Asymmetric orbital hybridization promotes polarization and covalent interactions between heavy metal cations and charged surfaces

The interactions between heavy metal cations and charged surfaces are major processes responsible for the deactivation/activation of heavy metals, attracting special interest in both agricultural and environmental issues. The traditional view argues that the interactions between heavy metal and hydroxyl surface are primarily controlled by electrostatic forces and coordination-covalent bonds, while the interactions between heavy metal and siloxane surface are solely determined by electrostatic forces. A theory of asymmetric orbital hybridization was applied in this study to investigate the interactions between metal cations and charged surfaces. New surface reactions, polarization-enhanced induction forces (PEIF) and polarization-induced covalent bonds (PICB) between heavy metal cations (Cu2+ and Zn2+) and O atoms of charged siloxane surfaces, were discovered, and their respective contributions were quantified using asymmetric orbital hybridization theory. Both PEIF and PICB for metal cations with spzdz2 hybridization are stronger than those with spz hybridization. For example, the proportion of covalent bonds in the overall adsorption energy is lower for Ca2+ with 3s3pz hybridization (28.92 %) compared to Cu2+ with 3s3pz3dz2 (32.77 %). The new surface reactions based on asymmetric orbital hybridization determine the adsorption selectivity of heavy metal cations and strongly affect their removal in wastewater. The removal efficiency of heavy metals can be regulated through the enhancements of hydroxyl groups, electric field strength, and orbital hybridization effects at the clay mineral surfaces.