基于MnO2共沉淀的海水中234Th的分析方法

利用室内模拟MnO₂共沉淀和直接β计数分析海水中²³⁴Th。实验中对模拟条件进行了优化，结果表明：共沉淀滤膜选择混合膜时，实验流程空白计数率稳定在(0.50±0.04)min^-1；共沉淀前若酸化，会破坏海水中U和CO₃^2-络合作用，导致U-Th分离效果减弱；共沉淀时最佳pH=10.0；海水中添加碳酸盐能明显降低海水中U的共沉淀效率；增大沉淀剂的用量，会一定程度改善U-Th的分离效果，每升海水中加入1.2mg KMnO₄和3.0mg MnCl₂·4H₂O时最佳。本实验方法已经将模拟条件的结论结合实际情况应用到东海天然海水中²³⁴Th的分析，该方法的全程回收率为（36.0±5.2)%(n=3)。

Analysis Method of 234Th in Seawater by MnO2 Co-Precipitation

This paper mainly focuses on analysis of ²³⁴Th in seawater by using MnO₂ co-precipitation and β counter. In lab study, several conditional experiments, such as the selection of filter membrane, pH, whether acidification before co-precipitation, carbonate concentration and the usage amount of MnO₂, were performed. If the mixed cellulose ester membrane was used for sample counting on gas flow β counter, it makes the blank count rate to be around (0.50±0.04) min^-1. Acidification can promote the precipitation of uranium because it can destory the complexation of uranium with CO₃^2-. At pH=10.0, the separation effect of ²³⁴Th from ²³⁸U solution is good. Moreover, carbonate can coordinate uranium and make uranium kept in solution. Besides, the efficiency is increased when more MnO₂is added. The best reagent amount is 1.2mg KMnO₄ and 3.0mg MnCl₂·4H₂O per liter of sample in this study. We have applied this method combined with the actual situation to analyze ²³⁴Th in seawater samples from East China Sea. The recovery of ²³⁴Th is around （36.0±5.2)%(n=3).