Solvent extraction behaviors of rhodium and palladium in nitric acid solutions using amide extractants

Ito, Kengo*; Morita, Misaki*; Araki, Yuta*; Kato, Chizu*; Fukutani, Satoshi*; Matsumura, Tatsuro  ; Fujii, Toshiyuki*

Rhodium (Rh) and Palladium (Pd) in high-level radioactive waste are primarily fission products. This Study focused on understanding the extraction behavior of these platinum group elements (PGEs) using the novel extractants -hexaoctylnitriloacetamide (HONTA) and alkyl diamideamine (ADAAM). Both extractants showed affinity for Pd, with distribution coefficients significantly exceeding 1, demonstrating their effectiveness in Pd separation. In contrast, the distribution coefficients for Rh were consistently below 10 , indicating low extraction efficiency from nitric acid. However, by leveraging the salting-out effect with calcium nitrate hydrate, a distribution coefficient of 570 for Rh was achieved using HONTA. To overcome the difficult back-extraction of PGEs with HONTA, experiments were conducted using HEDTA and thiourea. Back-extraction with HEDTA in high-concentration nitric acid (2M) resulted in 90% extraction of Pd, while thiourea-based back-extraction with nitric acid yielded over 40% extraction for Rh, with the maximum of 62.7% achieved using hydrochloric acid.