Flocculation behavior and interaction mechanism of carboxymethyl chitosan/chitosan-based graft flocculants on kaolinite (00−1) surface: Experiment and DFT calculation

Chitosan-based flocculants represent a highly promising alternative to industrial synthetic organic flocculants, offering exceptional environmental friendliness and efficiency. The carboxymethyl chitosan-graft-poly (acrylamide-dimethyl diallyl ammonium chloride) (CMCPD) and chitosan-graft-poly (acrylamide-dimethyl diallyl ammonium chloride) (CPD) were successful synthesized using UV-induced graft polymerization. The flocculation behavior and mechanism of these two chitosan-graft-copolymers on kaolinite suspension were investigated employing settling test and density functional theory calculation of the structural units of CMCPD and CPD on the kaolinite (00–1) surface. Experimental results indicated that the CMCPD had a greater settling rate of 19.47 m/h at the dosage of 80 g/t than CPD (12.96 m/h). The simulation results revealed distinct adsorption conformations of CMCPD and CPD units on the kaolinite (00–1) surface, characterized by an inclined posture and a horizontal posture, respectively. The stable adsorption of CPD can be attributed to the electrostatic attraction, which is evidenced by its higher adsorption energy (−4.74 eV) on the kaolinite (00–1) surface compared to CMCPD (−3.63 eV). It should be noted that both the adsorption energy and conformation play crucial roles in determining the flocculation efficiency of a flocculant. The inclined adsorption of CMCPD not only displayed electrostatic attraction but also enhanced its bridging ability, leading to a rapid increase in floc size and settling rate. The settling performance was significantly influenced by the bridging effect exerted by flocculants. This study holds great significance for the advancement of novel and efficient flocculant development.