Electroosmotic pumping in unit cells.

Electrodialysis in unit cells - the authors call it 'electro-osmotic pumping' (e.o.p.) - is described and analysed in terms of the transport coefficients characterising the membrane pairs. Bags made of three different types of cation- and anion-exchange membranes were used in actual experiments, namely, thin and thick polyethylene-based and Selemion membranes. Brine concentrations, volume flow, and current efficiencies were determined for different current densities, at a fixed value of the NaCI concentration of the feed. The graphs of brine concentration, cSUPb, vs. current density level off at high values of i, and cSUPb approaches a maximum plateau, cSUPbSUPmSUPaSUPxSUP' which depends only on the electro-osmotic coefficient of the membrane pair, cSUPbSUPmSUPaSUPx 1/2 FBETA. The smaller the ratio between osmotic and electro-osmotic water flows, the smaller the current necessery to reach this plateau. The average value of DELTAt for the different membrane pairs was determined from the membrane potential for a concentration difference similar to that obtained in the e.o.p. experiments at high current densities. It gives a good (lower) estimate of the actual coulomb efficiency of the process. The graphs of volume flow vs. effective current density become straight lines at high values of the current. Electro-osmotic coefficients and SIGMALSUPp's of the membrane pairs may be determined from the slope and the intercept at the ordinate-axis, of these lines. The results agree quite well with the values of these coefficients measured separately for the different membranes, and averaged for the membrane pairs. (A)