Mass transfer properties of chlorinated aromatic polyamide reverse osmosis membranes

Abstract

Water (A) and solute (B) permeability of aromatic polyamide (PA) reverse osmosis membranes (RO) were monitored under varying applied pressure, solute nature and concentration to assess their evolution after exposure of the membrane to free chlorine. Above a threshold value of 400ppmh HOCl water permeability was influenced by permeation conditions during both filtration of ultrapure water (UP water) and reverse osmosis of salts performed sequentially. Water permeability decreased during the filtration of UP water performed at a constant applied pressure of 60bar. During the reverse osmosis of an electrolyte solution, performed at a constant permeation flux of 31Lh−1m−2, A was observed to increase continuously with time, e.g. up to a factor of 3 after exposure to 3120ppmh HOCl, most severe dose used. Differences in the charge density of mono- and divalent cations did not influence the rate of increase of A with time, which was however shown to depend on salt flux and ascribed to a diffusion limited relaxation process presumed to occur within the dense hydrated PA network. The relative and opposite impact of applied pressure and of salt permeation highlighted the importance in distinguishing conditions under which the water permeability (A) of a chlorinated membrane is measured, whether during the filtration of UP water or of a salt.