Upgraded utilization of biochar after heavy metal (Ni) adsorption for CO2 capture

Biochar, with its well-developed pore structure and tunable surface chemistry, has been widely employed for heavy metal adsorption. However, the biochar after adsorption of heavy metals suffers from high risk of secondary pollution and difficulty in resource utilization. This study explores a feasible approach to converting it into an efficient CO2 adsorbent. Heavy metal ions (represented by Ni2+ in this study) are primarily captured via the mesoporous structure of biochar and can be stably anchored to the carbon skeleton after high temperature treatment. This process introduces alkali metal oxide sites on the biochar surface, which were confirmed as chemisorption centers for CO2 by in situ near-ambient pressure X-ray photoelectron spectroscopy (in situ NAP-XPS). Meanwhile, the adsorbed Ni2+ acts as in situ structural template, inducing the evolution of mesopores into narrow micropores, increasing the intermolecular forces between the pore walls and CO2. The synergistic enhancement effect of chemical and physical adsorption significantly improves the CO2 adsorption performance of biochar. The Ni-loaded biochar achieves a CO2 adsorption capacity of 4.49 mmol/g at 25 °C and 1 bar, and a CO2/N2 dynamic separation coefficient of 74.47 in multi-component breakthrough experiments. This study provides a green and sustainable approach that combines heavy metal pollution control with the development of CO2 capture materials, delivering dual energy and environmental benefits.