Highly efficient and selective recovery of Au(III) from aqueous solution by bisthiourea immobilized UiO-66-NH2: Performance and mechanisms

The design and synthesis of efficient adsorbents for recovery of precious metal from secondary resources are of great environmental and economic significance. In present study, an efficient and redox-active adsorbent (UiO-66-BTU) was fabricated by modifying Zr-MOF (Zirconium-based Metal Organic Framework) with bisthiourea for gold recovery in aqueous solution. It is indicated that Au(III) adsorbing onto UiO-66-BTU is an irreversible, favorable chemisorption process conformed to D-R model and pseudo-second-order model. The UiO-66-BTU showed significant adsorption ability (680.20 mg/g at pH 2.0) and high selectivity towards Au(III) in presence of various mixed ions in actual wastewater, which were mainly attributed to the Lewis soft-soft interaction, ion-exchange/electrostatic interactions, and redox reaction between Au(III) and S-containing functional groups. Notably, we demonstrated that the reduction of Au(III) to metallic gold as a dominant recovery mechanism involving the in situ confined nucleation, fusion and growth of AuNPs to branched Au precipitation, with Au(0) accounted for approximately 80% for the recycling products. Furthermore, the UiO-66-BTU membrane achieved similar high adsorption capacity and the membrane surface remains active to efficiently recovered metallic particles with an excellent repeatability at eight cycles of adsorption. The reduced Au particles formed isolated clusters with bigger size and regular hexagonal shape. Consequently, UiO-66-BTU has great potential for gold recycling in industrial practice.