Abstract: Landfill leachate contains many heavy metal ions, and under the long-term seepage of leachate, the capability of landfill liners to adsorb and retain heavy metals is gradually degraded due to their deterioration problems. To improve the long-term adsorption and retention capacity of the liner materials for heavy metals, in this study, biochar is added to loess, and Pb(NO₃)₂ is used as a pollution source to study the adsorption and retention capacity of the biochar-amended loess for Pb²⁺ through the seepage tests. The changes in mineral composition and functional groups before and after adsorption of Pb²⁺ on the biochar-amended loess are elucidated through the XRD and FTIR microscopic tests. The results indicate that the adsorption of Pb²⁺ of the loess is mainly through the interfacial precipitation of calcite minerals, resulting in the obligatory adsorption of cerussite and quartz minerals. The SEM test results show that the generation of cerussite attached to the surface of loess particles prevents the adsorption of Pb²⁺ on the loess, which leads to the removal efficiency of Pb²⁺ on the loess specimens at the late stage of the seepage tests is only 45%. The FTIR tests detect many oxygen-containing functional groups in the biochar-amended loess specimens, which provides evidence for the adsorption of Pb²⁺ by functional groups through complexation. The XRD tests confirm the predominance of phosphate minerals on the surface of biochar for the adsorption of Pb²⁺ through interfacial precipitation. The liner materials of the biochar-amended loess relieve the depletion of calcite minerals in the loess by preferential adsorption of Pb²⁺ through oxygen-containing functional groups and phosphate minerals, which prolongs the Pb²⁺ adsorption time, resulting in an increase of removal efficiency of Pb²⁺ to 85% at the late stage of the seepage tests.