The role of β, γ, δ-substituent of dialkylphosphinic acids in HREE extraction and separation behaviors

Heavy rare earth element (HREE) separation has always been a great challenge. The kind of dialkylphosphinic acids is recognized as the third generation of organophosphorous acid extractants. However, detailed relationship between β -substituent (especially when it changes from H to methyl, and further to ethyl) and their extraction behaviors and selectivity for HREEs have rarely been reported. Besides, there are also few reports on effect of γ , δ -substituent on HREE extraction and separation. In this paper, we synthesized five and collected two dialkylphosphinic acids with different β , γ , δ -substituents (from H to ethyl). These extractants were divided into two groups to discuss their extraction behaviors and selectivity for HREEs: β -substituent group (P208, INET-1, P218, USTB-1 and P227) and γ , δ -substituent group (P218, P2132 and Cyanex 272). To evaluate and compare their extractabilities, P507 was used as a reference and Tm, Yb, Lu were extracted with 0.01 mol/L of these extractants from single Tm, Yb and Lu solutions (∼4 × 10 −4  mol/L) at different initial pH. Besides, pH 0.5 Tm , pH 0.5 Yb and pH 0.5 Lu values and their differences (ΔpH 0.5 Tm–Yb , ΔpH 0.5 Yb–Lu , ΔpH 0.5 Tm–Lu ) for each extractant were given to roughly evaluate their separation performance for Tm, Yb and Lu. To further investigate the selectivity of these extractants for HREEs, Tb, Dy, Ho, Er, Tm, Yb and Lu were extracted from their mixed solution with initial pH of 4.00 at phase ratio A/O of 2:1, 1:1 or 1:2. Their HREE separation performance is mainly discussed based on the adjacent HREE separation factors ( β N+1/N ) and their average value ( β T b − L u ¯ ).