Preparation of yttria-stabilized ZrO2 nanofiltration membrane by reverse micelles-mediated sol-gel process and its application in pesticide wastewater treatment

8 mol% yttria-stabilized ZrO₂ (8YSZ) nanofiltration (NF) membranes were prepared from size-controlled spherical ZrO₂ nanoparticles with an average diameter of ˜10 nm and a particle roundness value greater than 0.90, and the nanoparticles were efficiently fabricated by a reverse micelles (RMs)-mediated sol-gel process. It was found that yttria doping not only suppressed the tetragonal to monoclinic (t-m) phase transition, ensuring the membranes integrity, but also decreased the tetragonal grain size, increased the specific surface area, narrowed the pore size distribution and thus optimized the NF performance. The as-prepared 8YSZ NF membranes with a thickness of ˜260 nm exhibited high NF performances, while the pure water permeability and molecular weight cut-off (MWCO) were 3.9-4.2 L m⁻² h^-1 bar^-1 and 800 ± 50 Da respectively. In the treatment of pesticide wastewater, the removal rate of carbofuran by 8YSZ NF membranes was more than 82%, while the maximum removal rate could reach 89%. Furthermore, the contaminated membranes could be restored as ever after alkali wash and low-temperature calcination, implementing multiple reuses.     

Effect of sol size on nanofiltration performance of a sol-gel derived microporous zirconia membrane

This paper reports the effect of sol size on nanofiltration performances of sol–gel derived microporous zirconia membranes. Microstructure, pure water flux, molecular weight cut-off (MWCO) and salt retention of zirconia membranes derived from zirconia sols with different sizes were characterized. Thermal evolution, phase compo-sition, microstructure and chemical stability of unsupported zirconia membranes (powder) were determined by thermogravimetric and differential thermal analysis, X-ray diffraction, nitrogen adsorption–desorption and static solubility measurements. Results show that nanofiltration performance of zirconia membranes is highly depen-dent on sol size. The sol with an average size of 3.8 nm, which is smaller than the pore size of theγ-Al2O3 support (pore size:5–6 nm), forms a discontinuous zirconia separation layer because of excessive penetration of sol into the support. This zirconia membrane displays a MWCO value towards polyethylene glycol higher than 4000 Da. A smooth and defect-free zirconia membrane with a MWCO value of 1195 Da (pore size:1.75 nm) and relative high retention rates towards MgCl2 (76%) and CaCl2 (64%) was successfully fabricated by dip-coating the sol with an appropriate size of 8.6 nm. Zirconia sol with an average size of 12 nm exhibits colloidal nature and forms a zirconia membrane with a MWCO value of 2332 Da (pore size:2.47 nm). This promising microporous zirconia membrane presents sufficiently high chemical stability in a wide pH range of 1–12.     

Application of piezoelectric quartz for self-cleaning membrane preparation

Separation membranes that are prepared from piezoelectric ceramics can generate ultrasound on-line to maintain surface cleanliness. Here, a lead-free piezoelectric support is presented consisting of quartz. The Na₂O and Al₂O₃ were employed as sintering aids to improve the mechanical performance of piezoelectric quartz support while avoid the formation of no-piezoelectric cristobalite. A ZrO₂-based thin microfiltration membrane layer was applied on the optimized support. The membrane structure, thus obtained had an average pore size and ultrasonic emission of 270 nm and 5.1 mV, respectively. The stationary permeance of the membrane in the treatment of oil-in-water emulsion was 163 L m^?2?h^?1?bar^?1 (LMH/bar). With the application of alternating voltages of 60 V and 100 V, the permeance increased to 198 and 225 LMH/bar, respectively, and the oil rejection was maintained above 97%. The in-situ ultrasound directly acted on membrane surface, so it removed more fouling resistance at lower power than the external one.     

Effect of boehmite sol on the performance of alumina microfiltration membranes

Precursor film method was used to prepare highly permeable ceramic microfiltration membranes in this work. The performance of alumina microfiltration membranes was improved by adding boehmite sol in the membrane precursor film. With increasing the boehmite sol content, the effective average pore size of the membrane was continuously decreased and the separation efficiency of the membrane was increased. These improvements were due to that the boehmite sol not only helped the dispersion of the α-Al₂O₃ powder in the membrane forming slurry, but also formed γ-Al₂O₃ in the gaps among the existed α-Al₂O₃ particles which was beneficial for membrane sintering and pore size decreasing. For the membrane prepared with 45 wt% boehmite sol, the effective average filtration pore diameter was 178 nm and the water permeance of the membrane reached 1691 Lm⁻²h⁻¹bar⁻¹, which was much better than the values reported before. Moreover, the reusability of the membrane was confirmed using a recycling test.     

Inkjet Printing of Microporous Silica Gas Separation Membranes

Inkjet printing was applied to manufacture silica‐based gas separation membranes, which were coated on a pore‐graduated alumina substrate with a mesoporous γ‐alumina interlayer. A silica sol diluted by 1‐propanol was used to print the membrane layer followed by thermal treatment in a rapid thermal processing furnace. The membrane thickness was varied between 30 and 110 nm by conducting one, two, and three coating steps. In the latter case, H₂ permeance in the range of 2.0 × 10^?8–3.3 × 10^?8 mol/s·m²·Pa combined with H₂/CO₂ selectivities in the range of 15–25 were achieved, proving the concept that inorganic gas separation membranes can be successfully processed by inkjet printing.     

Nano-Modification of the Polyvinyl Alcohol/Organic Acid-Modified Polyvinylidene Fluoride Thin-Film Composite Membrane and Its Application in the Nanofiltration Process

In this study, a novel thin-film nanocomposite (TFN) membrane is developed consisting of a cross-linked nano-modified polyvinyl alcohol (PVA) selective layer on an organic acid-modified polyvinylidene fluoride (PVDF) membrane. The nano-modification of the PVA layer is performed via incorporating different amounts of the amine-functionalized multiwalled carbon nanotubes (MWCNTs-NH₂) into the PVA matrix. The effect of citric acid on the chemical structure and morphology of the PVDF support is also investigated. The performance of the resultant membranes in the nanofiltration (NF) of MgSO₄ and acid yellow-17 aqueous solutions is also studied. The results indicate that the modification of the support with 0.5 wt% of citric acid increased the water permeance from 1.59 L m⁻² h⁻¹ bar⁻¹ (LMH/bar) for PVA/PVDF to 4.49 LMH/bar for the PVA/modified PVDF membrane. Furthermore, the optimum value of MWCNT-NH₂ (0.6 wt%) increases the permeance of the resultant TFN membrane to 4.94 LMH/bar while maintaining a high rejection. Interestingly, the incorporation of MWCNT-NH₂ into the PVA layer and citric acid into the PVDF solution results in a membrane with the highest permeance of 6 LMH/bar.     

Gas permeation properties of silica membranes with uniform pore sizes derived from polyhedral oligomeric silsesquioxane

The sol‐gel method was applied in the fabrication of homogenous polyhedral oligomeric silsesquioxane (HOMO‐POSS)‐derived silica membranes. Single gas permeation characteristics in a temperature range of 100–500^°C were examined to discuss the effect of silica precursor on amorphous silica networks. HOMO‐POSS‐derived membranes showed a CO₂ permeance of 1.1 × 10^?7 mol m^?2 s^?1 Pa^?1 with a CO₂/CH₄ permeance ratio of 131 at 100^°C, which is a superior CO₂/CH₄ separation performance by comparison with tetraethoxysilane (TEOS)‐derived silica membranes. Normalized Knudsen‐based permeance (NKP) was applied for quantitative evaluation of pore size. HOMO‐POSS‐derived membranes had loose amorphous silica structures compared to TEOS‐derived membranes and pore size was successfully tuned by changing the calcination temperatures. The activation energy for a HOMO‐POSS‐derived membrane fired at 550^°C with a uniform pore size of ～ 0.42 nm increased linearly with the ratio of the kinetic diameter of the gas molecule to the pore diameter, λ (=d_k/d_p), and showed a trend similar to that of DDR‐type zeolite membranes. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1733–1743, 2012     

Evaluating the chemical stability of metal oxides in SO3 and applications of SiO2-based membranes to O2/SO3 separation

The decomposition of sulfur trioxide to produce sulfur dioxide and oxygen using a catalytic membrane reactor is technology that promises to improve the economic viability of the thermochemical water-splitting Iodine-Sulfur (IS) process for large-scale CO₂-free hydrogen production. The chemical stability of membrane materials under SO₃, however, is a significant challenge for this strategy. In this study, microporous membranes with a layered structure that consisted of a membrane support prepared from α-Al₂O₃, an intermediate layer prepared from silica-zirconia, and a top layer prepared from bis (triethoxysilyl)ethane-derived organosilica sols, were examined for stability under SO₃ and for use in SO₃/O₂ separation. An α-Al₂O₃ support that features SiO₂–ZrO₂ intermediate layers with large pore sizes and a high Si/Zr molar ratio showed excellent resistance to SO₃, which was confirmed by N₂ adsorption, Energy Dispersive X-ray Spectroscopy (EDS), and Scanning Electron Microscopy (SEM). These membranes also demonstrated a negligible change in gas permeance before and after SO₃ exposure. Subsequently, in binary-component gas separation at 550°C, microporous organosilica-derived membranes achieved an O₂/SO₃ selectivity of 10 (much higher than the Knudsen selectivity of 1.6) while maintaining a high O₂ permeance of 2.5 × 10⁻⁸ mol m^–2 s^–1 Pa^–1.     

Zirconium carbide oxidation: Kinetics and oxygen diffusion through the intermediate layer

Oxidation of hot‐pressed ZrC was investigated in air in the 1073‐1373 K range. The kinetics were linear at 1073 K, whereas at higher temperature samples initially followed linear kinetics before undergoing rapid oxidation leading to a Maltese cross shape of the oxide. The linear kinetics at 1073 K was governed by inward oxygen diffusion through an intermediate layer of constant thickness between ZrC and ZrO₂ which was comprised of amorphous carbon and ZrO₂ nanocrystals. Diffusion of oxygen through the intermediate layer was measured to be 9 × 10^?10 cm² s^?1 using ¹⁸O as a tracer in a double oxidation experiment in ¹⁶O/¹⁸O. Oxidation at 1073 and 1173 K produced samples made of m‐ZrO₂ and either t‐ or c‐ZrO₂ with an adherent intermediate layer made of amorphous carbon and ZrO₂, whereas oxidation at 1273 and 1373 K produced samples with a voluminous oxide made of m‐ZrO₂ showing a gap between ZrC and the oxide. A substoichiometric zirconia layer was found at the gap at 1273 K and no carbon uptake was detected in this layer when compared with the top oxide layer. The loss of the intermediate layer and the slowdown of the linear rate constant (g m^?2 s^?1) at 1273 K compared to 1173 K was correlated with the preferential oxidation of carbon at the intermediate layer which would leave as CO and/or CO₂ leaving a gap between ZrC and substoichiometric zirconia.     

Preparation and characterization of Al3+-doped TiO2 tight ultrafiltration membrane for efficient dye removal

Al³⁺-doped TiO₂ (AT) tight ultrafiltration membrane with stable anatase phase was prepared by a modified sol-gel process using butyl titanate and aluminum chloride as the precursor and aluminum source respectively. The removal of Alizarin red-S was investigated by filtration experiment. A dip-coating process on homemade flat Al₂O₃ intermediate layer by TiO₂ sol followed by heat treatment was adopted to obtain the desired AT membrane. The addition of Al³⁺ inhibits the phase transformation of nanosized TiO₂ from anatase to rutile and restrains the growth of crystallite, resulting in the pore size of the separation layer reducing to 3.5 nm. The prepared AT1-500 membrane exhibits enhanced hydrophilicity with no cracks or pinholes, and shows a water permeability of 9.6 L m^-2 h⁻¹ bar⁻¹ and cut-off molecular weight (MWCO) of 4650 Da. The membrane demonstrated a retention rate of 96.9% for Alizarin Red-S (250 ppm) and maintained almost constant under repeated using.     

Pilot-scale spiral wound membrane assessment for THM precursor rejection from upland waters

The outcomes of a pilot-scale study of the rejection of trihalomethanes (THMs) precursors by commercial ultrafiltration/nanofiltration (UF/NF) spiral-wound membrane elements are presented based on a single surface water source in Scotland. The study revealed the expected trend of increased flux and permeability with increasing pore size for the UF membranes; the NF membranes provided similar fluxes despite the lower nominal pore size. The dissolved organic carbon (DOC) passage decreased with decreasing molecular weight cut-off, with a less than one-third the passage recorded for the NF membranes than for the UF ones.

The yield (weight % total THMs per DOC) varied between 2.5% and 8% across all membranes tested, in reasonable agreement with the literature, with the aromatic polyamide membrane providing both the lowest yield and lowest DOC passage. The proportion of the hydrophobic (HPO) fraction removed was found to increase with decreasing membrane selectivity (increasing pore size), and THM generation correlated closely (R² = 0.98) with the permeate HPO fractional concentration.     

Hydrothermal stability of fluorine-induced microporous silica membranes: Effect of steam treatment conditions

A fluorine-SiO₂ membrane was prepared using triethoxyfluorosilane (TEFS) as a Si precursor, and its hydrothermal stability was evaluated. The TEFS membrane calcined at 750°C had fewer Si-OH and Si-F groups in its network structure and showed H₂ permeance that was greater than 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ with H₂/N₂ and N₂/SF₆ permeance ratios of 10 and 210, respectively. This membrane performance was relatively stable under the temperature (< 500°C) used for steam treatment, regardless of the steam partial pressure (30, 90 kPa). On the other hand, when the steam treatment temperature was increased beyond 500°C, gas permeance decreased significantly and the membrane became highly selective for He and H₂ over smaller molecules (He/N₂: > 600, H₂/N₂: > 100). The relationship between the activation energy of H₂ and the permeance ratios (He/H₂, He/H₂O, H₂/H₂O) of a TEFS-derived membrane under steam treatment higher than 600°C resulted in a network pore size that approximated in conventional microporous SiO₂ membranes.     

Hydrogen Permeation Properties and Hydrothermal Stability of Sol–Gel‐Derived Amorphous Silica Membranes Fabricated at High Temperatures

The sol–gel method was applied to the fabrication of amorphous silica membranes for use in hydrogen separation at high temperatures. The effects of fabrication temperature on the hydrogen permeation properties and the hydrothermal stability of amorphous silica membranes were evaluated. A thin continuous silica separation layer (thickness = <300 nm) was successfully formed on the top of a deposited colloidal silica layer in a porous glass support. After heat treatment at 800°C for an amorphous silica membrane fabricated at 550°C, however, it was quite difficult to distinguish the active separation layer from the deposited colloidal silica layer in a porous glass support, due to the adhesion of colloidal silica caused by sintering at high temperatures. The amorphous silica membranes fabricated at 700°C were relatively stable under steam atmosphere (500°C, steam = 70 kPa), and showed steady He and H₂ permeance values of 4.0 × 10^?7 and 1.0 × 10^?7 mol·m^?2·s^?1·Pa^?1 with H₂/CH₄ and H₂/H₂O permeance ratios of ~110 and 22, respectively. The permeance ratios of H₂/H₂O for membranes fired at 700°C increased drastically over the range of He/H₂ permeance ratios by factors of ~3–4, and showed a value of ~30, which was higher than those fired at 500°C. Less permeation of water vapor through amorphous silica membranes fabricated at high temperatures can be ascribed to the dense amorphous silica structure caused by the condensation reaction of silanol groups.     

Effect of rheological properties of AlOOH sol on the preparation of Al2O3 nanofiltration membrane by sol-gel method

In order to directly prepare an High Flux Al₂O₃ nanofiltration membrane on an Al₂O₃ support with an average pore size of 4 μm, AlOOH sol was prepared with aluminum isopropoxide as the precursor, The effect of rheology on the dip-coating of AlOOH sol and the effect of viscoelasticity on the heat treatment of AlOOH gel film to prepare defect-free Al₂O₃ nanofiltration membrane were studied. The results indicate that AlOOH sol will increase its viscosity with the increase of the standing time. When the viscosity increases to a certain extent, the colloidal particles will gradually transform into gels, and change from Bingham fluid to Herschel-Bulkley pseudoplastic fluid. The thickness of the AlOOH gel film is related to the viscosity of the AlOOH sol. The flow viscosity of AlOOH sol should be about 0.0025～0.005 Pa·s, while the thickness of the AlOOH gel film after dip-coating is about 6.5～12 μm. The storage modulus and loss modulus of AlOOH gel film increase with the increase of aging time. Only when the storage modulus of the AlOOH gel is greater than the saturated vapor pressure of the solvent under normal pressure (0.1 MPa), it will not crack due to the evaporation of the sol during the heat treatment process. After the storage modulus exceeds 0.1 MPa, the surface of the heat-treated Al₂O₃ ceramic membrane is smooth and crack-free, the rejection rate for crystal violet dye is 99.8%, and its average pore size is 2.75 nm, that has the capability of nanofiltration. Due to the lack of intermediate layer, the pure water flux of the Al₂O₃ nanofiltration membrane is as high as 201.7 l.m^-2bar^-1h^-1, and the steady-state filtration flux is 48.7 l.m^-2bar^-1h^-1 when filtering 20 mg/l crystal violet solution. By controlling the rheological properties of AlOOH sol, a high flux Al₂O₃ nanofiltration membrane was prepared.     

Deposition of thin ultrafiltration membranes on commercial SiC microfiltration tubes

Porous SiC based materials present high mechanical, chemical and thermal robustness, and thus have been largely applied to water-filtration technologies. In this study, commercial SiC microfiltration tubes with nominal pore size of 0.04 μm were used as carrier for depositing thin aluminum oxide (Al₂O₃) ultrafiltration membranes. These ultrafiltration membranes were obtained by coating, drying and calcination of a colloidal suspension of boehmite particles. After calcination, the membrane material consisted of nanosized γ-Al₂O₃ crystallites and had a narrow pore size distribution with average pore size of 5.5 nm. Membrane thickness was tuned by repeating the coating of boehmite sol. By doing so, we were able to reduce the defect density on the membrane surface, as evidenced by SEM analysis and by the significant reduction of water permeance after depositing the second γ-Al₂O₃ layer. After five times coating, a 5.6 µm thick γ-Al₂O₃ layer was obtained. This membrane shows retention of ~75% for polyethylene glycol molecules with M_n of 8 and 35 kDa, indicating that, despite their intrinsic surface roughness, commercial SiC microfiltration tubes can be applied as carrier for thin ultrafiltration membranes. This work also indicates that an improvement of the commercial SiC support surface smoothness may greatly enhance permeance and selectivity of γ-Al₂O₃ ultrafiltration membranes by allowing the deposition of thinner defect-free layers.     

Zirconia ultrafiltration membranes on silicon carbide substrate: microstructure and water flux

This study reported the preparation of ZrO₂/SiC ceramic membrane with silicon carbide as the substrate and intermediate layers and zirconia as the selective layer. The substrate and intermediate layers were sintered by evaporation-condensation process at 2200 and 1900 ℃, respectively. After sintering, the intermediate layer presented layer thickness of 50 μm, pore size of 0.87 μm and pure water permeability of 2140 L/(m²·h). The selective lay was deposited on the silicon carbide substrate by dip-coating method and then sintered in the temperature range from 800 to 1000 ℃. For the membrane coated by one dip-coating cycle and sintered at 800 ℃, it presented average pores of 82 nm and water flux of 850 L/(m²·h). Due to the exclusion of low-melting oxides during sintering, the ZrO₂/SiC ceramic membrane can satisfy the separation and purification of chemical corrosion and high temperature wastewater.     

Mesoporous nanocrystalline zirconium oxide: novel preparation and photoluminescence property

A novel strategy involving the combination of soft-templating and solid–liquid method (CSSL) is presented to synthesize mesoporous nanocrystalline zirconia with high specific surface area, that is, the mesostructured zirconia hybrid is firstly synthesized via cooperative assembly between zirconium sulphate as inorganic precursor and 1-hexadecyl-3-methylimidazolium bromide (C₁₆mim⁺Br⁻) as the structure-directing agent, and subsequently ground with solid magnesium nitrate salt followed by heat-treatment in air. The resulting zirconia material after calcination at 600 °C possesses a wormlike arrangement of mesopores surrounded by tetragonal ZrO₂ nanocrystallites of ca. 2.3 nm. The BET surface area is 255 m²/g and the pore size is ca. 4.3 nm. However, no mesoporous structure exists in the obtained zirconia material via the simple soft-templating method at the same calcination temperature. Photoluminescence (PL) spectra of the obtained mesoporous nanocrystalline ZrO₂ show a strong emission peak at ca. 394 nm under UV excitation of 250 nm wavelength.     

Characteristics of ammonia permeation through porous silica membranes

A sol–gel method was applied for the preparation of silica membranes with different average pore sizes. Ammonia (NH₃) permeation/separation characteristics of the silica membranes were examined in a wide temperature range (50–400°C) by measurement of both single and binary component separation. The order of gas permeance through the silica membranes, which was independent of membrane average pore size, was as follows: He > H₂ > NH₃ > N₂. These results suggest that, for permeation through silica membranes, the molecular size of NH₃ is larger than that of H₂, despite previous reports that the kinetic diameter of NH₃ is smaller than that of H₂. At high temperatures, there was no effect of NH₃ adsorption on H₂ permeation characteristics, and silica membranes were highly stable in NH₃ at 400°C (i.e., gas permeance remained unchanged). On the other hand, at 50°C NH₃ molecules adsorbed on the silica improved NH₃‐permselectivity by blocking permeation of H₂ molecules without decreasing NH₃ permeance. The maximal NH₃/H₂ permeance ratio obtained during binary component separation was ～30 with an NH₃ permeance of ～10^?7 mol m^?2 s^?1 Pa^?1 at an H₂ permeation activation energy of ～6 kJ mol^?1. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Thin-film nanofibrous composite membranes containing cellulose or chitin barrier layers fabricated by ionic liquids

The barrier layer of high-flux ultrafiltration (UF) thin-film nanofibrous composite (TFNC) membranes for purification of wastewater (e.g., bilge water) have been prepared by using cellulose, chitin, and a cellulose-chitin blend, regenerated from an ionic liquid. The structures and properties of regenerated cellulose, chitin, and a cellulose-chitin blend were analyzed with thermogravimetric analysis (TGA) and wide-angle X-ray diffraction (WAXD). The surface morphology, pore size and pore size distribution of TFNC membranes were determined by SEM images and molecular weight cut-off (MWCO) methods. An oil/water emulsion, a model of bilge water, was used as the feed solution, and the permeation flux and rejection ratio of the membranes were investigated. TFNC membranes based on the cellulose-chitin blend exhibited 10 times higher permeation flux when compared with a commercial UF membrane (PAN10, Sepro) with a similar rejection ratio after filtration over a time period of up to 100 h, implying the practical feasibility of such membranes for UF applications.     

Development of stabilized zirconia–alkali salts dual membranes for carbon dioxide capture

Molten Na₂CO₃–K₂CO₃ (NKC, 56–44 mol%) eutectic compositions were vacuum-impregnated, at the eutectic temperature, into two porous ZrO₂:8.6 mol% MgO (magnesium-partially stabilized zirconia, MgPSZ) and ZrO₂:8 mol% Y₂O₂ (yttria-fully stabilized zirconia, 8YSZ) ceramics. Thermogravimetric analyses were performed in mixtures of that composition with MgPSZ and 8YSZ ceramic powders. Before impregnation, porosity was achieved in the two compounds by addition and thermal removal of 30 vol.% NKC. To ascertain the carbonates had filled up through the ceramic body, both sides of the parallel and fracture surfaces of the disk-shaped impregnated compositions were observed in a scanning electron microscope and analyzed by energy-dispersive X-ray spectroscopy. The electrical conductivity of the two ceramics, before and after impregnation, was evaluated by electrochemical impedance spectroscopy in the 5 Hz–13 MHz frequency range from approximately 530 to 740°C. The permeation of the carbonate ions through the membranes via the eutectic composition was assessed by the threshold temperatures of the onset of the carbonate ion percolation. The objectives were to prepare dual-phase membranes for the separation of carbon dioxide and for the development of carbon dioxide sensors.