Bentonite-stabilized CDA/CTA membranes: I. Improved long-term transport properties

Abstract

Organophilic-modified bentonites of montmorrillonit origin were incorporated into casting solutions of CDA/CTA blend membranes in concentrations up to 2000 ppm. Membrane preparation followed the well-known phase inversion process of Loeb—Sourirajan and Cannon—Saltonstall. Properties of membranes were characterized by short-term and long-term reverse osmosis test runs at conditions of 0.6 mol/l NaCl-solution, 25°C, 80 bar. The comparison of permeate fluxes and salt rejections after 1 hour and 200 hours of operation for the different membrane types yielded the following: Initial performance data were not influenced by filler addition, but flux declines with time were considerably lowered with increasing bentonite incorporation. Maximum permeate fluxes after a test duration of one year (0.308 m 3/m 2d) and integral product water amounts (107 m 3/m 2) were obtained by bentonite concentrations of 1000 ppm. As these membranes achieved the highest long-term salt rejections (97.5%), they were installed in an RO pilot plant on board NS Otto Hahn and comparatively investigated with undoped reference membranes. The permeate fluxes and salt rejections of the doped membranes were 0.305 m 3/m 2d and 94.5% respectively, compared to 0.195 m 3/m 2d and 92.5% for the undoped membranes. These improved long-term properties were interpreted by means of a double-layer membrane morphology and a flux-time correlation derived from it; the flux stabilization of bentonite-containing CA blend membranes resulted in an enhanced mechanical stability of the porous substructure.