Selective and sustainable recovery of Au3+ through complexation-reduction capture and mechano-assisted release by thermo-responsive poly (N-isopropylacrylamide-co-15-thiacrown-4 ether)@SiO2 nanoparticles

Alternative mining methods are becoming increasingly important towards sustainable and eco-friendly gold (Au) production. Herein, thiacrown ether (diallyl 15TCE4) with Au3+ recognition properties was combined with thermo-responsive N-isopropylacrylamide (NIPAM) on a silica (SiO2) support via ARGET-ATRP to afford a multi-functional Au3+ adsorbent P(NIPAM-co-15TCE4)@SiO2. The adsorbent was successfully prepared as confirmed by FTIR, NMR, TGA, EA, FE-SEM, HR-TEM, XPS, and DLS. The thermo-responsiveness of P(NIPAM-co-15TCE4)@SiO2 was observed at critical temperature T-c ~ 34.9 C. At T = 50 C (T > T-c) and pH = 2, the maximum Au3+ capacity q(m) = 100.60 mg g(-1) 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-0/Au3+. It is believed that partial Au3+ reduction is coupled with C-S-C oxidation to R2S = O in 15TCE4, which occurred within its cavity and/or between neighboring 15TCE4s that are closely held by the collapsed NIPAM at T > T-c. Meanwhile, desorption occurs at T < T-c through "mechano "-assisted Au release induced by the brush network movement as NIPAM component rehydrates at T = 27 C. Complete desorption was achieved even in a very mild lixiviant (0.1 M HCl/0.1 M TU) that eluted tightly-bound Au species and regenerated the adsorbent via S = O reduction to C-S-C. The adsorbent is highly selective with alpha = 9 - 43,053 for Au3+ against Pd2+, Pt2+, Cu2+, Pb2+, Zn2+ and Ni2+ present in a simulated mobile phone leachate. The reversible behavior of P(NIPAM-co-15TCE4)@SiO2 for Au3+ capture (T > T-c) and release (T < T-c) highlights the synergy between 15TCE4 and NIPAM, making it a reusable practical adsorbent with consistent performance for Au3+ recovery in complex feed wastes.