Gel-filled hollow fiber membranes for water softening

Three different gel-filled microporous poly(propylene) hollow fiber (HF) membranes were prepared by the incorporation of poly(4-vinylpyridinium salt) gels into the fiber walls. The gels were anchored in the pores of the membrane by cross-linking poly(4-vinylpyridine) with α,α′-dichloro-p-xylene, followed by quaternization using benzyl bromide. Different distributions of the gel were achieved by varying the preparation conditions. Energy dispersive X-ray (EDX) analysis and confocal microscopy were used to examine membrane morphology. The membranes were characterized by the amount of the incorporated gel (the gel polymer volume fraction), ion exchange capacity and wall thickness. The flux and sodium chloride rejection characteristics of the filled hollow fiber membranes were examined over a pressure range of 100–500 kPa. The flux of the fibers was significantly improved when the membranes had an asymmetric distribution of the gel through the wall of the fibers. The effect of the nature of the solute on separation was examined and the rejections followed the order MgCl2 > NaCl > Na2SO4. The performance characteristics of the membranes were quantitatively examined with mixed salt feed solutions containing salts of varying concentrations. The use of these membranes in water softening applications is discussed.