Analysis of lead(II) retention from single salt and binary aqueous solutions by a polyamide nanofiltration membrane: Experimental results and modelling

Abstract

The present paper reports the performance of a polyamide thin-film composite NF membrane (AFC 80) in the removal of highly polluting and toxic Pb(II) ions from single salt and binary aqueous solutions simulating real wastewaters. The effect of the operating variables (pH, transmembrane pressure and feed solution concentration) on the separation process was investigated. It was observed that the rejection of lead ions slightly increases with increasing the transmembrane pressure and slightly decreases with increasing the metal concentration in feed at constant pH of aqueous solutions. The maximum rejection of lead ions in single salt solution was 99.44% for a feed solution containing 50mg Pb/L at pH 5.7. In binary Pb–Cd aqueous solutions the metals rejections are higher than 98.5% for concentrations in the 15–80mg/L range, and the rejection order follows the order of metals hydration energies. The optimum operating conditions selected were: transmembrane pressure in 10–30bars range, feed solutions (polluted wastewaters) at pH 5.7 containing 50–80mg metal/L. The Spiegler–Kedem model ensures a very good description and prediction of the NF processes under study for a wide concentrations range. The model parameters showed that the transport mechanism of the solute is mainly convective, being almost totally hindered by the size of the metal ions which finally determine the salts retention. The AFC 80 membrane proved to be able to perform an effective removal of lead, and also of cadmium and nitrate ions, in very mild and economically advantageous conditions, leading to wastewaters suitable to be discharged into the environment. The structural characteristics of the AFC 80 membrane (effective pore radius and the thickness to porosity ratio) were estimated from uncharged solutes rejection experiments. The charge properties of the membrane surface were assessed by measuring the tangential streaming potential. It was found that the membrane becomes positively charged in lead nitrate solution, and this behaviour was attributed to adsorption of lead ions on the membrane surface. The membrane characteristics thus determined were correlated with the results of lead separation experiments and with the Spiegler–Kedem model parameters.