Influence\nof Chemical Cleaning on Physicochemical\nCharacteristics and Ion Rejection by Thin Film Composite Nanofiltration\nMembranes

Abstract

The impact of membrane cleaning with\nNaOH and HCl on the characteristics\nand associated changes in ion rejection was investigated in this study.\nNaOH affected the zeta potential of membranes with a greater concentration\nof carboxylic groups so that it was negative across the entire pH\nrange investigated. Exposure to NaOH led to swelling of the active\nlayer after each cleaning, especially for poly­(piperazineamide) membranes.\nA 23% increase in the effective pore radii for these membranes after\nNaOH cleaning for 18 h led to 25, 36, 53 and 62% decrease in the rejection\nof magnesium, calcium, sodium, and chloride ions, respectively. Sulfate\nrejection decreased only slightly even for poly­(piperazineamide) membranes\n(i.e., 7%) despite an appreciable increase in pore radii, which can\nbe explained by the impact of charge exclusion on ion rejection that\nwas enhanced by the 16% reduction in zeta potential. On the other\nhand, cleaning with HCl had a negligible impact on the zeta potential\nand performance of all membranes evaluated in this study. The increase\nin permeability after chemical cleaning was in agreement with the\ndecrease in rejection of inorganic ions and correlated well with the\neffective pore radii measured using the membrane potential technique.\nThe importance of charge exclusion in the rejection of inorganic ions\nwas highlighted by the observed differences in rejection and permeability\nvalues when testing membranes after NaOH cleaning.