Specific Cation/Anion Effects Of Soil Particle Aggregation And Aggregates Stability

As an important physical property of soil,the stability of soil aggregates not only profoundly affects the soil fertility function,but also is closely related to the movement of"soil,water,air,heat and solute".In the existing theories,the mechanisms of soil aggregates breakdown can be summarized as follows:closed air compression,uneven expansion,raindrop impact and physical and chemical dispersion.However,a series of studies show that,compared with external forces,the internal forces between soil particle on the mesoscale,namely,hydration force,electrostatic force and van der Waals attractive force,are critical to the stability of aggregates.The study also found that the interaction between soil particle is affected by electrolyte factors such as ion type,concentration and valence.Among them,different kinds of ions with the same valence can cause different effects when acting on the research system.People call this specific ion effects or Hofmeister effects.For a long time,it is generally believed that the difference of ionic radius and hydration radius among the isovalent ions leads to the difference of interface binding energy,which affects the interaction between charged particles,and then affects the aggregation of soil particle and the stability of aggregates.Recent research shows that the difference in the interface reaction between different cations and charged particles is mainly due to the non-classic polarization of cation（i.e.,the change of ion orbitals on the surface of charged particle）.This new discovery provides a new theoretical basis for the study of soil particle interaction,soil particle aggregation and the stability of aggregates in the presence of different cations.In addition,due to the net negative charge in the soil,the research on the specific effects of anions has been seriously ignored.However,anions may affect soil particle aggregation and aggregates stability in two ways:（1）anions can produce non-static adsorption on the surface of some soil colloidal particles,thus improving the surface charge density of soil particle;（2）Anion adsorption can neutralize/shield the positive electric field generated by the soil positively charged colloid,thus affecting the interaction of soil particle.Therefore,in this study,constant charge purple soil and variable charge red soil are used as experimental materials to determine the electrochemical properties in the presence of different cations and different anions,analyze the effects of different cation/anion interface reactions on the interaction force/energy of soil particle,and study the effects of soil particle interaction on soil particle aggregation and aggregates stability,the specific cation/anion effects of the soil particle aggregation and aggregates stability of two types of electric soil particle are clarified.The main results of this study are as follows:（1）The critical coagulation concentration of Na⁺and K⁺saturated constant charge soil particle is 524 and 99.2 mmol·L^-1 respectively,and the stability of aggregates follows K⁺>Na⁺.Based on the non-classic polarization of cations under the coupling effects of"electric field-quantum fluctuation",K⁺with stronger non-classic polarization will more fully shield the negative electric field near the surface,making the electrostatic repulsion between soil particle in the K⁺system less than that of the same soil Na⁺system,and then the soil particle in the K⁺system are easier to aggregate and the aggregates are more stable.（2）The critical coagulation concentration of Cl^-,NO₃^-and H₂PO₄^-saturated constant charge soil particle is 144,99.2,and 75.5 mmol·L^-1,respectively.The stability of aggregates follows Cl^-<NO₃^-<H₂PO₄^-.The negative charge density of soil surface is 0.265 C·m^-2,and in the presence of Cl^-,NO₃^-and H₂PO₄^-is 0.306,0.294 and 0.281 C·m^-2,respectively.In the presence of Cl^-,if the negative charge density of the soil surface is large and the electrostatic repulsion between particles is strong,the aggregation process is not easy to occur and the stability of aggregates are poor;In the presence of H₂PO₄^-,if the negative charge density of the soil surface is small and the electrostatic repulsion between particles is weak,the aggregation process is easy to occur and the aggregates are stable.In the asymmetric electric field of soil,the non-classic polarization effects of Cl^-with 3s3p orbitals of the outer electron is greater than that of NO₃^-with 2s2p orbitals of the outer electron,and Cl^-has stronger ability to attract the surface of the soil charged particles and enhance the negative electric field;The non-classic polarization of H₂PO₄^-increases the negative charge density while releasing H⁺and proton exchange reduces the negative charge density,which ultimately shows a weak ability to enhance the negative electric field.（3）The stability of Cl^-,NO₃^-and H₂PO₄^-saturated variable charge soil aggregates follows Cl^-≈NO₃^->>H₂PO₄^-.In the presence of Cl^-/NO₃^-,the aggregates do not break.The negative charge density of soil surface is 0.173 C·m^-2,and in the presence of Cl^-,NO₃^-and H₂PO₄^-is 0.187,0.180 and 0.218 C·m^-2,respectively.In the presence of H₂PO₄^-,the negative charge density of the soil surface is large,and the electrostatic repulsion between particles is strong,so the aggregates ars unstable;In the presence of Cl^-/NO₃^-,the negative charge density of the soil surface is small,and the electrostatic repulsion between particles is weak,and the aggregates are stable.In variable charge soils,the non-classic polarization of anions also exists,but specific adsorption is the key factor for increasing the negative charge density of soil surface by H₂PO₄^-.（4）Cl^-and NO₃^-increase the negative charge density of the constant charge soil surface by15.5%and 10.9%;and increase the negative charge density of variable charge soil surface by8.09%and 4.04%.Cl^-and NO₃^-show a stronger ability to increase the surface negative charge density in constant charge soil with stronger electric field.The non-classic polarization of ions is affected by the electric field.The stronger the electric field,the stronger the non-classical polarization.Therefore,this proves to some extent that the non-classic polarization of anions does occur in the electric field.On the contrary,H₂PO₄^-increase the negative charge density of the constant charge soil surface by 6.04%,and increase the negative charge density of variable charge soil surface by 26.0%,indicating that the specific adsorption of H₂PO₄^-in variable charge soil is dominant.Based on the above results,we draw the following conclusions:（1）The non-classic polarization of cations under the coupling effects of"electric field-quantum fluctuation"leads to the difference of the interface reactions between different cations and charged particles,which leads to the difference of the surface potential and the interaction between particles,and finally presents the specific cation effects of soil particle aggregation and aggregates stability.（2）The specific anion effects of soil aggregates stability in both constant and variable charge soils is due to the different ability of anions to interact with charged particles to increase surface negative charge density.We speculate that under the action of asymmetric electric fields,asymmetric hybrid orbitals of anions induce non-classical polarization,resulting in non-classical induced adsorption of anions,which will ultimately trigger or profoundly affect the stability of soil aggregates.（3）The non-classic polarization of anions under the influence of asymmetric electric field is related to the strength of external electric field.The greater the electric field strength,the stronger the non-classic polarization effects.Therefore,anions will have a stronger ability to increase the surface negative charge density in the soil with stronger electric field.