Sustainable Remediation of Chlorinated Phenols via Circular Bio-Economy: Valorising Spent Biochar through Zinc Chloride Co-Pyrolysis with Microcrystalline Cellulose

Managing exhausted adsorbents is a major challenge in water treatment. This study proposes a circular bio-economy strategy by upcycling spent, organically loaded biochar into high-quality Regenerated Carbon (RC). The RC was created through co-pyrolysis of a mixture of three used biochars, microcrystalline cellulose, and ZnCl 2 at 500°C. Characterisation revealed a turbostratic carbon structure, a moderate surface area of 567 m²/g, and a highly oxygenated, amphiphilic surface. Despite having a moderate surface area, RC achieved a high maximum adsorption capacity of 460.2 mg/g at 20 °C for 2,4-dichlorophenol (2,4-DCP), showing that surface chemistry plays a significant role alongside textural properties. Adsorption kinetics fit the Fractal-PFO model (n ≈ 0.56), suggesting diffusion-limited transport on a rough, energetically diverse surface. Equilibrium data matched the Liu isotherm, and thermodynamic analysis indicated an entropy-driven [ΔS° = +102.35 J/(K·mol)], endothermic physical process mainly influenced by the hydrophobic effect. The adsorption mechanism combines π-π electron donor-acceptor interactions, hydrogen bonding, and halogen bonding. Practically, RC maintained over 94% removal efficiency in complex simulated industrial effluents and achieved about 40% reversible capacity across multiple regeneration cycles with a NaOH/ethanol eluent. This Waste-to-Wealth strategy effectively converts hazardous spent biochar into a sustainable, highly efficient material for removing persistent chlorinated aromatics.