Metalloselective adsorption of Au(III), Pd(II), and Pt(IV) from acidic solutions by a thiourea-modified Cr-based metal-organic framework

The development of effective adsorbents is crucial for the sustainable recovery of noble metals from secondary resources. In this study, MIL-101(Cr)-Tu, a Cr-based metal‒organic framework (MOF) modified with thiourea, was successfully prepared by double-phase encapsulation followed by post-synthetic modification. At 298 K, the largest adsorption capacities 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. The uptakes of Au(III), Pd(II), and Pt(IV) increased with temperature. The adsorption data fit well to both the Langmuir and pseudo-second-order kinetic models, indicating that the adsorption process conformed to monolayer adsorption and chemisorption. Mechanistic analysis revealed that the 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 the thiourea groups owing to their “soft–soft” interactions. Moreover, Au(III) was reduced to Au(I) and Au(0), Pt(IV) was reduced to Pt(II), and the amino groups were oxidized to NO₂. 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 its great potential for the extraction of noble metals from secondary resources.