铀在北山地下水中的种态分布及溶解度分析

为了解铀酰离子在北山地下水中的吸附、扩散和迁移行为，利用地球化学计算软件PHREEQC，采用由OECD/NEA发布的最新铀的热力学数据，计算了铀在我国高放废物地质处置库重点研究区甘肃北山地下水中的种态分布，并分析了围岩中存在的方解石对铀溶解度的影响。计算结果表明，在北山地下水组成不变的前提下，在偏酸性条件下，铀主要以UO₂F⁺、UO₂SO₄、UO₂²⁺、UO₂F₂和UO₂(SO₄)₂^2-的形式存在，而在中性至弱碱性条件下，主要以 UO₂(CO₃)^4-₃、UO₂(CO₃)₂^2-、UO₂(OH)₃^-和UO₂(OH)₄^2-的形式存在。我国计划建造的高放废物处置库的设计深度为地下500～1000m，其水岩体系一般呈弱碱性。在这样的弱碱性水岩体系中，以阴离子形式存在的铀酰配合物具有较强的可移动性。当地下水的pH=7.56时，在Eh<24mV的条件下，铀主要以沥青铀矿的形式存在，而在更高的Eh条件下，则主要以UO₂²⁺与CO₃^2-和OH^-形成的阴离子配合物的形式存在。当地下水与空气接触时，O₂的存在会使Eh升高，此时铀的主要存在种态为UO₂²⁺及其各种配合物。当围岩体系中存在方解石时，在pH<8.0的条件下，铀在地下水中的溶解度会显著提高，而在更高pH条件下，方解石对铀的溶解度无明显影响。

Prediction of the Species and Solubility of Uranium in Beishan Groundwater

In order to understand the sorption, diffusion and migration behaviors of U in Beishan water-rock system, the speciation distribution of uranium in Beishan granite groundwater, along with its solubility influenced by calcite were calculated, using PHREEQC software with the most recent thermodynamic data published by OECD/NEA. The results suggest that under acidic conditions, uranium mainly occurs as UO₂F⁺, UO₂SO₄, UO₂²⁺, UO₂F₂ and UO₂(SO₄)₂^2-, while under nearly neutral to alkaline conditions, the dissolved uranyl predominantly exists as anionic UO₂(CO₃)^4-₃, UO₂(CO₃)₂^2-,UO₂(OH)₃^- and UO₂(OH)₄^2-, which are expected to enhance the mobility of uranium. Since the potential high level radioactive waste repository in China will be constructed at the depths of 500-1000 meters underground, the groundwater chemistry at such depths would be weakly alkaline, and the uranyl will be existed as anionic complexes that are mobile. When the pH of the groundwater is fixed at 7.56, the calculations demonstrate that uraninite is the main phase at Eh<24 mV. With the Eh elevation, uranyl and its various anionic complexes formed with CO₃^2- and OH^- become the predominant species. When the groundwater is exposed to air, uranyl and its various complexes are expected to be the predominant species due to the dissolution of O₂. In case the groundwater-host rock system contains calcite, uranium solubility can be enhanced significantly at pH<8.0, while the influence is negligible at higher pH.