Insights into the development of quaternized starch-derived polymers for simultaneous flocculation and bacterial inactivation: Mechanism and directed regulation

The regrowth of pathogenic bacteria in municipal sludge poses a significant environmental and health risk. Starch-derived polymers (St-g-PDMC) grafted with quaternary ammonium monomers have demonstrated dual functions of flocculation and bacterial inactivation. However, the mechanisms remain unclear, particularly the interplay between flocculation and bactericidal processes, hindering the optimization and application of such bactericidal flocculants. To directionally modulate the synergy between two functions, this study explored the structure-property relationship of St-g-PDMC with varying charge density (CD), chain length (CL), and chain density (CN) by adjusting the grafting ratio of monomers. The results revealed that polymers with high CD and CL effectively enhanced the efficiency, achieving an E. coli removal of 96.11% and bactericidal efficiency of 99.56%. Mechanism analysis revealed that the bactericidal process competed for active sites, weakening the flocculation dominated by charge neutralization, while high-molecular-weight intracellular substances synergistically enhanced adsorption bridging. The optimized polymer mitigated competition and promoted synergy, exhibiting enhanced diffusion, adsorption, and the ability to alter membrane permeability, as confirmed by molecular dynamics simulations. Finally, a polynomial model was developed to accurately predict the flocculation efficiency and optimal dosage, facilitating the application of quaternized bactericidal flocculants in complex aquatic environments by testing in real water.