and diffusion permeability of supports of an asymmetric and a composite NF membrane studied by concentration-step method

Fixed electric charge is believed to play an important role in the transfer of charged solutes in nanofiltration. Electrochemical measurements are useful tools for the determination of electrochemical perm-selectivity, which can be related to the fixed charge density by using a model. In the case of monolayer membranes, the electrochemical perm-selectivity is determined, in particular, through the measurements of stationary membrane potential. With composite/asymmetric NF membranes, the interpretation of this measurement is complicated by the membrane multi-layer structure. In the concentration-step technique, a membrane is equilibrated with an electrolyte solution and left in contact with this solution from the support side alone. The active membrane surface is suddenly touched by a pendant drop of solution of a different concentration, and the electrical response to this is tracked with a pair of reversible electrodes. In the very first moments after the touch, the whole concentration difference is located within the membrane active layer, and the initial electrical response is controlled by its electrochemical perm-selectivity. The characteristic time of relaxation of transient membrane potential is governed by the diffusion permeability of active layer and the porosity of and diffusivity in the membrane support. The concentration-step technique was used to study the electrochemical properties of active layers of a commercial polymer NF membrane (PES10) and a laboratory-made nano-porous ceramic membrane (provided by University of Twente) in KCl solutions of various concentrations. At pH 6, the fixed charge of polymer membrane was found to be negative whereas that of ceramic membrane was positive. The concentration of fixed charges ranged from 0.03 kmol/m3 to 0.17 kmol/m3 depending on the salt concentration.

The concentration-step method provides information on the electrochemical perm-selectivity of active layers of composite/asymmetric membranes. In combination with salt rejection data, it can also be used for the estimates of effective diffusion permeability of membrane supports. The electrical response at very short times is influenced by phenomena related to the poorly controllable processes of local wetting of membrane surface. Therefore, the information of interest is usually obtained by means of extrapolation from longer-time parts of time transients. For this extrapolation to be accurate, the characteristic relaxation time has to be, at least, one order of magnitude longer than the typical duration of short-time perturbations (ca. 50 ms). Therefore, the method is especially suitable for the characterization of membranes with not too high diffusion permeabilities of active layers.