Mathematical modeling of solvent extraction of uranium from sulphate media employing 2-ethylhexyl phosphonic acid-mono-2-ethylhexyl ester (PC88A) and its mixture with trioctylphosphine oxide (TOPO) as extractants

The extraction of U(VI) from sulphate medium with 2-ethylhexyl phosphonic acid-mono-2-ethylhexyl ester (PC88A, H₂A₂ in dimeric form) in n-dodecane has been investigated under varying concentrations of sulphuric acid and uranium. Slope analysis of uranium (VI) distribution data as a function of PC88A concentration suggests the formation of monomeric species, viz. UO₂(HA₂)₂. This observation was further supported by the mathematical expression obtained during non-linear least square regression analysis of U(VI) distribution data correlating the percentage extraction (%E) and the acidity (H _i). A mathematical model correlating the experimental distribution ratio values of U(VI) (D _U) with initial acidity (H _i) and initial uranium concentrations (C _i) was developed: D_\textU = 12.98( ±0.90)/{ C_\texti ^{- 0.75( ±0.05)} ×[ H_\texti ]² } D_{\text{U}} = 12.98( \pm 0.90)/\left\{ {C_{\text{i}}^{ - 0.75( \pm 0.05)} \times \left[ {H_{\text{i}} } \right]^{2} } \right\} . This expression can be used to predict the concentration of uranium in organic as well as in aqueous phase at any C _i and H _i. The extraction data were used to calculate the conditional extraction constant (K _ex) values at different acidities (2–7 M H⁺), uranium (0.02–0.1 M) and PC88A (0.2–0.6 M) concentrations. These studies were also extended for the extraction of U(VI) using synergistic mixtures of PC88A and TOPO from sulphate medium.     

An unusual extraction behavior of americium (III) and dioxouranium (VI) from picric acid medium using 18-crown-6 as neutral oxodonor

The extraction behavior of Am³⁺ and UO₂ ²⁺ is investigated employing chloroform solution of 18-crown-6 as the organic phase and picrate solution at pH 3.0 as the aqueous phase. In contrast to the commonly observed behavior, the extraction of Am³⁺ is preferred to that of UO₂ ²⁺. This unusual separation behavior is investigated as a function of several variable parameters such as crown ether concentration, picric acid concentration, inert electrolyte concentration, nature of diluent etc. Thermodynamic parameters are also evaluated.     

Chromatographic separation of carrier free 90Y from 90Sr using a diglycolamide based resin for possible pharmaceutical applications

N,N,N',N'-tetraoctyl diglycolamide (TODGA) has been used as the stationary phase in an extraction chromatography resin (XCR) material prepared for evaluating the uptake and the separation behaviour of (90)Y and (90)Sr from acidic feeds. Chromosorb-W was used as the solid support material while the feed solution was usually 4M HNO(3). The batch uptake studies have suggested almost no Sr(II) uptake while Y(III) uptake increased with acidity up to 4M HNO(3) beyond which a decrease in the K(d,w) values were observed. Column studies were carried out and breakthrough profiles were obtained for both Y(III) and Sr(II). No breakthrough of Y(III) was noticed even when >50 column volumes of the feed (carrier free (90)Y at 4 M HNO(3)) was passed through the column while about 20 column volumes were required for the breakthrough of Y(III) when the feed contained 1 g/L Y in 4 M HNO(3) spiked with (90)Y tracer. The reusability of the column was also studied which indicated in the deterioration of the column performance as shown by the sharp fall in the breakthrough volumes and was attributed to the probable leaching of the reagent from the support material. The role of absorbed dose was also investigated for Y(III) uptake. Separation of carrier free (90)Y tracer was carried out by loading the column with (90)Sr and eluting with 0.01M solutions of HNO(3) as well as EDTA. The purity of the product was ascertained by half-life method.     

Redox Behavior of Neptunium(V) in Tributyl Phosphate and <Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>-Dihexyl Octanamide Extractants Dissolved in <Emphasis Type="Italic">n</Emphasis>-Dodecane

Spectrophotometric investigations have been carried out on the disproportionation of Np(V) to form Np(IV) and Np(VI) in 1.1 mol⋅L⁻¹ solutions of tributyl phosphate (TBP) and in N,N-dihexyl octanamide (DHOA) in n-dodecane medium. The Np(V) was found to coordinate with Np(IV) in 1.1 mol⋅L⁻¹ TBP solution in n-dodecane to form a mixed valence “cation–cation” complex by bonding through an axial oxo group on Np(V). By contrast, this interaction was less prominent in the case of 1.1 mol⋅L⁻¹ DHOA solutions. The effect of 1-octanol, added as phase modifier, on the disproportionation behavior of Np(V) was also investigated. An attempt was made to calculate the disproportionation/reduction rate constants for Np(V) under the conditions of these studies. Absorbance measurements on the Np stripped from organic phases revealed the occurrence of Np(V) in the aqueous phase.     

Purification of americium from assorted analytical waste in hydrochloric acid medium

During the simultaneous extraction of plutonium and uranium using anion exchange chromatographic technique from analytical waste in hydrochloric acid medium, ²⁴¹Am which is invariably present in the plutonium bearing fuel samples remains in the effluent. A two step separation scheme was developed for the recovery and purification of Am from the assorted waste to facilitate the disposal of large volume of aqueous waste and the purified Am solution was utilized for spectroscopic investigation. The separation scheme involved solvent extraction using 0.1 M TODGA + 0.5 M DHOA for separation of americium from Fe, Pb, Ni and Na followed by extraction chromatographic technique using CMPO on inert support as stationary phase for separation of Ca from Am. A systematic study on the extraction behavior of Am from hydrochloric acid medium revealed that out of four extraction systems well known for actinide partitioning namely 0.1 M TODGA + 0.5 M DHOA, 1 M DMDBTDMA, 0.2 M CMPO + 1.2 M TBP and 30% TRPO, only 0.1 M TODGA + 0.5 M DHOA extracts americium from 7.5 M HCl feed acidity. A comparative study involving CMPO solvent extraction and column chromatographic technique revealed that elution of Am from column is satisfactory as compared to inefficient stripping of Am from organic phase in solvent extraction technique using 0.1 M HNO₃. The purity of the final solution was checked for 17 elements of interest and was found to be 98% pure, while the overall recovery of this two step separation scheme was found to be 95%.     

Characterization of high level waste for minor actinides by chemical separation and alpha spectrometry

The high level waste (HLW) generated from the reprocessing of the spent fuel of pressurized heavy water reactor has been characterized for the minor actinides. The radiation dose of the waste solution was reduced by radiochemical separation of cesium from HLW by solvent extraction with chlorinated cobalt dicarbollide dissolved in 20% nitrobenzene in xylene. Minor actinides (Np, Pu, Am, Cm) in the high level waste were assayed by alpha spectrometry following radiochemical separation. The gross alpha activity determined by liquid scintillation agrees well (within 10%) with the cumulative quantities of actinides determined by alpha spectrometry.     

Extraction studies of uranium into a third-phase of thorium nitrate employing tributyl phosphate and <Emphasis Type="Italic">N,N</Emphasis>-dihexyl octanamide as extractants in different diluents

Extraction behavior of 1 × 10⁻²–0.1 M U(VI) from aqueous phases containing 0.86 M Th(IV) at 4 M HNO₃ in 1.1 M tributyl phosphate (TBP) and 1.1 M N,N-dihexyl octanamide (DHOA) solutions in different diluents viz. n-dodecane, 10% 1-octanol + n-dodecane, and decahydronaphthalene (decalin) was studied. Third-phase formation was observed in both the extractants using n-dodecane as diluent. There was a gradual decrease in Th(IV) concentration in the third-phase (heavy organic phase, HOP) with increased aqueous U(VI) concentration [0.71 M (no U(VI))–0.61 M (0.1 M U(VI)) for TBP; 0.27 M (no U(VI))–0.22 M (0.1 M U(VI)) for DHOA]. The HOP volume in case of DHOA was ~2.2 times of that of TBP. Uranium concentration in HOP increased with its initial concentration in the aqueous phase [from 1.8 × 10⁻² M (0.01 M U(VI))–0.162 M (0.1 M U(VI)) for TBP; from 1.4 × 10⁻² M (0.01 M U(VI))–0.14 M (0.1 M U(VI)) for DHOA] suggesting that Th(IV) was being replaced by U(VI). An empirical correlation was developed for predicting the concentrations of uranium and thorium in HOP for both the extractants. No third-phase appeared during the extraction of uranium and thorium from the aqueous phases employing 10% 1-octanol + n-dodecane, or decalin as diluents, and therefore, were better choices as diluent for alleviating the third-phase formation during the reprocessing of spent thorium based fuels, and for the recovery of thorium from high-level waste solutions.