Marian Borkowski

Third Phase Formation Revisited: The U(VI), HNO3–TBP, n‐Dodecane System

Abstract In this work, the system U(VI), HNO3–tri‐n‐butylphosphate (TBP), n‐dodecane has been revisited with the objective of gaining information on the coordination chemistry and structural evolution of the species formed in the organic phase before and after third phase formation. Chemical analyses, spectroscopic and EXAFS data indicate that U(VI) is extracted as the UO2(NO3)2·2TBP adduct, while the third phase species have the average composition UO2(NO3)2·2TBP·HNO3. Small‐angle neutron scattering (SANS) measurements on TBP solutions loaded with only HNO3 or with increasing amounts of U(VI) have revealed the presence, before phase splitting, of ellipsoidal aggregates with the major and minor axes up to about 64 and 15 Å, respectively. The formation of these aggregates, very likely of the reverse micelle‐type, is observed in all cases, that is, when only HNO3, only UO2(NO3)2, or both HNO3 and UO2(NO3)2 are extracted by the TBP solution. Upon third phase formation, the SANS data reveal the presence of smaller aggregates in the light organic phase, while the heavy organic phase contains pockets of diluent, each with an average of about two molecules of n‐dodecane. #Work performed under the auspices of the U. S. Department of Energy, Office of Basic Energy Science, Division of Chemical Science (for the part performed at the Chemistry Division of ANL) and Division of Material Science (for the part performed at INPS), under contract No. W‐31‐109‐ENG‐38. §The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory (“Argonne”) under Contract No. W‐31‐109‐ENG‐38 with the US Department of Energy. The US Government retains for itself, and others acting on its behalf, a paid‐up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

Sans Study Of Third Phase Formation In The Th(iv)-hno3/tbp-n-octane System

Formation of a third organic phase at high metal loading in the extraction of tetravalent actinides by TBP in aliphatic diluents has been investigated mostly from the standpoint of the composition of the organic phase species before and after phase splitting. Very little is known of the structure and morphology of the organic phase species. In this work, a study of third phase formation upon either dissolution of Th(NO3)4 in 20% TBP in n-octane or Th(NO3)4 extraction from 1 M HNO3 by 20% TBP in n-octane is reported. Chemical analyses have shown that, under the conditions of this work, Th(IV) exists in the organic phase mainly as the trisolvate Th(NO3)4·(TBP)3. The third phase species also contains a small amount of HNO3, presumably hydrogen-bonded to the trisolvate complex. Small-angle neutron scattering measurements on TBP solutions loaded with only HNO3 or with increasing amounts of Th(IV) revealed the presence, before phase splitting, of large ellipsoidal aggregates with the parallel and perpendicular axes having lengths up to about 230 and 24 Å, respectively. Although the formation of these aggregates is observed in all cases, that is, when only HNO3, only Th(NO3)4, or both HNO3 and Th(NO3)4 are extracted by the TBP solution, the size of the aggregates is largest in the latter case. Formation of these aggregates is probably the main reason for phase splitting. #Work performed under the auspices of the U. S. Department of Energy, Office of Basic Energy Science, Division of Chemical Science (for the part performed at the Chemistry Division of ANL) and Division of Material Science (for the part performed at INPS), under contract No. W-31-109-ENG-38. Accordingly, the U.S. Government retains a non-exclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes.

Interpretation of Third Phase Formation in the Th(IV)–HNO3, TBP–n‐Octane System with Baxter's “Sticky Spheres” Model

Abstract Small‐angle neutron scattering (SANS) data for the tri‐n‐butylphosphate (TBP)–n‐octane, HNO3–Th(NO3)4 solvent extraction system, obtained under a variety of experimental conditions, have been interpreted using two different models. The particle growth model led to unrealistic results. The Baxter model for hard‐spheres with surface adhesion, on the other hand, was more successful. According to this model, the increase in scattering intensity in the low Q range observed when increasing amounts of Th(NO3)4 are extracted into the organic phase, has been interpreted as arising from interactions between small reverse micelles containing three TBP molecules. Upon extraction of Th(NO3)4, the micelles interact through attractive forces between their polar cores with a potential energy of up to about 2 kBT. The intermicellar attraction, under suitable conditions, leads to third phase formation. Upon phase splitting, most of the solutes of the original organic phase separate in a continuous phase containing interspersed layers of n‐octane. #The submitted article has been created by the University of Chicago as Operator of Argonne National Laboratory (“Argonne”) under Contract No. W-31-109-ENG-38 with the US Department of Energy. The US Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

FT-IR STUDY OF THIRD PHASE FORMATION IN THE U(VI) OR Th(IV)/HNO3, TBP/ALKANE SYSTEMS

ABSTRACT The infrared reflectance spectra of the third phases formed in the systems UO2(NO3)2/HNO3/20%TBP in n-dodecane and Th(NO3)4/HNO3/20%TBP in n-octane gave evidence for the presence in solution of a significant amount of weakly bonded molecular nitric acid. From the correlation between the ratio of the areas of the bands at 1672 cm−1 and 1648 cm−1, characteristic of weakly intermolecularly hydrogen-bonded nitric acid and nitric acid strongly bonded to TBP, respectively, the molecular HNO3 concentration was determined. The presence of these two bands in the spectra of the third phase samples provides evidence that only part of the HNO3 is directly and strongly bound to the TBP phosphoryl group. The ratio of the weakly intermolecularly hydrogen-bonded HNO3 to that bound directly to P=O group of TBP was much higher for the uranium than for the thorium third phases formed under comparable conditions. The estimated amounts of the weakly intermolecularly hydrogen-bonded HNO3 were about 47% and 30% of the total HNO3 present in the uranium and thorium systems, respectively. In the uranium third phase, the TBP hemisolvate of HNO3 (TBPċ2HNO3) was recognized as the predominant species with accompanying very small amount of monosolvate (TBPċHNO3). In the thorium system the hemisolvate of HNO3 was also present, but the monosolvate was found to be the major species. When the thorium concentration in the third phase was increased, a conversion of monosolvate into hemisolvate was observed. Analysis of the infrared spectra for both systems indicated that the nitrate anions form bidentate chelates with the studied metals. The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory (“Argonne”) under Contract No. W-31-109-ENG-38 with the U.S. Department of Energy. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.†Corresponding author. E-mail: borkowski@anchim.chm.anl.gov

Sans Study Of Third Phase Formation In The U(vi)-hno3/tbp-n-dodecane System

In spite of its technological importance, third phase formation in the extraction of hexavalent actinides from nitric acid solutions into alkane solutions of tri-n-butylphosphate (TBP) has received only limited attention. The focus of the few available literature works has been primarily centered on the composition of the third phase and on the stoichiometry of the metal complexes. Very little is known, on the other hand, about the structure and morphology of the third phase species of hexavalent actinides. In the present investigation, the formation of a third phase upon extraction of U(VI) by 20% TBP in deuterated n-dodecane from nitric acid solutions was studied. Chemical analyses have shown that U(VI) exists in the third phase as a species having the composition UO2(NO3)2·(TBP)2·HNO3. Small-angle neutron scattering measurements on TBP solutions loaded with only HNO3 or with increasing amounts of U(VI) have revealed the presence, both before and after phase splitting, of relatively large ellipsoidal aggregates with the parallel and perpendicular axes having lengths up to about 64 and 15 Å, respectively. The formation of these aggregates is observed in all cases, that is, when only HNO3, only UO2(NO3)2, or both HNO3 and UO2(NO3)2 are extracted by the TBP solution. Upon third phase formation, the SANS data reveal the presence of smaller aggregates in both the heavy and light organic phase. #Work performed under the auspices of the U. S. Department of Energy, Office of Basic Energy Science, Division of Chemical Science (for the part performed at the Chemistry Division of ANL) and Division of Material Science (for the part performed at INPS), under contract No. W-31-109-ENG-38. Accordingly, the U.S. Government retains a non-exclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes.

FT-IR SPECTROSCOPY OF NITRIC ACID IN TBP/OCTANE SOLUTION*

Infrared studies for the HNO3/0.73 M TBP n-octane system are reported. Two extracted species, TBP · HNO3 and TBP · 2HNO3, were identified in the organic phase. The concentration of the individual species was determined by the analysis of the vibrational band at ∼1650 cm−1. The band at 1648 cm−1 was assigned to the monosolvate TBP · HNO3 and the band at 1672 cm−1 to the hemisolvate TBP · 2HNO3. The infrared spectra revealed that with respect to the P═O bond, as well to each other, the HNO3 molecules in the hemisolvate are spectrally non-equivalent. The predominant structure of TBP · 2HNO3 involves the chain HNO3 dimer. Some ionic NO3 − and hydronium ions were identified in this system but only during formation of the monosolvate. The analyses performed in this system can serve for the characterization of HNO3 in related systems in the presence of metal species. *Work performed under the auspices of the office of Basic Energy Sciences, Division of Chemical Sciences, Department of Energy, under contract W-31-109-ENG-38.

Oxidative Leaching of Plutonium from Simulated Hanford Tank‐Waste Sludges

Abstract The behavior of surface‐sorbed plutonium during leaching of four Hanford tank waste sludge simulants with alkaline permanganate solutions has been investigated. The sludge simulants are representative of materials created as a result of the operation of the Bismuth Phosphate (sludges B and M), PUREX (sludge P), and REDOX (sludge R) processes at Hanford. Leaching with oxidants under alkaline conditions has been proposed as a means of reducing the chromium content of actual sludge samples prior to their vitrification. With identical amounts of plutonium deposited on each sludge sample, the percentage of radiotracer Pu leached from the sludges increased in the order P≥R>B. This order is the reverse of the chromium content of the sludges. The percentage of plutonium leached was independent of its initial oxidation state. The reverse trends of chromium content and plutonium leaching can be understood in terms of Pu re‐adsorption onto the MnO2 generated as Cr(III) (and other substrates) are oxidized by permanganate/manganate. The effect of chelating agents that are known to be present in tank wastes (EDTA, gluconate, glycolate, oxalate, and citrate) on KMnO4 reduction/MnO2 production and redeposition of plutonium was also studied and found to have varied effects on the process of oxidative dissolution/Pu redeposition. Overall, it appears that the MnO2 produced as a byproduct of oxidation helps to control Pu concentrations in the leachate phase. Oxidation of Pu to the hexavalent state without concomitant production of MnO2 leads to greater plutonium content in the leachate. The results are discussed, emphasizing the potential impact of oxidative leaching on plutonium mobilization from actual tank‐waste sludge. Work performed under the auspices of the Environmental Management Science Program (EMSP) of the U.S. Department of Energy at Argonne National Laboratory under contract number W‐31‐109‐ENG‐38

Actinide Complexes in Hydrometallurgical Separations: Observations on Complexation and Solvation

Though other approaches have positive features and could eventually supplant hydrometallurgical separations, solvent extraction and ion exchange (and related techniques) are technologically the most important methods for actinide processing and analysis, and likely will remain so for the next few decades. From uranium mining to actinide transmutation, increased understanding of the fundamental interactions between actinide cations, chelating agents, oxidizing and reducing agents, and the solvents (aqueous and non-aqueous) that serve as the medium for chemical manipulations is essential if these techniques are to advance in the 21st century. Research continues around the world in this field, principally in those countries actively involved in (or considering) actinide partitioning and recycle. In this presentation, the results of a variety of investigations designed to provide new insights into the nature of actinide interactions with solutes and solvents will be presented. Among the key issues discussed will be aspects of the thermodynamics and kinetics of actinide-ligand interactions, of structural features of actinide complexes in solutions, and of the nature of interactions of free actinide cations and their complexes with solvent molecules. Each of the systems discussed will have some significance in actinide separations science with primary emphasis on solvent extraction and ion exchange. Work performed under the auspices of the US Department of Energy Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences under contract number W-31-109-ENG-38.