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Biochar prepared from agricultural waste resource coconut shells was used as the original charcoal, which was oxidatively modified and characterized using batch adsorption tests before and after modification by SEM, FTIR, surface area, and elemental analyses. The removal capacity and adsorption mechanism of the modified biochar for Pb²⁺ and Cd²⁺ in water were investigated, and its adsorption kinetics and thermodynamics were discussed. The findings demonstrated that the specific surface area of potassium permanganate and nitric acid-modified coconut shell carbon (MHBC) is 3.02 times than that of the coconut shell carbon (BC). The kinetic data of adsorption of Pb²⁺ and Cd²⁺ on MHBC were more in accordance with the pseudo-second order kinetic model, indicating that chemical adsorption played a dominant role, and the adsorption rate gradually tended to balance with a decrease in solution ion concentration. The isothermal thermodynamic data of the adsorption of Pb²⁺ and Cd²⁺ by MHBC conformed to the Langmuir model. At pH = 5.5, the maximum adsorption capacities of Pb²⁺ and Cd²⁺ were 160.41 mg·g⁻¹ and 47.46 mg·g⁻¹, respectively. Thermodynamic parameter fitting showed that the process of adsorption of Pb²⁺ and Cd²⁺ by MHBC is a spontaneous endothermic reaction. The higher the temperature is, the stronger the adsorption capacity is. When the ions coexist, Pb²⁺ and Cd²⁺ have obvious competitive adsorption, and Pb²⁺ has a significant competitive advantage. Among them, MHBC has high adsorption and good reusability for Pb²⁺ and Cd²⁺, and has good practical application prospects.