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), MgCl 2 (1.0 and 3.5 M) and CaCl 2 (1.0 and 3.5 M) solutions with 9 ≤ pH m ≤ 13 (pH m = −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, MgCl 2 and CaCl 2 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 pH m ≈ 13 is however weaker than at pH m ≈ 9 due to the competition with the highly hydrolysed moiety prevailing at pH m ≈ 13, i.e. UO 2 (OH) 4 2− . The impact of citrate and gluconate in MgCl 2 and CaCl 2 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.

中文翻译：

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选定的水泥添加剂和模型化合物对 Nd(III)、Th(IV) 和 U(VI) 溶解度的影响：在碱性 NaCl、MgCl2 和 CaCl2 溶液中离子强度升高的筛选实验

Nd(III)、Th(IV) 和 U(VI) 的溶解度在以下选择的有机水泥添加剂和模型化合物存在下从欠饱和条件进行研究：己二酸、丙烯酸甲酯、柠檬酸、三聚氰胺、乙二醇、邻苯二甲酸和葡萄糖酸。实验是在Ar气氛中，在9≤pH m≤13（pH m = -log [H +]的NaCl（2.5和5.0 M），MgCl 2（1.0和3.5 M）和CaCl 2（1.0和3.5 M）溶液中进行的）。在所有系统中，溶液中有机配体的初始浓度均设置为 [ L ] 0 = 0.025 M，除非在特定情况下（例如己二酸、三聚氰胺和邻苯二甲酸），其中基质溶液中的配体浓度较低且受溶解度控制. 己二酸、丙烯酸甲酯、三聚氰胺、乙二醇和邻苯二甲酸不影响 Nd(III) 的溶解度，所研究的 NaCl、MgCl 2 和 CaCl 2 系统中的 Th(IV) 和 U(VI)。柠檬酸盐显着提高了 Nd(III)、Th(IV) 和 U(VI) 在 NaCl 系统中的溶解度。在NaCl系统中存在葡萄糖酸盐时，对Th（IV）和U（VI）观察到类似的效果。两种配体的 pH 值对复合物稳定性的影响是不同的。由于葡萄糖酸分子中有更多的醇基团，这种配体在可能涉及几个醇基团去质子化的高碱性条件下易于形成更稳定的复合物。然而，葡萄糖酸与 U(VI) 在 pH m ≈ 13 时的络合比在 pH m ≈ 9 时弱，这是由于与在 pH m ≈ 13 时普遍存在的高度水解部分（即 UO 2 (OH) 4 2- ）的竞争。柠檬酸盐和葡萄糖酸盐在 MgCl 2 和 CaCl 2 系统中的影响通常比在 NaCl 系统中弱，预计是由于与二元 Mg-L 和 Ca-L 复合物的竞争。然而，对于系统 Mg/Ca-Th(IV)-GLU 和 Ca-U(VI)-GLU，暗示可能形成三元配合物进一步提高溶解度。这些观察结果再次反映了柠檬酸盐和葡萄糖酸盐的络合特性的差异、后者配体中存在的醇基的关键作用以及相互作用基质阳离子的重要性。本研究中进行的筛选实验有助于确定有机水泥添加剂和模型化合物，它们可能会影响 An(III)/Ln(III)、An(IV) 和 An(VI) 在中高离子强度条件下的溶液化学(2.5 ≤ I ≤ 10.5 M)。