Effect Of Ions Adsorption On Water Infiltration In Illite Mineral Particles

In arid and semi-arid regions of northwest China,high-quality water resources are extremely scarce,which seriously hinders the high-quality development of regional agriculture.Understanding the mechanism of soil water infiltration has important scientific value.And it has practical significance for rational utilization and regulation of soil reservoir function and enhancement of soil water storage capacity.Over the years,researchers have proposed many models regarding soil water infiltration and solute transport from a macro perspective,and investigated the soil water infiltration and solute transport processes and their influencing factors.However,most of these works ignored the interfacial interactions and processes such as mineral particle-solution interactions,hydrogen bonding networks and ion adsorption and exchange.Thus,to understand the essential reason of macro phenomenon of soil water infiltration,it is necessary to further analyze the process of soil water infiltration from the micro scale.In this work,we used the widely distributed illite mineral in Northwest China as a representative to construct nanopore models.Molecular dynamics method was used to simulate the infiltration process of water in illite nanopores to investigate the effects of different pore sizes,cation types and concentrations on water infiltration,mineral-cation interfacial interaction and water molecule transport characteristics.The unsaturated water infiltration test of illite particles was set up with different concentrations of Na Cl solution（0mol/L,0.1 mol/L,0.5 mol/L,and 1 mol/L）and the same concentration（0.5 mol/L）of various salt solutions（Na Cl,KCl,and Ca Cl2）.In this work,the mechanism of mineral-ion interface interaction and its influence on unsaturated water infiltration process under different conditions were elucidated.Following this,it was combined with the water infiltration test of illite particles to further understand the water infiltration process as well as the underlying mechanism.The main conclusions are as follows:（1）The effect of small-sized nanopores on ions was more pronounced.Illite minerals show strong hydrophilicity,which leads to stronger adsorption forces compared to the surface tension.At the beginning of infiltration,a meniscus advanced frontier was formed with rapid migration of cations and water molecules.The overall infiltration velocity of the solution in each nanopore showed faster in the early stage and slower in the later stage.Illite nanopores exhibited significant filtering characteristics for ions,causes many ions gathered at the nanopore entrance.The small-sized（1.5 nm and 3 nm）nanopores showed more significant confinement and adsorption of Ca²⁺than Na⁺compared to large-sized（5 nm and 10 nm）nanopores.The diffusion coefficient of the solution in small-sized nanopores was higher than that in large-sized nanopores.（2）The concentration and type of cations had significant effects on water infiltration in illite mineral particles.The strong adsorption of illite minerals with high negative charge on high concentration（0.5 mol/L and 1 mol/L）of Na⁺was the main infiltration driver.Illite hydrophilicity was the main infiltration driver for low concentration solutions（0 mol/L and0.1 mol/L）.The number of outer-sphere Na⁺was more than that of inner-sphere Na⁺in low concentration solutions.Diffusion coefficient of Na⁺was higher in the high concentration solutions.The infiltration process of various cations has obvious difference.Numbers of high-valence Ca²⁺in the nanopores were significantly lower than those of other monovalent cations.The adsorption of illite minerals on Cs⁺with less number in monovalent ions was weaker,resulting in the lag of Cs⁺.Both cations formed obvious inner-sphere and outer-sphere adsorption.Number of outer-sphere Ca²⁺with higher hydration energy was more than that of outer-sphere Na⁺,K⁺,and Cs⁺.The diffusion coefficient of Ca²⁺was highest in the one-dimensional infiltration direction.Among the monovalent cations,K⁺with the highest number had a larger diffusion coefficient,and that of Cs⁺was the smallest.（3）Mineral-cations adsorption interaction was the driver of the unsaturated water infiltration in illite particles.Negatively charged illite particles showed stronger adsorption towards high content of Na⁺.Swelling and crushing properties of Na⁺on mineral particles were greatly weakened.Due to the stronger electrostatic adsorption between high-valent Ca²⁺and illite particles,the infiltration was blocked.As a common swelling inhibitor,K⁺can maintain the stability of the mineral pore structure and unblocked.Thus,K⁺infiltrated faster compared to other cations.Na⁺promotes the swelling of mineral particles compared to K⁺,resulting in a decrease of effective pores.This was also one of the main reasons why Ca²⁺with high hydration energy infiltrated was weakened.As mentioned above,water infiltration in illite mineral particles was explained by using molecular dynamics simulations combined with water infiltration experiments.Adsorption of cations and water molecules on mineral surfaces was significantly influenced by mineral nanopore size,the concentration and type of cations under unsaturated soil conditions.This resulted in the soil water infiltration process was altered.The results helped to further understand the related mechanism of soil water infiltration.It provides a theoretical basis for the efficient use of brackish water resources,etc.