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Studies On The Ion-exange Separation

STUDIES ON THE ION-EXANGE SEPARATION

OF BORON ISOTOPES

V. Atamaniouk, A. Gerasymchuk

V.I. Vernadskii Institute of General and Inorganic Chemistry, Ukrainian National Academy of Sciences, Palladin avenue 32-34, 03680 Kyev, Ukraine

The investigation of ion-exchange separation of boron isotopes 10B and 11B, on our view, is one of the important problems. Naturally occurring boron contains 19.8% 10B and 80.2% 11B. The absorption cross section of natural boron for thermal neutrons is 752 barns; for pure 10B and 11B, the corresponding values are 3837 and 0.005 barns, respectively. This means that isotopically pure 10B is five times more effective as a neutron shield than natural boron. The large cross section of 10B for thermal neutrons and the α particle emission subsequent to neutron capture have suggested the possible use of boron compounds (enriched in 10B) for neutron therapy of malignant tissue, especially for treatment of brain tumors and melanotic cancer.

The first experiment on ion-exchange separation of boron isotopes had been done using a strong-base anion exchange resin (Amberlite CG-400-I) in hydroxide form [1]. It was found that the lighter isotope boron-10 was enriched in the resin phase and that the values of the single-stage separation factors 1011S were 1.010 for aqueous boric acid, and 1.016 for boric acid in 8% glycerol-water solution. This process can be greatly improved by using a weak-base resin in free-base form or a strong-base resin in fluoride form, and thus dispensing with regeneration of the resin in the column [2,3]. The values of the separation factor obtained for a weak-base resins Diaion WA21, WA10 and WA30 were 1011S = 1.016 ÷ 1.019 [4]. It has Recently been shown [5], that on a boron-specific ion exchanger - N-methyl- D-glucamine type resin 1011S = 1027. However, in case of analysis of ion-exchange fractionation of boron isotopes there is a number of the undecided problems. So, pH-dependence of the boron isotope separation factors on N-methyl- D-glucamine resin has more reversed behavior, than on strong and weak base anion exchange resins. Such factors, as interrelation of a structure of boron species in a solution and in a phase of sorbent, influence of an ionic environment in a solution and

nature of sorbent on quantity of a sorption and isotope separation factors of boron 1011S are not carefully analysed.

The purpose of the present research is to determine the structural changes of boron species between the resin and the external solution phase using quantum mechanics calculations and calculations based on a statistical mechanics with quantum correction. These studies were carried out, using the techniques of ab initio quantum mechanics to describe trigonal planar molecules B(OH)3 and a tetrahedral anions B(OH)4¯. The basis 6 – 31 G in an unbounded Hartree – Fok’s method was used. The careful optimization of geometrical parameters and calculation of oscillatory spectra were made on the SCF level. The analysis of an electronic structure together with the analysis of valence oscillatory modes allows assuming, that the boron-isotope exchange reaction:

10B(OH)3 + 11B(OH)4‾ = 11B(OH)3 + 10B(OH)4‾

occurs in aqueous solutions of boric acid. The isotope effect between a trigonal planar structure B(OH)3 and a tetrahedral structure B(OH)4 as calculated from spectroscopic data on molecular vibrations, based on statistical mechanics with quantum corrections, is remarkable. The value of calculated equilibrium constant is 1.0194 at 298.1 K This means that 10B is enriched in the anionic species B(OH)4‾. The nature of particles in a phase of sorbent and in a solution treated depending on concentration and рН of a solution. The complex formation behavior between borates and active groups of resins was investigated.