Surface silylation and pore structure development of silica aerogel composites from colloid and TEOS-based precursor

Flexible aerogel-fiber composites were prepared by silylation and ambient drying of colloidal silica and tetraethylorthosilicate (TEOS)-based sol. After immersing glass fiber matrices into silica sol with colloid-based, colloid/TEOS-based, and TEOS-based silica sol, it was surface-modified in a trimethylchlorosilane/n-hexane solution and heat-treated at 230 °C in ambient atmosphere. Surface silylation of silica aerogel synthesized from colloid and TEOS-based silica sols showed different behaviors. For colloid silica gel, it was comprised of small sized mesopores because colloid-based silica gel has dense networks through great degrees of hydrolysis and condensation. On the contrary, TEOS-based aerogel was consisted of relatively large-sized pores because of comparatively lesser degree of hydrolysis and condensation. Through this study, we can know that the pore structures of silica aerogel could be controlled by choosing colloid or TEOS-based precursor and surface silylation reaction.     

Preparation and properties of novel environment-sensitive membranes prepared by graft polymerization onto a porous membrane

The present work is concerned with the preparation and some properties of novel environment-sensitive membranes. A porous poly (vinylidene fluoride) membrane (pore size 0.22 μm) was pretreated by air plasma; subsequently, hydrophilic monomers were graft polymerized on the treated surface. Since the filtration characteristics of the obtained membranes reflect the configuration of the grafted chains, these can be changed reversibly from ultrafilter to microfilter and vice versa in response to the membrane environment such as pH, solvent composition and ionic species. Grafted chains act as a sensor and a valve to regulate filtration characteristics. The poly(acrylic acid) grafted membrane for example is very sensitive to environmental pH. In the pH region of 1 to 5, the filtration rate sharply decreased with increasing solution pH, the filtration rate at pH 1.4 being about ten times higher than at pH 5.2. Together with this decrease in filtration rate, the membrane gained the ability of ultrafiltration of macromolecular solutes such as dextran (M_w = 2,000,000) and albumin (M_w = 67,000). In the pH region of 5.2 to 7.5, filtration rate and solute rejection did not depend on pH. The pH sensitivity is reversible and reproducible. Because of characteristics such as the drastic alteration in filtration rate and solute separation properties and the quick response to solution conditions, the environment-sensitive membranes developed here may find applications in various areas of membrane technology.     

Characterization of the pore area distribution in porous membranes using transport measurements

An approach originally proposed by Mason and coworkers has been applied to model porous membranes to show that transport measurements with small and large solutes can be used to distinguish between porous membranes with the same average pore size but different pore size distributions. In addtion, it is shown that such measurements can be used to account for membrane heteroporosity when predicting the sieving characteristics of a membrane. This is done by applying moment theory to results from flux measurements for a small solute at Pe ≈ 1 or to results from measurements of the reflection coefficient for a large solute at infinite Pe. No a priori assumptions about the nature of the distribution of pore areas are necessary.In this paper, the results from calculations performed with three different model membranes with log-normal pore size distribution are reported. These results show that one can begin to distinguish between membranes by measuring the hydraulic and diffusive permeability and performing at least one additional flux measurement — with either a small, non-hindered solute at Pe ≈ 1 or a large solute at infinite Pe. Results also show that a fairly narrow window can be placed on the sieving curve for a heteroporous membrane without performing any sieving measurements. This is an interesting and encouraging result because it means that many of the problems that arise from measuring and interpreting pore size distributions using more traditional techniques can be avoided by using small solute flux measurements to predict the separation characteristics of many porous membranes.     

Microfiltration process by inorganic membranes for clarification of TongBi liquor

Membrane separation is an alternative separation technology to the conventional method of filtration. Hence, it has attracted use in the purification and concentration of Chinese Herbal Medicine Extracts (CHMEs). The purpose of this work was to study the process of microfiltration of Tongbi liquor (TBL), a popular Chinese herbal drink, using ceramic membranes. Zirconium oxide and aluminum oxide membranes with pore mean sizes of 0.2 μm and 0.05 μm, respectively, are used for comparisons in terms of flux, transmittance of the ingredients, physical-chemical parameters, removal of macromolecular materials and fouling resistance. The results show that 0.2 μm zirconium oxide membrane is more suitable. The stable permeate flux reaches 135 L·h(-1)·m(-2), the cumulative transmittance of the indicator is 65.53%. Macromolecular materials, such as starch, protein, tannin, pectin and total solids were largely eliminated in retentate after filtration using 0.2 μm ZrO2 ceramic membrane, resulting in clearer TBL. Moreover, this work also reveals that continuous ultrasound could strengthen membrane process that the permeate flux increases significantly. This work demonstrates that the purification of CHME with ceramic membranes is possible and yielded excellent results.     

Synthesis of SiO2@SiO2 core-shell microspheres using urea-formaldehyde polymers as the templates for fast separation of small solutes and proteins

The monodisperse superficially porous core-shell silica microspheres (CSSMs) with controllable shell thickness and pore size were synthesized by an improved polymerization-induced colloid aggregation (PICA) approach for fast separation of small solutes and proteins.     

Heavy metals and color retention by a synthesized inorganic membrane

In this study, a new type of a double-layer ceramic membrane was used for the filtration of wastewater. The synthesized membrane consists of a macroporous substrate (with pore size of about 0.1 μm) prepared following the colloid filtration technique and a thin film functional layer (with pore size of about 10 nm) carried out according to the sol–gel preparation method.The ceramic membranes were tested for the removal of cadmium, zinc, Methylene Blue and Malachite Green from water under a pressure of 5 bar and a treatment time of 2 h. Liquid filtration and flow tests through these membranes resulted in a rejection rate of 100% for Methylene Blue and Malachite Green. This paper also presents the ability of the tubular membrane prepared to separate heavy metals (cadmium and zinc) from their synthetic aqueous solutions. The influence of the applied pressure, feed solute concentration, feed pH on the rejection of cadmium and zinc ions was studied. Retention rates of cadmium and zinc ions of 100% were observed for an initial feed concentration of 10⁻⁴ mol/L.     

Adsorption effects in rejection of macromolecules by ultrafiltration membranes

Rejection of adsorbing solutes by ultrafiltration membranes is not adequately described by the steric rejection theory [3]. Solute adsorption (fouling) changes the shape of the rejection curve. Typically, the measured curves are steeper than the theoretical curve. The shape of the curve can be predicted qualitatively from simple theoretical considerations. For adsorbing solutes, single-solute and multiple-solute ultrafiltration experiments give different results. Relative thickness of adsorbed solute layer in a membrane pore was found to depend on (1) solute size, (2) solute hydrophobicity, (3) pH and ionic strength for a protein solute, (4) solute concentration, and (5) time of adsorption. Large differences observed between water fluxes and fluxes of very dilute polymer solutions through the same membrane are also interpreted in terms of solute adsorption.     

Morphological changes of ethylene/vinyl acetate-based controlled delivery systems during release of water-soluble solutes

Thin slabs of theophylline and monomer albumin release systems were prepared by dispersing 212-300 μm and 300-25 μm particles respectively, of these bioactive agents in a methylene chloride solution of ethylene/vinyl acetate (EVAc) copolymer (40 wt% vinyl acetate), and evaporating the solvent at low temperatures according to the Langer—Folkman technique. Compositions containing 21.41 wt%, 31.04 wt% and 40.0 wt% albumin, and 19.32 wt% theophylline were prepared. Solute release experiments were performed in deionized water at 37 ± 0.1°C under perfect-sink conditions. The concentration of released solute was determined by measuring the absorbance of the UV spectra at 276 nm for albumin and 272 nm for theophylline. Both solutes could be released for long periods of time at controlled rates. The main mechanism of release was established to be solute dissolution and diffusion through the generated, waterfilled pore structure. Photomicrographs present the main features of this pore network. Mercury porosimetry was used to determine the pore volume and size of pores for freezedried slabs before, during and after the dissolution/diffusion/release process. Considerable pore collapse was observed and pore diameters of 8-650 μm were detected. In addition to solution diffusion through large pores, diffusion might occur through small constrictions between large pores or through a pore network of much smaller pores created in the matrix.     

Self-forming dynamic membrane for ultrafiltration of pineapple juice

The formation of self-forming dynamic membrane on a porous ceramic support was studied. Pineapple juice of 12° Brix concentration was used in the experiments which were carried out at 25°C by circulating the pineapple juice at the applied pressure of 100, 200, and 300 kPa and at cross-flow velocities of 1.30–2.95 m s⁻¹ through the ceramic membrane module for 1 h. The experimental data of flux and rejections showed that the dynamic membrane was well-formed after 30 min of circulation under the applied pressure of 300 kPa and at a cross-flow velocity of 2.0 m s⁻¹ in which the steady values of flux and rejections of macromolecules and sugars obtained from the filtration mode were 6.0×10⁻³ m³/m² h, 84–87% and 6%, respectively. The corresponding values for ultrafiltration by alumina membrane of MW cut-off 50,000, using equivalent conditions, were 15.8×10⁻³ m³/m² h, 91% and 10.5%. Ultrafiltration was found to be more promising. The stability of the self-forming dynamic membrane was acceptable when subjected to change of filtration conditions. The permeation flux increased with cross-flow velocity and decreased when the applied pressure was reduced. The resistances for filtration by dynamic membrane and by ultrafiltration were calculated. For a porous support of large pore sizes, an in-pore blockage of solutes which were smaller than the membrane pores reduced the pore volume and induced fouling. Internal fouling resistance (R_f) was, therefore significant and responsible for the values of flux and rejection and was approximately 70% of total resistance. While in ultrafiltration, in which membrane with a smaller pore diameter was used, R_f was only 20% but R_p, the polarized layer resistance, was as high as 60% of total resistance.     

Membrane microfiltration of oily water

Separation of oil in water emulsion was carried out by crossflow microfiltration using 3 types of microporous glass tubular membrane with different pore size of 0.27, 0.75, and 1.47 μm. The effect of pore size on permeate flux and oil rejection was investigated and the filtration mechanisms were analyzed based on various types of filtration models.     

Influence of the concentration of a low-molecular organic solute on the flux reduction of a polyethersulphone ultrafiltration membrane

Drastic flux reductions are sometimes encountered during ultrafiltration of solutes much smaller than the membrane pores. This usually occurs during ultrafiltration of hydrophobic, low-molecular solutes, such as fatty acids, alcohols and alkanes. The influence of the concentration of a carboxylic acid, octanoic acid, on the flux of a polyethersulphone membrane was studied in this investigation. The concentration was found to have a marked influence on the flux. The flux reduction was moderate at low concentrations, but became severe above a certain, critical concentration. Two flux-reducing mechanisms were evaluated; reduction of the effective pore radius by adsorption of solute molecules on the pore walls, and blocking of pores by capillary condensation. The adsorption of octanoic acid on a hydrophobic solid surface was studied by null ellipsometry.     

Crossflow microfiltration of oily water

Two α-alumina ceramic membranes (0.2 and 0.8 μm pore sizes) and a surface-modified polyacrylonitrile membrane (0.1 μm pore size) were tested with an oily water, containing various concentrations (250–1000 ppm) of heavy crude oil droplets of 1–10 μm diameter. Significant fouling and flux decline were observed. Typical final flux values (at the end of experiments with 2 h of filtration) for membranes at 250 ppm oil in the feed are ≈30–40 kg m⁻² h⁻¹. Increased oil concentrations in the feed decreased the final flux, whereas the crossflow rate, transmembrane pressure, and temperature appeared to have relatively little effect on the final flux. In all cases, the permeate was of very high quality, containing <6 ppm total hydrocarbons. The addition of suspended solids increased the final membrane flux by one order of magnitude. It is thought that the suspended solids adsorb the oil, break up the oil layer, and act as a dynamic or secondary membrane which reduces fouling of the underlying primary membrane. Resistance models were used to characterize the type of fouling that occurs. Both the 0.2 μm and the 0.8 μm ceramic membranes appeared to exhibit internal fouling followed by external fouling, whereas external fouling characterized the behavior of the 0.1 μm polymer membrane from the beginning of filtration. Examination of the external fouling layer showed a very thin hydrophobic oil layer adsorbed to the membrane surface. This oil layer made the membrane surface hydrophobic, as demonstrated by increased water-contact angles. The oil layer proved resistant to removal by hydrodynamic (shear) methods. By extracting the oil layer with tetrachloroethylene, followed by IR analysis, its average thickness at the end of a 2 h experiment under typical conditions was determined to be 60 μm for the 0.2 μm ceramic membrane and 30 μm for the 0.1 μm polymer membrane. These measured amounts of oil associated with the membrane at the end of the experiments are in good agreement with those determined from a simple mass balance, in which it is assumed that all of the oil associated with the permeate collected is retained on or in the membrane, indicating that the tangential flow did not sweep the rejected oil layer to the filter exit.     

Theoretical effects of macromolecule concentration and charge on membrane rejection coefficients

A hydrodynamic model for the convection of rigid, spherical solutes through cylindrical pores, which includes both steric and electrostatic interactions between pairs of solute particles and between solutes and the pore wall, has been developed to examine the effects of solute concentration and charge on solute rejection by membrane pores during ultrafiltration. Calculations have been performed for a wide range of charge conditions and the results are presented in terms of the membrane rejection coefficient at infinite dilution and a correction factor which accounts for the first-order effects of concentration. For pores and solutes of like charge, the rejection coefficient is predicted to decrease with increasing feed concentration or ionic strength.     

The effect of solute—membrane affinity on cyclic hydrocarbon—water transport in pressure-driven membrane separation processes

Experimental results for the pressure-driven membrane separation of cyclic hydrocarbons (1,3-cyclohexadiene, cyclohexene, and cyclohexane) from dilute binary aqueous solution using asymmetric cellulose acetate membranes are reported here. In these experiments, total solution fluxes are significantly lower than pure water fluxes at the same applied pressure; this flux reduction is attributed to strong solute—membrane affinity rather than to the osmotic pressure of either the bulk retentate or the boundary layer. An empirical parameter, Z, is used to describe flux reduction. A theoretically based friction parameter, B, is derived assuming the membrane can be represented as an ideal, finely porous membrane; this parameter indicates the influence of solute—membrane affinity on flow through the pores of the membrane. Both the empirical parameter Z and the theoretically based parameter B relate flux reduction to concentrations in the system. Both Z and B increase as solute—membrane affinity increases and decrease as membrane pore size increases. It is concluded that both the empirical flux reduction parameter, Z, and the theoretically based friction parameter, B, indicate the same system properties: solute—membrane affinity and membrane pore size.     

Nanofiltration of biogenic amines in acidic conditions: Influence of operation variables and modeling

Biogenic amines are present in some fermented and non-fermented beverages and can cause diseases. This study analyzes the feasibility of separating biogenic amines by nanofiltration in acidic medium. Solutions of chloride salts of three biogenic amines: putrescine, histamine and tyramine were filtered through a nanofiltration membrane with a 1000 Da molecular weight cut-off (MWCO) and a positive electrical charge at pH 3. Increasing the transmembrane pressure or cross flow velocity led to an increase in solute rejection and permeate flow. Moreover, a higher electrical charge or lower concentration of amine cations caused a larger rejection indicating that membrane-solutes repulsion governs the filtration process. Finally, the experimental results were analyzed using the classic Donnan–Steric pore model. Values of 0.83 nm and 5.4 μm were estimated for pore radius and membrane effective ratio thickness-porosity from the filtration of neutral solutes. Membrane volumetric charge density and the proton diffusivity inside the pores were estimated from the experimental results.     

Flux reduction of ultrafiltration membranes with different cut-off due to adsorption of a low-molecular-weight hydrophobic solute-correlation between flux decline and pore size

The flux of ultrafiltration membranes may be severely reduced when treating low-molecular-weight hydrophobic solutes even though the cut-off of the membrane is orders of magnitudes greater than the size of the solute molecules. In this investigation, the flux reduction was correlated to the membrane pore size using octanoic acid as a model substance. As a comparison, the pore size was also determined by measuring the retention of a dextran solution and by using the liquid–liquid displacement porometry method (LLDP). The membranes used were four asymmetric polysulphone and polyethersulphone membranes with nominal molecular weight cut-off (NMWCO) between 6 and 50 kDa. It is shown that the use of a low-molecular-weight hydrophobic solute may provide a rapid and simple method of characterising hydrophobic ultrafiltration membranes, both regarding their sensitivity to flux reduction due to adsorption, and their pore-size distribution.     

Fouling behavior of microstructured hollow fiber membranes in dead-end filtrations: critical flux determination and NMR imaging of particle deposition

The fouling behavior of microstructured hollow fibers was investigated in constant flux filtrations of colloidal silica and sodium alginate. It was observed that the fouling resistance increases faster with structured fibers than with round fibers. Reversibility of structured fibers' fouling was similar during silica filtrations and better in sodium alginate filtrations when compared with round fibers. The deposition of two different silica sols on the membranes was observed by NMR imaging. The sols had different particle size and solution ionic strength and showed different deposition behaviors. For the smaller particle-sized sol in deionized solution (Ludox-TMA), there was more deposition within the grooves of the structured fibers and much less on the fins. For the alkali-stabilized sol Bindzil 9950, which had larger particles, the deposition was homogeneous across the surface of the structured fiber, and the thickness of the deposit was similar to that on the round fiber. This difference between the deposition behavior of the two sols is explained by differences in the back diffusion, which creates concentration polarization layers with different resistances. The Ludox sol formed a thick polarization layer with very low resistance. The Bindzil sol formed a slightly thinner polarization layer; however, its resistance was much higher, of similar magnitude as the intrinsic membrane resistance. This high resistance of the polarization layer during the Bindzil sol filtration is considered to lead to quick flow regulation toward equalizing the resistance along the fiber surface. The Ludox particles were trapped at the bottom of the grooves as a result of reduced back diffusion. The fouling behavior in sodium alginate filtrations was explained by considering the size-dependent deposition within the broad alginate size distribution. The better reversibility of fouling in the structured fibers is thought to be the result of a looser deposit within the grooves, which is more easily removed than a compressed deposit on the round fibers.     

Theory of pressure-driven transport of neutral solutes and ions in porous ceramic nanofiltration membranes

Hindered transport theory and homogeneous electro-transport theory are used to calculate the limiting, high volume flux, rejection of, respectively, neutral solutes and binary electrolytes by granular porous nanofiltration membranes. For ceramic membranes prepared from metal oxides it is proposed that the membrane structural and charge parameters entering into the theory, namely the effective pore size and membrane charge density, can be estimated from independent measurements: the pore radius from the measured hydraulic radius using a model of sintered granular membranes and the effective membrane charge density from the hydraulic radius and the electrophoretic mobility measurements on the ceramic powder used to prepare the membrane. The electro-transport theory adopted here is valid when the membrane surface charge density is low enough and the pore radius is small enough for there to be strong electrical double layer overlap in the pores. Within this approximation the filtration streaming potential is also derived for binary electrolytes.     

Nanofiltration studies of larger organic microsolutes in methanol solutions

Multistep organic solvent-based pharmaceutical syntheses of larger organic microsolutes having molecular weights (MW) in the range of 300–1000 generally require athermal separation processes because the active molecules and the intermediates are thermally labile. To that end, nanofiltration (NF) of methanol solutions of three selected solutes, safranin O (MW 351), brilliant blue R (MW 826) and vitamin B₁₂ (MW 1355) has been studied in a batch stirred cell for a dilute solution of each individual solute at 3034 kPa (440 psig). The solvent-resistant membranes investigated and their manufacturer-specified molecular weight cut-offs (MWCO) are MPF-44 (250), MPF-50 (700) and MPF-60 (400). During an initial transient period, the solvent flux decreased with time and the solute rejection increased with time for every membrane reaching a steady state after about 12 h. This behavior resulting from membrane compaction and pore size reduction was partially reversible. Additional studies using higher feed solute concentrations (1 and 3 wt.%) show considerable reduction in solvent flux and increase in solute rejection; the effect appears to be far more than that due to an increase in osmotic pressure and possible reasons for such a behavior have been suggested. The observed solute rejection values are generally significantly lower than the manufacturer-specified MWCO values. Additional studies varying the feed solution pressure through the membrane MPF-60 indicate that the variation of the percent rejection of solutes safranin O and brilliant blue R with the solvent flux tends to follow the relation suggested by the Finely Porous Model.     

Use of gas-liquid porometry measurements for selection of microfiltration membranes

The process of crossflow microfiltration is hindered by the significant problem of fouling due to a pore size which favours penetration of the solutes. This leads to an internal fouling (adsorption and pore obstruction) which reduces permeate flux and makes any regeneration difficult. This study outlines a method of choosing an appropriate microfiltration membrane. Choice of membrane nature and pore size has been made in accordance with rapid dead-end filtration tests and the use of liquid-gas permporometry. Measuring pore size by porometry allows a choice of material which is non-adsorbent with regard to specific solutions to be microfiltered. Moreover, the internal fouling can be detected quickly by backflush washing after several minutes of dead-end filtration, and by measuring pore size distribution of the fouled membrane. Thus, choice of pore size will tend towards a membrane which bears slight internal fouling. The methodology described in this paper has allowed an appropriate choice of microfiltration membrane for use in recycling alkaline cleaning solutions in the dairy products industry.