Extraction of palladium from nuclear waste-like acidic solutions by a metal-organic framework with sulfur and alkene functions

We report a robust metal-organic framework (MOF) for convenient recovery of Pd(II) from acidic nitric solutions which emulate high-level liquid wastes (HLLW) generated from the reprocessing of spent nuclear fuel. The framework solid (ASUiO-66) was constructed from Zr(IV) ions and the multifunctional linker 2,6-bis(allylsulfanyl)terephthalic acid (H₂L), and features the well-known UiO-66 topology. Herein the robust Zr(IV)-carboxylate bonds impart structural strength to the host net, while the alkene and thioether units provide efficient and selective binding of the Pd(II) ions. For example, over 95% of the Pd(II) ions can be adsorbed from a simulated HLLW (1.0 M HNO₃, containing about 20 different types of metal elements), with Ag(I) being the only other metal ion taken up significantly by the ASUiO-66 sorbent. Moreover, the adsorbed Pd(II) species can be effectively stripped by a dilute solution of thiourea (0.01 M); and the regenerated framework solid can be used for additional cycles of Pd extraction, with the sorption capacity for Pd(II) being little changed (38-41 mg g^-1). The isotherm adsorption data fit well with the Langmuir model with a saturation capacity of 45.4 mg g^-1, being equivalent to each octahedral cage in the UiO-66 net containing roughly one Pd(II) ion. In a broader perspective, the alkene and thioether combination could be anchored onto other sorbent systems (e.g., porous polymers and resins) to impart versatile adsorption properties for the retrieval of noble metal ions.