We recently discovered a neutral dicalcium uranyl tricarbonate complex, Ca 2 UO 2 (CO 3 ) 3 (aq.), in uranium mining related waters [1]. We are now reporting a further validation of the stoichiometry and the formation constant of this complex using two analytical approaches with time-resolved laser-induced fluorescence spectroscopy (TRLFS) species detection: i) titration of a non-fluorescent uranyl tricarbonate complex solution with calcium ions, and quantitative determination of the produced fluorescent calcium complex via TRLFS; and ii) variation of the calcium concentration in the complex by competitive calcium complexation with EDTA 4− .Slope analysis of the log (fluorescence intensity) versus log [Ca 2+ ] with both methods have shown that two calcium ions are bound to form the complex Ca 2 UO 2 (CO 3 ) 3 (aq.). The formation constants determined from the two independent methods are: i) log β • 213 = 30.45 ± 0.35 and ii) log β • 213 = 30.77 ± 0.25. A bathochrome shift of 0.35 nm between the UO 2 (CO 3 ) 3 4− complex and the Ca 2 UO 2 (CO 3 ) 3 (aq.) complex is observed in the laser-induced photoacoustic spectrum (LIPAS), giving additional evidence for the formation of the calcium uranyl carbonate complex.EXAFS spectra at the L II and L III -edges of uranium in uranyl carbonate solutions with and without calcium do not differ significantly. A somewhat better fit to the EXAFS of the Ca 2 UO 2 (CO 3 ) 3 (aq.) complex is obtained by including the U-Ca shell. From the similarities between the EXAFS of the Ca 2 UO 2 (CO 3 ) 3 (aq.) species in solution and the natural mineral liebigite, we conclude that the calcium atoms are likely to be in the same positions both in the solution complex and in the solid.This complex influences considerably the speciation of uranium in the pH region from 6 to 10 in calcium-rich uranium-mining-related waters.

The actinide(IV) hexanuclear [M6(μ3‐O)4(μ3‐OH)4(HCOO)12(LT)6] complexes were prepared (LT = H2O or CH3OH). Their structures were investigated by single‐crystal X‐ray analysis and XAFS spectroscopy. HCOO– acts as a bridging ligand, which prevents the formation of polynuclear hydrolysis species like UIV hydrous oxide colloids at least up to pH = 3.25, and stabilizes the nanosized clusters in solution. The charge of the hexamer is balanced by the O/OH ratio of the μ3‐bridges.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

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Chemical speciation of U(VI) in natural seepage water and corresponding model solutions was investigated by time-resolved laser-induced fluorescence spectroscopy. Calculations of uranium speciation in this medium show that U0 2 (C0 3 )3~ and U0 2 (C0 3 )|" should be the major individual components. Due to the very low fluorescence intensity, the pure uranyl carbonato complexes could not be measured directly by TRLFS. However, a uranium fluorescence spectrum was recorded from seepage water samples. The TRLFS investigations show that the main component of uranium in this seepage water is a calcium uranium carbonato complex. The main fluorescence wavelengths of this complex are at 463.9, 483.6, 502.8, 524.3 and 555.4 nm. The fluorescence lifetime of the species is 64±17 ns. This study shows that the calcium content of the water has a great influence on the uranium speciation. For the first time, the existence of a calcium uranium carbonato complex -(Ca 2 [U0 2 (C0 3 ) 3 ] |

The coordination of the U(IV) and U(VI) ions as a function of the chloride concentration in aqueous solution has been studied by U L(III)-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The oxidation state of uranium was changed in situ using a gastight spectroelectrochemical cell, specifically designed for the safe use with radioactive solutions. For U(VI) we observed the complexes UO2(H2O)5(2+), UO2(H2O)4Cl+, UO2(H2O)3Cl2(0), and UO2(H2O)2Cl3- with [Cl-] increasing from 0 to 9 M, and for U(IV) we observed the complexes U(H2O)9(4+), U(H2O)8Cl3+, U(H2O)(6-7)Cl2(2+), and U(H2O)5Cl3+. The distances in the U(VI) coordination sphere are U-Oax = 1.76+/-0.02 A, Oeq = 2.41 +/- 0.02 A, and U-Cl = 2.71 +/- 0.02 A; the distances in the U(IV) coordination sphere are U-O = 2.41 +/- 0.02 A and U-Cl = 2.71 +/- 0.02 A.

SummaryThe uranyl(VI) malonate complex formation was studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS) at pH 4 and an ionic strength of 0.1 M NaClOThe measured fluorescence lifetimes of the 1:1 and 1:2 uranyl(VI) malonate complexes are 1.24 ± 0.02 µs and 6.48 ± 0.02 µs, respectively. The fluorescence lifetime of the uranyl(VI) ion is 1.57 ± 0.06 µs in 0.1 M perchloric media. The main fluorescence bands of the malonate complexes show a bathochromic shift compared to the uranyl(VI) ion and are centered at 494 nm, 515 nm and 540 nm for the 1:1 complexes and at 496 nm, 517 nm and 542 nm for the 1:2 complex. The spectra of the individual uranyl(VI) malonate complexes were calculated using a multi exponential fluorescence decay function for each intensity value at each wavelength, covering the entire wavelength range. Stability constants were determined for the complexes UO

Electrochemical, complexation, and electronic properties of uranyl(VI) and -(V) carbonato complexes in an aqueous Na2CO3 solution have been investigated to define the appropriate conditions for preparing pure uranyl(V) samples and to understand the difference in coordination character between UO22+ and UO2+. Cyclic voltammetry using three different working electrodes of platinum, gold, and glassy carbon has suggested that the electrochemical reaction of uranyl(VI) carbonate species proceeds quasi-reversibly. Electrolysis of UO22+ has been performed in Na2CO3 solutions of more than 0.8 M with a limited pH range of 11.7 < pH < 12.0 using a platinum mesh electrode. It produces a high purity of the uranyl(V) carbonate solution, which has been confirmed to be stable for at least 2 weeks in a sealed glass cuvette. Extended X-ray absorption fine structure (EXAFS) measurements revealed the structural arrangement of uranyl(VI) and -(V) tricarbonato complexes, [UO2(CO3)3]n- [n = 4 for uranyl(VI), 5 for uranyl(V)]. The bond distances of U-Oax, U-Oeq, U-C, and U-Odist are determined to be 1.81, 2.44, 2.92, and 4.17 A for the uranyl(VI) complex and 1.91, 2.50, 2.93, and 4.23 A for the uranyl(V) complex, respectively. The validity of the structural parameters obtained from EXAFS has been supported by quantum chemical calculations for the uranyl(VI) complex. The uranium LI- and LIII-edge X-ray absorption near-edge structure spectra have been interpreted in terms of electron transitions and multiple-scattering features.

Electrochemical and complexation properties of neptunium (Np) are investigated in aqueous perchlorate and nitrate solutions by means of cyclic voltammetry, bulk electrolysis, UV-visible absorption, and Np L(III)-edge X-ray absorption spectroscopies. The redox reactions of Np(III)/Np(IV) and Np(V)/Np(VI) couples are reversible or quasi-reversible, while the electrochemical reaction between Np(III/IV) and Np(V/VI) is irreversible because they undergo structural rearrangement from spherical coordinating ions (Np(3+) and Np(4+)) to transdioxoneptunyl ions (NpO2(n+), n = 1 for Np(V) and 2 for Np(VI)). The redox reaction of the Np(V)/Np(VI) couple involves no structural rearrangement on their equatorial planes in acidic perchlorate and nitrate solutions. A detailed analysis on extended X-ray absorption fine structure (EXAFS) spectra suggests that Np(IV) forms a decaaquo complex of [Np(H2O)10](4+) in 1.0 M HClO4, while Np(V) and Np(VI) exist dominantly as pentaaquoneptunyl complexes, [NpO2(H2O)5](n+) (n = 1 for Np(V) and 2 for Np(VI)). A systematic change is observed on the Fourier transforms of the EXAFS spectra for all of the Np oxidation states as the nitrate concentration is increased in the sample, revealing that the hydrate water molecules are replaced by bidentate-coordinating nitrate ions on the primary coordination sphere of Np.