Efficient removal of cadmium by zero-valent iron-coated hydrochar derived from biogas residues: Surface interaction and machine learning interpretation

• Fe@HC was fabricated by BR and Fe 0 loading; • The adsorption capacity of Cd(II) reached 496.51 mg/g with neutral-alkaline pH condition; • Cd(II) removal was mainly controlled by pore-filling, complexations, and reduction; • ANN model was employed to interpret and optimize Fe@HC parameters; • Efficiency of Fe@HC retained 79.88% after five regeneration cycles. Biogas residue (BR) is the major byproduct from anaerobic digestion, which was valorized by fabricating zero-valent iron-coated hydrochar (Fe@HC) in this work. The Fe@HC was then employed for cadmium removal from wastewater. Further, various techniques were applied to confirmed the successful zero-valent iron Fe loading onto mesoporous BR-derived hydrochar, revealing layered structures and abundant functional groups on Fe@HC. The composite achieved a high Cd(II) removal performance with adsorption capacity of 496.51 mg∙g −1 , according to synergistic mechanisms such as electrostatic attraction, precipitation, surface complexation, ion exchange, and chemical reduction. Besides, the artificial neural network (ANN) model was used to optimize Fe@HC synthesis and adsorption parameters (R 2 > 0.98), identifying the key factors of hydrothermal temperature (19.11%) and Fe 2 O 3 dosage (12.61%). All these results demonstrated a sustainable strategy for simultaneous BR reutilization and heavy metal immobilization, offering a promising pathway for machine learning application.