Ultrafiltration of salt solutions by ion‐excluding and ion‐selective membrances

Abstract

AbstractSalt‐filtering properties of several polymeric membranes were investigated at 40 atm. applied pressure. The ultrafilters were classified as ion‐excluding or ion‐selective membranes according to their ohmic resistance to the ions of the sodium chloride test solution. Membranes containing hydrophilic groups which have a high resistance to both cations and anions are classified as ion‐excluding. Polyelectrolyte membranes with high resistance to cations only are considered to be ion‐selective. In addition to cellulose acetate, which has been investigated previously, cellulose isobutyrate, cellophane‐supported polyvinyl methyl ketone, and cellophane‐supported polyvinyl acetate were found to be ion‐excluding membranes. The amorphous membranes, polyvinyl methyl ketone and polyvinyl acetate, do not reject salt alone; the cellophane appears to provide a supporting matrix which restricts the interchain distances in the interfacial region. Membranes composed of mixtures of the 1:1 copolymer of methyl vinyl ether and maleic anhydride with polyvinyl alcohol were durable, ion‐selective salt filters. Baking these membranes reduced swelling and flow rate and increased salt rejection. Those membranes containing high concentrations of the copolymer were flimsy and required bonding to a cellophane supporting membrane in order to obtain high salt rejection. The effects of process variables on the ultrafiltration properties of the ion‐selective membranes were also investigated. The salt rejection by either class of membrane depends on both a high concentration of the required functional groups and restricted interchain distances. Despite differences in the mechanism, each class behaves as a filter, in that the smaller the pores, the more effective the separation and the lower the flow rate.