Flocculation of both anionic and cationic dyes in aqueous solutions by the amphoteric grafting flocculant carboxymethyl chitosan-graft-polyacrylamide

A series of the amphoteric grafting flocculants, carboxymethyl chitosan-graft-polyacrylamide, has been prepared and employed for removal of both anionic and cationic dyes from aqueous solutions. •A series of amphoteric grafting flocculants (CMC-g-PAM) was prepared.•Flocculation of both anionic and cationic dyes by CMC-g-PAM was studied.•The flocculation mechanism of CMC-g-PAM was discussed in detail.•The grafted PAM contributed much to the bridging and sweeping flocculation effects. In the current work, a series of amphoteric grafting chitosan-based flocculants (carboxymethyl chitosan-graft-polyacrylamide, denoted as CMC-g-PAM) was designed and prepared successfully. The flocculants were applied to eliminate various dyes from aqueous solutions. Among different graft copolymers, CMC-g-PAM11 with a PAM grafting ratio of 74% demonstrated the most efficient performance for removal of both the anionic dye (Methyl Orange, MO) and the cationic dye (Basic Bright Yellow, 7GL) under the corresponding favored conditions (80mg/L of the flocculant at pH 4.0, and 160mg/L at pH 11.0). In comparison with its precursors, chitosan and carboxymethyl chitosan, CMC-g-PAM11 showed higher removal efficiencies and wider flocculation windows. More importantly, the graft copolymer produced notably more compacted flocs based on image analysis in combination with fractal theory, which was of great significance in practical water treatment. Furthermore, the flocculation mechanism was discussed in detail. The grafted polyacrylamide chains were found to contribute much to the improved bridging and sweeping flocculation effects, but reduced charge neutralization flocculation for the effect of charge screening.