A Cryptand-Based Covalent Organic Framework for Efficient Gold Recovery from Electronic Waste

Abstract

The increasing desire for gold and environmental burden of electronic waste (e-waste) demand robust adsorbents which enables the efficient extraction of trace Au(III) from complex acidic leaching solutions with high selectivity. Herein, we report the first cryptand-based covalent organic framework (COF), i.e., Cryptand-COF, which integrates macrocyclic cryptand units into an extended porous lattice to form two types of well-defined Au(III) adsorption sites. Computational simulations reveal an AB-stacked architecture that maintains accessible coordination pockets throughout the framework. Cryptand-COF exhibits an outstanding Au(III) uptake capacity of 1032 mg g−1 in a hydrochloric acid solution at pH 2. Excellent selectivity for gold adsorption is also demonstrated by this material in solutions containing high concentrations of co-existing metal ions , such as Fe(III), Cu(II), Co(II), Ni(II), Zn(II), Cr(II), Mn(II), Na(I), and Al(III). The framework exhibits an excellent recyclability in an acidic medium, retaining >90% of its initial Au(III) uptake after five ad-/desorption cycles. X-ray photoelectron spectroscopy suggests that Au(III) is captured and reduced in situ to Au(0) via cooperative redox involving N and O donor sites within the COF structure. Leaching studies using discarded central processing units further indicate that Cryptand-COF can selectively recover trace amounts of gold from electronic waste (e-waste) leachates with a high efficiency (>99%), underscoring its promise for use in practical e-waste recycling.