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.