Georges Belfort

The behavior of suspensions and macromolecular solutions in crossflow microfiltration

Abstract Although microfiltration is one of the oldest pressure-driven membrane processes, it is probably the least understood when it comes to the filtration of suspensions and macromolecules. Microfiltration is characterized by operation at low pressures, by high permeation fluxes, and by crossflow mode in flat or cylindrical geometries. The major limitation of microfiltration is membrane fouling due to the deposition and intrusion of macromolecules, colloids and particles onto and into the microporous membrane. In this review, we analyze the various components of this problem by focusing on the formation of cakes, the behavior of suspension flows and particle transport in simple geometry ducts, and on the formation and behavior of fouling layers including those resulting from macromolecules, colloids and particles. Some of the work we report on is very recent or is still in progress and needs independent verification. With this understanding, we hope that the reader will be able to use these concepts for analyzing other systems and for investigating new module designs.

Surface modification of ultrafiltration membranes by low temperature plasma II. Graft polymerization onto polyacrylonitrile and polysulfone

Low temperature plasma-induced surface modifications of polyacrylonitrile (PAN) and polysulfone (PSf) ultrafiltration (UF) membranes were studied. Treatment with water plasma and with He plasma drastically and almost permanently increased the surface hydrophilicity of PSf UF membranes. However, in contrast to the behavior of PAN UF membranes [23], the PSf surface pore structure was also changed as indicated by altered water permeabilities and reduced protein retentions. The lower permeability PSf membranes (nominal Mw cut-off 10 kD) showed slower but more extended conversion due to plasma excitation and stronger indications of pore etching effects in comparison with 30 kD cut-off membranes. Polymer peroxides on PAN and PSf membranes created by plasma excitation were monitored by the 2,2-diphenyl 1-picryl hydrazyl (DPPH) assay. Graft polymerization of hydrophilic monomers such as 2-hydroxy-ethyl methacrylate (HEMA) and acrylic or methacrylic acid onto PAN and PSf UF membrane surfaces was initiated via thermal decomposition of peroxides. The degree of modification could be adjusted by polymerization conditions. Graft polymer modified surfaces were characterized with the help of Fourier transform infrared attenuated total reflection (FTIR-ATR) and electron spectroscopy for chemical analysis (ESCA) spectra. The hydrophilic character of the modified surfaces was increased as compared to that of the parent membranes. With about 1–1.4 mmol/cm2 grafted HEMA, the contact angles (captive bubble technique; Θoctane/water) for PAN and PSf were reduced from 48 to 34° and from 92 to 43°, respectively. A clear dependency of PAN UF membrane water permeability on the amount of grafted monomer was observed. The monomer type influenced the water permeation flux per mole of grafted acrylate via specific swelling of the graft polymer layer in water. Hydrophilic PAN membranes, modified either by plasma treatment [23] or HEMA graft polymerization, showed significantly reduced fouling due to static protein adsorption, and improved protein UF performance. In particular, for water plasma treated PAN membranes with high initial retention, higher fluxes (up to 150%) with the same or even improved retentions were obtained. Hydrophilized PSf-g-HEMA membranes can provide improved performance in protein ultrafiltration over unmodified PSf UF membranes because pore etching effects are compensated for by the grafted layer yielding both improved filtrate flux (>30%) and protein retention of bovine serum albumin. Hence, plasma induced graft polymer modification of UF membranes can be used to adjust membrane performance by simultaneously controlling the surface hydrophilicity and permeability.

Surface modification of polysulfone ultrafiltration membranes and fouling by BSA solutions

Abstract Five different chemically modified versions of polysulfone were prepared via two different homogeneous chemical reaction pathways. They, together with the base polymer, were cast as membranes by a phase-inversion process. The surface energies of these membranes, as measured by contact angles, were used to characterize the different membranes. Streaming-potential measurements were obtained to probe the surface charge of the membranes. The surface roughness of each membrane was also determined by atomic-force microscopy. Each membrane was then exposed to deionized water, 0.08 g/l bovine serum albumin solution and deionized water using a standard filtration procedure to simulate protein fouling and cleaning potential. Both the chemistry and the size of the grafted molecules were correlated with respect to volumetric flux during ultrafiltration of protein solutions. Surface roughness seemed to be important for filtering pure water. Hysteresis between advancing and receding contact angles increased with hydrophilicity of the membrane surfaces. One possible explanation could be that surface reorientation was more likely with hydrophilic than with hydrophobic membranes. The membrane modified by direct sulfonation had the lowest surface energy and the shortest grafted chain length and exhibited the highest volumetric flux with BSA solution. It was also the easiest to clean and exhibited the highest initial flux recovery by stirring (91%) and backflush (99%) methods with deionized water. In most cases, backflushing rather than stirring was more effective in recovering the water flux.

Photochemical modification of 10 kDa polyethersulfone ultrafiltration membranes for reduction of biofouling

Poly(ether sulfone) 10 kDa ultrafiltration membranes were modified by photolysis using ultraviolet light and graft polymerization of hydrophilic monomers onto the membrane surface to create more hydrophilic and lower fouling membrane surfaces. The modified membrane surfaces were characterized by FTIR/ATR and captive bubble contact angle measurements to determine chemical and hydrophilicity changes during modification. The modified membranes were compared with an unmodified poly(ether sulfone) (control) membrane as well as a commercial regenerated cellulose and a low protein adsorbing poly(ether sulfone) membrane using a newly developed standardized filtration protocol with 1 wt% bovine serum albumin. The best performing modified membrane was with N-vinyl-2-pyrrolidinone and showed a 25% increase in hydrophilicity, a 49% decrease in bovine serum albumin fouling, and a 4% increase in bovine serum albumin retention compared to the unmodified poly(ether sulfone) membrane. While the regenerated cellulose membrane had the lowest fouling and the low protein adsorbing membrane had the highest flux of all tested membranes, the N-vinyl-2-pyrrolidinone-modified membranes had the best combination of low fouling and high flux.

A genetic system yields self-cleaving inteins for bioseparations

A self-cleaving element for use in bioseparations has been derived from a naturally occurring, 43 kDa protein splicing element (intein) through a combination of protein engineering and random mutagenesis. A mini-intein (18 kDa) previously engineered for reduced size had compromised activity and was therefore subjected to random mutagenesis and genetic selection. In one selection a mini-intein was isolated with restored splicing activity, while in another, a mutant was isolated with enhanced, pH-sensitive C-terminal cleavage activity. The enhanced-cleavage mutant has utility in affinity fusion-based protein purification. These mutants also provide new insights into the structural and functional roles of some conserved residues in protein splicing.

Development of a novel photochemical technique for modifying poly (arylsulfone) ultrafiltration membranes

Abstract A novel and general method for modifying hydrophobic poly (arylsulfone) ultrafiltration membranes to produce highly hydrophilic surfaces has been developed. This method consists of the direct UV irradiation of poly (arylsulfone) membranes in the presence of water or methanol soluble monomers. It was discovered that the poly (arylsulfone)s are intrinsically photoactive and that no photoinitiators are required for this process. Membranes were obtained in which polymeric segments derived from the hydrophilic vinyl monomers are directly bound to the poly (arylsulfone) chains by direct chemical bonds. A mechanism involving a photochemically induced free radical cleavage of the poly (arylsulfone) chains has been proposed. The effects of the irradiation conditions and the monomer concentration of three monomers: 2-hydroxyethyl methacrylate (HEMA), glycidyl methacrylate (GMA) and methacrylic acid (MAc) on the extent and depth of modification in the membrane were examined.

Surface modification of poly(ether sulfone) ultrafiltration membranes by low-temperature plasma-induced graft polymerization

Low-temperature helium plasma treatment followed by grafting of N-vinyl-2-pyrrolidone (NVP) onto poly(ether sulfone) (PES) ultrafiltration (UF) membranes was used to modify commercial PES membranes. Helium plasma treatment alone and post-NVP grafting substantially increased the surface hydrophilicity compared with the unmodified virgin PES membranes. The degree of modification was adjusted by plasma treatment time and polymerization conditions (temperature, NVP concentration, and graft density). The NVP-grafted PES surfaces were characterized by Fourier transform infrared attenuated total reflection spectroscopy and electron spectroscopy for chemical analysis. Plasma treatment roughened the membrane as measured by atomic-force microscopy. Also, using a filtration protocol to simulate protein fouling and cleaning potential, the surface modified membranes were notably less susceptible to BSA fouling than the virgin PES membrane or a commercial low-protein binding PES membrane. In addition, the modified membranes were easier to clean and required little caustic to recover permeation flux. The absolute and relative permeation flux values were quite similar for the plasma-treated and NVP-grafted membranes and notably higher than the virgin membrane. The main difference being the expected long-term instability of the plasma treated as compared with the NVP-grafted membranes. These results provide a foundation for using low-temperature plasma-induced grafting on PES with a variety of other molecules, including other hydrophilic monomers besides NVP, charged or hydrophobic molecules, binding domains, and biologically active molecules such as enzymes and ribozymes.

Enhanced performance for pressure-driven membrane processes: the argument for fluid instabilities☆

Abstract The performance of pressure-driven membrane processes may be significantly improved when unsteady fluid instabilities are superimposed on crossflow. The role of fluid mechanics, in particular unsteady secondary flows resulting from surface roughness, flow pulsations and centrifugal instabilities, coupled to solute mass transfer is discussed with respect to depolarization and defouling of membranes. Various possible mechanisms including wall shear rate and repeated renewal of the mass boundary layer are analyzed. The secondary flow pattern in a spiral crossflow filter has been visualized and shows a uniform velocity field with a steep gradient adjacent to the membrane surface. Unsteady flows of this type have been used with ultrafiltration and microfiltration membranes to show the efficacy of secondary flows. Significant dissipation with repeated renewal of the mass transfer boundary layer due to secondary flows is used to explain the multiple increase in membrane permeation rates.

Lateral migration of spherical particles in porous flow channels: application to membrane filtration

Abstract Lateral migration of spherical rigid neutrally buoyant particles moving in a laminar flow field in a porous channel is induced by an inertial lift force (tubular-pinch effect) and by a permeation drag force due to convection into the porous walls. The analysis of Cox and Brenner [7], for the particle motion in a nonporous duct is extended to include the effect of the wall porosity. Criteria are established under which the inertial and permeation drag force in the lateral direction can be vectorially added. Particle trajectories and concentrations profiles are calculated for a plane Poiseuille flow with one porous wall. For particles with radius of 1 μm, inertial and permeation drag forces are of comparable size under flow conditions often met in ultra- and hyperfiltration of dilute suspensions. For smaller particles the permeation drag force dominates.

Fluid mechanics in membrane filtration: Recent developments☆

Abstract Recent developments of the role of fluid mechanics in membrane filtration are presented. Understanding of membrane polarization and fouling is intimately related through mass transfer to fluid mechanics in membrane modules. After discussing the behavior of dissolved ions, macromolecules and collodial particles in cross-flow filtration, we describe several attempts to model both steady one and two phase flows and unsteady flows. For the latter, we consider both oscillating flows and flows with centrifugal instabilities. Moving cake and surface reaction models are also included. Several examples using these principles are taken from biotechnology and industrial effluent treatment.