Influence mechanism of hydrated cations on surface hydration of slime mineral particles

To elucidate the microscopic mechanisms underlying the impact of hydrated cations on the surface hydration of slime mineral particles (specifically, kaolinite and quartz, the primary minerals in slime), this study focused on constructing two common hydrated cations in slime water: Na(H₂O)₅⁺ and Ca(H₂O)₈²⁺. Using density functional theory, the adsorption of these two hydrated cations on the surfaces of kaolinite (001), ( 00\overline 1 ) and α-quartz (001), as well as their competitive adsorption with water molecules were simulated. The simulation results revealed that the adsorption energy of hydrated cations on all three surfaces was over 50% lower than that of water molecules. The adsorption stability on mineral surfaces was as follows: α-quartz (001) surface > kaolinite (001) surface > kaolinite ( 00\overline 1 ) surface. The adsorption energy of the competitively stable configuration was 34%–57% lower than that of a single hydrated cation on kaolinite and quartz. Additionally, the Ca(H₂O)₈²⁺ configuration exhibited a greater stability than the Na(H₂O)₅⁺ configuration under both adsorption conditions. When the hydrated cations adsorbed onto three surfaces, strong hydrogen bonds formed with surface, surpassing the strength of hydrogen bonds between water molecules and kaolinite/quartz surfaces. The hierarchy of hydrogen bonds between two hydrated cations on mineral surfaces was as follows: kaolinite (001) surface > α-quartz (001) surface > kaolinite ( 00\overline 1 ) surface. Under a competitive adsorption, the hydrogen bond between Na(H₂O)₅⁺ and mineral surface strengthened, while the bond between Ca(H₂O)₈²⁺ and mineral surface weakened. Although hydrogen bonding did not entirely correlate with changes in adsorption energy, electrostatic interactions in the adsorption configuration were identified. The electrostatic interaction in the single adsorption configuration of hydrated cations proved stronger than that in water molecular adsorption. Under a competitive adsorption, the electrostatic interactions between hydrated cations and mineral surfaces intensified, with Ca(H₂O)₈²⁺ demonstrating stronger interaction than Na(H₂O)₅⁺. Given the robust adsorption of hydrated cations on the surfaces of kaolinite and quartz, the dehydration of slime particles becomes more challenging. This could increase hydration repulsion between particles, resulting in a more stable dispersion of particles in slime water.