Experimental investigation and molecular dynamics simulation on the effect of ionic strength and pH on kaolinite flocculation using a novel cationic chitosan-based flocculant

This study employed a combination of settling tests and molecular dynamics (MD) simulations to investigate the effect of ionic strength and pH on the flocculation of chitosan-grafted cationic polyacrylamide (CPD) in kaolinite suspensions, and its adsorption on kaolinite (001) surfaces. At a CPD dosage of 75 g/t, the settling rate of kaolinite dropped from 12.95 m/h to 7.89 m/h with an increase in KCl concentration from 0 mM to 200 mM. MD simulations revealed that the radius-of-gyration (Rg) of CPD decreased from 17.04 Å to 15.43 Å and the self-diffusion coefficient (D) reduced from 0.0296 Å 2 /ps to 0.0104 Å 2 /ps. The interaction energy rose from 984 kcal/mol to 1416 kcal/mol and the distribution density of CPD became more concentrated. Such results indicated that the increased ionic strength improved the adsorption and electro-neutralization efficiency of CPD, reducing the turbidity of supernatant. However, this increase led to a reduction in the molecular size and mobility of CPD, weakened its bridging capacity and slowed kaolinite settling. For increasing pH, the settling rate of kaolinite initially increased and then decreased, reaching a maximum value of 15.14 m/h at pH 5. This trend was consistent with results of D. The bridging capacity of CPD was enhanced under acidic conditions but weakened at excessively low pH. This can be attributed to the excessively strong electrostatic interaction between CPD and kaolinite, hindered its migration. Our work would enhance the understanding of impacts of solution chemistry on the settling of suspended solids and facilitate the development of effective flocculants.