The Effect Of Ion-interface Reaction On Saturated Hydraulic Conductivity Of Saline Soil

Soil water movement is closely related to many aspects such as agricultural production and environmental protection.Saline-alkali soil is an important soil resource in the world.Due to the high content of soluble harmful salts and sodium ions in saline-alkali soil,soil humus has been lost,soil structure has been destroyed,and poor ventilation and water permeability have adversely affected water movement,seriously restricting the efficient development of agriculture.Since Darcy’s Law was proposed,researchers have been trying to describe soil water movement more accurately.However,due to the complexity of soil itself and the uncertainty of water movement,it is difficult to find a universally applicable mathematical model.Therefore,researchers began to focus on the study of soil internal mechanisms during water infiltration,attempting to explore the laws of soil water movement from the factors and mechanisms that affect soil water movement.Early studies have found that for the same soil,when the electrolyte type and concentration are different,soil water infiltration and transport show completely different characteristics.Electrolyte concentration and type do not directly affect soil water movement,but change the stability of soil aggregates by controlling the mesoscale interparticle interactions to affect soil water movement.When water containing different electrolytes is added to the same soil,the water infiltration rate can vary several times,which is known as the specific ion effect.Recent studies have shown that the coupling effect between electric field and quantum fluctuations is an important reason for specific ion effects.There are a large number of sodium ions in saline-alkali soil,which exhibit a strong dispersion ability due to weak quantum effects,making it difficult for water to infiltrate and leading to low saturated hydraulic conductivity,soil structure dispersion,and difficulty in salt-leaching.Soil saturated hydraulic conductivity is an important parameter of soil physical properties and an important indicator of soil saturated permeability.Moreover,saturated hydraulic conductivity is one of the key parameters for controlling soil salt leaching in saline-alkali soils.Therefore,studying saturated hydraulic conductivity is of great significance for controlling water and salt transport in saline alkaline soils and improving soil structure.In previous studies,the impact of particle interaction on soil water movement was conducted in conventional soils with low salt content,and the contribution of particle interaction to water infiltration was not evaluated in saline alkaline soils with high salt content,especially in dispersive sodium content.Based on previous research,this study quantitatively evaluated the impact of electrolyte type and concentration on the interaction between particles in saline soil by considering the ion quantum effect at the soil solid/liquid interface,and further elaborated the ion interface reaction mechanism of the interaction between particles on the saturated hydraulic conductivity of saline soil,providing a theoretical basis for improving the permeability of saline soil and promoting salt leaching of saline soil.This paper mainly studied the saturated hydraulic conductivity of saline soil under the conditions of single electrolyte,mixed electrolyte,and biochar-electrolyte coupling,and evaluated the significant contribution of interparticle interactions to the saturated hydraulic conductivity of saline soil.The following research results were obtained:（1）The saturated hydraulic conductivity of saline soil reduce with the decrease of infiltration electrolyte concentration,the increase of surface potential and electrostatic repulsion force.Whether in 1:1 or 1:2 type electrolytes,the presence of K⁺is more conducive to the water infiltration of saline soil,and the saturated hydraulic conductivity of potassium system is greater than that of sodium system;The saturated hydraulic conductivity of saline soil in the carbonate system is lower than that in the nitrate system because the p H of the infiltration liquid in the carbonate system is higher than that in the nitrate system.The increase in p H value will lead to an increase in soil cation exchange capacity（CEC）and surface charge density.As the p H value increases from 7 to 12,the CEC value of the soil increases from 2.64 to 5.70cmol_（+）kg^-1,and the soil surface charge density also increases by 2.3 times as the p H value increases.The increase in surface charge density increases the soil surface potential,thereby increasing the electrostatic repulsion between particles and slowing down the movement of soil water,resulting in a decrease in the saturated hydraulic conductivity.（2）The saturated hydraulic conductivity of saline soil under mixed electrolyte system is mainly controlled by the high counter ion content of effective valence.The saturated hydraulic conductivity of the electrolyte system mixed with four counter ions in equal proportion is shown as Na NO₃+Ca（NO₃）₂>Na NO₃+Ca（NO₃）₂+Mg（NO₃）₂>KNO₃+Na NO₃+Ca（NO₃）₂+Mg（NO₃）₂>Na NO₃+KNO₃.Electrolyte type and ionic strength determine the water infiltration of saline soil by controlling the interaction force between particles.The saturated hydraulic conductivity of saline soil can be uniformly described as a function of electrostatic repulsion energy,where the larger the repulsion energy,the lower the saturated hydraulic conductivity.Ion polarization at the soil solid-liquid interface regulates the magnitude of the repulsive force between particles,thereby controlling soil structural stability and water infiltration processes.（3）The coupling effect of biochar and electrolyte controls the saturated hydraulic conductivity of saline soil.Electrolytes affect water infiltration by controlling the interaction between particles.Biochar can effectively increase the specific surface area and surface charge amount of soil,reduce surface charge density and electrostatic repulsion force between soil particles,thereby improving the saturated hydraulic conductivity of saline soil.When 1%biochar was added,the soil surface charge density decreased by 0.03 C/m²,and the electrostatic repulsive force between soil particles significantly decreased;When the proportion of biochar is increased to 5%,the change in surface charge density is small,and the electrostatic repulsion force no longer decreases further.The significant increase in saturated water conductivity is mainly due to the increase in soil porosity caused by biochar,which promotes water infiltration.Therefore,biochar can simultaneously improve soil water infiltration by regulating soil particle interactions,improving soil structure,reducing soil bulk density,and increasing soil porosity.