(377l) Development of Highly Selective Composite Adsorbents for Selective Recovery of Gold (Au3+) and Silver (Ag+) Ions from Electronic Wastes (E-wastes)

Production of electrical and electronic devices with short life cycles and lack of suitable recycling technologies has led to the surging electronic waste generation worldwide. E-wastes such as spent mobile phones contain higher amount of precious metals compared to their mineral ore sources hence are attractive secondary sources for recovery and recycling. Many adsorbents derived from biomass, modified magnetic nanoparticles, and MOFs have been developed for the recycling/recovery of these valuable metals like gold (Au³⁺) and silver (Ag⁺) from e-wastes. However, most of these adsorbents are not selective towards Au³⁺ or Ag⁺. Meanwhile, crown ethers (CEs) are attractive macrocyclic complexing agents as their cavity structures can be tailored to accommodate the ionic size of a precious metal and their heteroatoms (i.e. O, N, or S) can be selected to afford their affinity towards Au³⁺ and Ag⁺.

In this study, 13-, 15-, 17- and 19- membered CEs with sulfur donor atoms (thia-CEs) were synthesized and demonstrated to form stable complexes with Au³⁺ and Ag⁺. The thia-CEs were prepared by intermolecular cyclization of bulky epoxide intermediates with 1,2-benzenedithiol. These thia-CEs were subsequently immobilized on porous polypropylene membrane (PP) and mesoporous silica (SBA-15) to fabricate thia-CEs @PPs and thia-CEs @SBA-15 composite adsorbents, respectively. The developed composite adsorbents (thia-CEs @PPs and thia-CEs @SBA-15) were thoroughly characterized by both computational (DFT) and experimental (high-resolution spectroscopy) methods such as FTIR, ¹H and ¹³C NMR, HRMS, TGA, BET, XRD, SEM and ICP-MS. The full characterization results showed that the composite adsorbents were successfully prepared. The immobilization of the thia-CEs onto solid support materials (PP and SBA-15) enhanced the reusability of the active ionophores (thia-CEs). Moreover, DFT result showed that the thia-CEs have different cavity sizes (13-thia-CE = 1.31−1.94 Ã…; 15-thia-CE = 2.19−2.52 Ã…, 17-thia-CE = 2.12−2.21 Ã… and 19-thia-CE = 2.27−2.43 Ã…) which are responsible for the metal-ion specificity. The 19-thia-CE@PP is highly selective towards Ag⁺ ions (2.30 Ã…), whereas the 15-thia-CE@PP is highly selective towards Au³⁺ ions (1.36 Ã…). Adsorption experimental result reveled that 19-thia-CE@PP has higher adsorption capacity (q = 206 mg Ag⁺/g) and selectivity towards Ag⁺(α_{Ag⁺/Mⁿ⁺}= 25 – 287) as compared to other cations (Mⁿ⁺= Cu²⁺, Ni²⁺, Zn²⁺, and Pb²⁺) present in leached PCB e-waste. Similarly, Au³⁺ adsorption experiments using thia-CEs@SBA-15 were conducted and preliminary results showed that the 15-thia-CE@SBA-15 has high selectivity towards Au³⁺ in the presence of Pd²⁺ and Pt⁴⁺ ions. Overall results demonstrate the suitability of the composite adsorbents (thia-CEs@PPs and thia-CEs@SBA-15) for valuable metal recovery applications. Furthermore, the synthesis strategy described in this work has great potential for a wide range of applications such as metal sensing, metal recovery and catalysis. This research was supported by the National Research Foundation of Korea (NRF) under the Ministry of Science and ICT (No. 2018R1D1A1B07047503, No. 2017R1D1A1B03028102, and 2017R1A2B2002109) and the Ministry of Education (No. 22A20130012051(BK21Plus)).