Yuanzineng kexue jishu (Dec 2023)
Separation of Ln(Ⅲ)/An(Ⅲ) by Using 2,2′-bipyridine and Bis(2,4,4-trimethylpentyl)dithiophosphinic Acid Synergistic System
Abstract
Bis(2,4,4-trimethylpentyl)dithiophosphinic acid (HA) has excellent performance in the separation of trivalent actinides and lanthanides, but it still has some problems in practical application, such as the high requirements towards the control of system acidity and the strict operation window. In order to solve these problems, the extraction/separation of trivalent actinides and lanthanides, Am(Ⅲ), Cm(Ⅲ), Nd(Ⅲ), and Eu(Ⅲ), was investigated in synergistic extraction system of HA with 2,2′-bipyridine (B). The effect of pH value of the aqueous phase, concentration of 2,2′-bipyridine, and temperatures on the extraction ability and separation selectivity was explored. It is found that 2,2′-bipyridine significantly contributes to the extraction capability and separation selectivity to the metal ions for HA. With 0.05 mol/L B and 0.5 mol/L HA in kerosene, the synergistic system gives the values of 2.52, 2.73, 3.46, and 3.79 for the half-extraction pH for Am(Ⅲ), Cm(Ⅲ), Nd(Ⅲ), and Eu(Ⅲ), respectively, which are lower about 0.6 than those using 0.5 mol/L HA-kerosene as organic phase without 2,2′-bipyridine. Accordingly, the significantly improved separation factors (SF) are calculated to be 5.64, 2.51×104 and 2.61×103 for SFAm/Cm, SFAm/Eu, and SFAm/Nd, respectively. With enough 2,2′-bipyridine in the organic phase, only one extracted complex, MA3B, is formed in the synergistic system. Enthalpies （ΔH-） for the exothermic extraction of Am(Ⅲ), Cm(Ⅲ), Nd(Ⅲ), and Eu(Ⅲ) are determined to be -58.78, -56.48, -26.69, and -20.26 kJ/mol, respectively. The extraction capabilities for Am(Ⅲ), Cm(Ⅲ), Nd(Ⅲ), Eu(Ⅲ) and separation factors for Am/Cm, Nd/Eu decrease with elevated temperatures. The system changes from endothermic reaction to exothermic reaction after adding 2,2′-bipyridine. The ΔH-An decreases more significantly than ΔH-Ln, which provides the evidence of the covalent stability of extracted complex of An(Ⅲ) in the synergistic system is stronger than the extracted complex of Ln(Ⅲ), and shows that the An(Ⅲ)/Ln(Ⅲ) separation selectivity of the synergistic system is better. And entropies (ΔS-) for the exothermic extraction of Am(Ⅲ), Cm(Ⅲ), Nd(Ⅲ), and Eu(Ⅲ) are determined to be -330.90, -327.72, -287.51, and -255.58 J/(mol·K), respectively. The entropies of the synergistic system significantly reduce compared with the system without 2,2′-bipyridine. The extraction reaction of the synergistic system is an enthalpy-driven reaction. Therefore, compared with the purified Cyanex 301 extraction system, the synergistic extraction and separation of trivalent lanthanides and actinides can be carried out under relatively low pH conditions by using 2,2′-bipyridine or similar synergistic extractant with purified Cyanex 301, which is conductive to industrial implementation. Furthermore, due to the good separation selectivity for americium over curium of this system, a simple step-by-step stripping can be used after the separation of trivalent lanthanide and actinide to effectively achieve the americium and curium separation, which has a promising industrial application prospects.
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