Cr-based metal–organic frameworks (MOFs) with high adsorption selectivity and recyclability for Au (Ⅲ): Adsorption behavior and mechanism study

Preparation of efficient adsorbents is of great significance for the recovery of precious metals from secondary resources. Herein, –NH2 was successfully grafted on MIL-101(Cr) via hydrothermal reaction to improve the adsorption capacity of Au (Ⅲ) in aqueous solution. MIL-101(Cr)–NH2 had a huge specific surface area (1016.50 m2‧g−1), which can provide more adsorption sites for Au (III). At 298 K, the maximum adsorption capacity of Au (III) by MIL-101(Cr)–NH2 was 792.43 mg∙g−1 and was higher than that by MIL-101(Cr) (140.83 mg∙g−1). The adsorption data was well fitted by Langmuir and pseudo-second-order models, confirming that the adsorption process conformed to single molecule adsorption and chemical adsorption. Mechanism study indicated that the adsorption of Au (III) by MIL-101(Cr)–NH2 was initiated with electrostatic attraction and promoted by reduction and inner complexation between AuCl4- and protonated amino groups. During adsorption, Au (Ⅲ) was reduced to Au (0) and Au (I) by –NH2. Density functional theory (DFT) calculation demonstrated that MIL101(Cr)–NH2 possessed higher binding affinity and more adsorption sites for Au (III) than MIL-101(Cr). Moreover, MIL-101(Cr)–NH2 exhibited excellent selectivity towards Au (III) and good recyclability, which is promising in the recovery of precious metals.