Concentration Polarization and Fouling during Ultrafiltration of Colloidal Suspensions and Hydrophobic Solutes

Abstract

Flux reductions experienced during ultrafiltration are due either to concentration polarization or fouling. It is usually difficult to distinguish between these two phenomena, but by using a turbulence-promoting module it is possible to determine the reversibility of a flux reduction, and thus distinguish between concentration polarization and fouling. By using a turbulence-promoting module, it is also possible to distinguish between different cases of fouling. In this paper, fouling caused by the deposition of material at the surface of the membrane is illustrated by results from tests with a silica sol, and fouling due to interactions in the membrane matrix is illustrated by results from ultrafiltration of a low-molecular organic solute.     

Resistance Analyses for Ultrafiltration in Tubular Membrane Module

Abstract

The resistance analyses for ultrafiltration of macromolecular solutions by the resistances-in-series model and the modified gel-polarization model, respectively, have been extended to the turbulent ultrafiltration system in this study. The experiments are carried out by ultrafiltrating dextran T500 solutions in a tubular membrane module with membrane material of ZrO₂-TiO₂/carbon. It is found that the permeate fluxes are predicted very well by these models for both laminar and turbulent systems, and the resistance caused by the concentration polarization/gel layer (R _p) can be reduced by increasing the crossflow velocity on the membrane surface. Analysis by the resistances-in-series model showed that, R _p decreases with flow velocity with exponents of 0.49 and 0.99 for the laminar system and the turbulent system, respectively.     

Adhesion of Nanoparticles Fouling Glass Surfaces

The fouling of glass surfaces by nanoparticles formed from corroding iron was studied. Simple experiments demonstrated that adherent films of nanoparticles were deposited by corrosion of iron particles on a glass surface and by flowing water past corroding particles and then onto a glass surface. The water collected from this experiment was found to contain about 2 parts per million (ppm) of particles 500 nm in diameter when tested by photon correlation spectroscopy. However, electron micrographs showed that the primary particles in the fouling film were 20 nm in diameter. This discrepancy was explained by a new theory of nucleation of the fouling films. The theory was confirmed by measuring the particle size of ferric hydroxide dispersions as a function of concentration and pH. It was shown that the 20 nm primary nanoparticles nucleated much larger stable aggregates (defined as nucleags), which were sensitive both to pH and to magnetic fields. In particular, as the pH rose above 6, flocculation occurred, and large unstable agglomerates were observed. The conclusion was that three types of particle could exist in the corrosion product of iron and water: nanoparticles, nucleags, and flocs.     

Influence of pH and Ionic Strength during Food Protein Ultrafiltration: Elucidation of Permeate Flux Behavior,Fouling Resistance,and Mechanism

Ultrafiltration (UF) has found wide application in the food industry. In this work, gelatin, as a model food protein, was used to study the influence of pH and ionic strength on fouling during ultrafiltration using 30 kDa polyethersulfone (PES) membrane. The results showed significant water flux reductions after filtration for all feed solution conditions. The maximum fouling rate was obtained near gelatin's IEP, suggesting corresponding electrostatically driven fouling. Addition of salt increased the flux at pH near the gelatin's IEP but it had a negative effect at pH above or below gelatin's IEP. Analysis by the resistance-in-series model suggested that concentration polarization, reversible and irreversible fouling has contributed to the overall fouling. Cake formation was identified as the dominant mechanism for flux decline.     

Characterization of Irreversible Fouling after Ultrafiltration of Thermomechanical Pulp Mill Process Water

Large volumes of wastewater with dissolved wood components are treated in wastewater treatment plants at thermomechanical pulp mills. It has been shown previously that hemicelluloses in these wastewater streams can be recovered by membrane filtration. A serious obstacle when treating lignocellulose process streams is fouling of the membranes. Fouling not only increases operating costs but also reduces the operating time of the membrane plant. When optimizing the membrane cleaning method, it is important to know which compounds cause the fouling. In this work fouling of an ultrafiltration membrane was studied. The fouling propensity of untreated process water and microfiltrated process water was compared. Fouled membranes were analyzed using scanning electron microscopy and attenuated total reflection Fourier transform infrared spectrometry. Acid hydrolysis of membranes exposed to untreated process water and microfiltration permeate revealed that 508 mg/m² and 37 mg/m² of polysaccharides, respectively, remained on the membranes even after alkaline cleaning.     

Modeling of Fouling in Three Ultrafiltration Cell Configurations: Swirl,Plane and Axial Annular

Membrane fouling is a complex phenomenon induced by various chemical or physical factors. Several models can be used in order to predict flux. In this paper, models extracted from the literature are compared with experimental data collected in our laboratory during ultrafiltration of bentonite suspensions for three different cell designs (a classical plane unit and two annular units, one fitted with a tangential inlet inducing a swirling decaying flow, the other generating a pseudo axial flow). Mass transfer coefficients are measured by means of an electrochemical method for the two axial cell designs and are further included in the gel model predicting the limiting ultrafiltration flux. In ultrafiltration, the particles retained by the membrane will accumulate in the immediate vicinity of its surface to form a layer of higher particle concentration involved in the gel model. This concentration is also determined experimentally. Nevertheless, the gel model is not able to predict the permeation flux during ultrafiltration of bentonite suspensions in different cell designs. A modification of the erosion model, which takes into account the cell configuration, is also presented.     

Effects of Ionic Environments on Bovine Serum Albumin Fouling in a Cross‐Flow Ultrafiltration System

The influence of electrostatic interactions on membrane fouling during the separation of bovine serum albumin (BSA) from solution was studied in a cross‐flow ultrafiltration system. Experiments were carried out at different pH values between 3.78 and 7.46; and for different ionic strengths between 0.001 M and 0.1 M. The changes in permeate flux, cake layer resistance, zeta potentials of BSA and polyether sulfone (PES) membranes, and electrostatic interaction energies, were evaluated. At all of the ionic conditions studied, PES membranes are negatively charged. However, BSA molecules are either negatively or positively charged depending on the ionic environment. Whereas the cake layer resistance decreased with increasing pH and ionic strength, the permeate fluxes increased. The calculated electrostatic energy was a minimum at the isoelectric point (IEP) of BSA. However, at this point, the cake resistances corresponding to fouling at each ionic strength, were not minimized. Below the IEP of BSA, the electrostatic forces were attractive, while above the IEP, repulsive electrostatic forces were dominant.     

Bench-scale evaluation of seawater desalination by reverse osmosis

In bench-scale tests of seawater reverse osmosis desalination it is important to carefully consider osmotic pressure effects and determine the extent of concentration polarization so that sources of flux variation—whether from fouling, compaction, or osmotic pressure changes—can be properly assessed. Rigorous modeling of concentration polarization is difficult because of the complex geometries and flow regimes in RO modules; typically, concentration polarization must be measured. However, concentration polarization measurement usually requires knowledge of membrane permeability, which can vary from coupon to coupon. In this study a method is presented to determine both the membrane permeability and the concentration polarization regime in a single test. The key to the test is to allow the salt concentration to vary over time in a predictable way and extract parameters from a model fitted to the flux data. The usefulness of this technique is highlighted by evaluating results from several seawater experiments. It was found that specific flux decline in the experiments was caused by changes in osmotic pressure and membrane compaction. RO fouling by seawater organic-matter was not significant for the several seawater samples tested.     

Silica sol modified phenolic resin and its effect on mechanical properties of Al2O3-SiC-C bricks

Phenolic resin(PF) is commonly used as a binder in Al₂O₃-SiC-C bricks; however, it reduces the strength and accelerates the damage after carbonisation under temperatures of 300–800 °C. To improve the medium-temperature mechanical properties of Al₂O₃-SiC-C bricks, the mechanism of silica sol modify PF was analyzed, and silica sol modified phenolic resin (PFS) was introduced into Al₂O₃-SiC-C bricks to explore the effect of PFS on the mechanical properties of Al₂O₃-SiC-C bricks. The results showed that the silanol groups (Si-OH) in the silica sol can react with the hydroxyl groups (-OH) in the PF to form a Si-O-C bond, which makes PFS have a higher residual carbon rate and greater thermal stability. Overall, the introduction of PFS enhances the mechanical properties of the Al₂O₃-SiC-C bricks. When the content of silica sol in PFS is 10 wt%, the cold modulus of rupture and compressive strengths increased by 62.6% and 22.1%, respectively.     

微滤和超滤过程中浓差极化和膜污染控制方法研究

本文较全面的介绍了微滤和超滤过程中浓差极化和膜污染的各种控制方法,并比较了各种方法的优缺点。     

Humic acid fouling in the membrane distillation process

This work investigates the extent of humic acid fouling during the membrane distillation process for water treatment. The effects of pH, ionic strength, and divalent ion on fouling were studied. The experiments were performed with a 0.22-μm PVDF flat-sheet membrane in a direct contact membrane distillation unit. Flux declines were negligible (less than 6%) for the ranges of humic acid concentration, ionic strength, and pH studied. The examination of the membrane surface by SEM revealed a thin deposit layer. The addition of divalent cations (Ca²⁺) into the solution considerably reduced flux when Ca²⁺ concentration exceeded the critical coagulation concentration. Ca²⁺ affected flux by forming complexes with humic acids and resulted in coagulation on the membrane surface. The normalized flux, J/J₀, was 0.57 after 18 h of operation when the CaCl₂ concentration was 3.775 mM. However, the deposit of humic acid coagulate on the membrane surface was loosely packed, and was rather easily removed. Rinsing of the fouled membrane with clean water and a 0.1 M NaOH solution gave 100% of flux recovery.     

A non-sintering fabrication method for porous Si3N4 ceramics via sol hydrothermal process

A non-sintering fabrication method for porous Si₃N₄ ceramics with high porosity and high mechanical strength was proposed. Strength of the porous ceramics can be obtained by silica sol mass transfer process in hydrothermal conditions rather than a traditionally controlled high temperature sintering process. Under hydrothermal circumstances, silica sol is continuously transferred to the necks of Si₄N₃ powder compact, depositing there and thus consolidating the ceramic skeleton. The key of the method to obtain homogeneous microstructure and mechanical strength is how to keep the silica sol from gelatin during hydrothermal procedure. The stabilization of silica sol and its affecting factors were studied. The results indicated that ultrasonic treatment makes alkali-catalyzed silica sol remain stable even in 200?℃ hydrothermal condition, which insures consecutive silica transportation. The effect of hydrothermal time on open porosity/mechanical strength of the porous Si₄N₃ ceramics were also thoroughly investigated. The porous Si₄N₃ ceramics with open porosity above 42% and flexural strength of 45?MPa were obtained without any high temperature sintering process. This method can be widely employed for the preparation of other porous ceramics as well.     

Pilot plant study of an ultrafiltration membrane system fordrinking water treatment operated in the feed-and-bleed mode

A pilot plant study of a polysulfone ultrafiltration (MWCO of 30 kD) tubular membrane process was conducted for the treatment of reservoir water. The membrane separation system was operated in the cross-flow filtration mode at 4.7 m/s and the feed-and-bleed mode for a long term of 4000 h without chemical cleaning and backwashing. The results showed that the behavior of permeate flux of the membrane system operated in the feed-and-bleed mode was similar to that of membrane systems with a periodic backwashing. At the beginning of filtration, bleeding of highly concentrated retentate caused a significant increase in permeate flux by 20%. However, as filtration progressed over time, the permeate flux of the fouled membrane was almost independent of the change in concentration of retained materials by bleeding the retentate. Three distinct stages in permeate flux decline were observed as follows: (1) sharp decrease from 120 to 30 1/m²/h in 1250 h due to a rapid build-up of a fouling layer, (2) gradual decrease to 15 1/m²/h in 2800 h due to the role of tangential shear induced by cross-flow velocity, and (3) stable permeate flux until 4000 h due to the establishment of a pseudo-steady-state condition. Permeate quality was stable, regardless of concentrating and diluting retentate in each cycle and fouling for a long duration of filtration. Rejection efficiencies for ultraviolet absorbance at 260 nm (UV₂₆₀) and dissolved organic carbon (DOC) were around 58% and 49%, respectively. The measured turbidity and concentration of suspended solids in bleed water agreed with calculated values from a simple mass valance, while the measured DOC and UV₂₆₀ of bleed water were much lower than calculated values.     

Fabrication of homogeneous mullite-based fiber porous ceramics with high strength and porosity

Silica sol is widely used in the preparation of mullite-based fiber porous ceramics (MFPCs), but it aggregates at the top surface of MFPCs during the drying process. This leads to the decrease in mechanical strength and porosity. To overcome the problem and fabricate homogeneous MFPCs, the sodium silicate solution and glass fibers were applied in the fabrication process of MFPCs. The effects of concentrations of sodium silicate solution and silica sol, amounts of glass fibers and sintering temperatures on the properties of prepared MFPCs were studied. The sodium silicate solution consolidated the silica sol and mullite fibers, forming a homogeneous structure and ensuring the even distribution of silica sol. Compared with other reported MFPCs, this process required low sintering temperature while maintaining high compressive strength (2.14 MPa) and porosity (75.93%). This study provides an effective method for preparing MFPCs with high strength, uniformity and porosity.     

Influence of pH and ionic strength on the metal profile of impregnation catalysts

In this study the influence of pH and ionic strength on the final metal distribution is investigated. After impregnation, catalysts are usually dried at temperatures between 50°C and 200°C. During this process, a redistribution of the metal occurs, which is a complex function of the drying conditions, the properties of the impregnating solution and the support material. In our model, transport in the gas and liquid phase is described by the dusty gas model, and the Nernst-Plank equation, respectively. The metal adsorption on the porous support is described by the Revised Physical Adsorption model developed by Agashe and Regalbuto (J. Colloid Interface Sci. 185 (1997) 174). In their model, the adsorption constant is a function of the pH and the ionic strength of the liquid solution. The results show that for a positively charged metal complex, the impact of drying is strong when the initial pH of the liquid solution is below the point of zero charge (PZC). In such a case, the metal accumulates at the particle surface when the convective flow is strong (high temperature), and at the particle center when the solute-metal diffusivity is high. A comparison of our model with a drying model that assumes a constant adsorption equilibrium constant shows that the variations of pH and ionic strength cannot be ignored when the initial adsorption constant is low and the pH below the PZC. In such cases, the adsorption constant increases over several orders of magnitude when pH and ionic strength effects are accounted for.     

Adsorption of Pb(II) from aqueous solution to MX-80 bentonite: Effect of pH, ionic strength, foreign ions and temperature

The adsorption of Pb(II) from aqueous solution to MX-80 bentonite was studied using batch technique under ambient conditions. Removal percent (%) and distribution coefficient (K_d) were determined as a function of shaking time, pH, ionic strength and temperature. The results showed that the adsorption behavior of Pb(II) on bentonite was strongly dependent on pH and ionic strength. The presence of complementary cations depressed the adsorption of Pb(II) on bentonite in the order of Li⁺ ≈ Na⁺ > K⁺ at pH 2–5. The adsorption isotherms were simulated by the Langmuir, Freundlich, and Dubinin–Radushkevich (D–R) models very well. The thermodynamic parameters (ΔH⁰, ΔS⁰, and ΔG⁰) for the adsorption of Pb(II) were determined at three different temperatures of 291 K, 308 K and 328 K. The adsorption reaction was exothermic and the process of adsorption was favored at low temperature. The results suggest that bentonite is suitable as a sorbent material for the recovery and adsorption of Pb(II) from aqueous solution.     

The role of hydrodynamic conditions and solution chemistry on protein fouling during ultrafiltration

This study investigates the effect of hydrodynamic conditions and solution chemistry on protein fouling during ultrafiltration. Drastic flux reduction was observed at high initial flux and/or low cross-flow velocity. A limiting flux existed during BSA filtration, beyond which membrane flux cannot be sustained. Further increase in pressure over the limiting value did not enhance the stable flux. The rate and extent of BSA fouling were also strongly dependent on the feedwater composition, such as BSA concentration, pH, and ionic strength. Foulant concentration had no effect on the stable flux, although the rate approaching to the stable flux increased proportionally with increasing foulant concentration. Fouling was most severe at the isoelectric point of BSA (pH 4.7), where the electrostatic repulsion between foulant molecules is negligible. Membrane fouling became less severe at pHs away from the isoelectric point. Increasing ionic strength at pH 3.0 promoted severe fouling likely due to electric double layer (EDL) compression. On the other hand, the flux behavior was insensitive to salt concentration at pH 4.7 due to the lack of electrostatic interaction. At a solution pH of 5.8, effect of ionic strength on long-term flux behavior was directly opposite to that on the transient behavior. While the long-term flux was lower at higher ionic strength due to EDL compression, the transient behavior was also affected by the BSA retention of the membrane.     

基于响应面法的硅溶胶注浆材料配比优化研究

硅溶胶注浆材料具有可注性好、凝胶时间可控、环保无毒、耐久性高等优点,但常规的硅溶胶固砂体抗压强度偏低,其推广应用受到了一定限制。本文通过单因素实验分别研究了胶凝剂种类、胶凝剂浓度、硅溶胶与胶凝剂质量比、注浆材料pH值对硅溶胶固砂体抗压强度的影响,同时通过响应面法研究了胶凝剂浓度、硅溶胶与胶凝剂质量比、注浆材料pH值交互作用对抗压强度的影响。试验结果表明:单因素对抗压强度影响的显著程度依次为胶凝剂浓度、注浆材料pH值、硅溶胶与胶凝剂质量比;双因素交互作用对抗压强度影响的显著程度依次为胶凝剂浓度和注浆材料pH值、胶凝剂浓度和硅溶胶与胶凝剂质量比、硅溶胶与胶凝剂质量比和注浆材料pH值。采用响应面法获得的硅溶胶固砂体抗压强度最优配比为胶凝剂质量浓度为7%,硅溶胶与胶凝剂质量比为7 ∶1,注浆材料pH值为7。响应面法的预测值和实测值误差较小,表明响应面法可用于硅溶胶注浆材料的配比优化。     

REVISITING ELECTROOSMOTIC FLOW: AN IMPORTANT PARAMETER AFFECTING SEPARATION IN CAPILLARY AND MICROCHIP ELECTROPHORESIS

Electroosmotic flow mobility (EOF) is the movement of bulk liquid that provides an opportunity to separate charged molecules, either positive or negative, and transport all neutral molecules to the detector as a single peak. EOF originates on the silanol groups of the fused-silica capillary wall and is usually responsible for ions moving in the opposite direction of the electrostatic attraction. The interaction of the silanol groups with the electrolyte buffer leads to the formation of an electric double layer. Understanding double-layer theory and EOF is the first necessary step towards understanding many of the experimental observations in capillary and microchip electrophoresis. In this work, we introduce and validate a method to measure the EOF on both coated and uncoated capillaries by measuring the current time history, which has led to enhanced precision of the EOF measurement. We have also used the introduced method to study the fundamental parameters, such as the effect of electric field, temperature, buffer ionic strength, and pH on electroosmotic flow.