Specific Ion Effects On 2:1-type Clay Aggregate Stability And Clay Migration Intensity

Soil particle migration often causes soil erosion,agricultural non-point source pollution and other environmental problems.Soil aggregates breakdown is considered to be the the first step in the process of soil particle migration.During the rainfall,the aggregates are dispersed into micro-aggregates or single soil particles,and these small particles are migrated by splash erosion and rain erosion,then soil erosion occurred.Therefore,controlling the aggregates breakdown is a direct and effective means to control the soil particle migration intensity.It is generally acknowledged that the aggregates breakdown during rainfall is mainly attribute to raindrops and other external effects.Recently,however,it is pointed out that,compared with the external effects on the classical erosion theory,the strong internal forces between soil colloidal particles are the main factors leading to the aggregate breakdown.In fact,the phenomenon of the soil "dry soil aggregated into a block,wet soil dispersed into grains" are all based on the external manifestations of the differences in the intrinsic interaction between soil particles.As the main component of organic matter in soil,humus is considered to be an important factor affecting the stability of soil aggregates.The size of humus particle was between 100-500 nm,which belonged to the typical soil colloid particles as the same as most soil minerals.Therefore,the aggregate format between humus and clay particle should be colloid aggregate,and the interaction between humus and clay will have an important impact on the stability of aggregates.The classical DLVO theory only considers the influence of ion concentration and valence on the particle interaction,while ignoring the difference between the same valence ions-specific ion effects.It has been shown that ions with the same valence displaying strong specific ion effects in the interaction of colloidal particles.So,this effects will also affect the interaction between humus and clay colloidal particles,and then affect aggregate stability.Therefore,this study will start from the interaction between colloidal particles,to explore the influence of interaction force between colloidal particles from different ion-surface interaction on humus-clay aggregate stability and clay migration intensity.Montmorillonite were chosen as the material and four cation species(K⁺,Na⁺,Ca²⁺,Mg²⁺)were employed to characterize the difference of ion-surface interactions.The aggregates stability is reflected by the aggregate breakdown intensity and clay migration intensity The results showed that:（1）The results of ion-surface interaction of Na⁺/K⁺ affecting on humus-clay aggregate stability showed that,regardless of whether aggregate contain humus,the ability of K⁺ to maintain aggregate stability were stronger than that of Na⁺.It was found that the electrostatic field strength for Na⁺ system was much higher than K⁺ system under the same concentration.Correspondingly,the electrostatic repulsive pressure between clay particles for Na⁺ system was much stronger than K⁺ system,therefore,the aggregate stability in Na⁺ system is much lower than K⁺ system.And the difference of the aggregate stability under the two system decreased with the increasing electrolyte concentration.Aggregate stability increased with the increase of humus in K⁺ system,Although the increase of humus also strengthened the aggregates stability in Na⁺ system,but not with the aggregate stability was proportional to the content of humus.By measuring the zeta potential of the particles,it was found that the zeta potential of the particles in the two system increased with humus increasing.The zeta potential for Na⁺ system was higher than that of K⁺ system under the same concentration,it was consistent with the surface potential of particles in Na⁺ and K⁺ system.When compared the influence of the ion non-classical polarization effect and humus on aggregate stability,cation strong polarization plays an important role in aggregate stability,its effect on increasing aggregates stability was much more important than humus.The coupling effect of cation strong polarization and humus on aggregate stability is much stronger than the effect of the two alone present.（2）In the experiment of aggregate breakdown,there were also differences between the ability of Ca²⁺ and Mg²⁺ to maintain the aggregate stability,the aggregate stability is higher under Ca²⁺ system.It was found that by considering the ion non-classical polarization effect the surface electric field strength and electrostatic repulsive force in the Ca²⁺ system were lower than that of the Mg²⁺ system under the same electrolyte concentration.The study also found that the present of humus in the Ca²⁺/Mg²⁺ system did not promote the aggregate stability,but promoted the aggregates dispersion.This seems to contradict with the existing theory,and the reasons remain to be further revealed.One possible reason is that Ca²⁺/Mg²⁺ may lead to the aggregation of humic molecules,thus reducing the contact area between humus and clay.The comparison of the influence of two valence cations and monovalent cations on the aggregates stability showed that the ability of the divalent cations to maintain the aggregates stability was stronger than that of the monovalent cations.（3）There were obvious specific ion effects on the clay migration intensity in Na⁺/K⁺ system.The clay migration intensity for Na⁺ system was higher than K⁺ system under the same electrolyte concentration.It was consistent with the experimental results of clay aggregate stability in Na⁺/K⁺ system.The stronger the electrostatic field intensity of montmorillonite was,the higher the electrostatic repulsive pressure between particles was,and the greater the clay migration intensity was.The study indicated that the clay migration intensity was mainly influenced by the electrostatic repulsive pressure from the electrostatic field strength of aggregates.But at low electrolyte concentrations,the electrostatic field may also affect the clay migration intensity through the hydration repulsive force.