Adsorptive dead-end filtration for removal of Cr(vi) using novel amine modified polyacrylonitrile ultrafiltration membranes

Abstract

The present study investigates the concept of combining the processes of ultrafiltration (UF) and ion exchange into one single filtration step. By this, particulate matter and dissolved toxic hexavalent chromium (Cr(VI)) ions are removed by adsorptive dead-end filtration. For this purpose, we modified a conventional polyacrylonitrile (PAN) UF membrane with two methods to generate weak-base and strong-base anion exchanger functional groups on the membrane surface and in its pore structure. The pristine PAN membranes were chemically modified to generate primary (PAN-NH) and quaternary (PAN-Q) amine groups. Pristine PAN, PAN-NH and PAN-Q membranes were characterized and compared regarding their chemical and mechanical properties. The modified PAN membranes were tested in chromium filtration experiments at pH 5.5, pH 7 and pH 8.5. The fully loaded membranes were regenerated by caustic solution for desorption of the chromium. The effect of the membrane modifications on the pure water permeability, contact angle and BET surface was negligible. However, the charge of the membrane was affected significantly seen by the reversal of zeta potential from negative to positive values after modification. The filtration experiments revealed that the total adsorption capacities of PAN-NH and PAN-Q membranes are 151 mg and 145 mg Cr(VI) per m² membrane surface after complete membrane saturation (c_feed = c_permeate), respectively. When compared with pristine PAN and PAN-Q membrane, the PAN-NH membrane has demonstrated high removal rates (>90%) for Cr(VI) in the first ten hours of filtration and thus more relevant for application-orientated membrane water treatment systems. Desorption of previously adsorbed Cr(VI) was achieved by filtering a regeneration solution (1 M NaCl and 0.01 M NaOH) through the loaded membranes. After regeneration, the adsorption capacity was almost completely recovered. The findings of this study verify that low-pressure amine-modified PAN membranes provide an effective and energy-efficient way to process Cr(VI) contaminated water for possible drinking water purposes.