铀、钍在碳酸钙-海水界面的附着行为研究

摘要

Using the constant addition system, the uptake of uranium and thorium at the interface of CaCO3-seaw-ater was investigated. Significant differences were observed not only between U and Th but also between aragonite and calcite. Partition coefficients of U (DU) between CaCO3 and seawater were determined to range from 1.5 to 3.5 for aragonite and 0.04 to 0.19 for calcite. The partitioning of U is very weak, but the value of DU was observed to increase with increasing precipitation rates of aragonite or calcite. It suggests that U was incorporated into CaCO3 precipitates via coprecipitation. However, because U(VI) is present in seawater in the form of 2 22 3UO (CO )n n , the incorporation of U(VI) could only be achieved by the occupation of defect sites. As compared to U, Th could be strongly gathered by both calcite and aragonite. The partition coefficients of Th (DTh) between CaCO3 and seawater range from 240 to 6330 for aragonite and 430 to 6160 for calcite. However, there is no correlation between DTh and CaCO3 precipitation rates. The uptake of Th at the interface of CaCO3-seawater was achieved simply via adsorption. Such behavior of Th(IV) should be related to dominance of Th(OH)4 in our solutions.%运用实验模拟的手段，对铀(U)、钍(Th)在碳酸钙-海水界面的附着行为进行了初步研究，通过测定相关的分异系数(D)定量评估了文石和方解石两种碳酸钙矿物对U和Th的捕集作用。实验结果显示，不仅 U 和 Th 之间存在显著差异，而且两种碳酸钙矿物之间也存在明显不同。U 介于碳酸钙和海水之间的分异系数(DU)分别在1.5~3.5(文石)和0.04~0.19(方解石)区间内变化，分异行为非常微弱但其分异系数随着碳酸钙沉淀速率的增加而逐渐上升。这表明U元素主要是以共沉淀的方式进入到文石或方解石沉淀中，但由于其在海水溶液中主要以2223UO (CO )n n的形式存在，因而只能以占据晶格缺陷的方式进入碳酸钙。与之相反的是，文石和方解石都对 Th 表现出极强的捕集能力，所测定的分异系数(DTh)分别在240~6330(文石)和430~6160(方解石)范围内变化，但DTh与碳酸钙沉淀速率之间无明显关联。Th 在碳酸钙表面的附着行为可界定为单纯的吸附作用，这一行为应与 Th 主要以 Th(OH)4的形式赋存于溶液中有关。