Describing the adsorption of PAM on coal/kaolinite surface in aquatic by combining experiments and MD simulation

Abstract

Hydration is an important factor influencing the adsorption of reagent molecules at the mineral interface. Understanding the micro-scale characteristics of hydration and the molecular adsorption at the interface is fundamental to improving the effectiveness of solid–liquid separation. In this paper, the flocculation processes of coal sludge in water and in kaolinite solutions were investigated, and the surface properties of the settling materials were analysed using Scanning Electron Microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results showed that as the concentration of polyacrylamide (PAM) increased, the flocculation effect showed a trend of first increasing and then decreasing. The kaolinite particles covered the coal surface, reducing the exposed area of the coal and the amount of PAM molecules adsorbed. Under the same conditions, the settling time of particles in kaolinite solutions is 60% longer than that in water. Further, molecular dynamics (MD) simulation was used to study the distribution of water molecules on the surface of coal and kaolinite, and the micro-scale configurations of PAM at the coal-water and kaolinite-water interfaces were analysed. The results showed that the coal surface was weakly hydrated, and PAM molecules can replace some of the water molecules near the coal surface. Kaolinite surface was strongly hydrated and the maximum density of the hydration layer is close to that of the kaolinite. PAM cannot cross the hydration layer with three layers of water molecules at the aluminium-oxygen octahedra (001) surface of kaolinite. However, there was only one layer of water molecules between PAM and silicon-oxygen tetrahedra (00–1) surface. When the kaolinite covered the coal surface, it was difficult for the PAM to replace the water at the surface, the 00–1 surface was the key dissociation surface to promote flocculation. This study illustrated microscopic insights into the effect of mineral hydration on the adsorption of flocculant molecules and may enrich the theory of mineral flocculation and sedimentation.