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

Abstract

• P(NIPAM- co -15TCE4)@SiO 2 with thermo-responsive and Au 3+ recognition properties was developed. • Au 3+ capture at T > T c involves complexation, reduction to Au + , and disproportionation to Au 0 /Au 3+ . • Au (Au 0 , Au + , Au 3+ ) release at T < T c is via “mechano”-assisted elution in a very mild lixiviant. • P(NIPAM- co -15TCE4)@SiO 2 is highly selective to Au 3+ against other metal ions in simulated electronic wastewater. • P(NIPAM- co -15TCE4)@SiO 2 is highly competitive and can be reused with consistent performance. Alternative mining methods are becoming increasingly important towards sustainable and eco-friendly gold (Au) production. Herein, thiacrown ether (diallyl 15TCE4) with Au 3+ recognition properties was combined with thermo-responsive N ‑isopropylacrylamide (NIPAM) on a silica (SiO 2 ) support via ARGET-ATRP to afford a multi-functional Au 3+ adsorbent P(NIPAM -co- 15TCE4)@SiO 2 . 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)@SiO 2 was observed at critical temperature T c ∼ 34.9 °C. At T = 50 °C ( T > T c ) and pH = 2, the maximum Au 3+ capacity q m = 100.60 mg g −1 was quickly achieved within 2 h. HR-TEM/EDS, WAXD and XPS results indicate that Au 3+ capture involves complexation, partial reduction to Au + and disproportionation to Au 0 /Au 3+ . It is believed that partial Au 3+ reduction is coupled with C-S-C oxidation to R 2 S = 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 α = 9 – 43,053 for Au 3+ against Pd 2+ , Pt 2+ , Cu 2+ , Pb 2+ , Zn 2+ and Ni 2+ present in a simulated mobile phone leachate. The reversible behavior of P(NIPAM -co- 15TCE4)@SiO 2 for Au 3+ 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 Au 3+ recovery in complex feed wastes.