Ion Separation Characteristics During Freezing-Thawing Of Brackish Water And Leaching Effect On Saline Soil

Soil salinization is one of the main causes of the decline in agricultural production in the arid and semi-arid regions of the northwest of China.Although there is a shortage of fresh water resources in the inland saline areas,underground brackish water is widely distributed.Combined with the characteristics of low temperatures in the inland winter,the extraction of brackish water for ice melt irrigation is expected to change the previous way of fresh water pressure salt,play a role in saving fresh water resources and improving saline soils.In addition to the water quality and quantity of brackish water,as well as the soil bulk and water content,the ion migration characteristics in the ice during freezing and the changes in the physicochemical properties of the soil during freezing also have an important influence on the infiltration of meltwater and the effect of salt leaching in freezing irrigation.In this paper,different aspects of the brackish water icing irrigation test are disassembled and indoor tests and theoretical analysis are carried out.The effect of freeze-thaw on water-holding characteristics and pore distribution of soils with different degrees of salinization,and the heterogeneous migration of different ions in soil water were investigated,revealing the influence mode of salinity and multi-component solute migration of saline soils by the coupling effect of freeze-thaw infiltration of brackish water and soil freeze-thaw,with a view to providing theoretical support and technical reference for sustainable utilization of brackish water and salinization prevention and control in arid and semi-arid areas.The main findings of this paper are as follows.（1）The ion separation characteristics of the freeze-thaw process in brackish water ice bodies and its microstructure development process were elucidated.The results show that during unidirectional freezing,soluble salts are driven by ice crystal repulsion and gravity,with the total dissolved salts（TDS）in the bottom ice layer being approximately three times higher than in the top ice layer.Specifically,after unidirectional freezing,45%of Na⁺and Cl^-were concentrated in the bottom layer,while around 30%of Ca²⁺was concentrated in the middle layer.During unidirectional thawing,desalination rates of over 80%were achieved after 14 hours.As the initial salinity increased,the connectivity of the brine channels became more pronounced.During the unidirectional melting,the percentage content distribution of Na⁺,K⁺,Cl^-and SO₄^2-showed two peaks of 30%and 50%respectively.the percentage content of Ca²⁺peaked only once,at 25%.（2）The heterogeneous transport characteristics of different ions in soil water by freeze-thaw action were elucidated.The results show that after freeze-thaw treatment,the water content of saline soils decreases under the same substrate suction and the water holding capacity of the soil decreases significantly when the substrate suction is less than 3000 cm.At the same time,according to the analysis of the soil equivalent pore size distribution,the soil microporosity（0.3～5μm）content decreased by 1-6%and the medium（30～75μm）and large porosity（≥75μm）content increased by about 2-3%.During freeze-thawing of soil water,the repulsion of salts by ice crystal formation increases the solute concentration gradient of the water discharged under all levels of suction,i.e.the concentration is smaller under low suction and larger under high suction.the concentrations of Na⁺,K⁺,Mg²⁺,SO₄^2-and Cl^-show more consistent characteristics before and after the freeze-thawing treatment with suction:after freeze-thawing,the ion The ion concentrations in soil water under all levels of suction after freeze-thaw treatment showed a clear trend from low to high,while Ca²⁺showed a decrease in ion concentration with increasing suction after freeze-thaw treatment.（3）The influence patterns of salt and multi-component solute transport in freeze-thaw saline soils by infiltration of brackish water icing meltwater and the leaching effect were revealed.The results show that the infiltration capacity of the soil was significantly increased under both freeze-thaw treatments,and the time taken for thawing to complete was reduced from 215 hours to 45-75 hours.The freeze-thaw treatment reduced the water-holding capacity of the soil,which resulted in a significant reduction in the water content of the surface soil under both freeze-thaw treatments compared to the non-freeze-thaw treatment,while the water content of the deeper soil increased significantly due to the increased infiltration capacity of the soil.The increased infiltration capacity of the freeze-thaw treatment also resulted in a more adequate leaching of salts from the deeper soil layers.Compared with the non-freeze-thaw treatment,the contents of total salt,Na⁺and Cl^-in the L4-L5 layer decreased significantly,and the leaching effect was improved,while the contents of SO₄^2-and Mg²⁺in the overall profile increased slightly,and the accumulation of Ca²⁺in L5 layer and K⁺in L1-L2 layer increased significantly.During melting,the leaching effect of total salt and Na⁺increased,but the accumulation of Ca²⁺in L5 layer and K⁺in L2 layer increased significantly.