SEPARATION OF COUNTERIONS DURING PRESSURE-DRIVEN TRANSPORT OF ELECTROLYTE MIXTURES ACROSS CHARGED POROUS MEMBRANES

It has been previously predicted theoretically that a substantial sepa ration of counterions will occur during the pressure-driven transport of electrolyte mixtures across charged porous membranes. This presumpt ion has been supported experimentally using negatively charged sulfona ted polysulfone and ternary electrolyte solutions with a simple colon like Cl-. In dilute solutions the selectivity has approached values be tween 8 and 10 and sometimes even greater. The experimental findings o f the relationship of selectivity to feed ionic strength and feed comp osition agree fairly well with the theory. Moreover, the theoretical p rediction is also supported by the correlation of selectivity with mob ilities of counterions and transmembrane volume flow. The less mobile counterions of symmetrical mixtures like KCl/LiCl have a rejection inf erior to the more mobile ones when porous charged membranes are used. The reverse effect was observed when more dense membranes of the same polymer and degree of substitution were employed. However, some deviat ions from normality were noticed when the KCl/MgCl2 mixture was used s uch as changes in the sign of the selectivity logarithm and the nonmon otonic dependences of the selectivity on the feed ionic strength and t ransmembrane volume flow. This leads to the assumption that the mobili ty of the Mg2+ ion in the membrane phase is lower than that of the Kion, which is just opposite of their bulks. This relative decrease in the Mg2+ ion's mobility has been interpreted in terms of stronger elec trostatic interactions with membrane fixed charges. over, the lack of anomalies when the LiCl/MgCl2 solution is used leads to the assumption that the mobility of the Mg2+ ion in the membrane phase is between th at of Li+ and K+ ions.