Transformation of cadmium-rich hazardous waste into a high-performance adsorbent via a synergistic gas-foaming and template-etching strategy

The substantial accumulation of cadmium (Cd)-laden adsorbent waste poses significant ecological risks. Transforming this waste into value-added materials presents a promising strategy to mitigate environmental challenges. Herein, a facile method converts spent Cd-bearing alkali lignin biochar (Cd-enriched ALB) into cadmium oxide (CdO-W) via one-step calcination in air for the highly efficient removal of tetracycline hydrochloride (TCH). This approach addresses the challenge of safe disposal and resource recovery of spent Cd-laden biochar. By utilizing the biochar matrix as a dispersive template to inhibit nanoparticle agglomeration and the CO 2 released during its pyrolysis as a synergistic gaseous foaming and etching agent, a hierarchical porous structure with a high specific surface area is constructed. The resulting CdO-W exhibits a remarkable adsorption capacity of 1260 mg/g for TCH at pH 7, which is 55.9% higher than that of chemically synthesized CdO-S (808 mg/g). This superior performance is attributed to the unique synthesis route where the biochar matrix acts as a dispersive template to prevent CdCO 3 nanoparticle agglomeration, while the CO 2 released during its pyrolysis synergistically acts as a gaseous foaming agent and etchant. This process creates a hierarchical porous structure with a high specific surface area, exposing abundant active sites. Mechanistic investigations reveal that adsorption proceeds primarily through coordination complexation, hydrogen bonding, and electrostatic interactions. This work provides a feasible path for the resource utilization of Cd-rich hazardous waste and offers insights for designing cost-effective, high-performance functional materials from other spent heavy-metal adsorbents.