Uranium isotope fractionation and application of uranium isotopes in environmental geosciences—a review

doi:10.13745/j.esf.sf.2022.10.44

Abstract

Abstract:

In recent years, uranium (U) isotopes as geochemical indicators play an increasingly important role in environmental studies. To accelerate the development and application of U isotopes in China, this paper performs a systematic review on U geochemistry, U isotope analysis and testing techniques, U cycling and isotopic composition of surface U, mechanism of U isotope fractionation, and research progress on and technical barriers to the application of U isotopes in environmental geosciences in the past two decades. Uranium as a redox-sensitive element, significant U isotope fractionation has been observed in natural and anthropogenic environments. Uranium isotopes have been successfully applied to trace U distribution, transport, and diffusion behavior in modern terrestrial epigenetic systems and to reconstruct the history of synergistic coevolution between the environment and living organisms over geological and historical periods. However, in general, U isotope research both at home and abroad is still in its infancy, mostly limited to qualitative analysis, and some application issues need to be solved. For example, U isotopes with certain valency are still not measurable, hindering a further understanding of the mechanism of U isotope fractionation during redox processes; other lackings include the means to deal with U contamination in groundwater; systematic, regional U isotope test data on epigenetic sediments; and analytical models for quantitative atmospheric source calculations. Among other issues, the mechanism of U isotope fractionation during carbonate rock formation is still unclear and the fractionation correction factor is difficult to determine. In paleoreconstruction, the use of U isotopes of black shale is problematic as accurate isotope data are difficult to obtain due to local signal interference, and the lack of isotope data on non-major geological events makes it impossible to recover the complete redox history of the Earth. Research on the geochemical mechanism of U isotope fractionation at high temperature and related application is scarce. Future directions on U isotope research and application include: improve testing techniques; increase precision in instrumental detection/analysis; conduct more systematic testing; establish comprehensive databases; deepen mechanistic understanding of isotopic fractionation; improve application quality; expand application to more fields; use multi-element tracers to improve tracer accuracy. This paper provides an essential reference for the application of U isotopes in environmental Earth sciences in China.

Key words: U isotope, U cycling, fractionation mechanism, environmental science, Earth sciences, U tracing

CLC Number:

P597.1
X14

LI Xi, ZHU Guangyou, LI Tingting, CHEN Zhiyong, AI Yifei, ZHANG Yan, TIAN Lianjie. Uranium isotope fractionation and application of uranium isotopes in environmental geosciences—a review[J]. Earth Science Frontiers, 2024, 31(2): 447-471.

Figures/Tables 20

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同位素	半衰期/Ma	放射性比/ (Bq·g^-1)	丰度比例/%
²³⁴U	0.248	2.31×10⁸	0.005 4
²³⁵U	703.8	8.00×10⁴	0.720
²³⁸U	4 468	1.24×10⁴	99.275

同位素	半衰期/Ma	放射性比/ (Bq·g^-1)	丰度比例/%
²³⁴U	0.248	2.31×10⁸	0.005 4
²³⁵U	703.8	8.00×10⁴	0.720
²³⁸U	4 468	1.24×10⁴	99.275

步骤	加酸介质	体积	目的
清洗柱	0.05 mol/L HCl	20 mL	清洗柱
平衡柱	3 mol/L HNO₃	6 mL
上样	3 mol/L HNO₃	10 mL
淋洗	3 mol/L HNO₃	40 mL	仅保留Th、U和Np
盐酸介质	1 mol/L HCl	6 mL
接Th,Np	5 mol/L HCl+ 0.1 mol/L Oxalic	20 mL	洗掉Th、Np
去草酸	5 mol/L HCl	10 mL
接U	0.05 mol/L HCl	25 mL	回收U

步骤	加酸介质	体积	目的
清洗柱	0.05 mol/L HCl	20 mL	清洗柱
平衡柱	3 mol/L HNO₃	6 mL
上样	3 mol/L HNO₃	10 mL
淋洗	3 mol/L HNO₃	40 mL	仅保留Th、U和Np
盐酸介质	1 mol/L HCl	6 mL
接Th,Np	5 mol/L HCl+ 0.1 mol/L Oxalic	20 mL	洗掉Th、Np
去草酸	5 mol/L HCl	10 mL
接U	0.05 mol/L HCl	25 mL	回收U

U同位素分馏		分馏机理	研究实例代表
氧化还原过程中 U同位素分馏	微生物还原	生物还原过程富集重同位素²³⁸U	Rademacher等^[76];Basu等^[77];Stylo等^[78];Basu等^[79];Stirling等^[80]
氧化还原过程中 U同位素分馏	非生物还原	无机非生物还原过程比较复杂,可富集重的²³⁸U,也可富集轻的²³⁵U,也可以不分馏	Stirling等^[13];Stylo等^[78];Brown等^[81]
吸附过程中U同位素分馏		吸附过程导致吸附剂富集轻同位素²³⁵U,同位素平衡分馏值约为-0.20‰;实验测定的同位素分馏与自然样品一致	Weyer等^[14];Goto等^[61];Brennecka等^[82]; Dang等^[83];Chen等^[84]
碳酸盐岩沉淀及成岩过程中U 同位素分馏	碳酸盐岩共沉淀及生物作用的U同位素分馏	碳酸钙共沉淀过程存在U同位素分馏,δ²³⁸U取决于溶解态U的中性组分含量,中性组分含量越高,则分馏系数越大;生物效应影响碳酸钙中U的分馏,U同位素分馏大小取决于钙化区的开放程度	Chen等^[66];Chen等^[67] ;Chen等^[85]
碳酸盐岩沉淀及成岩过程中U 同位素分馏	碳酸盐岩成岩过程中U同位素分馏	碳酸盐岩成岩作用导致分馏,但沉积后的成岩作用未导致进一步分馏;准同生白云石化不会导致进一步U同位素分馏	Romaniello等^[63];Tissot等^[68];Chen等^[85]; Hood等^[86];Herrmann等^[87];Hood等^[88];Dahl等^[89]