Non-steady-state membrane characterisation techniques

Nanofiltration (NF) is a relatively new membrane process suitable for the separation of solutes of close molecular weights. The rejection mechanisms of nanofiltration membranes have not yet been reliably identified. The electrostatic repulsion of coions by fixed membrane charge (Donnan exclusion) is considered one of the most probable rejection mechanisms. However, practically no direct information on the electrochemical and/or electrokinetic properties of NF membranes is available. The interpretation of conventional electrochemical and/or electrokinetic measurements with NF membranes is complicated by their multilayer structure. From steady-state measurements under linear conditions one can obtain only average membrane transport properties. Information on the constituent layers can be obtained from non-steady-state and/or non-linear measurements, alone. The purpose of this project is to explore the opportunities offered by linear non steady-state techniques. Volume flows and changes in solute concentration typically cannot be observed at sufficiently short times, when the system is still far away from a steady state. Therefore the only suitable response is usually electrical. The membrane system can be disturbed from equilibrium in three ways: chemically (by changing the solute concentration), hydraulically and electrically. Accordingly, one can observe transient membrane potential, transient filtration potential and electrical impedance. Estimates show that the characteristic relaxation time in all those measurements is the same and may be essentially longer than the classical time of diffusional relaxation (l2/D). Nonetheless, it may still be as short as 0.1 sec.. That calls for special designs for the set-up of rapid concentration and hydrostatic pressure changes. Those designs are discussed proceeding from the theory of linear non-steady-state membrane phenomena and available experimental data on transient membrane potential. Preliminary information on the ion transport numbers within active layers of several NF membranes could be obtained. However, modifications will have to be undertaken within this project to extend the measuring range towards sufficiently short times. The designs for two novel measuring techniques - transient filtration potential and modified Hittorf`s technique - are described, the corresponding test cells will be developed. Three types of NF membranes will be studied by the above-mentioned techniques and the classical (tangential) streaming potential method. From those experiments we shall be able to determine the following properties of membrane active layers: ion transport numbers, electrokinetic charge density, diffusional permeability and zeta-potential of their surface. Those parameters are necessary to improve the understanding and modelling of NF membrane performance.

The rejection mechanisms of NF membranes are essentially controlled by electrochemical properties. Their determination is complicated by the presence of membrane supports and is practically impossible in steady state. We are developing integrated sets of novel membrane characterization techniques whose crucial elements are non- steady-state. The interfaces between the active layers and supports of NF membranes are concentrationally polarised by electric current. When the current is switched off, the Ohmic component disappears immediately, while the relaxation of diffusional one is delayed. The relaxation pattern is controlled by the electrochemical and diffusional properties of active layers and supports. Two commercially available NF membranes (PES10 and Desal5 DK) have been studied in pH-neutral KCl solutions of various concentrations. From the chronopotentiogrammes in combination with the results of NF measurements, we could estimate the ion transport numbers and the fixed charge density within the active layers. The latter value was used to estimate the "Donnan" salt reflection coefficient. Its comparison with the experimentally observed one has revealed that the Donnan exclusion is not the dominant rejection mechanism for the Desal5 membrane, while for the PES10 membrane the salt rejection is essentially controlled by the fixed charge.