Chitosan/pectin graft copolymers for anionic dye flocculation: Preparation, characterization, flocculation mechanism and application assessment

To achieve cost-effective decolorization, two structurally tunable flocculants—CPDA, bearing a cyclic quaternary ammonium structure, and CPDM, possessing a comb-like quaternary ammonium architecture—were synthesized via a one-step grafting strategy. The effects of raw material mass ratio (CS: pectin: monomer), temperature, reaction time, and initiator dosage on the synthesis were systematically evaluated. The optimal mass ratios were determined to be 1: 1: 3 for CPDA and 1: 1: 1 for CPDM, with the remaining conditions held identical (temperature, 50 °C; reaction time, 3 h; APS dosage, 200 mg). To assess the effectiveness and applicability of CPDA and CPDM, the influences of flocculant dosage, initial pH, initial dye concentration, and coexisting inorganic ions on decolorization performance were investigated. Removal efficiencies of 87%–96% were achieved by both flocculants for the four anionic dyes examined, with CPDA exhibiting superior performance under acidic to neutral conditions and CPDM proving more effective under alkaline conditions. Mechanistic analysis indicated that electrostatic neutralization, charge patching, adsorption bridging, and network capture acted synergistically in the flocculation process. This study offers new insights into the controllable synthesis of chitosan-based flocculants and the understanding of their structure–activity relationships.