Effective and selective adsorption of Au (III), Pd (II), and Pt (IV) from aqueous solution by a thiourea-modified Cr-based metal-organic framework

The development of effective adsorbents is crucial to recovering noble metals sustainably from secondary resources. In this work, MIL-101(Cr)-Tu, a Cr-based metal-organic framework (MOF) modified with thiourea, was successfully prepared via double-phase encapsulation approach (DPEA) post-synthesis modification. At 298K, the utmost uptakes of Au (III), Pd (II) and Pt (IV) by MIL-101(Cr)-Tu were 1230.83±8.72, 330.41±7.29, and 315.58±13.67 mg·g-1, respectively. And the uptakes of Au (III), Pd (II) and Pt (IV) increased with temperature. The adsorption data was well fitted to Langmuir model and the pseudo-second-order kinetics model, indicating that the adsorption process conformed to monolayer adsorption and chemisorption. Mechanistic analysis revealed that amino and thiourea groups facilitated the adsorption of noble metals through coordination and electrostatic attraction. Notably, Au (III), Pd (II), and Pt (IV) interacted strongly with thiourea groups due to their “soft-soft” interaction. Moreover, Au (III) was reduced to Au (I) and Au (0), Pt (IV) was reduced to Pt (II), and the amino group was oxidized to -NO2. In the meantime, the adsorption of Pd (II) did not involve redox reactions. Additionally, MIL-101(Cr)-Tu exhibited strong selectivity for noble metals and excellent reusability, demonstrating great potential for the extraction of noble metals from secondary resources.