Carbonate Complexation of Neptunyl(VI) Ion

Kato, Yoshiharu; Kimura, Takaumi; Yoshida, Zenko; Nitani, Noriko

doi:10.1524/ract.1998.82.special-issue.63

Cited by 11 publications

(12 citation statements)

References 22 publications

(40 reference statements)

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“…Therefore, Si are stability constants of the Np(VI) tricarbonate and silicate complexes: we determined using the reference values of dissociation constants of carbonic acid, K 1 = 4.45 0 10 37 and K 2 = 4.69 0 10 311 , and metasilicic acid, K 1 = 2.2 0 10 310 and K 2 = 1.6 0 10 312[9], which were recalculated to the ionic strength of 0.1 by the Davies equation[10]. The b 3 value was taken from[11] (log b 3 = 20.41 at an ionic strength of 0.1). The stability constant of the Np(VI) silicate complex we calculated using measured values of pH and known concentrations of carbonate and silicate ions in the solution.…”

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Behavior of Np(VII, VI, V) in silicate solutions

et al. 2004

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Properties of Np(VII, VI, V) in silicate solutions were studied spectrophotometrically. In noncomplexing media, the Np(VII) cation transforms into the anionic species at pH 5.5!7.5. In the presence of carbonate ions, this rearrangement occurs at pH 10 !11.5, and in silicate solutions, at pH 10.5!12.0. These data show that Np(VII) cation forms complexes with carbonate and silicate ions, the latter being stronger. From the competitive reactions of Np(VI) complex formation with carbonate and silicate ions, the stability of NpO 2 SiO 3 complex was estimated (log > = 16.5) using the known stability constant of NpO 2 (CO 3 ) 3 4! . Complexation of Np(V) with SiO 3 2! ions was not detected by the methods used. It is known that in alkaline solutions Np(VII) exists as an anion NpO 4 (OH) 2 33 [133], and in acidic solutions, as a cation NpO 3 + [4] or Np(OH) 6 + [5]. The cationic form transforms into the anionic form in 1 M NaNO 3 or NaClO 4 at pH 5.537.5 [6]. In the presence of carbonate ions ([KHCO 3 ] + [K 2 CO 3 ] = 0.5 M), this transition occurs at pH 10 311.5. Thus, the range of the existence of the cationic Np(VII) form is extended by three pH units. Hence, we can conclude that Np(VII) cation forms a complex with carbonate ions [6]. There are no data on Np(VII) behavior in silicate solutions, in which both metasilicate SiO 3 23 and orthosilicate SiO 4 43 ions (and protonated species of the latter) can exist.In this work, the state of Np(VII) in silicate solutions was studied spectrophotometrically. In addition, complex formation of Np(VI) and Np(V) with silicate ions was studied. EXPERIMENTALChemically pure grade HClO 4 , ultrapure grade LiClO 4 and NaOH (the latter as a 17 M solution), analytically pure grade NaHCO 3 , recrystallzed pure grade NaClO 4 and chemically pure grade Na 2 CO 3 , and A grade Na 2 SiO 3 . 9H 2 O (Reakhim) were used. All solutions were prepared with double-distilled water and standardized by usual methods. The silicate solution was ozonized to oxidize traces of reducing agents and purged with purified air to remove the dissolved ozone.The 237 Np preparations were purified by anion exchange. The Np(IV) nitric acid solution was evaporated with HClO 4 until a white vapor appeared, and this procedure was repeated. The wet salts were dissolved in 0.01 M HClO 4 . Spectrophotometric analysis showed that neptunium was almost fully in the hexavalent state. A part of the Np(VI) solution was mixed with 4 M LiOH (final concentration of LiOH 0.7 M) or with 17 M NaOH and ozonized to obtain Np(VII). To another part of the Np(VI) solution, NaNO 2 was added, and the resulting Np(V) was precipitated with ammonia. After coagulation, the precipitate was centrifuged, washed with ice-cold double-distilled water, dissolved in the minimal amount of HClO 4 , and diluted with water. The concentrations of Np(VI) and Np(V) in the solutions prepared were determined complexometrically after reduction to the tetravalent state with hydroxylamine in 4 M HCl [7]. The Np(VII) concentration was determined spectrophotometricall...

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Behavior of Np(VII, VI, V) in silicate solutions

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“…1. Solubility data sets in ^ = 0.1 M Perchlorate medium are obtained for U(VI) under 100% CO2 atmosphere, while the Np(VI) data set is obtained from solutions equilibrated with a mixture of 80% COJlWo O2 after passing the gas through an ozonizer [36]. With exception of the Observation that Np(VI) solubilities at lg [COi"] > -8 are slightly lower than the corresponding U(VI) data, no general conclusion can be drawn with respect to characteristic differences in the actinyl(VI) systems by Visual inspection of Fig.…”

Section: Resultsmentioning

confidence: 99%

“…Kato et al [36] reported solubility data from three separate vessels. Hence, the data is interpreted by a model function where [Np(VI)],"m,i is the i-th measured total Np(VI) concentration, Ig^COz = -0.0969, the solubility product of the solid phase and y?ion the formation constant of the n-th carbonato species of Np(VI).…”

Section: Analysis Of Np(vi) Datamentioning

confidence: 99%

“…Hence, the data is interpreted by a model function where [Np(VI)],"m,i is the i-th measured total Np(VI) concentration, Ig^COz = -0.0969, the solubility product of the solid phase and y?ion the formation constant of the n-th carbonato species of Np(VI). Because there was no experimental evidence from spectroscopic investigations [37], formation of (Np02)2(OH)i"^ was not considered [36] in contrast to the U(VI) system, where (U02)2(OH)r is detected by UV-Vis spectroscopy [7,9]. The term is a stochastic disturbance.…”

Section: Analysis Of Np(vi) Datamentioning

confidence: 99%

“…species is not justified. An upper Umit of lg ß\<yi < 14.0 was given previously [36]. When comparing the model Eq.…”

Section: Analysis Of Np(vi) Datamentioning

confidence: 99%

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Comparative Analysis of Actinide(VI) Carbonate Complexation by Monte Carlo Resampling Methods

et al. 1999

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Uranium / Neptunium / Pwbability density / Non-normal data / Solubility / Steric constraints SunimarySelected data from solubility studies of UOJCOJCS) and NpOzCOjCs) in 0.1 M Perchlorate medium are analyzed by balanced Monte Carlo resampling techniques for evaluation of probability density distributions for the solubility products lg of AnOjCOjCs) (An = U, Np) and the formation constants lg ß\ou lg ß' m2 and lg ß\oj. of Solution species AnOjCO?, An02(C0,)i' and An02(C0,)r, respectively. Comparison is made on the basis of non-parametric statistics by KolmogorovSmimov test, Bayesian statistics and Wilcoxon-Mann-Whitney rank order test, thus avoiding assumption of normal probability densities for the variables. Variables for Np(Vl) are generally found lower than the respective variables for U(VI), in agreement with both the smaller effective Charge of Np(VI) compared to U(VI) and structural properties of actinyl(VI) carbonato species. The quality of data reported in literature is found quite scattered and insufficient to allow a statistically conclusive assessment of the properties of both systems. Large Standard deviations reported for analogous Pu(VI) data prevented further Interpretation. Hence, this work calls for further studies of these systems.

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