Characterisation of surface properties of ceramic membranes by streaming and membrane potentials

Synthesis and characterization of poly(arylene ether ketone) copolymers with pendant carboxylic acid groups for antifouling ultrafiltration membrane

This work focused on the synthesis of a series of poly(arylene ether ketone) random copolymers with various controlled pendant carboxylic acid groups (PEAK-COOH) by direct copolymerization method and the preparation of antifouling ultrafiltration membranes via the conventional immersion precipitation phase inversion method. The morphologies of the PEAK-COOH membranes were investigated by scanning electron microscopy. In addition, the surface hydrophilicity and charged property of the PEAK-COOH membranes were studied by water contact angle and membrane potential measurements, and the results indicated that the introduction of COOH on the polymer chains of membrane materials was really an effective way to negatively charge the membrane and enhance the hydrophilicity. The pure water flux and protein solution permeation through the prepared membranes were increased with the increase in the content of carboxylic acid groups. The cycle ultrafiltration experiments for protein solution revealed that nonspecific protein adsorption, especially irreversible protein adsorption, for the PEAK-COOH membranes, was significantly reduced, suggesting superior antifouling performance.

Influence of steric, electric, and dielectric effects on membrane potential

The membrane potential arising through nanofiltration membranes separating two aqueous solutions of the same electrolyte at identical hydrostatic pressures but different concentrations is investigated within the scope of the steric, electric, and dielectric exclusion model. The influence of the ion size and the so-called dielectric exclusion on the membrane potential arising through both neutral and electrically charged membranes is investigated. Dielectric phenomena have no influence on the membrane potential through neutral membranes, unlike ion size effects which increase the membrane potential value. For charged membranes, both steric and dielectric effects increase the membrane potential at a given concentration but the diffusion potential (that is the high-concentration limit of the membrane potential) is affected only by steric effects. It is therefore proposed that membrane potential measurements carried out at high salt concentrations could be used to determine the mean pore size of nanofiltration membranes. In practical cases, the membrane volume charge density and the dielectric constant inside pores depend on the physicochemical properties of both the membrane and the surrounding solutions (pH, concentration, and chemical nature of ions). It is shown that the Donnan and dielectric exclusions affect the membrane potential of charged membranes similarly; namely, a higher salt concentration is needed to screen the membrane fixed charge. The membrane volume charge density and the pore dielectric constant cannot then be determined unambiguously by means of membrane potential experiments, and additional independent measurements are in need. It is suggested to carry out rejection rate measurements (together with membrane potential measurements).

Electrokinetic and surface chemical characterizations of an irradiated microfiltration polysulfone membrane: Comparison of two irradiation doses

The effect of ionizing radiation on the surface and electrokinetic characteristic parameters for a porous membrane of pore size 0.2 mum is determined and correlated with the irradiation dose (10 and 80 J/kg). Changes in NaCl permeability and membrane system electrical resistance determined from diffusion and impedance spectroscopy measurements are consistent with the increase of membrane pore radii/porosity, in agreement with SEM micrographs and reported results. Low irradiation dose seems to clean the membrane surface of impurities, according to XPS results, but the increase of irradiation doses could affect surface roughness. Due to the relatively high pore radius, ion transport numbers are practically independent of radiation and dose, but irradiation slightly modifies the membrane solution interface by increasing its weakly electronegative character, which could be of interest in the ultrafiltration of proteins or macromolecules.

Influence of various monovalent cations and calcium ion on the colloidal fouling potential

The influence of various monovalent cations and of divalent calcium ions on colloidal fouling strength was investigated quantitatively on a bench-scale ultrafiltration device. A higher colloidal fouling potential (k) was consistently observed with lithium chloride compared to the same ionic strengths of chlorides of other monovalent cations (Na+, K+, and Cs+). This observation was attributed to the formation of an impervious layer around the colloidal particle by lithium ions that prevented the repulsive forces due to the interaction of the silica hairs formed on the particles in the presence of water. The impact of the divalent calcium ion on the fouling potential was more complex. The fouling potential first increased with calcium ion concentration and then decreased. The maximum value of fouling potential occurred at the ionic strength corresponding to the critical coagulation concentration, which decreased with increasing colloid concentration. The colloidal fouling potential was well correlated by a bilinear relationship with colloid concentration and ionic strength for all salts tested under the critical coagulation concentration.

Quantifying the effect of ionic strength on colloidal fouling potential in membrane filtration

Ultrafiltration experiments were conducted to study the fouling potential of colloidal suspensions under different ionic strengths and colloid concentrations. A linear relationship was found relating the colloidal fouling potential to the logarithm of the Debye-Huckel parameter, a characteristic for electrical double layers of colloids. This finding provided a useful quantitative linkage between the colloidal fouling potential and the water chemistry. Considering the linear dependence of colloidal fouling potential on the colloid concentration, a bilinear model was proposed to explain the coupling effects of colloid concentration and ionic strength of the suspension on the fouling potential. The model predictions of fouling potential were found to fit accurately with experimentally determined fouling potential values. Further analysis of the model showed that ionic strength can significantly affect colloidal fouling, for example, a 10-fold increase in ionic strength from 0.001 to 0.01 M for a given feed concentration has the same membrane fouling effect as doubling the feed concentration. The model allows for a quick and reliable assessment of fouling potential without even performing any experiments. This could then be used to design the membrane process or pretreatment stages required to mitigate membrane fouling.

Determining the Zeta Potential of Porous Membranes Using Electrolyte Conductivity inside Pores

The zeta potential is an important and reliable indicator of the surface charge of membranes, and knowledge of it is essential for the design and operation of membrane processes. The zeta potential cannot be measured directly, but must be deduced from experiments by means of a model. The possibility of determining the zeta potential of porous membranes from measurements of the electrolyte conductivity inside pores (lambda(pore)) is investigated in the case of a ceramic microfiltration membrane. To this end, experimental measurements of the electrical resistance in pores are performed with the membrane filled with KCl solutions of various pHs and concentrations. lambda(pore) is deduced from these experiments. The farther the pH is from the isoelectric point and/or the lower the salt concentration is, the higher the ratio of the electrolyte conductivity inside pores to the bulk conductivity is, due to a more important contribution of the surface conduction. Zeta potentials are calculated from lambda(pore) values by means of a space charge model and compared to those calculated from streaming potential measurements. It is found that the isoelectric points are very close and that zeta potential values for both methods are in quite good agreement. The differences observed in zeta potentials could be due to the fact that the space charge model does not consider the surface conductivity in the inner part of the double layer. Measurements of the electrolyte conductivity within the membrane pores are proved to be a well-adapted procedure for the determination of the zeta potential in situations where the contribution of the surface conduction is significant, i.e., for small and charged pores. Copyright 2001 Academic Press.