Microscopic Mechanism Of Soil Mineral Interface Reaction

All macroscopic processes in soils have the inherent microscopic basis.Previous studies show that the ionic interfacial reactions in soils have a profound impact on the mesoscopic soil particles interactions,which further affect the macroscopic processes such as formation and stability of soil aggregates,structure and pore conditions of soils,transport of water,heat and solutes,soil erosion and non-point source pollution of farmlands.Therefore,it is of critical importance to study the microscopic mechanisms of ion interface reactions and interactions between soil colloidal particles,which are insightful to the in-depth understanding of how different microscopic processes in soils affect the occurrence of macroscopic processes through the interaction of soil particles on the mesoscopic scale.Particles such as minerals,humus and microorganisms in soils generally carry a large amount of surface charges,which will adsorb counterions from the soil solutions and make soils electroneutral at the macroscopic level.However,due to thermal motion,these adsorbed counterions in vicinity of particle surfaces are distributed according to the Boltzmann law.As a result,the electric field formed by surface charges will not be fully screened by adsorbed counterions,and it reaches as high as¹0⁸ V·m^-1?water medium?or¹0¹⁰ V·m^-1?vacuum medium?with the distance of tens of nanometers away from soil surfaces.Recent studies have manifested that adsorbed ions exhibit the strong non-classical polarization in the external electric field,suggesting that the electric field around soil particles may alter the energy and quantum state of adsorbed ions'extra-nuclear electrons.The classical atomic structure theory considers only the influence of electric fields generated by nuclear charges and extra-nuclear electrons on the electronic structures of atoms.However,for charged interfaces such as soils,it is necessary to consider the influence of electric fields around soil particles on the electronic structure of ions/atoms?including surface atoms and adsorbed ions?.If the electric field around soil particles show the pronounced influences on the energy and quantum states of these atoms/ions'extra-nuclear electrons,further impacts will be caused for the surfaces of soil particles,atoms,ions and molecules around soil particles.As a result,the interactions between soil minerals,humus and microbial have been substantially regulated,and finally the soil macroscopic phenomena and processes have been fundamentally affected.This thesis employs the quantum mechanics theory,molecular dynamics simulations,periodic density functional theory,ion adsorption miscible displacement technique and dynamic light scattering technique to carry out the studies at the subatomic scale to the atomic/molecular scale,and then to the mesoscopic scale.Based on these studies,the multi-scale correlation analysis is conducted.The substantial orbital changes of surface atoms and adsorbed cations under the influence of electric fields have been revealed,and the new mechanism of interactions between soil mineral particles?including interface atoms,ions and water molecules?.This thesis also proposes a method to determine the soil mineral particles'interaction energy and clarify how this new mechanism influences and controls the interactions between soil particles.This thesis proposes a new theory of asymmetric hybridization of atoms/ions under the external electric fields.This new theory has been validated by molecular dynamics simulations,periodic density functional theory analysis and experimental determination of interface reaction energies.Adsorption of ions and water molecules and soil particles interaction is investigated and successfully interpreted by the orbital hybridization theory.The following results have been obtained thus far:?1?The asymmetric electric field formed by the surface charges of soil particles causes the significant changes of the ion/atomic orbitals and produces the asymmetric hybrid orbitals.The properties of hybridization orbitals of ions/atoms with 2s2p outer-shell orbitals(such as-N,-O,-F,Na⁺,Mg²⁺,etc.)and 3s3p outer-shell orbitals(such as-P,-S,-Cl,K⁺,and Ca²⁺,etc.)are completely different,resulting in the substantially different interface reaction activities and interfacial reactions.The reactions of Na⁺and K⁺at montmorillonite surfaces are exemplified and discussed using the asymmetric hybridization orbital theory.The main results are as follows:1)The interaction potential energy of Na⁺at montmorillonite surfaces originates from two sources:The classical coulomb potential energy and asymmetric hybridization enhanced dipole potential energy,while the interaction potential energy of K⁺at montmorillonite surfaces comes from three sources:The classical coulomb potential energy,the enhanced dipole action potential energy?2.067 times as Na⁺?and the covalent bonding potential energy?or polarization-induced covalent energy?caused by the asymmetric hybridization;2)No matter how strong the electric field is,Na⁺will not covalently interact with surface-O of montmorillonite.However,when the electric field is strong enough,K⁺will covalently interact with surface-O of montmorillonite.3)The potential energy of K⁺a montmorillonite surfaces is much higher than that of Na⁺.?2?The asymmetric hybridization orbital theory and theoretical speculations have been verified by measurement of the surface adsorption energies of Na⁺-and K⁺-montmorillonite by laser scattering and density functional theory?DFT?methods.The results of DFT analysis show that no matter how strong the electric field is,Na⁺does not covalently interact with surface-O of montmorillonite.When there is no external electric field or the external electric field is very weak,K⁺does not covalently interact with surface-O of montmorillonite.When the electric field is strong enough,K⁺interacts covalently with surface-O of montmorillonite,whereas its relative energy is merely-14.2 kJ·mol^-1.Therefore,the strength of this type of covalent bonding is significantly lower than that of classical covalent bonding.Accordingly,the results of DFT analysis are consistent with the theoretical predictions of asymmetric hybridization of ions and atoms in asymmetric electric fields at montmorillonite surfaces.According to the experimental data of aggregation activation energy measured by the light scattering technique,the adsorption energies of Na⁺and K⁺at montmorillonite surfaces indicate that the total potential energy of K⁺at montmorillonite surfaces is indeed much higher than that of Na⁺.The potential energy of Na⁺comes from two parts:the classical coulomb potential energy and the enhanced dipole potential energy due to the asymmetric hybridization.The interaction potential energy of K⁺at montmorillonite surfaces does originate from three sources:Classical coulomb interaction potential energy,asymmetric hybridization enhanced dipole interaction potential energy and polarization-induced covalent interaction energy.The potential energy of K⁺covalent bonding with the montmorillonite surface is by far lower than that of classical covalent bonding,which confirms the essential characteristics of polarization-induced covalent bonding as revealed by the quantum mechanics theory.In consequence,the dynamic light scattering experiments are consistent with the theoretical predictions of the asymmetric hybridization of ions and atoms under the asymmetric electric field.?3?Based on the asymmetric hybridization orbital theory,the measurement of van der Waals potential energy between soil particles has been established by using the dynamic light scattering technique.Firstly,the mathematical relationship between the average aggregation rate and the van der Waals potential energy of particles in complex systems is given.Theoretically,it is potential to measure the van der Waals potential energy by measuring the average aggregation rate using the dynamic light scattering technique.On such basis,we have successfully determined the van der Waals potential energy of multi-dispersed montmorillonite particles in four different electrolyte solutions.The results show that the van der Waals potential energy among montmorillonite particles obtained in NaCl,KCl,MgCl₂ and CaCl₂ solutions are-16564,-18801,-17355 and-18547 J·mol^-1,or the inter-particle potential energy calculated on basis of individual particles was-6.802,-7.588,-7.005 and-7.486 kT,respectively.The average value is-17817±857 J·mol^-1 or-7.22±0.304 kT,with an error of merely 4.22%.Although the method can be used to determine the van der Waals potential energy between particles under different electrolyte conditions,KCl solution should be the best choice.The method proposed in this paper is potentially applied to complex systems such as soils.?4?Based on the asymmetric hybridization orbital theory,this thesis studies the interaction between soil mineral surfaces and water molecules.By means of molecular dynamics simulations on the adsorption behaviors of water molecules at mica surfaces,we find that the water molecules at clay interfaces are significantly different from those of bulk aqueous solutions in terms of structure,dynamic properties and stability.With increase of surface charges,the hydrogen-bonding network gradually changes from hydrogen-bond between water molecules and water molecules to hydrogen-bonding between water molecules and mica surface-O atoms.Surface charge density plays a key role in the interfacial reaction of water molecules and mica.The interfacial water molecules at low surface charge densities are alike to the"liquid state",while the interfacial water molecules at high surface charge densities approach the"solid state".However,in classical theory,the activity of the lone pair electrons of clay surface-O atoms is too low to form hydrogen bonding with the interfacial water molecules.Nevertheless,the strong electric field at mica surfaces cause the outer-shell orbitals of surface-O atoms to be asymmetrically hybridized,resulting in the higher activity of their lone pair electrons.In this way,the lone pair electrons of surface-O atoms can partially enter into the empty orbital area of the H atoms of the interfacial water molecules,and hydrogen bonding interactions between surface-O atoms atom and interfacial water-H atoms are constructed.Accordingly,the asymmetric hybridization of O atoms of mica surfaces does play a key role in the interaction with the interfacial water molecules.?5?Molecular dynamics simulations for the adsorption of Pb²⁺/Na⁺,Pb²⁺/Cs⁺,Pb²⁺/Cd²⁺,Na⁺/Cs⁺at mica/water interfaces have been performed,and the results are consistent with the predictions of the asymmetric hybridization orbital theory.The results show that when the charge density of mica surfaces is sufficiently high,Pb²⁺,Na⁺,Cs⁺and Cd²⁺from single and binary electrolyte solutions are mainly adsorbed as inner-sphere.However,when the mica surface has a low charge density,adsorption of these ions occurs principally in the outer-sphere mode.When the external electric field is very weak,although the surface-O atoms have the lone pair electrons and heavy metal ions have the vacant orbitals,the heavy metal ions would rather not be coordinated with O atoms?inner sphere adsorption?.On the other hand,when the external electric field is strong enough,alkali metal ions can also have inner-sphere adsorption;although in the classical theory,Na⁺and Cs⁺can only have electrostatic adsorption at clay surfaces.Clearly,the results can be explained satisfactorily by means of the asymmetric orbital hybridization theory.?6?Based on the asymmetric hybridization orbital theory,adsorption kinetics of alkali metal ions on the surface of soil particles with variable charges?hydrated silica?and constant charges?montmorillonite?is probed.A new mechanism of ionic interfacial adsorption is detected,and the sources of ionic interfacial adsorption in different systems may differ significantly.Alkali metal ions have four different forces at montmorillonite surfaces:electrostatic force,non-classical polarization force,polarization-induced covalent force and dispersion force.The non-classical polarization and polarization-induced covalent bonding come from the asymmetric hybridization of outer orbitals of ions/atoms proposed in this thesis.However,when shifted to silica surfaces,the interaction of alkali metal ions contains only two classical forces,electrostatic force and dispersion force,indicating that the outer-orbital of silica-O atoms will not be asymmetrically hybridized.It is due to that the source of the negative electric field at silica surfaces is on the O atoms?since the negative charge comes from the dissociation of H atom of surface hydroxyl groups?,and hence this electric field is not an external electric field for surface-O atoms.In contrast,the negative electric field of layered aluminosilicate minerals is located in the center of tetrahedral or octahedral sites,so this electric field is an external electric field for surface-O atoms of clay minerals.?7?Through the adsorption of heavy metal ions and alkaline earth metal ions at montmorillonite particles surface and aggregation of montmorillonite particles in different electrolyte solutions,we find that in soil clay minerals systems,the asymmetric hybridization of the outer-shell orbitals of ions/atoms determines the interfacial reaction of all ions including H⁺,alkali metal ions,alkaline earth metal ions and heavy metal ions.It is the reaction caused by the asymmetric hybridization determines the interaction between soil minerals.Accordingly,this thesis takes clay minerals as the experimental material and systematically verifies the importance of the asymmetric hybridization of ion/atom outer-shell orbital in soil particles interaction.This thesis points out that due to the asymmetric hybridization of outer-shell orbital of ions/atoms that occurs in asymmetric electric fields at the surfaces of soil minerals,the interaction between alkali,alkaline earth metal and heavy metal ions and soil mineral surfaces may include:electrostatic force,classical polarization force,dispersion force,polarization induced covalent force and polarization enhanced coordination force.Based on the above results,we arrive to the following conclusions:1)The electric field at soil mineral surfaces changes the electron orbitals of ions/atoms,and the asymmetric hybridization orbitals take place;2)Based on the asymmetric hybridization theory,there are three new interactions arising in the interfacial reaction of ions/molecules:non-classical polarization force,polarization induced covalent force and polarization enhanced coordination force.In consequence,if electrostatic force and dispersion force are also included,this thesis expands the ionic interfacial force up to five sources;3)The asymmetric hybridization in the asymmetric electric field of soil particles dominates the reaction type,mode and intensity of the interaction between ions/molecules and the surface of soil particles and also controls the interaction type,mode and intensity among soil mineral particles.