Membrane Surface Coverage Research Articles

Numerical modelling and analysis of concentration polarization and scaling of gypsum over RO membrane during seawater desalination

Gypsum scaling commonly occurs over cross-flow RO membrane during seawater desalination. It blocks the pores of the semi-permeable membrane and results in substantial permeate flux decline in a shorter time span. The aim of the present study is to understand the gypsum scaling pattern and permeate flux decline through numerical modelling of nucleation, growth of gypsum crystal, and membrane surface coverage using a moment-based population balance method and CFD (computational fluid dynamics). We have performed numerical simulations of gypsum scaling in a roto-dynamic RO filtration system for different combinations of CaSO4 concentrations and feed pressure while NaCl concentration remains the same to match seawater salinity. We have found that the scaling starts where shear rate over membrane is the least and the growth of membrane surface blockage by scaling is controlled by feed pressure and CaSO4 concentration. For higher feed pressure permeate discharge initially is more but stronger concentration polarization causes more scaling and faster permeate discharge decline. Hence, for higher CaSO4 concentrations moderate feed pressure may be suitable but for lower CaSO4 concentrations higher feed pressure may be advantageous.

Visualization of Bacterial Colonization and Cellular Layers in a Gut-on-a-Chip System Using Optical Coherence Tomography.

Imaging of cellular layers in a gut-on-a-chip system has been confined to two-dimensional (2D)-imaging through conventional light microscopy and confocal laser scanning microscopy (CLSM) yielding three-dimensional- and 2D-cross-sectional reconstructions. However, CLSM requires staining and is unsuitable for longitudinal visualization. Here, we compare merits of optical coherence tomography (OCT) with those of CLSM and light microscopy for visualization of intestinal epithelial layers during protection by a probiotic Bifidobacterium breve strain and a simultaneous pathogen challenge by an Escherichia coli strain. OCT cross-sectional images yielded film thicknesses that coincided with end-point thicknesses derived from cross-sectional CLSM images. Light microscopy on histological sections of epithelial layers at the end-point yielded smaller layer thicknesses than OCT and CLSM. Protective effects of B. breve adhering to an epithelial layer against an E. coli challenge included the preservation of layer thickness and membrane surface coverage by epithelial cells. OCT does not require staining or sectioning, making OCT suitable for longitudinal visualization of biological films, but as a drawback, OCT does not allow an epithelial layer to be distinguished from bacterial biofilms adhering to it. Thus, OCT is ideal to longitudinally evaluate epithelial layers under probiotic protection and pathogen challenges, but proper image interpretation requires the application of a second method at the end-point to distinguish bacterial and epithelial films.

Biofouling in forward osmosis systems: An experimental and numerical study

This study evaluates with numerical simulations supported by experimental data the impact of biofouling on membrane performance in a cross-flow forward osmosis (FO) system. The two-dimensional numerical model couples liquid flow with solute transport in the FO feed and draw channels, in the FO membrane support layer and in the biofilm developed on one or both sides of the membrane. The developed model was tested against experimental measurements at various osmotic pressure differences and in batch operation without and with the presence of biofilm on the membrane active layer. Numerical studies explored the effect of biofilm properties (thickness, hydraulic permeability and porosity), biofilm membrane surface coverage, and biofilm location on salt external concentration polarization and on the permeation flux. The numerical simulations revealed that (i) when biofouling occurs, external concentration polarization became important, (ii) the biofilm hydraulic permeability and membrane surface coverage have the highest impact on water flux, and (iii) the biofilm formed in the draw channel impacts the process performance more than when formed in the feed channel. The proposed mathematical model helps to understand the impact of biofouling in FO membrane systems and to develop possible strategies to reduce and control biofouling.

Membrane-active Antimicrobial Peptides as Template Structures for Novel Antibiotic Agents

The increase of pathogens being resistant to antibiotics represents a global health problem and therefore it is a pressing need to develop antibiotics with novel mechanisms of action. Host defense peptides, which have direct antimicrobial activity (also termed antimicrobial peptides) or immune modulating activity, are valuable template structures for the development of such compounds. Antimicrobial peptides exhibit remarkably different structures as well as biological activity profiles with multiple targets. A large fraction of these peptides interfere physically with the cell membrane of bacteria (focus of this review), but can also translocate into the cytosol, where they interact with nucleic acids, ribosomes and proteins. Several potential interaction sites have to be considered on the route of the peptides from the environment to the cytoplasmic membrane. Translocation of peptides through the cell wall may not be impaired by the thick but relatively porous peptidoglycan layer. However, interaction with lipopolysaccharides of the outer membrane of Gram-negative bacteria and (lipo)teichoic acids of Gram-positive bacteria may reduce the effective concentration at the cytoplasmic membrane, where supposedly the killing event takes place. On a molecular level several mechanisms are discussed, which are important for the rational design of improved antimicrobial compounds: toroidal pore formation, carpet model (coverage of membrane surface by peptides), interfacial activity, void formation, clustering of lipids and effects of membrane curvature. In summary, many of these models just represent special cases that can be interrelated to each other and depend on both the nature of lipids and peptides.

Influence of protein bulk properties on membrane surface coverage during immobilization

Biomolecules immobilization is a key factor for many biotechnological applications. For this purpose, the covalent immobilization of bovine serum albumin (BSA), lipase from Candida rugosa and protein G on differently functionalized regenerated cellulose membranes was investigated.Dynamic light scattering and electrophoresis measurements carried out on biomolecules in solution indicated the presence of monomers, dimers and trimers for both BSA and protein G, while large aggregates were observed for lipase.The immobilization rate and the surface coverage on functionalized regenerated cellulose membranes were studied as a function of biomolecule concentration. Results indicated that the saturation coverage of BSA and protein G was concentration independent (immobilized protein amount of 2.40±0.03mg/g and 2.65±0.07mg/g, respectively). Otherwise, a different immobilization kinetics trend was obtained for lipase, for which the immobilized amount increases as a function of time without reaching a saturation value.Atomic force microscopy (AFM) micrographs showed the formation of monolayers for both BSA and protein G on the membrane surface, while a multilayer structure is found for lipase, in agreement with the trends observed in the related immobilization kinetics. As a result, the morphology of the proteins layer on the membrane surface seems to be strictly dependent on the proteins behavior in solution.Besides, the surface coverage has been described for BSA and protein G by the pseudo second order models, the results indicating the surface reaction as the controlling step of immobilization kinetics. Finally, enzyme activity and binding capacity studies indicated the preservation of the biomolecule functional properties.

Activity determinants of helical antimicrobial peptides: a large-scale computational study.

Antimicrobial peptides (AMPs), produced by a wide range of organisms, have attracted attention due to their potential use as novel antibiotics. The majority of these peptides are cationic and are thought to function by permeabilizing the bacterial membrane, either by making pores or by dissolving it (‘carpet’ model). A key hypothesis in the literature is that antimicrobial and hemolytic activity correlate with binding affinity to anionic and zwitterionic membranes, respectively. Here we test this hypothesis by using binding free energy data collected from the literature and theoretical binding energies calculated from implicit membrane models for 53 helical AMPs. We indeed find a correlation between binding energy and biological activity, depending on membrane anionic content: antibacterial activity correlates best with transfer energy to membranes with anionic lipid fraction higher than 30% and hemolytic activity correlates best with transfer energy to a 10% anionic membrane. However, the correlations are weak, with correlation coefficient up to 0.4. Weak correlations of the biological activities have also been found with other physical descriptors of the peptides, such as surface area occupation, which correlates significantly with antibacterial activity; insertion depth, which correlates significantly with hemolytic activity; and structural fluctuation, which correlates significantly with both activities. The membrane surface coverage by many peptides at the MIC is estimated to be much lower than would be required for the ‘carpet’ mechanism. Those peptides that are active at low surface coverage tend to be those identified in the literature as pore-forming. The transfer energy from planar membrane to cylindrical and toroidal pores was also calculated for these peptides. The transfer energy to toroidal pores is negative in almost all cases while that to cylindrical pores is more favorable in neutral than in anionic membranes. The transfer energy to pores correlates with the deviation from predictions of the ‘carpet’ model.

Microfiltration of algae ( Chlorella sorokiniana): Critical flux, fouling and transmission

Fouling and transmission studies of algae broth ( Chlorella sorokiniana) during cross-flow microfiltration were performed using direct microscopic observation technique. The flux stepping test was conducted for approximately 15 min interval and images were captured at the beginning and at the end of each flux step for further analysis of membrane surface coverage. The rate of membrane surface coverage was correlated to the rate of transmembrane pressure (TMP) rise. Results suggested that Direct Observation Through the Membrane (DOTM) technique was more sensitive to indicate the membrane fouling than TMP monitoring method. Deposition of microalgal cells started to occur even at a very low permeate flux. The layer of algal cells would eventually build up and might result in severe fouling. Despite this early deposition, TMP values remained low and would only increase when the fouling had become more pronounce. Lower fouling rate was achieved at higher cross-flow velocity. However, higher cross-flow velocity (CFV) also generated greater shear force which enhanced the transmission of extracellular polymeric substances (EPS). The particle removal due to bubble-induced shear was visualised with DOTM to provide better understanding. A significant reduction in fouling rate and near complete removal of microalgae could be achieved by applying continuous air bubbling. Based on TMP measurement, an intermittent aeration mode could achieve approximately identical performance as continuous bubbling mode. However, the recorded DOTM images indicated that intermittent bubbling mode could not completely maintain a cell-free surface.

Influence of conductive surface on adsorption behavior of ultrafiltration membrane

In this research the influence of conductivity on adsorptive behavior of PM30 ultrafiltration membrane was investigated using BSA solution as the feed. The conductive membrane was prepared from the originally nonconductive membrane by chemical polymerization of pyrrole as the conducting media on the membrane surface. Both Langmuir and Redlich–Peterson isotherms properly describe the quasi-equilibrium adsorption data which are produced using experimental results of flux and rejection. Higher capacity of protein adsorption was achieved using nonconductive in comparison with conductive membrane. Using nonconductive membrane, an excessive feasibility and spontaneity of BSA adsorption was observed based on the greater negative value of Gibbs free energy change ( Δ G ° ) which is a criterion for spontaneity of adsorption. Determination of filtration mechanism was conducted for elucidation the dominant adsorption region within the membranes i.e. membrane surface or internal pores. The filtration mechanisms for BSA solution using nonconductive and conductive membranes were surface cake deposition and intermediate (partial) blocking, respectively. First-order-kinetic model versus second-order-kinetic model indicated a superior interpretation of adsorption kinetics for both membranes; however, the required time to reach to the equilibrium for nonconductive membrane was slightly higher. All the distinctions in adsorption behavior of the conductive membrane originate from the repulsive potential field appears on the surface of the membrane during preparation. This electrostatic field acts as a barrier against the passage of the negatively charged proteins. Moreover, the partial coverage of membrane surface and internal pores with poly(pyrrole) may reduce the quantity of the active adsorptive sites on the membrane surface and matrix or presumably deactivates a part of the sites.

Pore-covered thermoresponsive membranes for repeated on-demand drug release

Reversible on/off-switching of bovine serum albumin (BSA) permeation through a thermoresponsive composite membrane with negligible permeation in the off-state is demonstrated. UV-photografting of poly( N-isopropylacrylamide) onto a poly(ethylene terephthalate) microfiltration membrane results in a hydrogel graft layer on the irradiated side of the membrane only. The amount of hydrogel grafted onto the membrane can be controlled by the amount of crosslinker. Above the lower critical solution temperature (LCST) of the hydrogel (on-state), the shrunken state of the graft layer appears to only partially cover the membrane, allowing BSA permeation through the uncovered pores. Provided the grafting degree is high enough, the swollen hydrogel covers the membrane completely below the LCST (off-state), thus preventing BSA permeation. The on-demand release mechanism proposed here is based on switching the membrane surface coverage rather than previously reported switches based on effective pore size or hydrogel mesh size. The main advantage of our mechanism is that higher fluxes can be achieved in the on-state, since permeation is not limited by pore-narrowing.

Real-time measurement of inorganic fouling of RO desalination membranes using ultrasonic time-domain reflectometry

Fouling is a major problem that limits the wider application of membranes for liquid separations. Progress towards improved methods for the control of this generally complex process would be facilitated by the development of additional techniques for the study of fouling under realistic operating conditions. This paper describes a novel approach to the real-time, noninvasive, in situ measurement of membrane fouling using ultrasonic time-domain reflectometry (TDR). Membrane fouling of commercial composite membranes was studied during reverse osmosis desalination of calcium sulfate solutions at three different axial velocities under conditions of constant feed pressure and concentration. The ultrasonic TDR measurements are compared with traditional indicators of fouling including flux decline, changes in permeate concentration, and magnitude of membrane surface coverage. Overall, the results show a good correspondence between the decline in the ultrasonic signal amplitude and the development of a fouling layer. In addition, the data suggest that ultrasonic TDR may be particularly responsive to fouling layer initiation and its subsequent removal during membrane cleaning. While the present results show promise, additional work is required to fully evaluate the capabilities of the technique and its applicability in commercial membrane separation systems.

Determination of pore size distribution in hollow fibre membranes

Isopropanol displacement under nitrogen pressure was used for the determination of pore size distribution in microfiltration polypropylene hollow fibres. Applying various assumptions about gas transport process two completely different characteristics of pore sizes were obtained. To verify these results an analysis of SEM images of the investigated membrane was conducted concerning its porous structure (pore diameters, surface occupied by pores). According to the SEM analysis the mean coverage of membrane surface by pore entrances should be about 20% of total area. For the distribution which accounted for pore evacuation according to Young–Laplace equation with contact angle θ=67° surprisingly dense coverage amounting to over 70% of total surface (by calculated total pore number over 10 13 per m 2) was predicted. Results for the distribution which accounted for gas bubble formation at the membrane surface (equivalent to θ=0°) fit into the expected range of pore numbers and membrane coverages (about 10 11 per m 2 and about 10%, respectively). It is concluded that the mechanism of bubble formation, determined by an actual pressure, liquid surface tension and pore size, is the crucial process while the value of contact angle θ does not play any role in the determination of pore size distribution.