Robust dialdehyde xylan-cross-linked aminated lignin network; high-capacity and renewable biological macromolecular platform for palladium recovery.

Abstract

Developing sustainable adsorbents from renewable resources is vital for strategic precious metal recovery. In this study, a largely biomass-derived network, dialdehyde xylan (DAX)-crosslinked polyethylenimine (PEI)-aminated lignin (DPAL), was synthesized through a green, water-based process. Xylan was oxidized to DAX as a bio-linker, while lignin was aminated to enhance palladium affinity. The DPAL network was formed via Schiff base reactions, establishing stable imine (C=N) linkages. DPAL exhibited exceptional structural stability with solubility below 5% across pH 2-10 and high surface hydrophilicity with a contact angle of 26.9°. Adsorption experiments revealed a maximum Pd(II) capacity of 256.1 mg/g at pH 2, consistent with Langmuir isotherm and pseudo-second-order kinetic models. Mechanistic analysis revealed a dual adsorption pathway: dominant electrostatic attraction between protonated amine groups and anionic Pd complexes, coupled with in situ reduction of Pd(II) to Pd(0). DPAL also exhibited high selectivity for Pd(II) over coexisting metal ions, and thermodynamic analysis confirmed that the adsorption was spontaneous and exothermic. Furthermore, DPAL retained over 75% of its initial adsorption efficiency after five consecutive cycles, and both crystalline PdO and metallic Pd(0) were successfully recovered from the spent adsorbent via calcination.