An activated carbon with high adsorption capacity was synthesized from walnut shell as a solid waste using different chemical reagents. It was used to mitigate chemical oxygen demand from municipal landfill leachate. The activated carbon synthesized with impregnation ratio of 4:1 (H3PO4 to char) at a temperature of 500 degrees C (ACH4-500) demonstrated the best textural properties based on Brunauer-Emmett-Teller analysis, i.e., specific surface area of 1,851.1m(2)/g, total pore volume of 1.03cm(3)/g, and mean pore diameter of 2.24nm and was selected for adsorption experiments. The maximum adsorption capacity and removal were 123.1mg/g and 84.7% under optimum condition, respectively. The equilibrium data were fitted with different model isotherms and Redlich-Peterson model showed the best match with experimental data. Kinetic data were well described by pseudo-second-order equation. Study of adsorption thermodynamics revealed spontaneous and endothermic nature of adsorption. In addition, the adsorbent showed satisfactory results, e.g., 100% removal of Pb2+, Cd2+, and Mn2+, in diminishing heavy metals as hazardous materials from landfill leachate.