Multifaceted optimization of CO2/H2O activation on the surface properties of food waste biochar: An efficient green adsorbent for the removal of Pb(II) from water

In this study, an adsorbent (CHBC) with good adsorption performance for Pb(II) was successfully prepared by CO₂/H₂O activation of food waste biochar. Specifically, the specific surface area of CHBC reached 144 m²·g⁻¹, which was 48 times higher than that of unactivated food waste biochar, and the adsorption of Pb(II) by CHBC was a spontaneous heat-absorption reaction, with a maximum Pb(II) adsorption capacity of 114.59 mg·g⁻¹, which can be well described by the pseudo-second-order kinetic model and the Freundlich isothermal adsorption model. The adsorption process of Pb(II) on CHBC is dominated by chemisorption, and the adsorption mechanism includes the ion-exchange reaction of Pb(II) with Ca²⁺ on CHBC, the precipitation reactions with phosphate and SO₃^2-, the complexation reactions with oxygen-containing functional groups such as –OH and C=O, the electrostatic attraction, and the Pb(II)-π interactions. The combination of these mechanisms results in an excellent adsorption capacity for Pb(II) for CHBC.