The interaction and mechanism between threonine-montmorillonite composite and Pb2+ or Cu2+: Experimental study and theory calculation

Abstract The maximum adsorption capacities of threonine-montmorillonite (Thr–Mt) composite to Pb2+ and Cu2+ in aqueous solution were higher than that of original sodium-montmorillonite (Na–Mt). Molecular dynamics (MD) simulation and Multiwfn wavefunction program based on density functional theory (DFT) were exploited to probe the interaction and mechanism between threonine-montmorillonite composite and Pb2+ or Cu2+. Moreover, for the first time, Independent Gradient Model (IGM) analysis was utilized to decode the interaction essence among Thr, Mt. and Pb2+ or Cu2+ to reveal the removal mechanism of Thr-Mt to Pb2+ and Cu2+. MD results indicated that the arrangement of Thr+ presented a preferred almost vertical orientation, which was consistent with the X-ray diffraction (XRD) result. There was hydrogen bonding between carboxyl group or hydroxy group of Thr+ and Mt. surface. Multiwfn calculation implied that the minimum points of ESP and ALIE were located near the O atoms of hydroxyl group and carboxyl group, which were most likely providing the lone pair electron to form a complex with the metal ion,which was accordance with the result of basin analysis. IGM analysis vividly revealed that O atoms of carboxyl group and hydroxyl group were binding sites, and the interaction between Pb2+ and Na–Mt or Pb2+ and Thr-Mt was stronger than that of Cu2+, which was well consistent with the experimental conclusion.