Abstract: Biochar, as a typical adsorbent, has shared the excellent performance in the remediation of various pollutants in water and soil. However, the pore structure of pristine biochar is composed of micropores (<2 nm), with fewer mesopores (2-50 nm) and macropores (>50 nm), resulting in the limited adsorption capacity for the heavy metal pollutants. Structural modification can be expected to enhance the adsorption performance of the biochar for its broad application. This study aims to synthesize a highly efficient calcium carbonate/biochar (Ca/BC) composite for Pb2+ adsorption. Sugarcane molasses and calcium carbonate (CaCO₃) were utilized as the precursors. A K₂CO₃-urea mixture was selected as the activating agent. The composite was prepared via hydrothermal carbonization followed by high-temperature pyrolysis activation. The Ca/BC composites were prepared with the coexisting micropores, mesopores, and macropores. Hydrothermal pre-carbonization was combined to reduce the foaming behavior of molasses during direct pyrolysis. High-temperature pyrolysis was followed for the pore formation and expansion. The surface morphologies and pore structure of Ca/BC were characterized using Scanning Electron Microscopy (SEM), Raman spectroscopy, and an Autosorb-iQ physisorption analyzer. The Pb2+ was selected as the target pollutant. A systematic investigation was made to explore the effects of the activation temperature, pH, and adsorbent dosage on the Pb2+ adsorption. The adsorption mechanisms of Pb2+ onto Ca/BC were determined after optimization. The results demonstrated that the high-temperature activation significantly enhanced the porosity of Ca/BC, the number of mesopores, specific surface area, and pore volume. The Pb2+ adsorption capacity of Ca/BC exhibited a strong positive correlation with the activation temperature. The equilibrium adsorption capacities of Ca/BC-500, Ca/BC-700, and Ca/BC-900 for Pb2+ reached 105.31, 142.74, and 195.46 mg/g, respectively, under optimal conditions (25 °C, pH value is 6.0, initial Pb2+ concentration=200 mg/L, and adsorbent dosage=1.0 g/L). The activation at 900 °C also yielded the optimal performance of the adsorption. The adsorption mechanism revealed that the Pb2+ uptake by Ca/BC was a spontaneous and endothermic process governed by chemisorption. Kinetic data were represented by the pseudo-second-order model (R²>0.998), indicating that the chemical interactions were the rate-limiting. Adsorption isotherms closely followed the Langmuir model (R²>0.991), indicating the monolayer adsorption onto energetically homogeneous sites. Multiple mechanisms greatly contributed to the Pb2+adsorption, including electrostatic attraction, pore filling, cation-π interactions, and ion exchange. The Ca/BC surface exhibited the increasing negative charge (confirmed by decreasing pHpzc from 4.66 to 3.24, as the activation temperature increased from 500 °C to 900 °C), thereby enhancing the electrostatic attraction of Pb2+ cations. Furthermore, the Pb2+ adsorption capacity increased with the specific surface area (from 134.43 to 294.83 m²/g) and pore volume (equilibrium capacity increased from 105.31 to 195.46 mg/g). Cation-π interactions were represented by the characteristic C=C bond vibrations (observed at 1597 and 1509 cm⁻¹ in FTIR spectra). The ion exchange occurred under weakly acidic environments, where Pb2+ replaced Ca²⁺ ions that were released from the CaCO₃ component of Ca/BC. Furthermore, the Ca/BC exhibited excellent structural stability and reusability. The regenerated Ca/BC-900 retained 85.47% of its initial Pb2+ adsorption capacity after four consecutive adsorption-desorption cycles, where 0.1 M HCl was selected as the eluent. Its robustness and practical potential were highlighted for the sustainable wastewater treatment in heavy metal remediation. The strong chemical bonding was attributed to the inherent resilience of the mineral-carbon composite framework during high-temperature activation. The abundant low-cost agricultural waste (molasses) with a mineral activator (CaCO₃ and K₂CO₃-urea) can be expected to serve as an economically viable and environmentally sound approach for the high-performance adsorbents.