Thia-crown Ether based Composite Adsorbents for Selective Recovery of Gold (Au 3+ ) and Silver (Ag + ) ions from Electronic Wastes

Global electronic waste (E-waste) is increasing due to the fast-technological advancement of electrical and electronic devices with short life cycles, and lack of suitable recycling technologies. Inappropriate disposal of these waste materials causes severe environmental impacts. Furthermore, E-waste is attractive for recycling of precious metals like gold and silver, even compared to their mineral ore sources. Many lixiviants and adsorbents have been developed for the recycling of these valuable metals. However, most of the adsorbents are not selective towards specific precious metal ions. 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 the right affinity towards the target ion. In this work, CEs with sulfur donor atoms or thia-CEs (TCEs) were synthesized and proved to form stable complexes with Au3+ and Ag+. The TCEs were prepared by intermolecular cyclization of bulky epoxides with 1,2-benzenedithiol. The TCEs were immobilized on porous polypropylene membrane (PPM) and silica (SBA-15) to enhance their reusability. Different types of composite adsorbents were fabricated containing TCEs with different cavity sizes to render them metal-ion specific.The composite adsorbents were prepared through immobilization of TCEs onto the surface of PPM and SBA-15, respectively. The developed composite adsorbents (TCE@PPMs and TCE@SBA-15) were thoroughly characterized by high-resolution spectroscopic methods such as NMR, HRMS,FTIR,TGA, SEM, XRD and BET. Adsorption experiments reveal the high selectivity of the composite adsorbents towards Au3+((αAu3+/Pd2+= 1.22)and Ag+(αAg+/Mn+= 8 - 177) as compared to other cations (Mn+) present in leached spent mobile PCBs, such as Cu2+, Ni2+, Zn2+, and Pb2+. Overall results demonstrate the suitability of the composite adsorbents (TCE@PPM and TCE@SBA-15) for valuable metal recovery applications. This work was supported by National Research Foundation of Korea (NRF) funded by the Ministry of Education (2009-0093816 and 22A20130012051 (BK21Plus)).