# Analysis of Factors for Compacted Clay Liner Performance Considering Isothermal Adsorption
###### Abstract  
The concentration profiles and breakthrough curves of the 2 m thick compacted clay liner (CCL) given in the specification were compared, considering three different adsorption isotherms (upper convex, linear, and lower concave). In addition, the effects of transport parameters, sorption isotherms, and source concentrations on pollutant migration were analyzed. The results showed that the dimensionless breakthrough curves of different source concentrations considering the linear adsorption isotherm coincided with each other, as the partition coefficient of the linear adsorption isotherm was constant. For the lower concave isotherm, the migration of a large source concentration was slowest, because the partition coefficient of the lower concave isotherm increased with an increase in concentration. For the upper convex isotherm, the migration of a large source concentration was fastest, because the partition coefficient decreased with an increase in concentration. The effects of the nonlinear isotherms on the shape of the outflow curve were similar to the effects of a change in the hydrodynamic dispersion (\(D_{\text{h}}\)): the concentration front of the upper convex isotherm was narrower, which was similar to the effect of a reduction in \(D_{\text{h}}\) (i.e., \(P_{\text{L}}\)), and the concentration front of the lower concave isotherm was wider and similar to the effect of an increase \(D_{\text{h}}\) (i.e., \(P_{\text{L}}\)). Therefore, the diffusion and adsorption parameters were fitted separately in the study, in case the nonlinear adsorption behavior was mistakenly defined as linear adsorption.  
C 2021 119735 10.3390/app11209735 10.390/app11209735 10.390/app11209735 10.390/app12021 15 October  
concentration and infinite far-zero gradient of the outflow was suitable for simulating the outflow concentration at the bottom of the soil column. All these studies are based on the linear sorption property of the pollutant on the material.  
Isothermal adsorption lines can be classified into three types: the straight type, the upper convex type, and the lower concave type, according to the change in the slope of the curve closest to the origin of the isothermal adsorption line [19; 20; 21]. Many studies in the literature assumed that the adsorption behavior in the tests was linear, and therefore the test results were fitted based on a linear adsorption model [1; 2; 3; 16; 17; 22]. However, most of the batch test results reported in the literature show that the isothermal adsorption curves of heavy-metal ions are mostly nonlinear [1; 23; 24; 25; 26]. [PERSON] et al. [27] simulated the breakthrough curve with a retardation factor that gradually decreased over time, i.e., considering nonlinear adsorption as linear adsorption. [PERSON] et al. [28] presented analytical solutions considering a piecewise linear adsorption. [PERSON] et al. [29] presented approximate analytical solutions for four nonlinear adsorptions. [PERSON] et al. [30] discussed the effect of the retarding factor of the nonlinear adsorption model on the migration of instantaneous injection-type pollutants. [PERSON] et al. [31] used numerical methods to analyze the effect on pollutant migration of considering nonlinear adsorption as linear adsorption.  
Many theoretical analyses and experiments have been conducted on the performance of the compacted clay liner (CCL) [11; 13; 32; 33]. However, the effect of the isothermal line type on the CCL's performance has rarely been discussed. In this paper, the effects of transport parameters, isothermal line type, and source concentrations on the CCL's performance were investigated synthetically. This research could provide an insight into the migration characteristics of different isothermal line types and the regression of migration parameters.