地下水中镭的固液分配系数及其测定方法研究

近十多年来天然放射性镭(Ra)同位素在水环境、水资源研究领域越来越受到重视,Ra同位素应用在海底地下水排泄估算、湖泊水均衡计算、地下水年龄研究、地下水地表水污染源解析等方面取得了重要的研究成果,这得益于Ra同位素测试技术的发展和对Ra地球化学性质认识的深化。Ra的固液分配系数(K_d)是影响Ra在土壤和含水层中迁移的关键参数,正确认识K_d是利用Ra同位素精准示踪水文过程特别是地下水循环过程的决定性因素。本研究对前人报道的地下水中Ra的K_d值分布范围进行了综合分析,并探讨K_d值的主要影响因素。发现天然环境下K_d值的变化范围为0.1~1×10⁶ mL·g^-1,最大值和最小值之间相差7个数量级。水的盐度、pH值和沉积物中铁(Fe)的含量等对K_d具有显著影响。总结了Ra的K_d值的确定方法,把不同研究者提出的方法归纳为氡(Rn)平衡法、热动力学吸附平衡计算法、实验法和离子交换法,分析了各种方法的优缺点。 提出建立精准的水溶液中Ra的α离子输入反冲项测定方法,优化分析计算流程,提供准确获得Ra的K_d值的技术思路。本研究成果能够为Ra水文地球化学性质研究及其在水文过程中的应用提供参考和技术支撑。

Distribution Coefficient of Ra in Groundwater and Its Determination Technique

In the past decade, the Ra isotope has attracted increasingly more research attention in the field of water environment and water resources. Important findings have been made on the application of Ra isotopes in estimating seabed groundwater discharge, calculating lake water balance, study of groundwater age and analysis of groundwater pollution source. This is due to the development of Ra isotope analytical technology and the enhanced understanding of Ra geochemical behavior. The Ra distribution coefficient (K_d) is a key parameter affecting the Ra migration in soil and aquifer, and its understanding is pivotal for accurate tracing of hydrological processes, especially groundwater cycle. We compiled a range of K_d value and analyzed the main influencing factors. We found that the K_d value in natural environment varies from 0.1-1×10⁶ mL·g^-1, extending across seven orders of magnitude between the maximum and minimum. The water salinity, pH, and Fe content in sediments have significant impact on K_d. The K_d determination methods were summarized and divided into four types, including the (1) “Rn” equilibrium, (2) thermodynamic adsorption equilibrium calculation, (3) experimental method, and (4) ion exchange equilibrium method. The pros and cons of each method were analyzed. We proposed the establishment of an accurate determination method of ion input recoil term in aqueous solution and to optimize analysis and calculation process, which is the technical idea of obtaining K_d value accurately. Our review and analysis can provide reference and technical support for studying the hydrogeochemical properties of Ra and its hydrologic applications.