(377l) Development of Highly Selective Composite Adsorbents for Selective Recovery of Gold (Au3+) and Silver (Ag+) Ions from Electronic Wastes (E-wastes)
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 = 1.31â1.94 Ã ; 15-thia-CE = 2.19â2.52 Ã , 17-thia-CE = 2.12â2.21 Ã and 19-thia-CE = 2.27â2.43 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+(Î±Ag+/Mn+= 25 â 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)).