Modeling Of Colloid Migration Process In Soil Aquifers

Groundwater is one of China’s important sources of water supply,and its pollution prevention and control have significant implications for the country’s economic development.Extensive field measurements and research have shown that colloids were widely present in groundwater,with extremely complex physical and chemical properties,unique features such as large surface area and multiple adsorption sites.On the one hand,colloids themselves can cause groundwater pollution.On the other hand,colloids in the soil can lead to blockages and affect the migration of groundwater pollutants,which has a significant impact on groundwater quality.Therefore,developing colloid migration models and researching their mechanisms can effectively promote the prevention and control of groundwater pollution.The simulation of colloid migration processes includes two main challenges:one is that colloids themselves are involved in multiple physical and chemical processes(such as blocking,adsorption and desorption),resulting in many existing models having many artificial parameters;the other is that soil heterogeneity affects the migration of colloids,causing their breakthrough curve to exhibit a tailing phenomenon.Addressing these two issues,this article proposes a blocked adsorption convection-diffusion equation based on existing experimental data,and a time fractional derivative convection-diffusion blocked adsorption model based on fractional calculus theory,to describe the migration of colloids in heterogeneous soil.The main results and conclusions are as follows:(1)A plugging adsorption advection-dispersion equations(PA-ADE)is proposed for the simulation of the migration of colloids in underground aquifers,and the equation is solved using differential method.The plugging term of the model takes into account various physicochemical factors such as colloid particle size,p H,ion strength,flow rate.That affect colloid blockage,and it has only one fitting parameter,the plugging coefficient(K_p),which making the model more accurate.(2)Analyzing the parameters of the PA-ADE model,it was found that K_p mainly affects the peak value of the breakthrough curve(BTC),while the retardation factor(R_d)mainly impacts the colloid concentration peak and the time to reach the peak,both of which have clear physical meanings.To address colloid migration in heterogeneous soils,a time fractional order plugging adsorption advection-dispersion equations(PA-TFADE)was proposed.The fractional order(α)accurately describes BTCs tail behavior due to soil heterogeneity and the shape of the penetration curve peak.(3)Through existing experimental data to confirm the applicability of model,both PA-ADE and PA-TFADE.The results showed that the fitting results of PA-TFADE(with R~2values greater than 0.98)were significantly better than those of PA-ADE(with R~2 values greater than 0.91).Both models can effectively describe the influence of colloid particle size,ion strength,flow rate,and vegetation type on the colloid penetration curve peak.However,PA-TFADE is more suitable for describing the tail behavior of colloid migration in the later stages of the soil aquifer.