Study On The Interaction Of Black Soil Mineral–Humic Acid Mixed Colloid By Atomic Force Microscope

Soil colloidal particles are the basic materials that form soil aggregates and structures.The surface properties of soil colloids and the interaction of colloidal particles significantly affect both soil properties and soil fertility.However,due to the heterogeneity of soil system,the microcosmic and complexity of soil colloid migration process,and limited to the constraints of traditional research methods,it is difficult to clearly,completely and truly capture the interaction process and mechanism of soil colloid at the micro scale.Because the atomic force microscope（AFM）has nanometer-level resolution and can measure the interaction force between particles,it helps to study the properties of soil colloids more intuitively and clarify the mechanism of soil colloids interaction.In this study,the surface morphology of black soil mineral colloid in gas/solid interface are directly observed by Atomic Force Microscope,and analyze the the size of colloid and its condensate.The surface force and adhesion force of the colloid are measured at the gas/solid interface and solid/liquid interface,and the force spectrum was characterized.At the same time,natural humic acid colloid was added to observe the effect of humic acid on the morphology and surface force.The main conclusions are as follows:（1）Black soil mineral colloid and black soil mineral-humic acid colloid have similar coagulation characteristics in the four cation systems of K⁺,Na⁺,Ca²⁺and Mg²⁺,that is,with the increase of ion concentration,the black soil mineral colloid particles increases,and the particles aggregate horizontally first and then aggregate vertically.Among them,in terms of particle size,the aggregation of monovalent ions increases slightly in the transverse direction,and the aggregation of divalent ions occurs at least three times in the transverse direction;In terms of particle height,there is a significant increase in particle height in both monovalent and divalent electrolyte solutions.The particle height of monovalent ions is at least 1 times that of non added humic acid,and the particle height of divalent ions is at least 4 times superimposed in the longitudinal direction.（2）The effects of different base ions on the coagulation of black soil mineral colloid and black soil mineral-humic acid mixed colloid are studied by AFM.By the AFM,the four given cations showed that strong ion specific effect is found in the aggregation of black soil mineral colloid particles and varies in the sequence K⁺>Na⁺;Ca²⁺>Mg²⁺.（3）Through the study of the surface force of black soil mineral colloid by AFM,it is found that in the monovalent ion system,humic acid plays a role of increasing the surface adhesion at the gas/solid interface,promoting the aggregate process of black soil mineral-humic acid mixed colloid,while reducing the surface adhesion at the solid/liquid interface and hindering the aggregate process of black soil mineral-humic acid colloid.Under the condition of gas-solid,the maximum surface attraction and adhesion of black soil mineral colloid and black soil-humic acid colloid are K⁺>Na⁺;Ca²⁺>Mg²⁺;Under solid-liquid conditions,the surface force of colloid has a long-range repulsive force approaching 0 at a distance of about 3-8 nm.The analysis of the surface force of black soil mineral colloid at the distance of 6 nm shows that there is K⁺>Na⁺;Ca²⁺>Mg²⁺in the shielding of electric field by different cations.In conclusion,the interaction process of black soil mineral-humic acid mixed colloid is affected by electrolyte type,electrolyte concentration and surface electric field.The results can provide theoretical support for revealing the mechanism of the formation and stability of soil organic-inorganic complexes.