Department of Earth and Atmospheric Sciences, University of Nebraska-Lincoln, Lincoln, NE, USA.
Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA, USA.
J Hazard Mater. 2021 Jun 15;412:125089. doi: 10.1016/j.jhazmat.2021.125089. Epub 2021 Jan 8.
Uranium minerals are commonly found in soils and sediment across the United States at an average concentration of 2-4 mg/kg. Uranium occurs in the environment primarily in two forms, the oxidized, mostly soluble uranium(VI) form, or the reduced, sparingly soluble reduced uranium(IV) form. Here we describe subsurface geochemical conditions that result in low uranium concentrations in an alluvial aquifer with naturally occurring uranium in soils and sediments in the presence of complexing ligands under oxidizing conditions. Groundwater was saturated with respect to calcite and contained calcium (78-90 mg/L) with elevated levels of carbonate alkalinity (291-416 mg/L as HCO). X-ray adsorption near edge structure (XANES) spectroscopy identified that sediment-associated uranium was oxidized as a uranium(VI) form (85%). Calcite was the predominant mineral by mass in the ultrafine fraction in uranium-bearing sediments (>16 mg/kg). Groundwater geochemical modeling indicated calcite and/or a calcium-uranyl-carbonate mineral such as liebigite in equilibrium with groundwater. The δC (0.57‰ ± 0.15‰) was indicative of abiotic carbonate deposition. Thus, solid-phase uranium(VI) associated with carbonate is likely maintaining uranium(VI) groundwater levels below the maximum contaminant level (MCL; 30 µg/L), presenting a deposition mechanism for uranium attenuation rather than solely a means of mobilization.
在美国，铀矿物普遍存在于土壤和沉积物中，平均浓度为 2-4mg/kg。铀在环境中主要以两种形式存在，一种是氧化的、大部分可溶的铀(VI)形式，另一种是还原的、难溶的铀(IV)形式。在这里，我们描述了在氧化条件下存在络合剂时，土壤和沉积物中存在天然铀的情况下，导致冲积含水层中铀浓度较低的地下地球化学条件。地下水处于方解石饱和状态，含有钙（78-90mg/L）和高碳酸盐碱度（291-416mg/L 以 HCO 计）。X 射线吸收近边结构（XANES）光谱分析表明，沉积物中与铀结合的铀为铀(VI)形式（85%）。在含铀沉积物的超细颗粒中（>16mg/kg），方解石是主要的矿物。地下水地球化学模型表明，方解石和/或钙铀碳酸盐矿物（如钙铀石）与地下水处于平衡状态。δC（0.57‰±0.15‰）表明是无生命的碳酸盐沉积。因此，与碳酸盐结合的固相铀(VI)可能将地下水中的铀(VI)水平维持在最大污染物水平（MCL；30μg/L）以下，为铀衰减提供了一种沉积机制，而不仅仅是一种迁移手段。
