Extraction of Th(Ⅳ) From Nitric Acid-Sodium Nitrate Media With Bis(p-Tert-Butylphenyl)Dithiophosphinic Acid

Thorium, a crucial and prospective nuclear material, whether enriched and extracted from ores or recovered from the complex composition of nuclear waste, is imperative for alleviating pressure on energy resources and sustaining ecological balance. A variety of methods are currently applied within the domain of thorium isolation and purification, among which solvent extraction has the distinction of being the most commonly used and recognized extraction method in the nuclear field. Nevertheless, identifying a suitable extractant continues to pose a significant challenge. Dithiophosphinic acid has been the subject of considerable research interest due to its exceptional ability to selectively separate trivalent actinide ions, An(Ⅲ), from lanthanide ions, Ln(Ⅲ). However, the extraction of tetravalent actinide ions has received comparatively less attention. In order to further evaluate the potential application of those extractants, the extraction behavior of Th(Ⅳ) from nitric acid-sodium nitrate media was investigated using bis(p-tert-butylphenyl)dithiophosphinic acid(HL) in xylene as the organic phase. The effects of HL concentration in the organic phase on the extraction of Th(Ⅳ) were studied, as well as the NaNO₃ concentration in the aqueous phase, the pH of the equilibrated aqueous phase, and the temperature, to determine the composition of the extracted complex and further to reveal the extraction mechanism. In addition, the selectivity for Th(Ⅳ) in coexistence with U(Ⅵ) and Ln(Ⅲ) was analyzed to evaluate its potential application in the advanced nuclear fuel cycle. The extraction is governed by the cation exchange mechanism, described by the equation of \mathrmT\mathrmh^4++4\mathrmH\mathrmL_\left(\mathrmo\right) \rightleftharpoons\;\;\;\;\;\;K_\mathrme\mathrmx \mathrmT\mathrmh\mathrmL_4_\left(\mathrmo\right)+4\mathrmH^+ , with the extracted complex species of ThL₄. At 25 ℃, the extraction equilibrium constant(K_ex) is measured as 10^−7.84, while ΔH^ϴ is 25.8 kJ/mol and ΔS^ϴ is −66.4 J/( mol•K). Moreover, the determined mutual separation factors of Th(Ⅳ), U(Ⅵ) and Ln(Ⅲ) increase with the increasing pH within the experimental pH range. When the pH of the equilibrium aqueous phase is 3.11, the determined SF_{Th(Ⅳ)/Eu(Ⅲ)}, SF_{Th(Ⅳ)/Nd(Ⅲ)}, and SF_{U(Ⅵ)/Th(Ⅳ)} are 4.2×10⁴, 1.1×10⁴, and 1.0×10², respectively. The high selectivity for U(Ⅵ), Th(Ⅳ) and Ln(Ⅲ) demonstrates its potential application in the separation of uranium, thorium and rare earths, providing new insights for the development and exploitation of nuclear energy resources.