Atomic Mechanism of Polyacrylamide Adsorption on Kaolinite and Humic Acid–Kaolinite Complexes: Experimental and Molecular Dynamics Studies

Polyacrylamide (PAM) is extensively used in soil improvement. Its interaction with soil clay minerals and organic matter influences numerous physicochemical and biochemical processes, including carbon cycling. Nevertheless, the environmental behavior of PAM in soil and its underlying mechanisms are not fully understood. This research employed a combination of molecular dynamics (MD) simulations and a series of experimental techniques to investigate the interaction of PAM with kaolinite and humic acid (HA)-kaolinite (Kao) complexes, elucidating the adsorption mechanism of PAM on kaolinite and HA-Kao complexes at the atomic scale. Adsorption experiments demonstrated that the HA-Kao complexes had an 8.51% higher adsorption capacity for PAM than kaolinite alone; MD simulation results also showed that the interaction energy of PAM adsorbed on HA-Kao complexes was 6.23% higher than that of kaolinite alone, indicating that the HA-Kao complexes were more favorable for PAM adsorption. The FTIR, XPS experiments, and MD simulation results indicate that hydrogen bonding is a significant mechanism for the adsorption of PAM on kaolinite and HA-Kao complexes. Humic acid collaboratively enhanced the adsorption of PAM, and its hydroxyl and carboxyl groups offered additional active sites for PAM adsorption, facilitating the formation of a greater number of hydrogen bonds (12.89% higher than kaolinite alone), shorter hydrogen bond lengths, and larger hydrogen bond angles, which are beneficial for the adsorption of PAM. This work presents a novel perspective for understanding the adsorption and migration behavior of PAM in the soil environment.