Selective extraction of Pd(II) using 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetra[(2-ethylthio)ethoxy]thiocalix [4]arene as extractant

In this study, a novel method for selective recovery of palladium from chloride systems has been proposed based on supramolecular chemistry. 5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetra[(2-ethylthio)ethoxy]thiocalix[4] arene (TTCA), which is expected to have excellent complexation ability to Pd(II) was synthesized and further used as the extractant for palladium uptake from the aqueous phase. The effects of various conditions on the complexation process between TTCA and Pd(II) were systematically investigated through liquid-liquid extraction experiments. The Pd(II) extraction efficiency of up to over 99 % was reached at Cl- concentration of 0.1 mol/L, TTCA/Pd(II) mole ratio of 3:1, and reaction time of 2.0 h. The remaining palladium in the aqueous phase after extraction was below 0.1 ppm. The results of Job's method and 1H NMR titration showed that TTCA and Pd(II) were mainly complexed in 2:3 during the extraction process. TTCA showed good selectivity toward Pd(II) in the presence of completing ions. Particularly for the most competing element of platinum, the Pd-Pt separation coefficient reached 4 x 104 level. Moreover, palladium can be effectively stripped from the loaded organic phase by ammonia. The extraction efficiency of Pd(II) maintained at a high level of around 96 % after five extraction -stripping cycles, indicating the excellent reusability of TTCA in Pd(II) extraction from chloride systems.