Modeling of Pu(IV) Extraction from Acidic Nitrate Media by Tri-n-butyl Phosphate

Abstract

A thermodynamic model of the distribution of Pu(IV) between aqueous solutions of nitric acid and lithium nitrate and 30 % (by volume) TBP in n-dodecane was developed. The mean activity coefficients of the hydrogen ion, nitrate ion, and water were calculated using Bromley's method of activity coefficients. The computation of the distribution ratios is based on a critical evaluation of the speciation of Pu(IV) under the solution conditions used. Five Pu(IV) species, Pu 4+ , Pu(OH) 3+ , Pu(OH) 2 2+ , Pu(NO 3 ) 3+ , and Pu(NO 3 ) 2 2+ , were considered to be present in (0.1 to 4) mol • L -1 aqueous solutions of HNO 3 . Because of the various extraction capabilities of the different oxidation states of Pu, disproportionation of Pu(IV) is the main factor controlling the distribution of Pu at low acidity. Two different Pu(IV) solvate adducts Pu(NO 3 ) 4 • TBP 2 and Pu(NO 3 ) 4 • TBP 2 • HNO 3 were considered as extracted species over a wide range of experimental conditions, and their extractions constants were determined. The correlation between experimental and calculated data produced a reasonable fit. To determine the extraction constant of hydrolyzed Pu(IV) species for low acid concentrations, additional experimental data on the kinetics of disproportionation of tetravalent plutonium in two phase systems would be necessary.

Unified Framework for Modeling Reactive Extraction of Metals: Illustration on Plutonium(IV) Extraction with Tri-n-butyl Phosphate

“…The equilibrium constants at zero ionic strength for these reactions and the interaction parameters between the different positive ions (Pu 4+ , hydrolyzed Pu species, and Pu–nitrate complexes in eqs –) and nitrate ion in the activity coefficient models have to be provided. Although Tkac et al considered the same set of reactions in their model for Pu­(IV) extraction, the interaction parameter for Pu 4+ and nitrate ion used in their work was drawn from Neck et al, which was considered equivalent to the behavior of Th 4+ . Berg et al provided values for the equilibrium constants at zero ionic strength for the formation reactions of Pu-nitrate complexes (reactions R5 and R6 listed in Table ).…”

“…An alternative approach is to develop a theoretical model for the distribution coefficient in terms of the equilibrium constants of different ionic reactions occurring in the system and concentrations of acid, metal concentration, and the amount of solvent used. ,,, For developing such a model, all the reactions between ionic species and the different organo-metal complexes have to be specified. Additional approximations are made in order to derive an analytical expression for the distribution coefficient in terms of equilibrium constants, parameters of activity coefficient models, and other known quantities.…”

“…It is easier to obtain reliable parameters from experimental data for the subsystems. In general, sufficient experimental data for the overall system are not available for estimating all the parameters of the thermodynamic model. By dividing into appropriate subsystems, experimental data available for these sub-systems are effectively used in estimating the parameters of the overall model. Some of the subsystems (such as the aqueous nitric acid subsystem or the two-phase aqueous HNO 3 and TBP subsystem) are common to several metal extraction processes, ,,,− and therefore, the thermodynamic models for these subsystems have a general utility. …”

“…Some of the subsystems (such as the aqueous nitric acid subsystem or the two-phase aqueous HNO 3 and TBP subsystem) are common to several metal extraction processes, ,,,− and therefore, the thermodynamic models for these subsystems have a general utility.…”

Measurement and modeling for vanadium extraction from the (NaVO3+H2SO4+H2O) system by primary amine N1923

“…Besides, the Margules model and the UNIFAC model were also used to calculate the activity coefficient in organic phase [20]. Paulenova et al determined the extraction constants of hydrolyzed Pu(IV) species for low acid concentrations by TBP with thermodynamic model [21]. Most researchers used the Pitzer equation to calculate the activity coefficient in the aqueous phase.…”