Effects of Organic Degradation Products on the Sorption of Actinides

Impact of selected cement additives and model compounds on the solubility of Nd(III), Th(IV) and U(VI): screening experiments in alkaline NaCl, MgCl2 and CaCl2 solutions at elevated ionic strength

The solubility of Nd(III), Th(IV) and U(VI) was studied from undersaturation conditions in the presence of selected organic cement additives and model compounds: adipic acid, methyl acrylate, citric acid, melamine, ethylene glycol, phthalic acid and gluconic acid. Experiments were performed under Ar atmosphere in NaCl (2.5 and 5.0 M), MgCl2 (1.0 and 3.5 M) and CaCl2 (1.0 and 3.5 M) solutions with 9 ≤ pHm ≤ 13 (pHm = −log[H+]). Initial concentrations of organic ligands in solution were set constant in all systems to [L]0 = 0.025 M, except in specific cases (e.g. adipic acid, melamine and phthalic acid) where the ligand concentration in the matrix solutions was lower and controlled by solubility. Adipic acid, methyl acrylate, melamine, ethylene glycol and phthalic acid do not impact the solubility of Nd(III), Th(IV) and U(VI) in the investigated NaCl, MgCl2 and CaCl2 systems. Citrate significantly enhances the solubility of Nd(III), Th(IV) and U(VI) in NaCl systems. A similar effect was observed for Th(IV) and U(VI) in the presence of gluconate in NaCl systems. The impact of pH on the stability of the complexes is different for both ligands. Because of the larger number of alcohol groups in the gluconate molecule, this ligand is prone to form more stable complexes under hyperalkaline conditions that likely involve the deprotonation of several alcohol groups. The complexation of gluconate with U(VI) at pHm ≈ 13 is however weaker than at pHm ≈ 9 due to the competition with the highly hydrolysed moiety prevailing at pHm ≈ 13, i.e. UO2(OH)4 2−. The impact of citrate and gluconate in MgCl2 and CaCl2 systems is generally weaker than in NaCl systems, expectedly due to the competition with binary Mg-L and Ca-L complexes. However, the possible formation of ternary complexes further enhancing the solubility is hinted for the systems Mg/Ca-Th(IV)-GLU and Ca-U(VI)-GLU. These observations reflect again the differences in the complexation properties of citrate and gluconate, the key role of the alcohol groups present in the latter ligand, and the importances of interacting matrix cations. The screening experiments conducted within this study contribute to the identification of organic cement additives and model compounds potentially impacting the solution chemistry of An(III)/Ln(III), An(IV) and An(VI) under intermediate to high ionic strength conditions (2.5 ≤ I ≤ 10.5 M). This shows evident differences with respect to investigations conducted in dilute systems, and thus represents a very relevant input in the safety assessment of repositories for radioactive waste disposal where such elevated ionic strength conditions are expected.

Protonation and complexation of isosaccharinic acid with U(VI) and Fe(III) in acidic solutions: potentiometric and calorimetric studies

Protonation and complexation of α-isosaccharinic acid with U(VI) and Fe(III) have been studied in acidic solutions att=25 °C andI=1.0 mol dm-3NaClO4. From the potentiometric titrations, the protonation constant of the carboxylate group is calculated to be 3.65±0.05 and the data are consistent with the presence of three and four successive mononuclear complexes for U(VI) and Fe(III), respectively. The formation constants of the complexes, logβjfor the reactions of M+L=MLjwherej=1-3 for U(VI),j=1-4 for Fe(III) and L stands for isosaccharinate, are determined to be 2.91±0.15 (UO2L), 5.37±0.07 (UO2L2), 7.25±0.18 (UO2L3), 5.06±0.17 (FeL), 8.51±0.15 (FeL2), 11.00±0.16 (FeL3), and 12.99±0.17 (FeL4). From the calorimetric titrations, the enthalpy of protonation of the carboxylate group is determined to be -(7.94±0.03)kJ mol-1, similar to that of otherα-hydroxycarboxylates. The enthalpies of complexation between U(VI) and isosaccharinate are quite small: Δ H1= -(1.0±1.0)kJ mol-1, Δ H2=1.4±1.8 kJ mol-1and Δ H3=-(6.2±3.0)kJ mol-1, typical of the interactions between carboxylates and hard-acid cations. The complexation between U(VI) and isosaccharinate is mainly entropy-driven. In comparison, the enthalpies of complexation for FeL3and FeL4are large and exothermic, contributing significantly to the stability of the complexes.

Adsorption of lanthanum to goethite in the presence of gluconate

We have examined the effect of the gluconate anion, an analogue for cellulose degradation products, on the adsorption of trivalent lanthanum (La3+)#to goethite. Lanthanum is investigated as an analogue for the trivalent actinides. Batch pH adsorption edge experiments were used to quantify the adsorption of La3+in the absence of gluconate and in solutions where gluconate was present at a 1:1 mole ratio to lanthanum. Using available thermodynamic data, it is calculated that lanthanum is primarily present in solution as the free La3+ion at pH values up to 8.5 in the absence of gluconate. Above pH 8.5, solid La(OH)3precipitates from solution. In the presence of gluconate, complexation decreases the free La3+concentration in solution. The fraction of La3+complexed increases, from 3% to 50%, as the concentrations of La3+and gluconate were increased. Very little effect on the adsorption of La3+to goethite was observed in the presence of gluconate below pH 7. At pH values above 7, however, gluconate doubled the maximum amount of La3+adsorbed when present at concentrations that saturated the goethite adsorption sites. The presence of gluconate did not appear to inhibit the formation of La(OH)3(s) at pH 8.5 and milli molar lanthanum concentrations. Adsorption to the goethite surface was represented with a surface complexation approach using the diffuse double-layer model. Intrinsic binding constants for the surface complexes were estimated from the pH adsorption edge data using the computer code FITEQL 4.0 and visual curve fitting. Two surface reactions were used to fit the adsorption data in the absence of gluconate: 1) a strong binding site with no proton release and 2) a much higher concentration of weak binding sites with release of two protons per La3+adsorbed. In the presence of gluconate, a third surface complex was needed that involved a ternary complex of two lanthanum atoms with one gluconate molecule.

Review of the complexation of tetravalent actinides by ISA and gluconate under alkaline to hyperalkaline conditions

Isosaccharinic (ISA) and gluconic acids (GLU) are polyhydroxy carboxylic compounds showing a high affinity to metal complexation. Both organic ligands are expected in the cementitious environments usually considered for the disposal of low- and intermediate-level radioactive wastes. The hyperalkaline conditions imposed by cementitious materials contribute to the formation of ISA through cellulose degradation, whereas GLU is commonly used as a concrete additive. Despite the high stability attributed to ISA/GLU complexes of tetravalent actinides, the number and reliability of available experimental studies is still limited. This work aims at providing a general and comprehensive overview of the state of the art regarding Th, U(IV), Np(IV), and Pu(IV) complexes with ISA and GLU. In the presence of ISA/GLU concentrations in the range 10(-5)-10(-2) M and absence of calcium, An(IV)(OH)x(L)y complexes (An(IV)=Th, U(IV), Np(IV), Pu(IV); L=ISA, GLU) are expected to dominate the aqueous speciation of tetravalent actinides in the alkaline pH range. There is a moderate agreement among their stability, although the stoichiometry of certain An(IV)-GLU complexes is still ill-defined. Under hyperalkaline conditions and presence of calcium, the species CaTh(OH)4(L)2(aq) has been described for both ISA and GLU, and similar complexes may be expected to form with other tetravalent actinides. In the present work, the available thermodynamic data for An(IV)-ISA/GLU complexes have been reviewed and re-calculated to ensure the internal consistency of the stability constants assessed. Further modelling exercises, estimations based on Linear Free-Energy Relationships (LFER) among tetravalent actinides, as well as direct analogies between ISA and GLU complexes have also been performed. This approach has led to the definition of a speciation scheme for the complexes of Th, U(IV), Np(IV) and Pu(IV) with ISA and GLU forming in alkaline to hyperalkaline pH conditions, both in the absence and presence of calcium.