Post-Synthetic Modification of a MOF via Continuous Flow Methods for Gold E-Waste Recycling

This work represents a pioneering effort in utilizing continuous flow methods for the post-synthetic modification of a nanoporous metal-organic framework (Fe-BTC or MIL-100(Fe)) with short-chain redox-active oligomers (poly-p-phenylenediamine, PpPDA). The Fe-BTC/PpPDA composite has been previously demonstrated to rapidly and selectively extract gold from complex liquids. Thus, in the present study, Fe-BTC/PpPDA was synthesized on a 250 g scale and tested in eight industrially relevant solutions used for leaching metals from electronic waste. These leachates (e. g., cyanide, thiourea, and aqua regia) exhibited varying effectiveness, pH, and gold speciation, which led to significant differences in the composite's performance during gold extraction. Notably, Fe-BTC/PpPDA performed best in leaching solutions containing [AuCl₄]^- species. Subsequently, Fe-BTC/PpPDA was structured into spherical beads using a novel microdroplet technique in continuous flow. These structured adsorbents were then placed in a column and assessed for gold recovery from real e-waste solutions containing [AuCl₄]^- species. The composite reached a capacity of ~600 mg of gold per gram before breakthrough and a capacity of ~900 mg of gold per gram at a gold recovery rate of ~60 %. The selectivity of the composite was calculated to be 972, 262, and 193 for Au/Ni, Au/Co, and Au/Fe, respectively. Also, in situ X-ray absorption near edge spectroscopy (XANES) was employed to monitor the gold oxidation state, providing the first evidence of gold reduction occurring on a timescale relevant to flow-through experiments. It was confirmed that the redox-active oligomers facilitate the reduction of Au³⁺ to metallic Au during extraction, enhancing the composite's capacity and selectivity. Additionally, Fe-BTC/PpPDA outperformed several commercial resins commonly used in gold recovery. Considering the scalability of the composite and its outstanding performance in realistic solutions, this work suggests a promising future for MOF/polymer composites in selective metal recovery from waste streams. Furthermore, the continuous flow methods used for post-synthetic modification of the MOF may pave the way for more scalable production in the future.