Thiol-functionalized cellulose adsorbents for highly selective separation of palladium over platinum in acidic aqueous solutions

The selective separation of palladium (Pd) over platinum (Pt) remains a challenge in metal recycling, because of their similar electronic configuration and chemical properties. This study explored their selective separation using three new adsorbents by grafting sulfur-donor ligands onto cellulose, namely 2-aminothiophenol (Cell-AP), 2-mercaptopyridine (Cell-MP), and 2-mercaptobenzothiazole (Cell-MBT). These adsorbents exhibited excellent adsorption capacity for Pd(II), reaching 163.3 ± 2.5 mg/g (at pH 1), 92.5 ± 1.2 mg/g (at pH 1), and 27.5 ± 0.5 mg/g (at pH 2.5), respectively. Isotherm studies showed that the adsorption process followed Freundlich isotherm, indicating a multilayer adsorption that takes place on a heterogenous surface. Kinetic results fit best with pseudo-second-order and intra-particle diffusion model, suggesting that the rate-limiting process involves chemisorption and intra-particle diffusion. Thermodynamic studies showed that adsorption by Cell-AP and Cell-MP was a spontaneous exothermic reaction. The materials also showed superior selectivity of Pd(II) over Pt(IV) with a maximum separation factor (SF_Pd/Pt) of 64.50, which improves over most previous studies. The mechanism of selectivity was further investigated by XPS, FTIR, SEM and DFT calculations, which showed that PdCl₄^2- has smaller size, shorter HOMO-LUMO gap, and lower binding energy to ligands as compared to PtCl₆^2-. This was used to explain the higher affinity for Pd(II) over Pt(IV) of these sulfur-modified adsorbents. This work provides a practical approach for the selective separation of Pd over competing ions that could improve the efficiency for overall metal recovery or recycling processes.