Polyacrylic Acid Composite Membranes for Brackish Water Desalination

Abstract

In recent years, considerable interest has developed concerning the use of membranes containing fixed charged groups for water desalination applications[1–6]. The transport properties of these charged membranes have been indicated to be qualitatively consistent with the Donnan ion exclusion mechanism[7]; i.e., salt rejection is a decreasing function of concentration, divalent coions are rejected better than monovalent coions, and monovalent counterions are rejected better than divalent counterions. Earlier studies in this laboratory[8] involved the preparation and properties of polyacrylic acid (PAA) films cast directly onto a finely porous support membrane composed of mixed cellulose esters, cellulose nitrate-cellulose acetate (CN-CA). These studies showed that in dilute salt solutions the PAA-CN-CA composite membranes exhibited a desalination performance comparable to asymmetric CA membranes of the Loeb-Sourirajan type[9]. In general, (1) water flux and salt rejection were increasing functions of pressure, (2) rejection was a function of feed concentration and charge on the counterion, and (3) the membrane properties were relatively independent of pH above about pH 6, with increasing water flux at lower pH values accompanied by declining NaCl rejection.