北山地下水中三元铀酰配合物的化学种态分析

Chemical Species Analysis of Ternary Uranyl Complexes in Groundwater of Beishan

获知地下水环境中放射性核素的化学种态,对于理解其吸附、迁移和扩散行为至关重要。利用水文地球化学模拟软件PHREEQC,计算了BS03(pH=9.43)和BS05(pH=7.51)两种北山深部地下水中铀(铀酰)的种态分布,重点分析了pH、温度和初始铀浓度等因素的影响。研究结果表明,铀在弱酸性条件下,主要以UO_(2)CO_(3)(aq)和(UO_(2))_(2)CO_(3)(OH)_(3)^(-)存在;在中性至弱碱性条件下,主要以CaUO_(2)(CO_(3))_(3)^(2-)和Ca_(2)UO_(2)(CO_(3))_(3)(aq)存在,且碳酸铀酰钙的含量远高于碳酸铀酰镁。随着温度的升高,Ca_(2)UO_(2)(CO_(3))3(aq)、CaUO_(2)(CO_(3))_(3)^(2-)和UO_(2)(CO_(3))_(3)^(4-)逐渐分解。当初始铀浓度为4×10^(-4)~1×10^(-1)mmol/L时(t=25℃),北山深部地下水中铀的优势种态为Ca_(2)UO_(2)(CO_(3))_(3)(aq)(摩尔分数x>62%)和CaUO_(2)(CO_(3))_(3)^(2-)(x≈35%)三元配合物。研究结果对于分析关键放射性元素铀在高水平放射性废物处置库中的释放和迁移机制具有重要作用。

Due to anthropogenic and long-term geological factors,radionuclide leakage may occur in deep geological repositories of radioactive waste when groundwater infiltrates a failed waste canister.The released radionuclides can migrate to near and far field regions through groundwater,posing a serious threat to the ecological environment and human health.As a consequence,knowledge of radionuclide speciation in groundwater is essential for understanding their transport behavior.Uranium is one of the key elements of significant concern for deep geological disposal of high-level radioactive waste.In this study,the uranium species in groundwater from boreholes BS03(pH=9.43)and BS05(pH=7.51)located at the Beishan area(Gansu province)were calculated by the hydrogeochemical software PHREEQC(version 3).The effects of pH,temperature,and initial uranium concentrations on ternary uranyl species are presented.The results indicate that UO_(2)CO_(3)(aq)and(UO_(2))_(2)CO_(3)(OH)_(3)^(-)are the dominant species in a mildly acidic condition,while CaUO_(2)(CO_(3))_(3)^(2-)and Ca_(2)UO_(2)(CO_(3))_(3)(aq)are the major species in neutral and weakly alkaline conditions.In BS05 groundwater,the concentration of Ca^(2+)(5.11 mmol/L)is about three times higher than that of Mg^(2+)(1.58 mmol/L).However,the concentration of calcium uranyl carbonate is estimated to be about 250 times higher than that of magnesium uranyl carbonate(MgUO_(2)(CO_(3))_(3)^(2-)).With an increase of temperature,Ca_(2)UO_(2)(CO_(3))_(3)(aq),CaUO_(2)(CO_(3))_(3)^(2-)and UO_(2)(CO_(3))_(3)^(4-)concentrations progressively diminish.At uranium concentrations between 4×10^(-4)and 1×10^(-1)mmol/L(25℃),the dominant uranium species,in order of decreasing concentration are:Ca_(2)UO_(2)(CO_(3))_(3)(aq)(x>62%)>CaUO_(2)(CO_(3))_(3)^(2-)(x≈35%)>MgUO_(2)(CO_(3))_(3)^(2-)/UO_(2)(CO_(3))_(3)^(4-)(x≈1%),irrespective of the concentrations of Ca^(2+),Mg^(2+),and Na^(+).In the near-neutral and slightly alkaline conditions,the results suggest that the formation of neutral Ca_(2)UO_(2)(CO_(3))_(3)(aq)and anionic CaUO_(2)(CO_(3))_(3)^(2-)in Beishan groundwater reduces the distribution coefficient between uranium and environmental geo-medium,thereby enhancing the mobility and migration potential of uranium.The research findings contribute to understanding the transport and release mechanisms of uranium,as well as interactions between uranium species and engineered and natural barriers.