Nanostructured Gelatin Aerogel for Selective Recovery of Aqueous Pd(II) Based on Coordination to the Peptide Backbone

Recovering and recycling valuable metal compounds from secondary resources by inexpensive bio-based functional materials is vital for realizing a circular economy. Gelatin as a polypeptide shows a strong natural affinity for binding metal ions. In this study, high-porosity aerogels were prepared from glutaraldehyde cross-linked gelatin gels. These nanostructured aerogels are stable in highly acidic solutions (pH = 1.0–2.5) and selectively bind aqueous Pd(II) under industrially relevant conditions. By fine-tuning the cross-linker content, exclusive Pd(II) binding was achieved in the presence of competing Pt(II), Pt(IV), Cu(II), Ni(II), Cd(II), Hg(II), Au(III), Fe(III), and La(III) ions, among others. Based on FT-IR and X-ray photoelectron spectroscopy (XPS) results, the sorption mechanism is the reversible surface complexation of Pd(II) on the gelatin skeleton. The Pd(II) ions form coordination bonds with deprotonated amide nitrogens and adjacent carbonyl oxygens in the peptide backbone of gelatin. In the case of Pd(II), this mode of complex formation is preferred even at highly acidic pH, which explains the high selectivity of the gelatin-based sorbent. The sorption capacity of the optimum aerogel formulation is ca. 250 mg/g, and the equilibrium is set in 1 h. Palladium can quantitatively be recovered by washing the saturated aerogel with a 5 mM solution of the inexpensive amino acid methionine, also pointing out the practical reversibility of the binding process. The practical effectiveness of the sorbent was demonstrated by recovering the palladium content of a spent industrial catalyst.