Skierszkan Elliott K, Schoepfer Valerie A, Fellwock Matthew, Lindsay Matthew B J
Department of Earth Sciences, Carleton University, 2115 Herzberg Laboratories, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada.
Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada.
Environ Sci Technol. 2024 Sep 13. doi: 10.1021/acs.est.4c05594.
Uranium is a toxic and pervasive geogenic contaminant often associated with organic matter. Its abundance and speciation in organic-rich permafrost soils are unknown, thereby limiting our ability to assess risks associated with uranium mobilization during permafrost thaw. In this study, we assessed uranium speciation in permafrost soil and porewater liberated during thaw using active-layer and permafrost samples from a study area in Yukon, Canada where elevated uranium concentrations occur in bedrock and groundwater. Permafrost contained 1.1-28 wt % organic carbon and elevated uranium (range 7.6-1040 μg g, median 25 μg g) relative to local bedrock. The highest soil uranium concentrations were encountered in catchments hosting uranium-enriched bedrock and correlated positively with soil organic carbon. X-ray absorption spectroscopy, micro-X-ray fluorescence, and electron microscopy analyses revealed that solid-phase uranium predominantly occurs as uranium(VI) associated with soil organic matter. Extended X-ray absorption fine structure (EXAFS) analyses suggested the presence of uranium(VI) coordinated with carbon, consistent with bidentate-mononuclear uranyl complexation on carboxyl groups. Permafrost thaw produced circumneutral pH porewater (pH 6.2-7.5) with elevated dissolved uranium (0.5-203 μg L). Geochemical modeling indicated that calcium-uranyl-carbonate complexes dominated the dissolved uranium speciation. This study highlights that permafrost soil can mobilize uranium upon thaw and that uranium fate is linked to dynamic biogeochemical reactions involving organic carbon and groundwater chemistry.
铀是一种有毒且普遍存在的地质源污染物，常与有机物相关联。其在富含有机物的永久冻土中的丰度和形态尚不清楚，从而限制了我们评估永久冻土融化期间铀迁移相关风险的能力。在本研究中，我们使用来自加拿大育空地区一个研究区域的活动层和永久冻土样本，评估了永久冻土土壤及融化过程中释放的孔隙水中的铀形态。该研究区域的基岩和地下水中铀浓度升高。永久冻土含有1.1 - 28 wt%的有机碳，相对于当地基岩，铀含量升高（范围为7.6 - 1040 μg/g，中位数为25 μg/g）。在含有富含铀基岩的集水区中，土壤铀浓度最高，且与土壤有机碳呈正相关。X射线吸收光谱、微X射线荧光和电子显微镜分析表明，固相铀主要以与土壤有机物相关的铀(VI)形式存在。扩展X射线吸收精细结构（EXAFS）分析表明存在与碳配位的铀(VI)，这与羧基上的双齿单核铀酰络合一致。永久冻土融化产生了pH值为中性的孔隙水（pH 6.2 - 7.5），溶解铀含量升高（0.5 - 203 μg/L）。地球化学模型表明，碳酸铀酰钙络合物主导了溶解铀的形态。本研究强调，永久冻土土壤在融化时能够迁移铀，并且铀的归宿与涉及有机碳和地下水化学的动态生物地球化学反应有关。
