Highly efficient and acid-resistant metal-organic frameworks of MIL-101(Cr)-NH2 for Pd(II) and Pt(IV) recovery from acidic solutions: Adsorption experiments, spectroscopic analyses, and theoretical computations

Cr-based metal-organic frameworks (MOFs) of MIL-101(Cr)-NH2 was post-synthesized from nitro-functionalized MIL-101(Cr) (MIL-101(Cr)-NO2) through a reduction process. Adsorption behaviors and interactions of MIL-101(Cr)-NH2 and MIL-101(Cr)-NO2 with platinum group metal (PGM) anions of Pd(II) (PdCl42-) and Pt(IV) (PtCl62-), were investigated through batch adsorption experiments, spectroscopic analyses, and theoretical computations. According to adsorption kinetics and isotherms, the uptakes of Pd(II) and Pt(IV) by in MIL-101(Cr)-NH2 were found to be much higher than their uptakes by MIL-101(Cr)-NO2. The abundant protonated amine groups (BDC-NH3+) in MIL-101(Cr)-NH2 were verified to be the main adsorptive binding sites by XPS and FTIR spectroscopy, and FE-SEM imageries. Additionally, BDC-NH3+ shows extremely high affinities (b value) and binding energies (Ebind) for PdCl42- and PtCl62- through electrostatic attraction, resulting in much higher adsorption capacities of MIL-101(Cr)-NH2 for these PGMs as compared to those of MIL-101(Cr)-NO2. Moreover, the MOFs' Cr nodes without terminal -OH indicated positive electrostatic potentials, and certain values of Ebind for PGM anions. Thus, the few-amount cationic Cr sites could also make little contributions to the adsorption of PGM anions in MIL-101(Cr)-NH2 or MIL-101(Cr)-NO2. Furthermore, the perfect regeneration and reusability of MIL-101(Cr)-NH2 over five of adsorption-desorption cycles, suggesting its potential in practical applications.