厌氧条件下绿锈形态转变对地下水中铀迁移转化的影响与机理研究

Uranium is a highly mobile, cancerogenic and toxic radionuclide in groundwater, and the remediation of uranium contaminated groundwater is imperative for uranium mining, milling and refining sites in China. Uranium is quite soluble as the U(VI) species in groundwater and is immobilized when reduced to sparingly soluble U(IV). The fate and transport of uranium is therefore often controlled by redox processes and in particular the reduction of U(VI) to U(IV). Chemical and biotic reduction approaches are proposed to remediate the uranium contaminated groundwater. Recent studies have observed U(VI) can be rapidly removed from groundwater via reduction by green rust (GR), a new reductant, which is ubiquitously found in groundwater. Nevertheless, GR is highly reactive and thus can transform to other iron (oxyhydr)oxides in groundwater. This transformation is believed to affect the fate and transport of uranium in groundwater, but the magnitude of the impact and the mechanism are poorly understood and need further study. In this study, we hypothesize that (i) GR can transform to other iron (oxyhydr)oxides under anoxic conditions, and (ii) U(VI) could be sequestered via incorporation by GR transformation products. The objectives of this study are to identify the influence and potential pathways/mechanisms of GR transformation on uranium fate and transport in groundwater under anoxic conditions, and evaluate their relative significance for uranium remediation. This research will employ a powerful arsenal of techniques to investigate the reaction kinetics and mechanism, based on the knowledge of mineralogy, aqueous environmental geochemistry and chemical kinetics. The specific research tasks of this proposal are to: (1) characterize the GR transformation process under anoxic conditions and determine its transformation products using XRD, TEM and M?ssbauer Spectroscopy; (2) monitor the reaction kinetics between GR/its transformation products and U(VI) via spectrophotometer and ICP-MS, respectively, and examine reaction products by FTIR, XPS, XRD and TEM; (3) analyze and fit the experimental data using MINEQL and SCIENTIST softwares, determine how adsorption, reduction and incorporation processes affect the transformation of uranium, and explore the reaction mechanisms. This study is expected to figure out an optimum reaction pathway for uranium long-term immobilization, which could provide the basis for future development of new remediation strategies for uranium.

铀污染地下水的修复，是我国铀矿冶地区亟待研究解决的重大而紧迫的环境问题。近来美国学者发现一种新型还原剂- - 绿锈，它可快速还原去除地下水中的放射性污染物铀（U），但其反应活性极强，会发生形态转变，这种转变如何影响铀的迁移转化，目前尚不清楚，亟需深入研究。本研究旨在查明厌氧条件下绿锈形态转变对地下水中铀的迁移和宿命的影响。本研究将验证如下假说：1）在典型厌氧地下水环境条件下，绿锈可通过形态转变形成其它铁（羟基）氧化物；2）绿锈形态转变将影响铀的环境行为，铀可能嵌入矿物晶格而被长期固定。本研究拟采用野外调查、实验研究与计算模拟相结合的方法，系统研究绿锈的形态转变过程，分别监测绿锈及其转变产物除U(VI)的速率和程度, 探讨其反应机理，筛选出最有利于长期固定U(VI)的反应途径。研究结果将查明绿锈及其转变产物在影响铀污染物迁移转化方面的机理，为我国铀污染地下水的长远治理提供科学依据和技术支持。

铀污染地下水广泛分布于全球27个国家和地区，直接影响地下水水质并威胁人的身体健康，是世界上亟需研究解决的重大环境问题。本项目以新发现的铀还原剂——绿锈为研究对象，与绿锈还原铀研究的开创者美国Edward O’Loughlin研究员合作，通过批式实验，采用动态磷光分析仪（KPA）、X-射线衍射（XRD）、透射电镜（TEM）和同步辐射X-射线吸收光谱（XAS）等先进技术手段，研究了氯化绿锈、硫酸盐绿锈和碳酸盐绿锈在地下水条件下的形态转变情况及其对铀污染物去除的影响，分析了Fe(II)和Fe(III)在除铀过程中所起的作用，监测了生物成因和化学成因碳酸盐绿锈除铀的反应动力学过程并探讨其反应机理。结果表明在厌氧地下水条件下，碳酸盐绿锈最稳定，硫酸盐绿锈次之，氯化绿锈最不稳定，这三种绿锈之间的转变关系如下：碳酸盐绿锈在含Cl-或SO42-的地下水条件下基本不能转变为氯化绿锈或硫酸盐绿锈；硫酸盐绿锈在含Cl-的地下水条件下基本不能转变为氯化绿锈，但在含CO32-的地下水条件下可以转变为碳酸盐绿锈；氯化绿锈在含CO32-或SO42-的地下水条件可以分别转变为碳酸盐绿锈或硫酸盐绿锈。在有氧地下水条件下，这三种绿锈均可转变为磁铁矿、针铁矿和纤铁矿等其它铁矿物。碳酸盐萃取反应动力学和反应产物分析表明还原作用是铀去除的主导机制，铁络合实验表明Fe(II)和Fe(III)在除铀反应中可能扮演电子穿梭体的角色，促进铀还原反应的进行，在铁络合剂存在时，铀的还原速率明显减慢；在碳酸根和钙离子共存的地下水环境条件下，生物成因和化学成因碳酸盐绿锈均能快速还原去除地下水中的铀污染物，U(VI)被还原为U(IV)，但生物成因和化学成因碳酸盐绿锈的铀还原产物的赋存形态存在显著差异，这直接影响铀污染物的环境稳定性。上述结果深化了地下水环境条件下绿锈与铀相互作用的认识，为利用绿锈修复铀污染地下水提供了科学依据。