Modeling of flux decline during crossflow ultrafiltration of colloidal suspensions

Mass transfer during crossflow ultrafiltration is mathematically expressed using the two-dimensional convective–diffusion equation. Numerical simulations showed that mass transfer in crossflow filtration quickly reaches a steady-state for constant boundary conditions. Hence, the unsteady nature of the permeate flux decline must be caused by changes in the hydraulic boundary condition at the membrane surface due to cake formation during filtration. A step-wise pseudo steady-state model was developed to predict the flux decline due to concentration polarization during crossflow ultrafiltration. An iterative algorithm was employed to predict the amount of flux decline for each finite time interval until the true steady-state permeate flux is established. For model verification, crossflow filtration of monodisperse polystyrene latex suspensions ranging from 0.064 to 2.16 μm in diameter was studied under constant transmembrane pressure mode. Besides the crossflow filtration tests, dead-end filtration tests were also carried out to independently determine a model parameter, the specific cake resistance. Another model parameter, the effective diffusion coefficient, is defined as the sum of molecular and shear-induced hydrodynamic diffusion coefficients. The step-wise pseudo steady-state model predictions are in good agreement with experimental results of flux decline during crossflow ultrafiltration of colloidal suspensions. Experimental variations in particle size, feed concentration, and crossflow velocity were also effectively modeled.     

Flux decline in crossflow microfiltration and ultrafiltration: experimental verification of fouling dynamics

The theory of fouling dynamics in crossflow membrane filtration is compared with ultrafiltration experiments with suspensions of 0.12 μm silica colloids. It has been experimentally verified that colloidal fouling in crossflow filtration is a dynamics process from non-equilibrium to equilibrium and that the steady state flux is the limiting flux. With the cake concentration c_g identified from an independent experiment and the specific cake resistance calculated by Carman–Kozeny equation, the time-dependent flux and the time to reach steady state in the experiments of this study are correctly predicted with the theory of fouling dynamics.     

Assessment of an ultrafiltration technique for the fractionation of radionuclides associated with humic material

Experiments have been carried out with well characterised laboratory solutions to establish the physico-chemical behaviour of a suite of radionuclides in the presence of humic and fulvic acids using single flat membrane ultrafilters. Because of the uncertainties associated with the methodology, an approach has been adopted using mass balance determinations to assess the performance of the ultrafiltration process and facilitate the interpretation of the fractionation data. The size distribution of radionuclide-ligand complexes are reported and the results are discussed in the context of interpreting environmental data with more confidence.     

A liquid porosimetry technique for correlating intrinsic protein sieving: Applications in ultrafiltration processes

The understanding of variation in sieving properties of membranes is of great importance for the successful development of ultrafiltration applications. A liquid porosimetry technique is presented to quantify the sieving variation among several polyethersulfone ultrafiltration membranes. Observed sieving coefficients were measured with proper precautions taken to control and minimize fouling. These data were translated to intrinsic sieving coefficients using a stagnant film model. The intrinsic membrane sieving coefficient correlated well with the liquid porosimetry data. This liquid porosimetry technique can distinguish between membranes of different molecular weight cut-off and is sensitive enough to capture slight changes in the sieving coefficient of variants of the same cut-off membrane. This technique has several attractive features: it is non-destructive, independent of the module configuration and relatively simple to perform. Two potential applications of this technique are also examined: (1) quantification of the effect of membrane variation on high performance tangential flow filtration (HPTFF) for protein separations and (2) development of a membrane integrity test to ensure batch-to-batch consistency. This technique has the potential for use in membrane quality control, membrane selection, and validation of industrial ultrafiltration processes.     

In situ characterization by SAXS of concentration polarization layers during cross-flow ultrafiltration of Laponite dispersions

The structural organization inside the concentration polarization layer during cross-flow membrane separation process of Laponite colloidal dispersions has been characterized for the first time by in situ time-resolved small-angle X-ray scattering (SAXS). Thanks to the development of new "SAXS cross-flow filtration cells", concentration profiles have been measured as a function of the distance z from the membrane surface with 50 μm accuracy and linked to the permeation flux, cross-flow, and transmembrane pressure registered simultaneously. Different rheological behaviors (thixotropic gel with a yield stress or shear thinning sol) have been explored by controlling the mutual interactions between the particles as a result on the addition of peptizer. The structural reversibility of the concentration polarization layer has been demonstrated being in agreement with permeation flux measurements. These observations were related to structure of the dispersions under flow and their osmotic pressure.     

Indicator method for investigating the acid-base properties of suspension particles

The determination of the concentration of acid-base centers on the surface of suspensions from indicator adsorption requires precision measurements of changes in pH of solutions before and after the adsorption of the corresponding indicator.     

Spatial and temporal in situ evolution of the concentration profile during casein micelle ultrafiltration probed by small-angle X-ray scattering

Understanding the mechanisms involved in structural development in the vicinity of membrane constitutes a considerable challenge in the improvement of ultrafiltration process in industrial applications. In situ small-angle X-ray scattering (SAXS) performed with custom-made ultrafiltration cell has permitted the structural arrangement to be probed and concentration profiles to be obtained in deposited layers during frontal filtration of casein micelle suspension. SAXS allowed the structure of the accumulated layers of casein micelles between 280 microm and 1 mm from the membrane surface to be followed at length scales from a few nanometers to about 100 nm. These results have been combined with hydrodynamic measurements (permeation flux) and rheological investigations. Under frontal filtration, the time dependence of concentration at different distances from the membrane surface has been obtained. This temporal evolution is marked by an exponential increase of the concentration followed by a slower growth which has been associated with a change in the rheological behavior of the suspension from Newtonian to shear thinning behavior.     

Self-assembly prismatic aggregates formed during the calcination of ZnO powders: in situ monitoring by ETA technique and their photocatalytic properties

This paper describes a unique phenomenon occurred during the calcination of ZnO powders, i.e., the ZnO particles self-assembled to form prismatic aggregates with a clear edges and faces. Field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) were used to characterize the particle morphology and crystal structure of the calcined sample. The emanation thermal analysis (ETA) technique was used to monitor the changes of ZnO particle surface and subsurface microstructure irregularities and the occurrence of interparticle compaction phenomena under in situ conditions of heating and cooling. It was assumed from the ETA results that the driven force of the self-assembly of ZnO particles towards prismatic aggregates originated from the solid state diffusion and migration of grain boundaries. The photocatalytic tests indicated that the prismatic aggregates of ZnO calcined at 800 degrees C demonstrated a highest photocatalytic activity for acetaldehyde decomposition because of the enhancement of the surface-exposed high-active crystal face of (101 0).     

Free-solvent model shows osmotic pressure is the dominant factor in limiting flux during protein ultrafiltration

In protein ultrafiltration (UF), the limiting flux phenomenon has been generally considered a consequence of the presence of membrane fouling or the perceived formation of a cake/gel layer that develops at high operating pressures. Subsequently, numerous theoretical models on gel/cake physics have been made to address how these factors can result in limiting flux. In a paradigm shift, the present article reestablishes the significance of osmotic pressure by examining its contribution to limiting flux in the framework of the recently developed free solvent osmotic pressure model. The resulting free-solvent-based flux model (FSB) uses the Kedem–Katchalsky model, film theory and the free solvent representation for osmotic pressure in its development. Single protein tangential-flow diafiltration experiments (30 kDa MWCO CRC membranes) were also conducted using ovalbumin (OVA, 45 kDa), bovine serum albumin (BSA, 69 kDa), and immuno-gamma globulin (IgG, 155 kDa) in moderate NaCl buffered solutions at pH 4.5, 5.4, 7 and 7.4. The membrane was preconditioned to minimize membrane fouling development during the experimental procedure. The pressure was randomly selected and flux and sieving were determined. The experimental results clearly demonstrated that the limiting flux phenomenon is not dominated by membrane fouling and the FSB model theoretically illustrates that osmotic pressure is the primary factor in limiting flux during UF. The FSB model provides excellent agreement with the experimental results while producing realistic protein wall concentrations. In addition, the pH dependence of the limiting flux is shown to correlate to the pH dependency of the specific protein diffusion coefficient.     

Study of the growth process of in situ polyaniline deposited films

Polyaniline (PAni) thin films were deposited onto BK7 glass substrates using the in situ deposition technique. The control of the time and the aniline concentration in the PAni polymerization reaction on the film deposition allowed us to prepare films with different thickness, down to approximately 25 nm. The film growth process was monitored by measuring the UV-vis spectra and the AFM height profiles of the film surface. The curves of adsorption kinetics were analyzed with the Avrami's model, yielding an exponent n=3, thus indicating nucleation of spheroids at the initial stages of polymerization that grow through a diffusion process. AFM images of the surface height profiles corroborate this hypothesis, with spheroids growing with no preferred orientation during the in situ deposition.     

Micromechanical cantilever technique: a tool for investigating the swelling of polymer brushes

Polymer brush coatings are well-known for their ability to tailor surface properties in a wide range of applications from colloid stabilization to medicine. In most cases, the brushes are used in solution. Consequently, efforts were expended to experimentally investigate or theoretically predict the swelling behavior of the brushes in solvents of different qualities. Here, we show that the micromechanical cantilever (MC) sensor technique is a tool to perform time-resolved physicochemical investigations of thin layers such as polymer brushes. Complementary to scattering techniques, which measure the thickness, the MC sensor technique provides information about changes in the internal pressure of the brushes during a swelling and deswelling process. We show that the kinetics of both swelling and deswelling are dependent on solvent quality. Comparing the measured data with its thickness evolution, which was calculated based on the Flory-Huggins theory, we found that only the first 10% of the thickness increase of the polymer brush results in a significant pressure increase inside the polymer brush layer.     

Development of enhanced ultrafiltration methodologies for the resolution of racemic benzoin

In the scope of achieving the separation of chiral molecules, enzyme enhanced ultrafiltration (EEUF), a new method based on polymer enhanced ultrafiltration (PEUF), utilizing apoenzymes as ligands, was developed. Benzoin was chosen as the model chiral molecule. Bovine serum albumin (BSA) and apo form of benzaldehyde lyase (BAL) (E.C. 4.1.2.38) were used as chiral ligands in PEUF and EEUF experiments, respectively. In order to bind to the target enantiomer well, the addition of ligand to the benzoin solution was followed by ultrafiltration. With the use of BSA as ligand, adaptation of PEUF for chiral target molecules and process parameter optimization was carried out; whereas, in EEUF studies the effect of ligand concentration was focused on. In PEUF experiments, although total benzoin retention values reached to 48.7% and 41.3% at pH 10, for 15% (v/v) PEG 400 and 30% (v/v) DMSO cosolvents, respectively; obtained enantiomeric excess (ee) % values were all less than 20%. In EEUF experiments, at BAL concentrations greater than 158 ppm, total benzoin retention and ee% remained constant at ca. 75% and 60%, respectively. On the other hand, at 61 ppm BAL concentration, total benzoin retention was kept at ca. 75%, but ee% decreased to ca. 30%, probably due to the nonspecific binding of benzoin to DNA and other proteins. Thus, the method developed enzyme enhanced ultrafiltration, functioned with its intended purpose effectively in chiral separation.     

Study of cellulose pyrolysis using an in situ visualization technique and thermogravimetric analyzer

A technique has been developed to study cellulose pyrolysis by in situ visualization of cellulose transformation in a quartz capillary under a microscope using a CCD camera monitoring system and Raman spectroscopy. The processes and temperature of cellulose transformation during pyrolysis reaction can be observed directly. In situ visualization of reaction revealed that how oil is generated and expulsed concurrently from cellulose during pyrolysis. The in situ visualization result is the first direct evidence to show cellulose pyrolysis transformation. Pyrolysis characteristics were investigated under a highly purified N₂ atmosphere using a thermogravimetric analyzer from room temperature to 500 °C at the heating rate of 5 °C/min. The results showed that three stages appeared in this thermal degradation process. Kinetic parameters in terms of apparent activation energy and pre-exponential factor were determined.     

Spatial distribution of foulants on membranes during ultrafiltration of protein mixtures and the influence of spacers in the feed channel

Using matrix assisted laser desorption ionisation mass spectrum (MALDI-MS), this study reports the observations of the fouling distribution and composition along the membrane channel after the membranes were subjected to ultrafiltration of protein mixture solution in a crossflow module with and without spacer. In the fouling layer on a fully retentive membrane, the protein components with high molecular weight has higher presentation after 2 h of filtration and the presentation reduced to be lower than the smaller components after 6 h of filtration due to protein exchange and displacement phenomena in deposition layer caused by the differences in structure and diffusivity of different components. The protein exchange and replacement in the deposition layer was also observed on partial retentive membrane using a sequential fouling procedure. Fouling distribution along the membrane channel with spacer inserted in the module was more uniform and the flux was higher than that without spacer despite higher protein deposition observed in some cases.     

Bioactive glasses as potential radioisotope vectors for in situ cancer therapy: investigating the structural effects of yttrium

The incorporation of yttrium in bioactive glasses (BGs) could lead to a new generation of radionuclide vectors for cancer therapy, with high biocompatibility, controlled biodegradability and the ability to enhance the growth of new healthy tissues after the treatment with radionuclides. It is essential to assess whether and to what extent yttrium incorporation affects the favourable properties of the BG matrix: ideally, one would like to combine the high surface reactivity typical of BGs with a slow release of radioactive yttrium. Molecular Dynamics simulations show that, compared to a BG composition with the same silica fraction, incorporation of yttrium results in two opposing effects on the glass durability: a more fragmented silicate network (leading to lower durability) and a stronger yttrium-mediated association between separate silicate fragments (leading to higher durability). The simulations also highlight a high site-selectivity and some clustering of yttrium cations, which are likely linked to the observed slow rate of yttrium released from related Y-BG compositions. Optimisation of yttrium BG compositions for radiotherapy applications thus depends on the delicate balance between these effects.     

Development of an LC-MALDI method for the analysis of protein complexes

In this study, a two-dimensional LC-MALDI-TOF/TOF method has been developed for analyzing protein complexes. In our hands, the method has proven to be an excellent strategy for the analysis of protein complexes isolated in pull-down experiments. This is in part because the preservation of the chromatographic separation on a MALDI target yields an "unlimited" amount of time to obtain MS/MS spectra, making it possible to probe more deeply into complex samples. A brief statistical analysis was performed on the data obtained from the LC-MALDI-TOF/TOF system in order to better understand peptide fragmentation patterns under high-energy collision conditions. These statistical analyses provided some insight into how to evaluate the quality and accuracy of the database search results derived from the TOF/TOF-based analysis. The potential of the method was demonstrated by the successful identification of all the known penicillin-binding proteins in E. coli isolated using a drug-based pull-down with ampicillin as the bait. The performance of the LC-MALDI-TOF/TOF system was compared with that of an equivalent 2D LC-ESI-MS/MS approach, in the analysis of a protein bait-based pull-down. Regardless of the number of peptides identified in the ESI versus MALDI approach, the two approaches were found to be complementary. When the data is merged at the peptide level, the combined result gives higher Mascot scores and an overall higher confidence in protein identification than with either approach alone.     

Length dependency of flux and protein permeation in crossflow microfiltration of skimmed milk

Crossflow microfiltration of skimmed milk to fractionate casein micelles and whey protein was investigated regarding length dependency of flux and whey protein permeation using a 1.2 m long, 0.1 μm tubular ceramic membrane. A special module consisting of four sections was constructed allowing to assess the effects of membrane length online by measuring flux and permeation of the whey protein β-lactoglobulin as a function of local processing conditions. It was found that under the applied filtration parameters (mean transmembrane pressure Δp_TM,m = 0.5 bar; temperature ? = 55 °C; wall shear stress τ_w = 115 Pa) main parts of the membrane were controlled by a deposit layer. In consequence, the transmission of the whey protein β-lactoglobulin increases from 38% to 87% from membrane inlet to membrane outlet. Results show that a local optimum for protein fractionation exists regarding membrane resistance and process conditions.     

Numerical techniques for investigating an excited state of the anisotropic Heisenberg model

An analysis of the anisotropic Heisenberg model is carried out by solving the Bethe ansatz solution of the model numerically as a function of finite N. A brief introduction to the infinite chain limit is presented and the energy for a few limiting cases of the anisotropy parameter are evaluated. Numerical results for the infinite chain are given which can be compared with the case of finite increasing N. It is shown that the calculation can be extended to the case of an excited state of the model.