Investigating the performance and mechanism of a novel amphoteric chitosan-based flocculant to enhance kaolinite settling

Green and efficient flocculants are crucial for the effective treatment and recycling of mine wastewater. Herein, the flocculation performance and mechanism of a novel amphoteric chitosan-based flocculant CMC-g-CPAM on kaolinite suspensions were investigated through settling tests, zeta potential measurements, floc size distribution, quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM) measurements. Settling tests results revealed that CMC-g-CPAM displayed exceptional flocculation performance, achieving a settling rate of 18.21 m/h for kaolinite at natural pH, higher than that of chitosan-grafted-cationic acrylamide Chi-g-CPAM (11.19 m/h). The in-situ adsorption layer of CMC-g-CPAM on silica and alumina surfaces was more rigid than that of Chi-g-CPAM. Subsequently, more kaolinite particles were flocculated by CMC-g-CPAM, and formed a denser floc layer. The adsorption amounts of CMC-g-CPAM and Chi-g-CPAM on silica surfaces were found to be greater than those observed on alumina surfaces at natural pH. This phenomenon was related to the different surface charges of silica and alumina, electrostatic interactions between the polymers and surfaces. From AFM measurements, an attraction between the CMC-g-CPAM functionalized probe and the functional substrate as they approached each other and a weak adhesion force among CMC-g-CPAM (0.31 nN over 50 nm) were observed. These indicated the presence of intermolecular interactions among CMC-g-CPAM, which contributed to the formation of compact flocs. In contrast, an obvious repulsion was detected when the Chi-g-CPAM functionalized probe approached the functional substrate, inhibited the entanglement of polymer chains. Meanwhile, the interactions between multiple molecules and the substrate displayed a large adhesion force (2.10 nN over 150 nm), attributed to the less rigid adsorption layer of Chi-g-CPAM. Our findings would provide new insights into the design of flocculants and effective wastewater treatment.