Inorganic Nanofiltration Membranes Research Articles

Ultrahydrophilic Inorganic Nanosheet-Based Nanofiltration Membranes for High Efficiency Separations of Inorganic Salts and Organic Dyes.

Two-dimensional (2D) inorganic nanomaterials have garnered extensive attention in the fabrication of inorganic nanofiltration membranes due to their unique structures and properties. In this study, we developed a facile process for fabricating large-scale ultrahydrophilic nanofiltration membranes using layered titanate H1.07Ti1.73O4·nH2O nanosheets (HT-ns). A drying deposition process was used to fabricate HT-ns membranes on a poly(tetrafluoroethylene) (TF) substrate. To enhance the bonding strength between the substrate and the deposited HT-ns membrane, the substrate surface was modified with a Cu2+-adsorbed silane monomolecular layer, connecting a negatively charged HT-ns membrane and a positively charged substrate surface. The fabricated HT-ns membrane exhibited an excellent rejection performance for inorganic salts and dye molecules. The ultrahydrophilicity of HT-ns membrane with a low water contact angle of 31° results in an ultrafast water permeance, which is approximately 6 times higher than that of a simple graphene-based nanofiltration membrane. The results open a new avenue to a new category of ultrahydrophilic nanofiltration membranes.

Permeation Properties of Ions through Inorganic Silica-Based Membranes.

The development of inorganic membranes has mainly found applicability in liquid separation technologies. However, only a few reports cite the permeation and separation of liquids through inorganic nanofiltration membranes compared with the more popular microfiltration membranes. Herein, we prepared silica membranes using 3,3,3-trifluoropropyltrimethoxysilane (TFPrTMOS) to investigate its liquid permeance performance using four different ion solutions (i.e., NaCl, Na2SO4, MgCl2, and MgSO4). The TFPrTMOS-derived membranes were deposited above a temperature of 175 °C, where the deposition behavior of TFPrTMOS was dependent on the organic functional groups decomposition temperature. The highest membrane rejection was from NaCl at 91.0% when deposited at 200 °C. For anions, the SO42− rejections were the greatest. It was also possible to separate monovalent and divalent anions, as the negatively charged groups on the membrane surfaces retained pore sizes >1.48 nm. Ions were also easily separated by molecular sieving below a pore size of 0.50 nm. For the TFPrTMOS-derived membrane deposited at 175 °C, glucose showed 67% rejection, which was higher than that achieved through the propyltrimethoxysilane membrane. We infer that charge exclusion might be due to the dissociation of hydroxyl groups resulting from decomposition of organic groups. Pore size and organic functional group decomposition were found to be important for ion permeation.

Bioinspired Modification of Layer-Stacked Molybdenum Disulfide (MoS2) Membranes for Enhanced Nanofiltration Performance.

Inorganic nanofiltration membranes with high flux are urgently needed in water purification processes. Herein, polydopamine (PDA)-modified layer-stacked molybdenum disulfide (MoS2) nanofiltration membranes (NFMs) were fabricated via a pressure-assisted self-assembly process. The separation performance of the as-prepared membranes with various MoS2 loadings at different dopamine polymerization times was evaluated. The pure water permeance of PDA-modified MoS2 NFMs, with MoS2 loading of 0.1103 mg/cm2 at 4 h modification, could reach 135.3 LMH/bar. The rejection toward methylene blue could reach 100% with molecular weight cutoff approximately 671 Da and a high permeability of salts. Furthermore, the resultant membrane also exhibited a satisfactory long-term stability toward dye solution and antifouling property toward bovine serum albumin. This work may give inspiration to the development of inorganic membranes with high performance, especially high pure water permeance, for water-related processes.

Experimental investigation of inorganic nanofiltration membranes prepared by atomic layer deposition and sol-gel methods for saltless water softening

This study explored saltless water softening with inorganic Nanofiltration (NF) membranes prepared by Atomic Layer Deposition (ALD) and a Tetraorthosilicate (TEOS) sol-gel process applied by the slip cast method. The experimental investigations were performed on different material-based inorganic NF membranes, including titanium dioxide (TiO2), alumina (Al2O3), silver (Ag) and iron oxide (Fe2O3). The experimental results demonstrated that saltless water softening can be achieved with high flux through the viable rejection of Ca2+ with an initial rate between 7.1% and 34.1% in a single pass flow through filter in a solution of 0.5 g L–1 CaCl2 at neutral pH. A maximum rejection of 28.7% and 29.5% was achieved for Ca2++ at pH 4 and pH 10, respectively. The highest hardness rejection was achieved with a 25% waste water volume. In the recirculating loop tests with the same solution, a rejection rate of 9% was achieved within 15 min at a flow rate of 0.3 L min–1.

Separation of lanthanides(III) by inorganic nanofiltration membranes using a water soluble complexing agent

First separation of Ln(III) using an inorganic nanofiltration membrane has been achieved. Ln(III) ions are strongly repelled by the ZrO 2/TiO 2 membrane. When diethylenetriaminepentaacetic acid (DTPA) is used as water soluble complexing agent, the complexes formed with Ln(III), surprisingly passes through the membrane. The stability constant of DTPA with Eu 3+ is higher than the one with La 3+. Thus [EuDTPA] 2− complex is formed at lower pH than [LaDTPA] 2− and so these two elements can be separated. With an equimolar mixture of Eu 3+ and La 3+, a La 3+/Eu 3+ separation factor of 2.5 is obtained at pH 1.9. Other polyaminocarboxylic acids have been used such as EDTA and HEDTA and similar phenomenon occur. The best La 3+/Eu 3+ separation factor has been obtained with HEDTA. In that case, the La 3+/Eu 3+ separation factor is 7.5 at pH 2.3. All these ligands have been also evaluated with a TiO 2 nanofiltration membrane and results are comparable.

Coupled series–parallel resistance model for transport of solvent through inorganic nanofiltration membranes

In this work, permeation of a number of different classes of solvents through commercial TiO 2 and ZrO 2 ceramic nanofiltration membranes was measured. A wide variety in permeate flux levels were observed. Solvent permeation was found to be a function of solvent characteristics (viscosity, surface tension, effective molecular diameter and dielectric constant) along with membrane characteristics (surface tension, average pore size diameter and dielectric constant). This paper also presents a new mathematical model based on parallel convection–diffusion theory which relates permeation of solvent molecules to three different resistances in the surface and the main body of the membrane. Transport resistances are defined by easily measurable solvent and membrane properties as indicated above. The model was tested on a series of permeability experiments of water, acetonitrile, methanol, methyl ethyl ketone, ethylacetate, ethanol, tetrahydrofuran, 1-propanol, 1-butanol, toluene, cyclohexane and n-hexane, carried out with HITK275 and HITK2750 membranes (Inoceramic, Germany). Compared to the existing models, which are able to make predictions only for a given class of solvents (polar or apolar), the new model can surprisingly fit the results of the permeation study for all solvent classes, and even for water. The maximum deviation from experimental results was calculated as 4.8%.

NaCl rejection by an inorganic nanofiltration membrane in relation to its central pore potential

The effects of pH and ionic strength on the performance of an iron-modified silica inorganic nanofiltration (NF) membrane were evaluated. Inorganic NF membranes rely heavily on the charge exclusion mechanism to promote electrolyte rejection. However, the charge characteristics on the membrane surface are altered with changes in feed solution pH and ionic strength conditions. Another important aspect of charge exclusion is the need for double layer overlap within the pore, but the attenuation of the potential with distance is rarely evaluated to determine the extent of this potential overlap. In this paper, the Gouy-Chapman model was used to calculate the potential distribution across the pore and evaluate the magnitude of the potential at the center of the pore for a range of pore sizes. A quantitative relationship was then built to relate the potential at the pore center with the rejection of NaCl by the membrane under various feed solution conditions. The potential at the center of the pore could be decreased by increasing the ionic strength of solution and/or by decreasing the pH. A decrease in the potential at the pore center led to a decrease in membrane rejection of NaCl. Using the rejection and potential data acquired through this investigation, an operational pore diameter was defined that coincided well with the pore diameter calculated using nitrogen adsorption isotherms. Other physical traits of the iron-modified silica material are also evaluated to give a complete description of the membrane's attributes.

Retention of heavy metal ions with nanofiltration inorganic membranes by grafting chelating groups

Abstract New membranes with complexing abilities were obtained by grafting titanate coupling agents bearing ethylenediamine groups (EDA, strong chelating ligand towards Cu 2+ ) or pyrophosphate groups (PP, weak chelating ligand towards Cu 2+ and Zn 2+ ) on inorganic nanofiltration membranes. The retention of 10 mmol l −1 chloride solutions of Cu 2+ and Zn 2+ by the unmodified NF membrane (no complex formation) and the PP grafted membrane (formation of weak complex) was about 50% which is roughly the retention of Ca 2+ (no complex formation) with the EDA grafted membrane. Complex formation with the PP grafted membrane does not appear strong enough to enhance Zn 2+ and Cu 2+ retention. The retention of Cu 2+ was enhanced up to 80–90% with the EDA grafted membrane, due to the occurrence of formation of a strong complex with EDA groups. Contrary to resin process where a breakthrough curve is expected, a steady-state of Cu 2+ retention was observed, whereas Cu 2+ in the feed solution was about 50 times the membrane capacity to bind the metal cation.

Permeation of Liquids through Inorganic Nanofiltration Membranes

Inorganic nanofiltration membranes were prepared using silica–zirconia composite sols by the sol–gel method and applied for the permeation experiments of nonaqueous solutions. Pure solvent permeabilities of methanol, ethanol, and 1-propanol were measured at temperatures from 25 to 60°C. For membranes having pore sizes as large as 70 nm, the permeation mechanism of alcohols obeys the viscous flow mechanism irrespective of types of alcohols. On the other hand, the permeation mechanism through porous silica–zirconia membranes having pore diameters of 1 to 5 nm was found to be different from the viscous flow mechanism; small molecules showed larger permeabilities than large molecules. Activation energies of solvent permeability were found to be larger for large molecules and for membranes of smaller pore size.

Temperature effect on transport performance by inorganic nanofiltration membranes

AbstractThe effect of temperature on nanofiltration performance was examined using three inorganic membranes with a molecular‐weight cutoff of approximately 200, 600, and 2,000, respectively. The inorganic porous membranes were prepared from silica‐zirconia colloidal sols and used in nanofiltration experiments for neutral solutes over a temperature range of 20 to 60°C. The rejection of solutes decreased with an increase in temperature for the membranes, while the permeate volume flux increased. Three transport coefficients—refection coefficient, solute permeability, and water permeability—were obtained using the Spiegler‐Kedem equation, which accounts for the contribution of convection and diffusion to solute flux. As a result, the reflection coefficient corresponding to the fraction of solutes reflected by the membrane in convective flow was almost constant, irrespective of experimental temperature. Solute permeabilities, however, increased with temperature. The dependency was larger for larger solutes and membranes with smaller pore diameters. Therefore, the hindered diffusion of solutes through micropores was indicative of an activated process. Moreover, pure water permeability, after correction for the temperature effect on viscosity, also increased with experimental temperature.

Silica–zirconia membranes for nanofiltration

Inorganic nanofiltration membranes were fabricated from silica–zirconia composite colloidal sol (molar ratio Si/Zr=9/1) using a sol–gel process. Molecular weight cut-off (MWCO) was successfully controlled between 200 and 1000 Da by regulating the colloidal diameters of sol solutions in the final coating stage. The pure water permeabilities ranged from 0.15×10 −11 to 1.5×10 −11 m 3 m −2 s −1 Pa −1. Pore size and pore size distribution were estimated based on the dynamic method of humid air permeation, and found to be from 1.0 to 2.9 nm. The MWCO obtained from NF experiments using neutral organic solutes corresponds well with the pore diameters estimated from the dynamic permeation method. Silica–zirconia membranes were found to be stable in aqueous solution for periods in excess of four months.

Ionic interactions in nanofiltration of beta casein peptides

Nanofiltration (NF) membrane technology shows interesting potentials for separating organic components on the basis of solute charge and size in the range of 300-1000 g mol-1. Separation properties of two inorganic NF membranes were studied with a set of 10 small peptides (molecular mass range: 300-900 g mol-1; 3 < pI < 10) contained in a well-characterized tryptic beta casein hydrolysate. Peptides transmission strongly depended on ionic interactions in the system. Physicochemical conditions such as ionic strength and especially pH were crucial to the separation, because the membrane and peptides showed amphoteric properties. Thus, the three categories of peptides (acid, basic, neutral) were separated according to their pI because of presumed concentration gradients of charged peptides at the membrane: positive for basic peptides and negative for acid peptides. At optimum pH 8 this led to high transmissions of basic peptides (even over 100%), intermediate transmissions for neutral peptides, and low transmissions for acid peptides. The addition of multicharged cationic and anionic species in the hydrolysate induced a markedly enhanced selectivity when the polyelectrolyte was a membrane coion and a complete reversion of selectivity when it was a membrane counterion. Copyright 1998 John Wiley & Sons, Inc.