M.I. Vázquez

A study of temperature effect on chemical, structural and transport parameters determined for two different regenerated cellulose membranes

Chemical, structural and electrochemical characterizations of two cellophane membranes depending on both cellulose content (20 and 25 kg/m 2 ) and thermal treatment (60 ◦ C during 4 h), have being carried out. Different transport parameters were determined by measuring water and salt fluxes, membrane potential and impedance spectroscopy using NaCl solutions at different concentrations. Chemical degradation of the membrane surfaces due to membrane heating was obtained from XPS spectra. Electrochemical results shown that treated and untreated samples behave as weak cation-exchangers, but small differences in their electrical parameters (membrane fixed charge concentration, cation transport number and electrical resistance) due to both the thermal treatment and the amount of cellulose were obtained. Results also indicate a more open structure for the membrane with higher content of cellulose. The influence of concentration/polarization layers on salt permeation was also studied by measuring salt diffusion at different stirring rates. Moreover, variation of water permeability with temperature for untreated samples seems indicate changes in the membrane structure around 40 ◦ C, which was confirmed by differential scanning calorimetry (DSC) thermograms.

Chemical surface, diffusional, electrical and elastic characterizations of two different dense regenerated cellulose membranes

A comparison of NaCl transport across two dense cellulosic membranes from different suppliers is presented. Hydraulic and diffusional permeabilities were determined from volume flow-applied pressure and concentration-time relationships, while cation transport number and membrane conductivity were determined from electromotrice force and impedance spectroscopy measurements, respectively. Chemical surface differences between both membranes are correlated to transport parameters and morphology, but differences in elastic properties of both membranes might also be considered in order to get a more complete picture of membrane behaviors and to obtain structural-transport parameters correlations.

Morphological, Chemical Surface, and Diffusive Transport Characterizations of a Nanoporous Alumina Membrane

Synthesis of a nanoporous alumina membrane (NPAM) by the two-step anodization method and its morphological and chemical surface characterization by analyzing Scanning Electron Microscopy (SEM) micrographs and X-Ray Photoelectron Spectroscopy (XPS) spectra is reported. Influence of electrical and diffusive effects on the NaCl transport across the membrane nanopores is determined from salt diffusion measurements performed with a wide range of NaCl concentrations, which allows the estimation of characteristic electrochemical membrane parameters such as the NaCl diffusion coefficient and the concentration of fixed charges in the membrane, by using an appropriated model and the membrane geometrical parameters (porosity and pore length). These results indicate a reduction of ~70% in the value of the NaCl diffusion coefficient through the membrane pores with respect to solution. The transport number of ions in the membrane pores (Na+ and Cl−, respectively) were determined from concentration potential measurements, and the effect of concentration-polarization at the membrane surfaces was also considered by comparing concentration potential values obtained with stirred solutions (550 rpm) and without stirring. From both kinds of results, a value higher than 0.05 M NaCl for the feed solution seems to be necessary to neglect the contribution of electrical interactions in the diffusive transport.

Effect of polarisation layers on salt permeability across membranes with different structures

The influence of concentration-polarisation on salt transport across three membranes with different structures, an ultrafiltration (PPS), an asymmetric nanofiltration (PES-10) and a symmetric for dialysis (C-5), was studied. Salt diffusion measurements were carried out in a dead-end cell, at different stirring rates of the solutions at both sides of the membrane (0 ti (rpm) I 1100) under the same NaCl concentration gradient (A,X’= 5x lo-’ M’). Salt permeability for the membrane system (P,) at each stirring rate, which includes both the membrane itself and the polarisation layers, was determined by the fitting of the corresponding concentration vs. time curves. The permeability of the solute in the membrane (P,v”*) and the thickness of the solution layers at a given stirring celerity, d, were estimated by the fit of P, as a function of the solution stirring rate. Differences found between PJand 9”’ for the studied samples clearly show the different influence of concentration-polarisation layers on salt diffusion depending on the membrane structure. In fact, salt permeability through the C-5 membrane was at least one order of magnitude higher than for the two other samples, while P,“’ is around 2.5 times higher for the ultrafiltration membrane than for the nanofiltration one. A comparison of the thickness of polarisation layers at a given stirring celerity shows the following values: d(C-5)=25 pm, d(PPS) = 1 mm and (PES-10) = 6 mm. On the other hand, a comparison of the membrane permeability obtained for the cellophane membrane in the dead-end cell (3~10~~ m/s) and that obtained in a tangential flow cell (5x 1 Omh m/s), shows rather good agreement, which can be considered as a proof of validity of the results.