Effect Of Asymmetric Hybridization Of Constant Charge Surface On Adsorption Kinetics Of Bivalent Metal Ions

Ionic interface reaction has a profound impact on the occurrence of many macroscopic processes in soil,such as soil erosion,soil aggregate fracture and colloidal condensation in soil,etc.Therefore,only through the study of ionic interface can we fundamentally understand the mechanism of macroscopic processes from the perspective of microscopic mechanism.It is well known that a large amount of electric charges on the surface of soil/mineral particles can form a strong electric field of the order of 10⁸～10⁹V m^-1on the surface,which researchers believe is the fundamental reason for its various physical and chemical properties.Recently,it has been found that the long-range strong electric field generated by the surface charge of soil/mineral particles is asymmetric.This asymmetric strong electric field leads to a new type of hybridization in the outer orbitals of the atoms/ions on the surface of soil/mineral particles,which leads to a new interfacial bond cooperation:strong polarization and polaria-induced covalence.Based on the asymmetric hybrid orbital theory,the adsorption kinetics of bivalent metal ions Mg²⁺（2s2p）,Cd²⁺（5s）and Pb²⁺（6p）on the surface of constant charge soil/mineral under different electrolyte concentrations were studied.The effect of asymmetric hybridization on the adsorption behavior of divalent metal ions on the surface of montmorillonite/purple soil was investigated.According to the experimental results,the following conclusions are obtained:（1）The ion-exchange adsorption kinetics experiments of Mg²⁺,Cd²⁺and Pb²⁺on the surface of constant-charge montmorillonite/purple soil particles were carried out under different electrolyte concentrations.It was concluded that Pb²⁺>Cd²⁺>Mg²⁺appeared in the equilibrium adsorption capacity under the same electrolyte concentration.The trend indicates that exists adsorption selectivity of Pb²⁺>Cd²⁺>Mg²⁺on the surface of montmorillonite/purple soil particles with the same valence of three metal ions.Based on the asymmetric hybrid orbital theory,the interaction types of three bivalent metal ions on the surface of montmorillonite/purple soil particles are analyzed.It is found that Mg²⁺（2s2p）,Cd²⁺（5s）and Pb²⁺（6p）have different outer orbitals,so the new bonding types generated by asymmetric hybridization are different.Mg²⁺（2s2p）only has strong polarization effect except electrostatic effect.In addition to electrostatic interaction,Cd²⁺（5s）and Pb²⁺（6p）also have strong polarization and polarization enhancing coordination,and the polarization enhancing coordination effect of Pb²⁺is greater than that of Cd²⁺.In addition,the new bond capacity of the three metal ions under asymmetric hybridization decreased with the increase of electrolyte concentration.（2）By analyzing the adsorption kinetics parameters of Mg²⁺,Cd²⁺and Pb²⁺on two different surface adsorption materials with constant charge（montmorillonite/purple soil）and comparing the ratio of the equilibrium adsorption capacity of three metal ions on the surface of montmorillonite/purple soil particles with the corresponding adsorption material CEC value（metastable adsorption degree）under the same electrolyte concentration,It is concluded that the adsorption capacity of Mg²⁺,Cd²⁺and Pb²⁺on the surface of purple soil particles is stronger than that of montmorillonite particles.It is found that the absolute value of surface potentialφ₀of K⁺-purple soil particles is greater than that of K⁺-montmorillonite particles in the same ion type system under different electrolyte concentrations.Moreover,the charge density,σ_d,and electric field intensity,E_d,on the surface of K⁺-purple soil particles are greater than those on the surface of K⁺-montmorillonite particles when the three metal ions reach adsorption equilibrium.（3）By analyzing the distance（d value）between the cations of Mg²⁺+Li⁺and Cd²⁺+Li⁺to the surface of montmorillonite particles when they reach adsorption equilibrium,it is found that the d value of Mg²⁺and Cd²⁺in the two mixed systems decreases with the increase of electrolyte concentration,and under the condition of the same electrolyte concentration,The values of Mg²⁺and Cd²⁺d were both d _Mg>d _Cd;However,the d value of the monovalent ion Li⁺in the two mixed systems decreased first and then increased with the increase of electrolyte concentration.（4）By comparing the equilibrium adsorption capacity of Li⁺on the surface of montmorillonite particles between Mg²⁺+Li⁺and Cd²⁺+Li⁺,it is found that the equilibrium adsorption capacity of Li⁺in Mg²⁺+Li⁺system is larger than that in Cd²⁺+Li⁺system under the same electrolyte concentration.By calculating the adsorption energy of Li⁺,it is concluded that Cd²⁺has a stronger ability to reduce the electrostatic adsorption energy of Li⁺than Mg²⁺,indicating that cations with stronger asymmetric hybridization effect have greater competitive advantages in the adsorption process.Meanwhile,the adsorption behaviors of Mg²⁺and Cd²⁺on the surface of montmorillonite particles in the mixed system and the single system were compared.It was found that the adsorption capacity of Mg²⁺and Cd²⁺in the mixed system was weaker than that in the single system due to the influence of Li⁺.