Reductive Gold Recovery From Complex Matrices Using Dimercaptothiadiazole Functionalized UiO-66-NH2 Metal-Organic Framework

Abstract

Creating proficient adsorbents for recovering precious metals from secondary resources is of significant environmental and economic significance. In this study, the Metal-Organic Framework (MOF) UiO-66-NH2-DMTD made up by modifying Zr-MOF (Zirconium-based Metal-Organic Framework) with 2,5-Dimercapto-1,3,4- thiadiazole (DMTD) has been proposed as a promising redox-active catalyst capable of recovering gold (Au) from complex liquid matrices, including digested incinerated bottom ash (IBA), a digested CPU processor, and simulated wastewater matrices prepared using deionized water. The recovery process is achieved via a double mechanism (i.e., chelation and reduction). The redox reactions among Au (III) and functional groups containing thiol, Lewis soft–soft interactions (HSAB theory), and ion-exchange/electrostatic interactions. Importantly, this study demonstrated the reduction of Au (III) to metallic gold as a fundamental step in achieving the adsorption process. It is suggested that the recovery of Au (III) nanoparticles on UiO-66-NH2-DMTD is irreversible due to a chemisorption process confirmed through the pseudo-second-order model. UiO-66-NH2-DMTD shows an increase in the experimental adsorption capacity of gold that reaches 878.5 mg/g at a pH of 3 and temperature of 338 K. Outstandingly, it has a high ability to selectively capture Au (III), even when coexisting ions are present at five times higher concentrations in spiked Han River water, and ten times higher in IBA leached solution. This unique selectivity is attributed to a combination of electrostatic interactions, coordination, and the reduction mechanism reaction between Au (III) and the sulfide group encompassed in the DMTD functional groups. Moreover, the recyclability and recovery tests demonstrated the potential for the catalyst to be reused, demonstrating notable stability and enhancing the industrial applicability. The adsorption capability for gold ions shows a minimal reduction in the recovery rate even after eight recycling processes. Furthermore, the practical application was further tested in experiments involving liquid matrices with leached Au, Cu, and Ni extracted from a CPU processor and also in spiked Han River water, highlighting the current practical viability of UiO-66-NH2, particularly with the DMTD ligand functionalized into the basic structure. These results collectively demonstrated that the UiO-66-NH2-DMTD is a practical, sustainable adsorbent for efficient and selective Au (III) recovery in complex liquid matrices.