The pH dependence of Am(III) complexation with acetate: an EXAFS study

Abstract

The complexation of acetate with Am(III) is studied as a function of the pH (1-6) by extended X-ray absorption fine-structure (EXAFS) spectroscopy. The molecular structure of the Am(III)-acetate complexes (coordination numbers, oxygen and carbon distances) is determined from the raw k(3)-weighted Am LIII-edge EXAFS spectra. The results show a continuous shift of Am(III) speciation with increasing pH value towards the complexed species. Furthermore, it is verified that acetate coordinates in a bidentate coordination mode to Am(III) (Am-C distance: 2.82 ± 0.03 Å). The EXAFS data are analyzed by iterative transformation factor analysis to further verify the chemical speciation, which is calculated on the basis of thermodynamic constants, and the used structural model. The experimental results are in very good agreement with the thermodynamic modelling.

An EXAFS spectroscopic study of Am(III) complexation with lactate

“…For all samples, 10-11 oxygen atoms at a radial distance (R) of 2.47 Å are found in the first coordination sphere. These values are in good agreement with reported data for Am(III) and other trivalent actinides in aqueous solution (Allen et al, 2000;Brendebach et al, 2009;Frö hlich et al, 2015). Between pH 3.0 and 5.3, the coordination number with respect to carbon increases continuously from 1.0 and 2.2, whereas at higher pH the coordination number decreases due to the formation of hydroxo species as expected from the thermodynamic calcu- Table 3 Speciation of Am(III) calculated with Visual MINTEQ (version 3.0) for the experimental conditions given in Table 1 using the thermodynamic constants given in Table 2. Amounts of species are given in % (species < 1% not shown).…”

Role of the Hydroxo Group in the Coordination of Citric Acid to Trivalent Americium

“…These calculated distances are consistent with Am‐O distances reported in the Cambridge Structural Data base (CSD); Am‐COO – (monodentate) distances range from 2.38 to 2.55 Å, , and Am‐OH 2 distance is 2.513 Å . These values are also consistent with the one determined for Eu‐lactate or Am‐formate systems from DFT. According to DFT calculations, the Am‐O 2.1–2.2 Å short bond length found when fitting the EXAFS spectra for the Am‐citric‐pH‐1 system can only be attributed to the deprotonated hydroxyl function bonding to the americium ion.…”

“…Considering the tricarballylic system, in addition to these O dist and C shells, the best fit was obtained with a split of the first O shell (0.14 Å and 0.11 Å between the two O shells for the 1:1 and 1:2 complexes respectively). This effect (O shell split, C shell arising at about 3.3 Å) has been already reported for other Am(III) carboxylate systems such as Am(III) acetate or Am(III) formate . Moreover, this effect seems to increase from 1:1 to 1:2 complexes with significantly more C atoms in the 1:2 complex.…”

Advancing understanding of actinide(iii) (Ac, Am, Cm) aqueous complexation chemistry

“…One of many examples includes actinides dissolved in buffered ammonium acetate and acetic acid solutions, (NH 4 )O 2 CMe (aq) :HO 2 CMe (aq) . Beyond a few focused studies, [2][3][4][5][6][7][8][9] this stock solution is poorly characterized because of the inherent challenges associated with handling highly radioactive actinides. Interpreting spectroscopic results from actinides in this complicated aqueous environment is also challenging in comparison to well-dened organic solutions as well as when frozen in the solid-state.…”