A combined experimental and theoretical investigation on the recovery of palladium from nuclear waste solutions using phosphonate based solvents

In this study, phosphonate-based solvents, namely dibutylbutyl phosphonate (DBBP) and diamylamyl phosphonate (DAAP), were evaluated for the recovery of Pd from nitric acid medium and their performance was compared with the phosphate-based solvent tributyl phosphate (TBP). The extraction efficiency of DBBP and DAAP was examined as a function of nitric acid concentration in the range of 0.01–6 M, with maximum Pd extraction observed at 0.5 M HNO3. The distribution ratios of Pd for DBBP, DAAP and TBP are 41, 33 and 11, respectively at 0.5 M HNO3 which shows phosphonate based ligands shows higher extraction efficiency than the phosphate based ligands. The nature of the extracted species was characterized using UV–Vis and FTIR spectroscopy. Back-extraction of Pd from the loaded organic phase was investigated using various complexing agents, and thiourea was found to be the most effective one and the stripping efficiency was found to be about 30.63% and 38.89% for DBBP and DAAP, respectively. The extraction behavior of Pd using neat phosphonates was also studied in the presence of Ru and Rh, revealing preferential extraction of Pd, followed by Ru, with negligible extraction of Rh. To support the experimental findings, density functional theory (DFT) calculations were performed to estimate the complexation energies of Pd(II)nitrate complexes. The results showed stronger complexation for phosphonates (~–34 kcal/mol) compared to phosphates (~–31 kcal/mol), which consistent with experimental findings. Energy decomposition analysis indicated that enhanced orbital interactions are responsible for the higher stability of phosphonate complexes, demonstrating their suitability for Pd extraction from high-level waste solutions.