Zheng Zuoping, Tokunaga Tetsu K, Wan Jiamin
Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA.
Environ Sci Technol. 2003 Dec 15;37(24):5603-8. doi: 10.1021/es0304897.
The high stability of calcium uranyl carbonate complexes in the circumneutral pH range has a strong impact on U(VI) sorption in calcareous soils. To quantify this influence, sorption of U(VI) to soils in the presence of naturally occurring calcium carbonate was investigated by conducting batch experiments in which either U(VI) concentration or solution pH was varied. Two soils containing different calcium carbonate concentrations were selected, one from Oak Ridge, TN, and another from Altamont Pass, CA. The results show that the presence of calcium carbonate in soils strongly affects U(VI) sorption. Higher concentrations of soil calcium carbonate lead to a pronounced suppression of the pH-dependent sorption curve in the neutral pH range because of the formation of a very stable neutral complex of calcium uranyl carbonate in solution. A surface complexation model considering both strong and weak sites for ferrihydrite and ionizable hydroxyl sites for clay minerals was compared with experimental results, and U(VI) binding parameters were reasonably estimated. Fair agreement was found between the model predictions and sorption data, which span a wide range of U(VI) concentrations and pH. The results also show that appropriate solution-to-solid ratios need to be used when measuring distribution coefficients in calcareous soils to avoid complete CaCO3 dissolution and consequent dilution of calcium uranyl carbonate complexes.
碳酸铀酰钙络合物在接近中性的pH范围内具有高稳定性，这对钙质土壤中U(VI)的吸附有强烈影响。为了量化这种影响，通过进行批次实验来研究在天然存在碳酸钙的情况下U(VI)对土壤的吸附，其中U(VI)浓度或溶液pH值是变化的。选择了两种含有不同碳酸钙浓度的土壤，一种来自田纳西州橡树岭，另一种来自加利福尼亚州阿尔塔蒙特山口。结果表明，土壤中碳酸钙的存在强烈影响U(VI)的吸附。由于在溶液中形成了非常稳定的中性碳酸铀酰钙络合物，较高浓度的土壤碳酸钙导致中性pH范围内pH依赖性吸附曲线受到明显抑制。将考虑了铁水合物的强、弱位点以及粘土矿物的可电离羟基位点的表面络合模型与实验结果进行了比较，并合理估计了U(VI)的结合参数。在模型预测和吸附数据之间发现了良好的一致性，这些数据涵盖了广泛的U(VI)浓度和pH范围。结果还表明，在测量钙质土壤中的分配系数时，需要使用合适的液固比，以避免碳酸钙完全溶解以及随之而来的碳酸铀酰钙络合物稀释。
