica nanofiltration(NF) membranes were prepared by dip-coating a 1,2-bis(triethoxysilyl)ethane(BTESE)-derived separation layer on tubular TiO_2 support, for efficient separation of LiC l and MgCl_2 salt solutions. We found that the membrane calcinated at 400 °C(M1–400) could exhibit a narrow pore size distribution(0.63–1.66 nm) owing to the dehydroxylation and the thermal degradation of the organic bridge groups. All as-prepared membranes exhibited higher rejections to LiCl than to MgCl_2, which was attributed to the negative charge of the membrane surfaces. The rejection for LiCl and MgCl_2 followed the order: LiCl N MgCl_2, revealing that Donnan exclusion effect dominated the salt rejection mechanism. In addition, the triplecoated membrane calcined at 400 °C(M3–400) exhibited a permeability of about 9.5 L·m~(-2)·h~(-1)·bar~(-1) for LiCl or MgCl_2 solutions, with rejections of 74.7% and 20.3% to LiCl and MgCl_2,respectively, under the transmembrane pressure at 6 bar. Compared with the previously reported performance of NF membranes for Mg~(2+)/Li+separation, the overall performance of M3–400 is highly competitive. Therefore, this work may provide new insight into designing robust silica-based ceramic NF membranes with negative charge for efficient lithium extraction from salt lakes.