Polarization induced covalent bonding: A new force of heavy metal adsorption on charged particle surface

Highlights: • Polarization induced covalent bonding has been found in heavy metal adsorption. • Electrostatic and the covalent bonding forces jointly drive the adsorption. • The energies from the two forces have been quantitatively distinguished. • The polarization induced covalent bonding energy is lower than that of a classic one. • The covalent bonding energy can be adjusted and controlled. Classically, stable covalent bonding cannot occur between heavy metal cations and clay surface O atoms. However, the classical theory ignores the effect of the electric field arising from clay surface charges on the orbitals of surface O atoms. This article studies the adsorption behavior of heavy metal cations (Pb²⁺, Cd²⁺, Cu²⁺, and Zn²⁺) on charged montmorillonite surfaces from a new theoretical foundation based on the quantum mechanics analysis of surface O atoms in this electric field, which reveals that polarization-induced covalent bonding is a strong adsorption force. The strength of polarization-induced covalent bonding can be controlled by regulating the energy of the lone-pair electrons of surface O atoms, which depends on solution pH, electrolyte type, electrolyte concentration, temperature or dielectric constant of medium, etc. The mathematic relationship between the energy of lone-pair electrons of surface O atoms and electric field arising from surface charges was established through quantum mechanics analysis; and correspondingly the mathematical relationship between the polarization-induced covalent bonding energy and surface potential also was established for different heavy metal cations. The finding of the new adsorption force will have important impact on both theoretical research and removal/deactivation approaches of heavy metal cations.