A pH-responsive PAA-grafted-CNT intercalated RGO membrane with steady separation efficiency for charged contaminants over a wide pH range

Nanofiltration offers an effective solution in wastewater treatment by size sieving and particularly electrostatic interaction (Donnan effect). However, its separation performance fluctuates with pH changes because both membrane charge and contaminate ionization are sensitive to pH. Therefore, it is significant to fabricate a nanofiltration membrane with steady separation efficiency over a wide pH range. In this study, a novel pH-responsive reduced graphene oxide membrane intercalated by polyacrylic acid-grafted-carbon nanotube (RGO-gCNT membrane) was designed elaborately. Benefited from the pH-responsive property of polyacrylic acid, the pore size of RGO-gCNT membrane can be narrowed in acid solution and enlarged in basic solution. And the pore sizes at pH 3.0 and pH 8.2 are calculated to be about 0.8 and 1.3 nm, respectively. The results of pH influence on phosphate separation show that phosphate rejection rate of the non-responsive RGO-CNT membrane decreases sharply from 91.6% to 11.1% when the pH changes from 8.2 to 3.0. However, the pH-responsive RGO-gCNT membrane can retain 92.7% and 74.2% of the feed phosphates at pH 8.2 and 3.0, respectively. This novel pH-responsive RGO-gCNT membrane provides reliable separation efficiency over a wide pH range and gives new insight to design smart membranes.