Highly Efficient and Selective Palladium Recovery from High-Level Liquid Waste with a Pyrazinamide Ligand: Synthesis, Solvent Extraction, and Complexation Studies

Abstract

Efficient and selective palladium (Pd) recovery from high-level liquid waste (HLLW) constitutes a crucial task for both precious resource recovery and environmental remediation, but remains challenging to achieve high efficiency, sufficient selectivity, and fast kinetics. Herein, we report a new pyrazinamide ligand (denoted as PA) as the first example of employing pyrazine-based ligand for Pd2+ separation with impressive efficiency. Using an ionic liquid C8mimNTf2 as diluent, the developed pyrazinamide-ionic liquid (PA-IL) extraction system exhibited attractive merits for Pd2+ separation such as a wide range of workable acidities from 0.2 M to 5 M HNO3, ultra-fast extraction kinetics with equilibrium time of 3 min, and good Pd2+ selectivity over 16 ions coexisting in simulated HLLW and other non-metallic anions. Slope analysis indicated that the extraction mechanism involves the cation exchange of C8mim+ and the coextraction of NO3- anions in the course of Pd2+ extraction. Results from a set of spectroscopic investigations and single crystal X-ray diffraction analysis suggest the formation of complexes with stoichiometry of both 1:1 and 2:1 (PA/Pd2+) in solution and in solid state, of which the formation of 2:1 complex is the most thermodynamically favored according to DFT theoretical calculations. This work provides a highly promising pyrazinamide-based extraction system for efficient and selective Pd2+ separation from HLLW, and brings more understanding about the complexation and coordination behaviors between N-based ligands and Pd2+ ions.

Keywords: high-level liquid waste, palladium recovery, solvent extraction, pyrazine-based ligand