Study On Soil Particle Interaction As Considering Hofmeister Effects And Its Effects On Soil Water Infiltration

Soil particle interactions,mainly including the soil particle aggregation and dispersion process,deeply affect such as the formation and stabilization of soil aggregates,transport of nutrients and contaminants,soil permeability and soil erosion.A dispersive soil with poor porosity usually has reduced air and water movement,which results in the degradation of soil properties and soil fertility.Instead,soil with good aggregate structure could supply sufficient water,oxygen and nutrient elements to plant roots and promote crop production.Therefore,from the perspective of agricultural production and ecological environmental protection,it is particularly necessary to have a deeper and better understanding of soil particle interaction.Currently,the classical theory of Derjaguin,Landau,Verwey,and Overbeek?DLVO?is considered as a crucial theory to describe particle interactions,with the Schulze-Hardy rule used to predict colloidal aggregation behaviour.Although the classical DLVO theory could give a quantitative and satisfying description of particle interactions affected by ionic concentration and ion valences in electrolyte solutions,it's shown to be flawed because it is unable to explain Hofmeister effects.Hofmeister effects,the phenomena that ions with the same valence display different behaviors are ubiquitous in soils and have a significant impact on soil particle interactions.For example,it is well known that Na⁺is much more deleterious to soil structural stability than K⁺,whereas Ca²⁺was reported to accelerate the aggregation of soil particles more effectively compared to Mg²⁺.Essentially,the observed Hofmeister effects in soil particle aggregation or dispersion behaviors in cases of different electrolytes arise from the differences in ion-surface interaction for various ions.Hence,to correctly estimate the electrostatic repulsion and van der Waals attraction between soil particles in the presence of Hofmeister effects and for a further purpose of predicting colloidal aggregation behavior,a quantitative characterization of the ion-surface interaction energy of different ions is needed.On the other hand,the soil water infiltration at macroscale and the soil particle interactions at mesoscale are closely interrelated,but they were always separately investigated.Recent studies have shown that soil surface charges along with soil interparticle forces play a crucial role in soil aggregates stability and soil pores,which further affects soil water infiltration.Therefore,the linkage and comparison between the two can aid us quantitatively elucidate how soil particle interactions affect soil water infiltration process.In this study,soil particle interactions were investigated from the perspective of Hofmeister effects.Firstly,six cations(Li⁺,Na⁺,K⁺,Cs⁺,Mg²⁺,Ca²⁺)were selected and the cationic absolute effective charge coefficients were introduced towards a quantitative characterization of the differences in cation-surface interaction energy?Chapter 3?.Secondly,approach to estimation of Hamaker constant as taking Hofmeister effects into account was suggested,and correct estimation of the van der Waals attraction was achieved?Chapter 4?.Thirdly,A theoretical relationship between cation-surface interaction energy and aggregation activation energy was further established,and it can successfully predict CCC values for different electrolytes,which made up for the deficiency of classical DLVO model of Schulze-Hardy rule that the Hofmeister effects are not considered?Chapter 5?.Finally,quantitative calculation of soil interparticle forces and column experiments were combined to reveal the close relationship between mesoscopic soil particle interactions and macroscopic soil water infiltration?Chapter 6?.The main results of this study were as follows.1.The differences in cation-surface interaction energy of several common mono-/di-valent metal cations were quantitatively characterized by the introduced cationic absolute effective charge coefficients,?_i.The monovalent Li⁺,Na⁺,K⁺,Cs⁺as well as the divalent Mg²⁺,Ca²⁺were selected to reflect the Hofmeister effects,and the?_i values of aforementioned cations were calculated from the relative effective charge coefficients obtained from binary cation exchange equilibrium.For monovalent cations,their?_i values increased as?_Li?1.063?<?_Na?1.180?<?_K?1.942?<?_Cs?2.864?;for divalent cations,the?_i values increased as?_Mg?1.425?<?_Ca?2.005?.Theoretically,for equivalent cations,a cation with larger value of?_i will bear a stronger interaction with charged surfaces,and as a result,a higher adsorption rate and equilibrium adsorbed quantity when reaching thermodynamic equilibrium would be observed.The experimental results of cation adsorption kinetics and adsorption equilibrium showed that both the adsorption rates and the adsorbed quantity of five cations on K⁺-montmorillonite indeed decreased in the order Cs⁺>>Na⁺>Li⁺,and Ca²⁺>Mg²⁺,which was consistent with the above theoretical expectations.It also shows that?_i can successfully explain the differences in cation adsorption behaviors on montmorillonite,and the differences in cation-surface energy essentially led to the different adsorption affinity of cations with charged surfaces and the obseved Hofmeister effects in cation adsorptions.2.Approach to estimation of Hamaker constant considering Hofmeister effects based on the dynamic light scattering?DLS?technique was suggested.The Hamaker constant is an important material parameter for both the description of particle interaction and the prediction of colloidal stability.Existing methods based on the classic DLVO theory ignore the Hofmeister effects and would derive flawed results.In this chapter,approaches to the estimation of Hamaker constant with and without consideration of the Hofmeister effects were respectively put forward,where the Hamaker constant was obtained based on the mathematic relationship between electrostatic repulsive and van der Waals attractive interaction at critical coagulation concentration?CCC?.It indicated that the montmorillonite particles aggregation kinetics in the presence of Li⁺,K⁺and Cs⁺exhibited remarkable Hofmeister effects and the CCC values show:Li⁺?277.2 mM?>K⁺?80.3 mM?>Cs⁺?27.2 mM?.Without consideration of Hofmeister effects,completely distinct Hamaker constants for the same material were obtained from aggregation kinetics of the three cations?6.70,17.4 and 35.7×10^–2020 J for Li⁺,K⁺and Cs⁺system,respectively?.Obviously it is unacceptable because the obtained Hamaker constant values should be the same for the same montmorillonite.In contrast,by taking the Hofmeister effects into account,consistent Hamaker constants?6.20,6.09and 6.75×10^–2020 J for Li⁺,K⁺and Cs⁺system,respectively?could be derived and reached in good agreements with results reported in literature.3.Particle aggregation is deeply affected by Hofmeister effects,whereas aggregation behavior in the presence of Hofmeister effects predicted by the classic DLVO model didn't get satisfying results.In this chapter,description of colloidal particles aggregation in the presence of Hofmeister effects based on a theoretical relationship between cation-surface interaction energy and aggregation activation energy was established,and validity of the suggested theory was confirmed by the published experimental data of montmorillonite particles aggregation in solutions of LiNO₃,KNO₃,CsNO₃,Mg?NO₃?₂ and Ca?NO₃?₂.In the presence of Hofmeister effects,the differences in adsorption ability of the involved five cations were quantitatively characterized by the defined additional cation-surface interaction energy,and we found that the additional interaction energy for Li⁺,K⁺,Cs⁺,Mg²⁺and Ca²⁺on montmorillonite surface were respectively 0.063,0.942,1.864,0.850 and 2.010 times larger than the classic Coulomb interaction energy.As taking those additional cation-surface interaction energies into account,the CCC values for the presence of different cations were theoretically calculated by the suggested model,and the predicted CCC values matched the experimental results very well.The theoretically predicted CCC values in montmorillonite aggregation for KNO₃,CsNO₃,Mg?NO₃?₂ and Ca?NO₃?₂ were 78.8,29.9,6.48,and 3.12 mM respectively,and the corresponding measured CCC values were 80.3,27.2,7.99,and 2.38 mM respectively.However,the classic DLVO model of Schulze-Hardy rule failed to predict the pronounced shifts in CCCs for different cations of the same valence.This study indicated that in the presence of Hofmeister effects,the different additional adsorption energies of cations resulted in different adsorption affinities for charged surface.This essentially led to the differences in electric fields around particles and energy barriers for particle aggregation,and thus different CCC values.4.Soil interparticle forces calculations and column experiments were combined to elucidate the effects of soil particle interaction forces on water infiltration in a permanently soil?purple soil?and a variably charged soil?yellow soil?by using different concentrations of KNO₃ solutions(0.0001,0.001,0.01 and 0.1 mol L^–1),and some interesting results in soil water infiltration were discovered.It indicated that,?1?big differences in surface charge properties were detected between the permanently charged purple soil and the variably charged yellow soil.The surface charge number,surface potential and surface electric feld strength of the purple soil were all larger than those of the yellow soil;?2?four forces in soil:long rang van der Waals attractive,electrostatic repulsive,hydration repulsive and osmotic repulsive forces,co-determined soil water infiltration rate for both two tested soil,and the quantitative description of the four forces affecting soil water infiltration has been given;?3?at high electrolyte concentration(?0.1 mol L^-1)the osmotic repulsive force became more important in soil water infiltration,and yet at low electrolyte concentration the electrostatic repulsive force became more important;?4?soil surface charges and soil electrolyte concentrations through their influence on soil particle interaction forces to affect water infiltration.The surface charges of soil particles can generate an electric field around soil particles and further produce a strong electrostatic repulsive force between soil particles.The concentrations of electrolyte in soil solutions regulates the repulsive forces between soil particles,thus regulating soil structural stability and water infiltration process.In summary,we get the following conclusions:?1?The different additional cation-surface interaction energies of the same-valent cations can be quantitatively characterized by the introduced absolute effective charge coefficients of cations.?2?The different cation-surface interaction essentially bring about the differences in a series of properties and process on soil-water interface,such as ion adsorption affinity,particle aggregation/dispersion,and water infiltration.?3?Soil water movement at macroscale is actually determined by the nature and strength of soil particle interactions at mesoscale,while soil particle interactions are profoundly influenced by the Hofmeister effects of cation-interface interaction.