Elsevier

Journal of Alloys and Compounds

Volumes 444–445, 11 October 2007, Pages 647-651
Journal of Alloys and Compounds

Fluid dynamics simulation of highly loaded anion-exchange chromatography of Np(IV) based on adsorption isotherm determined by 237+239Np

https://doi.org/10.1016/j.jallcom.2007.05.101Get rights and content

Abstract

In order to investigate the optimum condition for anion-exchange chromatography for purification and recovery of actinide(IV) constituting transuranium elements, a convective-diffusion equation model treating mass balance and Langmuir-type kinetics in porous system, which was developed for thorium(IV) by us, was applied to neptunium(IV). Absorption isotherm of neptunium(IV) to anion-exchange (MSA-1) resin was carried out by using 237+239Np in 6N HNO3 media and hereby parameters of the Langmuir-type kinetics were determined as k0 = 2.5 × 103 and smax = 1.0. Accompanied with the fluid dynamics parameters already determined for the column system used for 227+232Th, elution curves of neptunium(IV) at highly loaded condition were estimated by the numerical calculation. According to the result, the loading of more than 10% of resin capacity leads to rapid breakthrough and severe tailing of neptunium which lowers purity and yield in the purification procedure. This numerical calculation will serve as a valuable measure to figure out column operation conditions for purification and recovery of transuranium elements.

Introduction

Metals of transuranium elements with very high purity are essential for reliable studies of the solid-state physics of transuranium compounds, especially of physical phenomena properties such as the de Haas–van Alphen effect where the infinitesimal quantity of impurities interfere the observation of the signal [1], [2], [3]. For neptunium compounds, the purity of prepared compounds is governed by the purity of the starting metal which is produced from the supplied neptunium oxide whose purity is as low as 99.9%. In order to prepare gram-amount neptunium metals, our research group developed a new method which only requires an aqueous acidic solution of the starting metal ion [4], [5]. This method is especially suitable for preparation of highly radioactive neptunium metal at high purity. Due to the cost of man-made element and the strict management under the nonproliferation policy, there is an increased need for purification, reuse and recovery of the neptunium element from used compounds or residues. The most suitable method for the extensive purification prior to the metal preparation is the anion-exchange chromatography because the stable formation of hexanitrate anion An(NO3)62− is characteristic to the actinide (An) chemistry [6].

The distribution constants KD of Np(IV) to anion-exchange resin in nitric acid was determined to show the maximum value of KD > 103 at the nitric acid concentration of 5–10N [7]. At tracer scale the large value of KD enables us to purify neptunium(IV) easily due to its exclusively large distribution to resin in nitric acid. However, with increasing amount of loading the value of KD decreases and the purification becomes difficult. For example of thorium(IV) chromatography, when the amount of thorium loaded onto the column is increased, peak position moved forward and distribution coefficient decreased substantially [8]. Simultaneously, the elution curves changed from Gaussian-type curves to frontal peaks with tailings.

In preparatory chromatography, it is essential to figure out column operation condition by which all of high throughput, high purity and high yield are achieved [9]. Experimental optimization of the column operation condition for neptunium, however, is prohibitedly difficult due to long retention time and large cost due to large amount (150–200 times of column volume) of radioactive effluent. Therefore, it is great helpful to use numerical calculation of elution curves for the determination of the column operation condition of the preparatory anion-exchange chromatography. For organic compounds such as medicinal compounds, the numerical calculation of elution curves for the estimation of the column operation condition is widely used for purification [10]. Therefore, large efforts have been devoted to study of the numerical calculation based on adsorption isotherm for non-linear chromatography at highly loaded state [9], [11], [12]. However, to the best of our knowledge, no theoretical study on the preparatory ion-exchange chromatography of inorganic compounds has been reported.

Recently, we have developed a theory and a numerical calculation method of preparatory anion-exchange chromatography of thorium(IV) hexanitrate anion [13], which are based on fluid dynamics in porous systems [14], [15] (Scheme 1). The dimensionless and normalized form of the advection-diffusion equation is given byCT+1εεST=F0ε2CZ2UεCZwhere F0 is the Fourier number and T, Z, U, C and S are the dimensionless parameters of time, axis along column, flow rate, concentrations in mobile and in stationary phases, respectively. The mathematical model of adsorption equilibrium and kinetics proposed by James [16] was modified by Bain [17]. The adsorption site consisting of cation exchange site (R+) is treated not as two molecules (R+ + R+) but as a single molecule (R22+) which react with the hexanitrate anion of thorium(IV) [6] to produce the adsorbed product [Th(NO3)6]R2. The adsorption kinetics is thus assumed to be the first order and adsorption (ks) and desorption (kd) rate constants can be defined and the Langmuir-type isotherm is obtained as [17], [18], [19]:1s=1smax1k0c+1was combined with Eq. (1), where k0 = ks/kd is the distribution coefficient, smax is the maximum adsorption concentration. In order to apply this numerical model to preparatory chromatography of neptunium(IV), the parameters of the Langmuir-type isotherm (k0 and smax) for neptunium(IV) are required to be determined corresponding to a combination of specific resin and specific nitric acid concentration and fine tuning of the parameters of D and ks for neptunium(IV) in specific column with specific mobile phase are required. Considering the similarity among actinide(IV), the values of D and ks for the neptunium(IV) system can be assumed to be identical to those for the thorium(IV) system used in the previous study[13].

In the present study, adsorption isotherm of neptunium(IV) to anion-exchange resin in highly loaded conditions was determined in a distribution experiment carried out by using 237+239Np in 6N HNO3 media and parameters of the Langmuir-type kinetics were determined as k0 = 2.5 × 103 and smax = 0.5. Numerical calculations of elusion curves of neptunium(IV) were carried out on the basis of this adsorption isotherm, as well as fluid dynamics parameters already determined for the column system used for 227+232Th. Finally, elution curves of anion-exchange chromatography at heavily loaded conditions of neptunium(IV) were calculated numerically and discussed.

Section snippets

Preparation of 237Np(IV) stock solution in 6 M HNO3

In a glove box with subatmospheric pressure, neptunium dioxide (237NpO2) 8.3558 g was dissolved in 6 mol dm−3 nitric acid and the solution volume was 57 mL. A portion of the neptunium(V) solution (0.750 mL; 4.1 × 10−4 mol) was transferred to a vial container. The 237Np solution was heated and its volume was reduced to 20 mL. Addition of aqueous ammonia leaded to formation of hydroxide precipitation of neptunium(V) (brownish-red, soggy precipitate). After matured for 30 min, the precipitation was

Adsorption isotherm

This is the first determination on the distribution to the MSA-1 macroporous resin. Distribution coefficients (KD) determined along with elapsed time are shown in Table 1. The value of KD at tracer scale is 2.5 × 103, which can be compared with the value of other anion-exchange resin of >103 to Dowex 1 × 4 resin at 5–10N of nitric acid concentration [7]. This value of KD is stable up to 0.47 day but decreases to 2.0 up to 0.91 day. This decrease in the distribution constants may be attributed to

Acknowledgements

We would like to thank Prof. H. Yamana of Kyoto University for his encouraging discussions and to Mr. K. Shirasaki for his helpful assistance in this work. We wish to acknowledge valuable discussions on numerical calculations with Prof. Y. Niibori of Tohoku University and Mr. H. Iwase of Nippon Sheet Glass, Co., Ltd. This work was performed also at the Irradiation Experimental Facility, IMR, Tohoku University.

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