# Analysis of Factors for Compacted Clay Liner Performance Considering Isothermal Adsorption
## 4 Conclusions  
The concentration profiles and outflow curves of the 2 m thick compacted clay liner (CCL) given in the specification were calculated, considering three different sorption isotherms (upper convex, linear, and lower concave). The effects of transport parameters, sorption isotherms, and source concentrations on pollutant migration were analyzed. The results showed that the effect of source concentrations on pollutant migration was different for different adsorption models. Source concentration values had no effect on the migration of the linear adsorption model, but for the Langmuir model, migration was slow for small source concentrations, and for the Freundlich model, migration was slow for large source concentrations. Because the partition coefficient of the linear isothermal adsorption curve was constant and did not change with concentration, the upper convex (Langmuir model) isothermal adsorption curve had a larger partition coefficient at smaller concentrations, and the lower concave (Freundlich model) isothermal adsorption curve had a smaller partition coefficient at smaller concentrations. With a small partition coefficient, the adsorption was low and the concentration front was deep; conversely, the concentration front was shallow with a larger partition coefficient.  
When the adsorption capacity corresponding to the source concentration was the same as in the linear model, the effect of the nonlinear adsorption model on the shape of the outflow curve was similar to the effect of a change in diffusion (\(D_{\text{h}}\)): the concentration front corresponding to the upper convex model was narrower, which was similar to the effect of \(D_{\text{h}}\) (i.e., \(P_{\text{L}}\)) reduction, and the concentration front corresponding to the lower concave model was wider, which was similar to the effect of an increased \(D_{\text{h}}\) (i.e., \(P_{\text{L}}\))). These results were verified using a column test. Therefore, the nonlinear adsorption behavior may be mistakenly defined as linear adsorption by fitting the diffusion parameters and adsorption parameters at the same time. For this reason, univariate analysis was recommended for analysis of the results. In other words, the diffusion and adsorption parameters should be fitted separately in the study.  
When the flow rate was small, the effect of \(D_{\text{d}}\)* on the curve was large, and the effect of \(\alpha\) was small, while there were opposite effects when the flow rate was large. The adsorption  
Figure 23: Three types of sorption isotherms used in fitting.  
Figure 22: Comparison of the measured breakthrough curve and the fitting breakthrough curves.  
had a significant effect on the outflow curve, so it is very important to obtain the correct adsorption parameters for the prediction of the CCL's performance.  
Conceptualization, X.Z. and J.Y.; methodology, X.Z. and H.W.; software, H.W.; validation, X.Z. and Y.L.; formal analysis, X.Z. and H.W.; investigation, X.Z. and J.Y.; resources, X.Z.; data curation, J.Y.; writing--original draft preparation, X.Z. and H.W.; writing--review and editing, X.Z. and Y.L.; visualization, X.Z. and H.W.; supervision, H.W.; project administration, X.Z.; funding acquisition, X.Z and H.W. All authors have read and agreed to the published version of the manuscript.  
This work was supported in part by the National Natural Science Foundation of China (Grant No. 41702329), in part by the Scientific Research Fund of Hunan Provincial Education Department (Grant No. 17B097), in part by Department of Natural Resources of Hunan Province (Grant No. 2020-15), in part by the Zhejiang Provincial Natural Science Foundation (No. LQ18E080001), and in part by the Ningbo City Science and Technology Project for Public Benefit (No. 2019C50014).  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.