Thermodynamic studies of actinide complexes. 1. A reappraisal of the solution equilibria between plutonium(IV) and ethylenediaminetetraacetic acid (EDTAH4) in nitric media

Abstract

A detailed reevaluation of the complexation equilibria of plutonium(IV) with the polyaminocarboxylic sequestering agent EDTA 4À has been performed in 1 M (H,K)NO 3 media at 298 K by means of visible absorption spectrophotometry and glasselectrode potentiometry at millimolar concentration levels. The high binding affinity even under strongly acidic conditions supports the exclusive formation of the neutral Pu(EDTA) complex over the range 0.01 [H + ] 0.9 M with an apparent formation constant of log b 110 ¼ 25.8(1) at 0.9 M HNO 3 . Extrapolation to zero ionic strength using the SIT approach provides the first ever-reported NEAeTDB compliant estimate of b 0 110 ðlog b 0 110 ¼ 32:2ð3ÞÞ. A global modeling procedure of the potentiometric data collected for three different metal over ligand ratios (1:1, 1:1.5, and 1:2) ascertained the formation of four additional species for pH values comprised between 2 and 6.5, namely [Pu(EDTA)(OH)] À , [Pu(EDTA) 2 H 2 ] 2À , [Pu(EDTA) 2 H] 3À , and [Pu(EDTA) 2 ] 4À . The visible absorption spectra for the first three complexes have also been determined. Data recorded above pH 3.5 (for a 1:1 molar ratio) or 6.5 (for a 1:2 molar ratio) were finally excluded from the global fitting procedure as speciation calculations suggested the precipitation of amorphous Pu(OH) 4(am) according to the latest critical value quoted in the literature for its solubility product. To cite this article:

Influence of humic acid on plutonium sorption to gibbsite: Determination of Pu-humic acid complexation constants and ternary sorption studies

“…The uncertainty associated with Pu-EDTA complexation is not accounted for in the FITEQL model and therefore the reported uncertianty in the Pu-HA complexation constant is underestimated. Similar to the modeling effort described in this work, where the inclusion of a hydrolyzed species was essential for accurately predicting experimental data, species such as [Pu(EDTA)(OH)] − and [Pu(EDTA)(OH) 2 ] 2− play a dominant role in systems containing both Pu and EDTA; yet are not well understood ( [25,27]). Therefore, every effort must be made to utilize the best available constants for studies of this nature, as these constants represent a potential source of error.…”

“…When examining Pu(IV)-EDTA complexation, Boukhalfa et al [25] included the species [Pu(EDTA) 2 H 3 ] − while Meyer et al [27] included [Pu(EDTA) 2 H 2 ] 2− . Meyer et al [27] used an iterative process of excluding data points, based on the theoretical solubility of Pu(OH) 4 (am), which resulted in a truncated dataset (deemed insufficient for inclusion in this study).…”

“…Meyer et al [27] used an iterative process of excluding data points, based on the theoretical solubility of Pu(OH) 4 (am), which resulted in a truncated dataset (deemed insufficient for inclusion in this study). However, for comparison purposes, the Meyer et al [27] Pu-EDTA stability constants were used to model the global Pu data.…”

“…Meyer et al [27] used an iterative process of excluding data points, based on the theoretical solubility of Pu(OH) 4 (am), which resulted in a truncated dataset (deemed insufficient for inclusion in this study). However, for comparison purposes, the Meyer et al [27] Pu-EDTA stability constants were used to model the global Pu data. The resulting Pu-HA stability constant (HL3 basis) of 6.72 is in good agreement with the value of log = 6.76 ± 0.14 calculated based on the Pu-EDTA stability constants from Boukhalfa et al [25].…”

Thermodynamics of Np(IV) complexes with gluconic acid under alkaline conditions: sorption studies

“…Linear free-energy relationship (LFER) are often used to assess the reliability of stability constants along the An(IV) series [17][18][19]. The log * β 0 1,4,1 value determined in this work was compared with other data available in the literature for An(IV)(OH) 4 GLU − complexes.…”