Highly selective and effective gold recovery from electronic wastewater by synergistic effect between covalent organic framework (COF) and MoS2: Performance and mechanisms

The development of high-efficiency adsorbents for recovering gold from electronic waste has significant economic and environmental benefits. In this work, the efficient and selective adsorbents (MoS2-TpTa-0.5, MoS2TpTa-1.0 and MoS2-TpTa-1.5) were fabricated by combining TpTa, a COF material obtained by condensing 2,4,6Triformylphloroglucinol (Tp) and 4,4 ''-Diamino-p-terphenyl (Ta), with MoS2 for gold adsorption from electronic wastewater. The interaction between MoS2-TpTa and Au(III) occurred through an irreversible monolayer chemical adsorption mechanism, exhibiting adherence to both the pseudo-second order kinetics model and the Langmuir adsorption isotherm model. Remarkably, the MoS2-TpTa-1.5 composite exhibited an impressive maximum adsorption capacity of 2564.80 mg g-1 at an initial pH of 5.0. In addition, it exhibited selectivity for Au(III) recovery from a simulated solution that included coexisting ions of Pb(II), Cu(II), Co(II), Mn(II), Ni(II), Mg(II), Zn(II), and Li(I). Meanwhile, the MoS2-TpTa composite can completely adsorb Au(III) from the lixivium of discarded circuit boards from mobile phones in as little as 5 min. In addition, MoS2-TpTa demonstrated good repeatability over five adsorption cycles. SEM, XPS and XRD characterization were employed to study the adsorption mechanism, revealing the coordination between Au(III) and N, O, and S functional groups. Subsequently, the presence of S2- in the material resulted in the conversion of Au(III) ions to gold nanoparticles through reduction. The gold particles generated relatively large isolated clusters with a mean diameter reaching an astonishing 0.31 mu m, exhibiting a regular hexagonal shape. This study proposes the combination of COF materials with metal sulfides to obtain composite materials with excellent adsorption performance, high selectivity and recyclability, holding substantial potential in practical electronic waste treatment for gold recovery.