The Effect Of Ion-surface Interacitons On Soil Water Infiltrability

Soil water infiltration is an important process in water cycle system on earth,which not only affects the storage of soil water and occurrence of overland runoff,but also directly affects the resource utilization of precipitation,soil erosion and occurrence of non-point source pollution in farmland.Therefore,the soil water infiltration is an important issue in agriculture and environment science.Soil is an unusually complex system,current models of soil water movement exist many defects,the main fault of classical models is that the soil is regarded as a rigid structure,i.e.the soil structure is unchanged in water movement process.It is well known that the soil pore structure strongly changes when water infiltrate in soil,and then strongly affects the soil water movement rate and soil water transfer.Early studies found that electrolyte style and concentration strongly affect the soil water movement rate and transfer for a given soil,but the interaction mechanism is not clear.Subsequent researches found electrolyte style and concentration determine the stability of soil aggregates.Thereby we can deduce electrolyte style and concentration may affect the soil water movement through affecting the soil pore structure.Current studies found the electrolyte in soil solution is diluted when water enter in soil,which changes the electric double layer（EDL）structure at soil particle-water interface,soil electric field strength in EDL rapidly increases,and the electrostatic repulsive force sharply increases,which results in the destruction of soil structure.Therefore,the EDL theory for description electrostatic properties of soil particle and DLVO or extended DLVO theories for description interactions of soil particles may describe soil water movement.However,the rate of soil water movement in KCl solution is obvious different to that in Na Cl solution,this phenomena is specific ion effects or Hofmeister effects.There are many researches for the Hofmeister effects including soil science.The latest researches indicate that the coupling effects between electric field from the surface charges of particles and quantum fluctuation of outer shell electrons of ion.Therefore,in this study,the effects of ion interfacial reactions on the EDL structure and interactions between particles were investigated,which then influence the soil water movement.Firstly,the thickness of sliding layer at EDL in single and mixture electrolytes was theoretically derived,and the effects of EDL structure on surface potential and electrostatic repulsive force were quantified.Secondly,the interactions between particles were quantified based on the calculated the surface potential,the relationship between the rate of soil water movement and interaction forces between soil particles was investigated.Thirdly,based on the ion specificity of water movement experiments,the important effects of Hofmeister energy from the coupling effects between electric field and quantum fluctuation on soil water movement were revealed.Fourthly,the dynamic light scattering technique and industrial CT scanning technique were used to characterize the effects of particle interactions on the aggregation/dispersion process and the pore size of the soil particles,which further explain the effects of particle interactions on soil water infiltration.Based on above four aspects of the works,the influence mechanism of particle interactions on the interfacial properties and soil water movement was elucidated.The main results of the present study are as follows:（1）The thickness of sliding layer at EDL in single and mixture electrolytes was theoretically derived,and the effects of EDL structure on surface potential and electrostatic repulsive force were quantified.Based on the analytical solutions of Gouy-Chapman equations in different single electrolyte solutions,the theoretical expressions of the thickness of sliding layer were derived.The results showed that the thickness of sliding layer in 2:1 electrolyte solution was much smaller than in 1:1 electrolyte solution for montmorillonite and purple soil system.The shear plane was located far from the Stern plane but close to the Gouy plane,which is a new knowledge for EDL structure.Based on the new knowledge,the surface potential of particle,electric field strength and electrostatic repulsive force could be correctly characterized,the surface potential of montmorillonite in various single electrolyte solutions was approximately 3 to 6 times larger than the corresponding zeta potential.The electric field strength and electrostatic repulsive force calculated from the zeta potential is much lower than the that calculated from the Stern potential.For colloidal particles of purple soil,the electric field and electrostatic repulsive force could reduce 1-2 and 4 order of magnitude respectively.Therefore,based on the new knowledge of position of shear plane,the surface potential could be correctly characterized,and then the particle interactions could be correctly described,finally the effects of particle interactions on soil water infiltration could be investigated.（2）The theoretical expressions of thickness of sliding layer in mixed electrolyte solutions were derived,and the effects of ion interfacial reactions on the EDL structure and electric properties were quantified.Based on the analytical solutions of Gouy-Chapman equation in mixed electrolyte solutions,the theoretical expressions of thickness of sliding layer in mixed electrolyte solutions were derived.Similar to 1:1 electrolyte solution,the position of shear plane was also far from Stern plane but closed to Gouy plane,and the effects of bivalent cation on the thickness were larger than that of monovalent cation.In mixed electrolyte solutions,the calculated Stern potential is much larger than the zeta potential,the former was approximately 4-5 times as large as the latter.The present part of the work provide a theoretical foundations for description the soil water movement process in mixed electrolyte system.（3）The quantification relationship between interactions between particles and soil infiltration was established based on the ion interfacial reactions.Through the calculations of the electrostatic repulsive force,van der waals attraction force and hydration force,we found the soil electrostatic repulsive determines the stability of soil aggregates and soil infiltration.The electrostatic repulsive force based on the zeta potential is much lower than that based on the surface potential,which indicated that the position of shear plane is indeed far from Stern potential.Soil electrostatic repulsive force induced the breakdown of soil aggregates,the released small particles block up soil pore,so the soil water movement rate decreased.Correspondingly,when the electrostatic repulsive force decreased,the interactions of particles is net attraction force,the soil aggregates was not broken,the soil pore was not blocked,and soil infiltration increased.The theoretical calculations showed that the critical points for the net interaction forces in Mg Cl₂ and Na Cl solutions agreed with the measured values and were 0.005 and 0.1 mol L^-1,respectively.The calculated swelling strength in the Mg²⁺ system was much weaker than in Na⁺ system,and the experimentally observed infiltrability in Mg²⁺ system was much higher than that in Na⁺ system for soil with the same non-breaking aggregates.Therefore,we can infer that ion interfacial reactions determine the soil electric field,and affect the soil aggregate swelling or breaking,then affect the soil infiltration.（4）The Hofmeister effects on soil infiltration were systematically analyzed,and the its origin may be the coupling effects between electric field and quantum fluctuation.In Li⁺,Na⁺,K+,and Cs+ ion solutions,the soil infiltration showed strong Hofmeister effects.For a given soil sample,the maximum soil infiltration rate in the four alkali metal ions are: 1.8 cm h^-1,4.3 cm h^-1,5.2 cm h^-1 and 13.0 cm h^-1 respectively.The DLVO force plus the hydration force between adjacent soil particles determines the soil water infiltration rate.Specifically,both DLVO and hydration forces exhibited strong specific ion effects,which could be quantified by Hofmeister energy: 0.06Fφ（0）,0.18Fφ（0）,0.94Fφ（0）and 1.86Fφ（0）in Li⁺,Na⁺,K+,and Cs+ ion solutions.Only the DLVO forces and hydration force including specific ion effects could correctly explain the Hofmeister effects on soil water infiltration.The hydration forces may result from the surface hydration of soil particles and cationic hydration of the double layer.Most importantly,the hydration pressure between adjacent soil particles was complexly influenced by electrolyte concentration.Both the Debye length and the cationic distribution were strongly influenced by the Hofmeister energies.（5）The dynamic light scattering technique and industrial CT scanning technique were used to characterize the effects of particle interactions on the aggregation/dispersion process and the pore size of the soil particles based on the ion interfacial reactions.The results showed that the CCC values of purple soil colloids in Na⁺ and Mg²⁺ ion solutions are 91.6 mmol L^-1 and 4.83 mmol L^-1,respectively;Na⁺ >> Mg²⁺.The CCC values of purple soil colloids in Li⁺,Na⁺,K+ and Cs+ ion solutions are 280.9,91.6,47.8 and 5.2 mmol L^-1,respectively;Li⁺ > Na⁺ > K+ > Cs+.The volume of soil pore with > 1 mm occupied 50.4% of all pore volume in Mg²⁺ system,that with > 1 mm only occupied 1.43%;The volume of soil pore with > 1 mm occupied 40.2% of all pore volume in Na⁺ system,that with > 1 mm only occupied 1.06%.The former with > 1mm was 1.42 times as large as the latter.In Li⁺,Na⁺,K+,and Cs+ ion solutions,the volume of soil pore with > 1 mm occupied 22.8%,40.2%,56.4%和 59.9% of all pore volume,that with > 1 mm only occupied 0.32%,1.06%,1.57%,和1.88%.The soil pore with > 1 mm in Cs+ ion solution are 13.7,5.22 and 2.70 times as large as Li⁺,Na⁺ and K+ respectively.According to the data of soil particles aggregation/dispersion and pore state,the inner mechanism of the effects of particle interactions under ion interfacial reactions on the soil water infiltration could be evaluated.In summary,based on the above-mentioned results,the main conclusions are:（1）The theory of calculating the thickness of shear plane in solutions with single and mixed electrolyte composition was established;it was demonstrated that the Stern potential must be adopted to quantitatively describe the influence of particles interaction forces on soil water infiltration.（2）Soil surface potential determined the soil water infiltration rate.The electrolyte concentration and type in bulk solution could adjust soil electric field and affect soil particles interaction,which controlled soil colloidal particles coagulation/dispersion and the releasing of soil micro-particles,thus changing soil porosity,and finally soil water infiltration.（3）There were strong specific ion effects on soil infiltration.Through the coupling effects of electric field originating from particle surface charges and ionic quantum fluctuation,the Hofmeister energy affected the DLVO forces and hydration force,which further resulting in differences in soil aggregate stability and soil porosity,and finally soil infiltration.Therefore,based on the concept that soil water infiltration process was controlled by particles interaction,it is possible to develop a new“internal controlling” technology to improve soil infiltration;that is through adjusting soil particles interactions to control the rate of soil water infiltration.