The Effect Of Colloid On The Permeability And Solute Transport Of Saturated Porous Media And Its Numerical Simulation

The infiltration of colloids in an issue associated with agriculture water management,groundwater recharge,geotechnical engineering,chemical engineering and environmental engineering.For instance,the sediment load of Yellow River is high and the use of it as irrigation water source would significantly affect soil permeability and infiltration rate.Similarly,reduced efficiency of artificial recharge of excessively exploited groundwater is expected because of the fine particles in recharge water that narrow or block the infiltration pathways.In addition,settlement of fine particles in soil pores of sand filter or filter bed decreases water purification efficiency during water treatment.Large particles in water can be easily filtered with pre-treatment technologies,but it is challenging to separate small particles or biomass including colloid.These small particles can be carried by the water flowing through the soil matrix and may deposit or precipitate due to a series of physical and chemical processes.The deposition and precipitation of small particles reduce soil permeability and change solute transport characteristics.Therefore,it is of great scientific and engineering significance to reveal the influence of colloidal particles on the permeability of porous media and the solute transport characteristics based on it.At present,there were many reports about the migration mechanism of colloids into porous media and the change of permeability at home and abroad.However,the research usually studied the influence on the permeability of media or the migration mechanism of colloidal particles.There are few studies on the effect of the permeability of media and the transport behavior of salt cation and colloidal particles when the permeability of media changed.In this paper,quartz sand and silica micro powder solution were used as porous medium and external infiltration colloid,NaCl,CaCl₂ and MgCl₂ were used as co migration salt solution.The relative permeability coefficient?K/K₀,K₀ was initial relative permeability coefficient,K was relative permeability coefficient at different times?of porous medium was used as the characterization index.The indoor quartz sand column outflow experiments were conducted to investigate the effect of colloidal particles and comigration of colloidal particles and salt cations on the permeability of saturated porous medium.The researches could provide scientific basis and theoretical reference for solving the problems encountered in engineering practice such as muddy water irrigation,artificial groundwater recharge.This paper was mainly based on the indoor soil column outflow experiment.The research content could be divided into three parts.Firstly,under the conditions of different colloidal concentrations(0.1?0.2 and 0.5 g·L^-1),colloidal particle sizes?2.6?6.5 and 13?m?,medium particle sizes?240?362 and 513?m?and head differences?5?10 and 20 cm?,the effect of colloidal particles on the permeability coefficient of saturated porous media and the mechanism of colloidal particles migration and deposition in the medium were discussed.Secondly,mixing NaCl,CaCl₂ and MgCl₂ with colloidal particles(0.2 g·L^-1)in different concentrations(0.1?1 and 3 g·L^-1)into a mixed solution for infiltration.Then,analyzing the effect on the permeability coefficient of saturated porous media,and discussing the transport mechanism of colloidal particles and salt cations under different conditions of permeability of saturated porous media.Thirdly,the transport behavior of salt cations in saturated porous media was simulated under different conditions using Hydrus-1D software.The main conclusions are as follows:?1?Under different colloidal concentrations,colloidal particle sizes,porous media particle sizes and head differences conditions,colloids infiltration could affect the permeability of saturated porous media,also affected the migration and deposition behavior of colloidal particles.Colloidal particles caused pore plugging and changed the internal structure of porous media when deposited in porous media,which leading to media permeability decreasing and affecting the migration behavior of colloidal particles.The migration and deposition of colloidal particles were affected by the concentration of colloidal particles,the particle size of colloidal particles,the particle size of porous media and the head difference.Under the same other conditions,the permeability of porous media decreased to a greater extent when the concentration of colloidal particles,porous particles and head differences increased.The effect of different colloidal particle sizes on the overall permeability of saturated porous media was not obvious,because the location of colloidal particles flocculated or sedimented in porous media was different.?2?Under different colloidal concentrations,colloidal particle sizes,porous media particle sizes and head differences conditions,colloid infiltration would affect the migration and deposition behavior of colloidal particles.Tthe migration behavior and deposition effect of colloidal particles were consistent with the decrease of permeability of porous media,through cmparing the change of the concentration of colloidal particles in the effluent with time and the deposition of colloidal particles in the media after the experiment.Based on different literature data,it was considered that K/K₀ would increase when the relative diameter ratio d_p/D_p of colloidal particles and porous media was more than 0.018 and the infiltration concentration was less than 0.5 g·L^-1.?3?When different concentrations of salt solutions and types?NaCl,CaCl₂ and MgCl₂?migrated together with colloidal particles,the concentration and type of salt cations would affect the permeability coefficient of saturated porous media,and then affected the migration behavior of salt cations and colloidal particles.When the salt solution of different concentrations migrating together with the colloidal particles,the salt cation promated the colloidal particles to be flocculed into large particles,which caused deposition and plugging of pores in the process of infiltration.The greater the ionic strength was,the more obvious the flocculation was,thus the greater the decreased of permeability of the porous media.Compared with different types of salt cations,it could be found that the larger the ionic valence state,the greater the degree of agglomeration of colloidal particles.When the ionic valence state was the same,the smaller the hydrated ion radius,the stronger the hydration ability,and the higher the agglomeration ability is.Therefore,the degree of agglomeration of colloidal particles was Mg²⁺>Ca²⁺>Na⁺.With the decrease of the permeability of saturated porous media,the time for various types of salt cation breakthrough curve to reach the peak concentration gradually increased.The peak concentration of ion breakthrough curve increased with the increase of ion concentration due to the lack of colloidal particles and media adsorption sites,?4?The local equilibrium adsorption?LEA?and one site adsorption model Model?OSM?were used to simulate the migration behavior of bromine ions and salt cations of different concentrations and types in saturated porous media.The model fitting effect was good except for some cases according to the inversion parameters.Therefore,the permeability of porous media sand column in space was divided into sections and the permeability of each section changed with time were taken as average value according to the experimental monitoring data,which could accurately simulate the migration process of salt cations of different concentrations in porous media with variable permeability saturation.