Abstract: Landfill leachate contains a significant amount of heavy metal ions. Over time, the ability of the landfill lining material to adsorb and block these heavy metals gradually deteriorates due to its own degradation. To enhance the long-term adsorption and blocking capacity of the lining material for heavy metals, this study introduced biochar to natural loess. The adsorption and blocking ability of the biochar-modified loess for Pb2+ was investigated using a seepage adsorption test with Pb(NO3)2 as the pollutant source. Additionally, changes in mineral composition and functional groups before and after Pb2+ adsorption on loess and biochar-modified loess were examined using XRD, FTIR, and other microscopic tests. The results indicate that Pb2+ adsorption on loess samples primarily occurs through the interfacial precipitation of calcite minerals, resulting in the obligatory adsorption of cerussite and quartz minerals. SEM analysis reveals the formation of white cerussite attached to the surface of loess particles, significantly hindering the adsorption of Pb2+ and leading to a removal efficiency of only 45% in the late stage of the osmotic adsorption test. FTIR analysis detected a significant presence of oxygen-containing functional groups in biochar-modified loess samples, providing evidence for the absorption of Pb2+ through complexation with these functional groups. XRD analysis confirmed that phosphate mineral adsorption on the surface of biochar was the dominant mechanism in Pb2+ interfacial precipitation. The biochar-modified loess lining material exhibits preferable adsorption of Pb2+ through multiple mechanisms, prolonging the adsorption time of calcite Pb2+ in loess, and improving the removal efficiency to 85% in the late stage of the seepage adsorption test.