Star polymer-assembled adsorptive membranes for effective Cr(VI) removal

There is a strong demand for materials that can effectively remove highly toxic hexavalent chromium (Cr(VI)) ions from wastewater. Herein, we present a new class of Cr(VI)-adsorptive membranes assembled with a Cr(VI)-adsorptive and cross-linkable star polymer. The poly(acryloyl hydrazide)-branched star polymer (PAH-SP) with numerous amine groups enabled rapid and high-capacity Cr(VI) adsorption and was suitable for assembly into a selective layer via interfacial polymerization (IP). The IP-assembled PAH-SP (PAH-TFC) membrane synergistically rejected Cr(VI) over the entire pH range via a combination of adsorption (under acidic conditions) and membrane filtration (under basic conditions). Furthermore, the PAH-TFC membrane was more resistant to oxidation by acidic Cr(VI) than commercial reverse osmosis (RO) and nanofiltration membranes because of the self-sacrificing effect of its abundant amine groups. Consequently, PAH-TFC exhibited significantly higher Cr(VI) rejection (∼99.5% at pH 3) under acidic conditions than a representative commercial RO membrane (Cr(VI) rejection of ∼55% at pH 3) with similar water permeance, while demonstrating comparable Cr(VI) rejection under basic conditions. Furthermore, PAH-TFC displayed excellent regeneration ability, maintaining its Cr(VI) rejection above the commercial RO level by proper base/acid rinsing.