Exceptionally amino-quantitated 3D MOF@CNT-sponge hybrid for efficient and selective recovery of Au(III) and Pd(II)

Designing materials for selective recovery of precious metals is a critical challenge in response to recent outbreaks of electronic wastes owing to economic and environmental concerns. We introduced high-performance hybrid materials (MCNTs) comprising of six-amino groups based on hexaaminobenzene metal-organic framework (HAB-MOF) and carbon nano-tube (CNT) sponge for selective recovery of Au(III) and Pd(II). Among all MOFs ever reported in the literature, the particular synthesized 30-MCNT (30 mg HAB-MOF on CNT sponge) exhibited the highest adsorption capacity (qe) of 1832 mg/g (45 °C, pH 3.0) for Au(III) and 1651 mg/g (45 °C, pH 1.0) for Pd(II), which can be regarded as a benchmark in the MOF family. The highly selective nature of 30-MCNT was examined in the presence of ten different metal ions and the results showed that the Au(III)/Pd(II) adsorption efficiency was decreased by only 5–10%. Moreover, the Au(III) and Pd(II) recovery was maintained ∼ 100% from the initial solution with metal concentration of 200 mg/L. The superior performance was mainly attributed to the role of abundant amino groups in the hybrid framework, developing several types of interactions with Au(III) and Pd(II) ions such as hydrogen bonding, electrostatic interactions, acid-base affinity and π-π inner sphere complexation. Furthermore, the remarkably high surface area and robust chemical and mechanical stability of CNT-sponge provided an exceptional support for impregnation of HAB-MOF to avail greater active sites and easier handpicked recycle. Density functional theory (DFT) simulation and X-ray photoelectron spectroscopy (XPS) studies were employed to explore the interaction mechanisms during the adsorption processes. Owing to facile and environment-friendly synthesis, excellent recovery and recycling, the material can be possibly considered for industrial application in precious metal recovery.