高铀水库沉积物-上覆水中铀的迁移富集机制与污染来源解析

It has been found from the early comprehensive investigation in a uranium industrial area, northern Guangdong that the uranium showed signficant enrichment in the surface water and sediments due to excessive discharge of U-containing wastewater, and uranium exhibited horizontal long distance migration and vertical enrichment. The transformation of uranium valence state will directly affect the content, speciation and fate of uranium in water environment, which plays a crucial role in understanding uranium migration, enrichment and geochemical behaviors. It is of great practical significance to distinguish the complex uranium pollution sources in the background area with extremely high anomalies. The project, which is intended to study the downstream reservoir near a uranium hydrometallurgy plant in northern Guangdong, aims to: 1) focus on the interaction mechanism of valence variation and isotopic composition of U in sediments (pore water) - overlying water profiles by using synchrotron radiation and multiple collector inductively coupled plasma mass spectrometry; 2) examine the migration and enrichment mechanism of U in sediments (pore water) - overlying water ; and 3) establish the two-terminal mixture model of uranium isotope which can be applied to quantitatively analyze and trace the natural and anthropogenic source of highly enriched uranium in reservoir sediment and contribution respectively. Consequently, this project contributes to promote the relatively weak research on uranium isotope, enriches the method of uranium migration and enrichment process in the water environment, and provides important theoretical basis for the examination and distinguishment of uranium pollution source, pollution dispute arbitration and environmental justice.

申请人所在团队前期对粤北铀矿区研究发现，由于含铀废水超标排放，造成地表水与沉积物中铀的异常富集，水中铀呈现横向长距离扩散与垂向富集特征。铀价态变化直接影响铀在水环境介质中的含量、形态与归宿，是铀迁移富集与地球化学行为研究的关键环节；铀特高异常背景区域的复合型铀污染源的定量解析，具有重要的现实意义。本项目拟以粤北铀矿水冶厂下游水库为研究对象，采用同步辐射技术、多接收道电感耦合等离子体质谱高精度测试技术，重点研究沉积物(孔隙水)-上覆水剖面的铀价态变化与铀同位素组成的内联机制；从铀同位素组成的角度，研究铀在水库沉积物(孔隙水)-上覆水中的迁移富集机制；建立铀同位素二端元混合模型，定量解析水库沉积物中高度富集铀的自然来源与人为来源的各自贡献份额。本项目有助于推进相对薄弱的铀同位素研究，丰富铀在水环境介质中迁移富集过程的研究方法，也可为铀污染源甄别解析、污染纠纷仲裁与环境司法提供重要理论依据。

铀矿水冶区地表水沉积物中铀(U)地球化学过程及其铀同位素组成知之甚少。本项目利用铀、钍同位素组成研究了广东某铀矿区水库上覆水及其沉积物剖面中铀的地球化学行为及其来源解析。结果表明，沉积物中总铀含量(207.3~1117.7 mg/kg)和稀酸提取态铀含量(90.3~638.5 mg/kg)明显偏高，具有较高的铀污染和活化再释放风险。U/Th同位素活度比测试数据表明，由于含铀废水的排放，特别是铀尾矿渗滤液的排放，所有采样的矿区水库沉积物都受到了明显的U污染，而U/Th同位素活度比的变化主要受U的地球化学行为(主要是氧化还原过程和吸附作用)的影响。矿区水库沉积物的铀同位素组成(δ238U)随沉积物剖面深度变化较大，变化范围为-0.62~-0.04‰。δ238U与稀酸提取态铀的关系表明，沉积物中铀同位素分馏受基岩风化(自然活动)、铀尾矿渗滤液(人为活动)及后续生物地球化学过程的共同控制。本项目的研究结果表明：U-Th同位素能在复杂物源和多种环境驱动力共同影响的背景下，深入解析环境介质中铀的地球化学过程和富集机制的有力手段。迄今为止，对于长期开采和水冶活动影响下沉积物中的铀等金属地球化学行为及其同位素组成的研究工作尚十分欠缺，当前项目的完成正好补充了这方面的基础数据与认识。