Synergistic Co-pyrolysis of Fenton sludge biochar for fluoride removal: Strategy optimization and cooperative adsorption mechanisms

Fluoride-containing wastewater posed significant environmental and health risks, and the high costs of current defluoridation adsorbents highlight the pressing need for cost-effective solutions. To achieve the advanced treatment of fluoride-containing wastewater and the resource utilization of solid waste, a pyrolytic-hydrothermal carbon adsorbent was developed from Fenton sludge (FS) and rice straw (RS). FS/RS-1:2BC, synthesized at 600 °C pyrolysis and 200 °C hydrothermal treatment with a 1:2 mass ratio, exhibited the highest fluoride ions (F⁻) adsorption capacity of 10.57 mg g⁻¹. Physicochemical characterization revealed FS/RS-1:2BC consisted of porous graphitic carbon from RS and α-Fe₂O₃ from FS, with abundant oxygen-containing groups and a large surface area for F⁻ adsorption. Adsorption kinetics and thermodynamic fitting indicated the adsorption of F⁻ was primarily relied on chemical interactions (chemical precipitation, ligand complexation, ion exchange, electrostatic attraction) between F⁻ and the homogeneous monolayer surface, with physical adsorption and intraparticle diffusion as secondary mechanisms. FS/RS-1:2BC exhibited excellent stability and applicability, with a broad pH acceptance range (pH 3.5–8). It maintained 9.58 mg g⁻¹ adsorption capacity under weakly alkaline and achieved around 90 % adsorption efficiency in coexisting anions. Regeneration experiments indicated that FS/RS-1:2BC retained 81.9 % adsorption capacity after five regeneration cycles. The development of FS/RS-1:2BC not only provided a cost-effective adsorbent for low-concentration fluoride removal from tailwater but also established a new approach for resource utilization of solid wastes.