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Abstract

Thermal decomposition of neat TBP, acid-solvates (TBP·1.1HNO3, TBP·2.4HNO3) (prepared by equilibrating neat TBP with 8 and 15.6 M nitric acid) with and without the presence of additives such as uranyl nitrate, sodium nitrate and sodium nitrite, mixtures of neat TBP and nitric acid of different acidities, 1.1 M TBP solutions in diluents such as n-dodecane (n-DD), n-octane and isooctane has been studied using an adiabatic calorimeter. Enthalpy change and the activation energy for the decomposition reaction derived from the calorimetric data wherever possible are reported in this article. Neat TBP was found to be stable up to 255 °C, whereas the acid-solvates TBP·1.1HNO3 and TBP·2.4HNO3 decomposed at 120 and 111 °C, respectively, with a decomposition enthalpy of −495.8 ± 10.9 and −1115.5 ± 8.2 kJ mol−1 of TBP. Activation energy and pre exponential factor derived from the calorimetric data for the decomposition of these acid-solvates were found be 108.8 ± 3.7, 103.5 ± 1.4 kJ mol−1 of TBP and 6.1 × 1010 and 5.6 × 109 S−1, respectively. The thermochemical parameters such as, the onset temperature, enthalpy of decomposition, activation energy and the pre-exponential factor were found to strongly depend on acid-solvate stoichiometry. Heat capacity (C p), of neat TBP and the acid-solvates (TBP·1.1HNO3 and TBP·2.4HNO3) were measured at constant pressure using heat flux type differential scanning calorimeter (DSC) in the temperature range 32–67 °C. The values obtained at 32 °C for neat TBP, acid-solvates TBP·1.1HNO3 and TBP·2.4HNO3 are 1.8, 1.76 and 1.63 J g−1 K−1, respectively. C p of neat TBP, 1.82 J g−1 K−1, was also measured at 27 °C using ‘hot disk’ method and was found to agree well with the values obtained by DSC method.

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Abstract  

High-level liquid waste from fast reactor fuel reprocessing stream contains significant quantities of lanthanides and trivalent minor actinides. The lanthanides and minor actinides (MA) have been separated from the fast reactor high-level liquid waste (FR-HLLW) using TRUEX solvent, which is a mixture of 0.2 M octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO)-1.2 M tri-n-butylphosphate (TBP) in n-dodecane. A new stripping composition, 0.1 M HNO3 and 0.1 M citric acid (CA), has been employed for back extraction of them from the TRUEX solvent. In order to separate lanthanides from actinides present in the strip solution, the extraction behavior of 241Am(III) and (152+154)Eu(III) from CA–HNO3 medium by a solution of bis-2-ethylhexylphosphoric acid (HDEHP) in n-dodecane has been studied. Separation factors (SF = D Eu/D Am) has been reported as a function of various parameters such as pH, concentrations of HDEHP, diethylenetriamine-N,N,N′,N′′,N′′′-pentaaceticacid (DTPA), 1-octanol and TBP in this paper.

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Summary  

A systematic study on the extraction of U(VI) from nitric acid medium by tri-n-butylphosphate (TBP) dissolved in a non-traditional diluent namely 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) ionic liquid (IL) is reported. The results are compared with those obtained using TBP/n-dodecane (DD). The distribution ratio for the extraction of U(VI) from nitric acid by 1.1M TBP/bmimPF6 increases with increasing nitric acid concentration. The U(VI) distribution ratios are comparable in the nitric acid concentration range of 0.01M to 4M, to the ratios measured using 1.1M TBP/DD. In contrast to the extraction behavior of TBP/DD, the D values continued to increase with the increase in the concentration of nitric acid above 4.0M. The stoichiometry of uranyl solvate extracted by 1.1M TBP/IL is similar to that of TBP/DD system, wherein two molecules of TBP are associated with one molecule of uranyl nitrate in the organic phase. Ionic liquid alone also extracts uranium from nitric acid, albeit to a small extent. The exothermic enthalpy accompanying the extraction of U(VI) in TBP/bmimPF6 decreases with increasing nitric acid and with TBP concentrations.

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Cereal Research Communications
Authors: M. Sivasamy, M. Aparna, J. Kumar, P. Jayaprakash, V.K. Vikas, P. John, R. Nisha, K. Srinivasan, J. Radhamani, S.R. Jacob, S. Kumar, Satyaprakash, K. Sivan, E. Punniakotti, R.K. Tyagi, and K.C. Bansal
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