(372e) Selective Recovery of Au3+ via Complexation-Reduction Capture and Release By Thermo-Responsive P(NIPAM-co-15TCE-4)@SiO2 Nanoparticles.

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 as compared to their mineral ore sources. Hence, recovery of valuable metals like gold (Au) from waste resources has high environmental and economic significance. Many adsorbents derived from biomass, modified magnetic nanoparticles, and MOFs have been developed for the recycling/recovery of these valuable metals like gold (Au³⁺) from e-wastes. However, most of these adsorbents are not selective towards Au³⁺ from its co-exiting metal ions such as Pd²⁺ and Pt⁴⁺. 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⁺. Herein, a multi-functional Au³⁺-selective thermo-responsive adsorbent, P(NIPAM-co-15TCE4)@SiO₂, was prepared from thiacrown ether (15TCE4) with Au³⁺ recognition properties and a thermo-responsive N‑isopropylacrylamide on a silica (SiO₂) support via the Activators ReGenerated by Electron Transfer; Atom Transfer Radical Polymerization (ARGET-ATRP) method. The adsorbent was successfully prepared as verified by NMR, FTIR, TGA, EA, FE-SEM, XPS, HR-TEM, and DLS. The thermo-responsiveness of P(NIPAM-co-15TCE4) @SiO₂ was observed at critical temperature T_c ~ 34.9 ^◦C. P(NIPAM-co-15TCE4)@SiO₂ showed Au³⁺ capture (T > T_c) and release (T < T_C) by varying the system temperature above and below its phase transition (LCST, T_c) point. At T = 50 ^◦C (T > Tc) and pH = 2, the maximum Au³⁺ capacity q_m was quickly achieved within 2 h. HR-TEM/EDS, WAXD and XPS results indicate that Au³⁺ capture involves complexation, partial reduction to Au⁺ and disproportionation to Au^o /Au³⁺. Adsorption results show competitive and selective Au³⁺ uptake of P(NIPAM-co-15TCE4)@SiO₂ with K_D ~16,276 mL g^-1 and α = 9 – 43,053 for Au³⁺ against Pd²⁺, Pt⁴⁺, Cu²⁺, Pb²⁺, Zn²⁺ and Ni²⁺ present in a simulated mobile phone leachate. Meanwhile, desorption occurs at T < T_c by the brush network movement as NIPAM component rehydrates at T = 27 ^◦C. Complete desorption was achieved even in a very mild eluent that eluted firmly bound Au species and regenerated the adsorbent via S = O reduction to C-S-C. The reversible behavior of P(NIPAM-co-15TCE4) @SiO₂ for Au³⁺ capture (T > Tc) and release (T < Tc) highlights the synergy between 15TCE4 and NIPAM, making it a reusable practical adsorbent with consistent performance for Au³⁺ recovery in complex feed wastes.

This study was supported by the National Research Foundation of Korea (NRF) funded by The Ministry of Science and ICT (2020R1A2C1003560, 2021R1A2C2093746, and 2021R1H1A2008284), Basic Science Research Program through the Ministry of Education (2020R1A6A1A03038817).