Nidal Hilal

Characterisation of nanofiltration membranes for predictive purposes - use of salts, uncharged solutes and atomic force microscopy

Abstract An asymmetric nanofiltration membrane (Hoechst, PES5) has been characterised by three different techniques: modelling of the rejection of simple salts, modelling of the rejection of uncharged solutes and atomic force microscopy. Interpretation of experimental data for the rejection of three salts having common co-ion (LiCl, NaCl, KCl) with model calculations allows a characterisation of the membrane in terms of three parameters: an effective pore radius (rp), the ratio of effective thickness over porosity ( Δx A k ) and an effective charge density (X). Interpretation of experimental data with model calculations for uncharged solutes (Vitamin B12, raffinose, sucrose, glucose, glycerin) allows a characterisation in terms of rp and Δx A k . Atomic force microscopy (AFM) allows direct determination of surface pore radius rps and surface porosity Aks. The AFM images provide direct confirmation of the presence of discrete surface pores in such membranes. Further comparison of the characterisation obtained with salts and that obtained with uncharged solutes shows that it is better to describe the transport through such membranes as occurring through discrete pores rather than using an homogenous description of the membrane structure. It is also shown that the complexity of a “space-charge” description of the electric field distribution in the nanometre dimension pores of such membranes is not warranted. Direct experimental evidence of the charging mechanism of the membranes is provided. Overall characterisation parameters suitable for predictive purposes are suggested.

Methods Employed for Control of Fouling in MF and UF Membranes: A Comprehensive Review

Abstract Membrane‐based processes are very susceptible to flux decline due to concentration polarization and fouling problems and the concept of fouling control via process optimization, membrane surface modification, and cleaning have been the focus of research in wastewater and water treatment. MF and UF membranes are utilized in many areas of industry. The main sector of application includes water and wastewater. There has been emphasis on the various methods used to reduce and, where possible, eliminate fouling. This review is a comprehensive insight into the wide range of techniques used in the control of fouling in both MF and UF membranes. It also addresses the amount of research that has gone into the various techniques used and the results achieved after experimental work.

Atomic force microscope studies of membranes: Surface pore structures of Cyclopore and Anopore membranes

Non-contact atomic force microscopy has been used to investigate the surface pore structure of Cyclopore and Anopore microfiltration membranes in air. Three Cyclopore membranes and three Anopore membranes of different pore sizes were studied. Excellent high resolution images were obtained. Analysis of the images gave quantitative information on the surface pore structure, in particular the pore size distribution. Non-contact AFM is an excellent means of obtaining such information for microfiltration membranes.

Polymeric membranes: Surface modification for minimizing (bio)colloidal fouling

This paper presents an overview on recent developments in surface modification of polymer membranes for reduction of their fouling with biocolloids and organic colloids in pressure driven membrane processes. First, colloidal interactions such as London-van der Waals, electrical, hydration, hydrophobic, steric forces and membrane surface properties such as hydrophilicity, charge and surface roughness, which affect membrane fouling, have been discussed and the main goals of the membrane surface modification for fouling reduction have been outlined. Thereafter the recent studies on reduction of (bio)colloidal of polymer membranes using ultraviolet/redox initiated surface grafting, physical coating/adsorption of a protective layer on the membrane surface, chemical reactions or surface modification of polymer membranes with nanoparticles as well as using of advanced atomic force microscopy to characterize (bio)colloidal fouling have been critically summarized.

A new technique for membrane characterisation: direct measurement of the force of adhesion of a single particle using an atomic force microscope

Abstract An Atomic Force Microscope (AFM) has been used to quantify directly the adhesive force between a colloid probe and two polymeric ultrafiltration membranes of similar MWCO (4000 Da) but different materials (ES 404 and XP 117, PCI Membrane Systems (UK)). The colloid probe was made from a polystyrene sphere (diameter 11 μm) glued to a V shaped AFM cantilever. Measurements were made in 10 −2 M NaCl solution at pH 8. It was found that the adhesive force at the ES 404 membrane was more than five times greater than that at the XP 117 membrane. As it allows direct quantification of particle/membrane interactions, this technique should be invaluable in the development of new membrane materials and in the elucidation of process behaviour.

Photochemical modification of membrane surfaces for (bio)fouling reduction: a nano-scale study using AFM

Biofouling, due to microbial growth on membranes, is a common problem during the operation of water treatment membrane plants. It leads to an increase in operation and maintenance costs due to the deterioration of membrane performance and ultimately shortening membrane life. In an attempt to develop membranes with lower fouling properties in this paper we used the photoinduced grafting technique for the modification of membrane surfaces. Two different hydrophilic monomers: 2-acrylamido-2-methyl-l-propanesulfonic acid (AMPS) and quaternary 2-dimethylaminoethylmethacrylate (gDMAEMA) were photografted to the surface of commercial polyethersulfone (PES) microfiltration membranes (Millipore). The modified membranes were characterised using atomic force microscopy (AFM) by visualisation and the measurement of pore size, pore size distribution and surface roughness. A direct quantification of the force of adhesion using silica colloid probe technique and comparisons with unmodified samples were also made. The membrane affinity to biofouling was tested in the presence of Escherichia coli bacteria. It was found that the number of bacterial cells able to proliferate from countable colonies was much less for qDMAEMA-grafted samples compared with unmodified PES membranes. Thus these modified membranes could be potentially more resistant to biofouling.

Visualisation of an ultrafiltration membrane by non-contact atomic force microscopy at single pore resolution

Abstract Non-contact atomic force microscopy (AFM) has been used to investigate the furface pore structure of a polyethersulfone ultrafitration membrane of specified molecular weight cut off (MWCO) 25 000 (ES625, PCI Membrane Systems). Excellent images at up to single pore resolution were obtained. This is the first time that AFM images of a membrane at such high resolution have been presented. Analysis of the images gave a mean pore size of 5.1 nm with a standard deviation of 1.1 nm. The results have been compared to previously published studies of membranes of comparable MWCO using contact AFM and electron microscopy. Non-contact AFM is a powerful means of studying the surface pore characteristics of ultrafiltration membranes.

Kinetics of wetting and spreading by aqueous surfactant solutions.

Interest in wetting dynamics processes has immensely increased during the past 10-15 years. In many industrial and medical applications, some strategies to control drop spreading on solid surfaces are being developed. One possibility is that a surfactant, a surface-active polymer, a polyelectrolyte or their mixture are added to a liquid (usually water). The main idea of the paper is to give an overview on some dynamic wetting and spreading phenomena in the presence of surfactants in the case of smooth or porous substrates, which can be either moderately or highly hydrophobic surfaces based on the literature data and the authors own investigations. Instability problems associated with spreading over dry or pre-wetted hydrophilic surfaces as well as over thin aqueous layers are briefly discussed. Toward a better understanding of the superspreading phenomenon, unusual wetting properties of trisiloxanes on hydrophobic surfaces are also discussed.

An atomic force microscopy study of the adhesion of a silica sphere to a silica surface—effects of surface cleaning

Abstract An atomic force microscope (AFM) has been used to quantify directly the adhesive interactions between a silica sphere and a planar silica surface. Electrostatic double-layer interactions have also been quantified through analysis of approach curves. The surfaces of the sphere and planar surface were treated prior to measurements either by ethanol washing or by plasma treatment. Adhesion forces were then measured in 0.01 M NaCl solutions at pH 3 and 8. The adhesion force did not vary greatly with pH for a given cleaning procedure. However, the magnitudes of the adhesion forces were substantially less for the plasma treated surfaces. The adhesion forces did not vary systematically with the loading force. Agreement of the adhesion measurements with theory (DLVO, using a non-retarded Hamaker constant based on the latest interpretation of spectroscopic data for water) was good for the ethanol treated surface at pH 3 — conditions where double layer interactions are negligible. However, the plasma treated surface at pH 3 showed adhesion an order of magnitude lower than calculated. In contrast, adhesion at pH 8 was in both cases greater than theoretical expectations, though the lower adhesion for the plasma treated surface was in quantitative agreement with the increased electrostatic double–layer interactions induced by plasma treatment. The results show that the adhesion of such surfaces is a complex phenomenon and that non-DLVO interactions probably play a substantial overall role.

Surface modified microfiltration membranes with molecularly recognising properties

Polyvinilidene fluoride (PVDF-phob and PVDF-phil) and polyethersulfone (PES) microfiltration membranes were surface modified with a thin layer of molecular imprinted polymer (MIP). This material is selective to adenosine 3:5-cyclic monophosphate (cAMP) via photoinitiated copolymerisation of 2-(dimethylamino)ethyl methacrylate as a functional monomer and trimethylopropane trimethacrylate as a cross-linker in the presence of cAMP in ethanol/water solutions. The specific and non-specific template binding of MIP during filtration of aqueous solutions of cAMP was studied for membranes with different degrees of modification. It was concluded that the ability of MIP membranes to bind cAMP is a result of both the specific size and shape of recognising sites in addition to the correct position of the functional groups involved in the template binding through ionic and hydrogen binding interactions. Profile imaging atomic force microscopy and scanning electron microscopy were used to visualise surfaces and cross-sections of MIP membranes. The main advantages of this approach for MIP membrane preparation are very fast MIP layer synthesis and the possibility to obtain MIP composite membranes by controlled deposition on different kind of polymeric supports. Atomic force microscopy in conjunction with the coated colloid probe technique has been used to measure interactions between a silica sphere coated with imprinted polymer and porous supports.