Selective and efficient adsorption of Au (III) in aqueous solution by Zr-based metal-organic frameworks (MOFs): An unconventional way for gold recycling.

Recycling precious metals from secondary resources is of great environmental and economic significance. In this study, the Zr-based MOFs UiO-66-NH2 was synthesized and used to adsorb Au (III) in aqueous solution. The ultrafine particle size (∼50 nm), excellent crystallinity and huge specific surface area (1039.2 m2 ·g-1) were verified by transmission electron microscope (TEM), powder X-ray diffraction (PXRD) and surface area analysis. About 50 % Au (III) was adsorbed within 6 min and the maximum adsorption capacity at 298 K reached up to 650 mg·g-1, showing superiority to traditional adsorbents. The general order kinetics model and Liu equation were suitable to describe the adsorption process, which was spontaneous, endothermic and driven by the increasing system entropy. Electrostatic attraction between -NH3+ and Au (III) anions and inner complexation to Zr-OH played a vital role in adsorption. Au (Ⅲ) was reduced to Au° by amino groups via redox reaction certified by X-ray photoelectron spectroscopy (XPS), PXRD and high-resolution transmission electron microscopy (HRTEM) analysis. Moreover, UiO-66-NH2 displayed high selectivity, robust stability and excellent reusability, making it an ideal candidate for gold recycling in industrial practice.