Waste–to–waste valorization: Sustainable palladium recovery from real spent catalytic converter leachates using chicken feathers

The increasing demand for platinum group metals (PGM), mainly driven by their extensive use in automotive catalytic converters, has heightened the need for efficient and green recovery methods. Simultaneously, the poultry industry generates significant amounts of keratin-rich chicken feathers (CF), which are commonly disposed of by incineration or landfilling, raising environmental concerns. This study explores the use of CF as a sustainable and cost-effective biosorbent for the selective recovery of Pd from synthetic multimetallic solutions and HCl-based spent autocatalytic converters leachates. Adsorption experiments revealed that the optimal Pd uptake (14.10 ± 0.31) mg.g^-1 was achieved at 0.2 M HCl (pH ∼ 0.79), while 2 M HCl provided the highest selectivity towards Pd over other metals. Adsorption kinetics followed a pseudo-second-order model, indicating chemisorption as the dominant mechanism, and Langmuir isotherm analysis yielded a Pd adsorption capacity of (19.61 ± 1.31) mg.g^-1. Desorption tests demonstrated ∼100% Pd recovery using 0.2 M thiourea + 0.5 M HCl solution. Characterization of the CF by Raman and solid-state UV-Vis spectroscopy, FTIR, SEM-EDX, and XPS confirmed the interaction of Pd with disulfide bonds and protonated amino groups as Pd-chloride complexes. Bench-scale trials on HCl-based leachates with an optimized CF to leachate ratio achieved near-complete Pd separation in a single adsorption step with high purity, highlighting the robustness and selectivity of this method under realistic conditions. Pd was successfully desorbed from CF by acidified thiourea solution, offering added value for catalytic applications.