Mechanistic insights into La(III) extraction using HDEHP: toward effective rare earth recovery from radioactive waste

Abstract

Abstract The rare earth element (REE) recovered from radioactive wastewater is of great significance for the resource sustainability, and radioactive waste minimization and management. In this study, the light REE La selected as an example, the extraction behavior of La(III) has been studied systematically using bis(2-ethylhexyl) phosphate (HDEHP) as extractant, optimizing operational parameters and elucidating the underlying mechanism for REE recovery. Optimal conditions yielded an extraction ratio exceeding 98 %, achieved at the contact time of 30 min, phase ratio of 1:1, and HDEHP concentration of 0.1 mol L −1 . Increasing HCl concentration was observed to suppress extraction, whereas elevated temperatures benefited the performance supported by thermodynamic analysis this was a spontaneous (Δ G = −8.80 kJ mol −1 ) and endothermic process. Mechanistic investigations via FT-IR spectroscopy and slope method analysis confirmed La(III) coordination with the P=O double bond of HDEHP, forming stable complex with stoichiometric ratio of 3:1 (HDEHP:La 3+ ), and the La(III) was extracted through cation exchange. These findings not only deepen the theoretical understanding of La(III) extraction, but also provide guidelines for the REEs recovery processes in the radioactive wastewater treatment.