U同位素分馏行为及其在环境地球科学中的应用研究进展

doi:10.13745/j.esf.sf.2022.10.44

摘要/Abstract

摘要：

近年来,研究铀(U)及其同位素有关的地球化学指标在地球环境科学领域发挥着越来越重要的作用。为加快我国U同位素发展和应用,本文系统回顾了近20年来U及其同位素的地球化学行为、U同位素分析测试技术、U循环与地表U同位素组成、U同位素分馏机理以及U同位素在环境科学领域的应用进展与技术壁垒。综述表明:U作为氧化还原敏感元素,在自然和人为活动中,U同位素存在显著分馏现象。U同位素已初步成功应用于示踪现代陆地表生环境系统中U的分布、迁移和扩散行为,重建地质历史时期环境与生命协同演化过程等地球环境科学领域。但总体而言,国内外U同位素研究工作仍处于起步阶段,多数局限于定性分析,在应用中也存在一些问题亟待解决,例如:特殊价态的U同位素仍无法测量,这制约了进一步对氧化还原过程中U同位素分馏机理的认识;地下水U污染处理手段缺失,表生沉积物的U同位素测试数据缺乏系统性和区域性,大气定量源计算解析模型还未完全建立;碳酸盐岩成岩过程中的U同位分馏机理尚不清楚,其分馏校正因子难以确定;黑色页岩包含海水和沉积物混杂的化学信号,难以准确扣除局部无效分馏信号;对非重大地质事件时期的关注较少导致无法恢复地球完整的氧化还原历史;U同位素高温地球化学分馏机理与应用基本处于空白等。未来,进一步改善U同位素测试技术,提高仪器检测分析精度,进行更系统的U同位素测试,建立各类型U同位素数据库,深化U同位素分馏机理认识,筑牢U同位素应用基石,尝试U同位素在更多领域的探索,以及将U同位素与其他氧化还原系统相关的示踪剂联合使用以提高示踪的准确性和可靠性,是U同位素研究的主要方向。本文对进一步推动我国U同位素在环境地球科学领域中的应用具有深远意义。

关键词: U同位素, U循环, 分馏机理, 环境科学, 地球科学, U示踪

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

中图分类号:

P597.1
X14

李茜, 朱光有, 李婷婷, 陈志勇, 艾依飞, 张岩, 田连杰. U同位素分馏行为及其在环境地球科学中的应用研究进展[J]. 地学前缘, 2024, 31(2): 447-471.

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.

图/表 20

图1 铀(U)元素基本性质

Fig.1 Basic properties of uranium(U) element

图2 自然界中U的形态(据文献[11]修改)

Fig.2 The form of U in nature. Modified after [11].

表1 天然U同位素相关信息

Table 1 Natural U isotope information

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

表2 利用U-TEVA树脂分离U步骤(据文献[16]修改)

Table 2 Separating U steps with U-TEVA resin. Modified after [16].

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

图3 地质历史时期的全球U同位素循环的示意图(图据文献[15] 修改) a—24 亿年前全球U循环;b—24亿年~6亿年前的全球U循环;c—6亿年前至现今的全球U循环。 AOC代表蚀变洋壳; MORB代表洋中脊玄武岩; OIB代表洋岛玄武岩; Arc代表大洋岛弧。

Fig.3 Schematic diagram of the global U isotope cycle during geological history. Modified after [15].

图4 现代海洋U循环及同位素组成(据文献[16]修订) U同位素平衡关系为δ238U河流=δ238U强还原×f强还原+δ238U弱还原×f弱还原+δ238U原生碳酸盐岩×f原生碳酸盐岩+δ238U铁锰结壳×f铁锰结壳+δ238U蚀变洋壳×f蚀变洋壳,f为各个储库所占比例。

Fig.4 Modern marine U cycle and isotopic composition. Modified after [16].

表3 U同位素分馏机理

Table 3 U isotope fractionation mechanism

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]

图5 微生物还原U同位素实验(数据引自文献[13,76⇓⇓-79]) a—微生物的参与能够使有机质降解为CO2,同时将U6+还原为U4+;b—前人利用微生物生物还原U6+,所测还原沉积物中δ238U值。

Fig.5 Microbial reduction of U isotope experiments. Data from [13,76⇓⇓-79].

图6 非生物还原U同位素(据文献[78]修改)

Fig.6 Nonbiological reduced U isotope. Modified after [78].

图7 吸附过程U同位素分馏(数据来源于文献[14,61,82⇓-84,97]) a—吸附过程中U同位素分馏;b—自然海洋中铁锰结核的U同位素分馏。

Fig.7 U Isotope fractionation in adsorption process. Data from [14,61,82⇓-84,97].

图8 现代海相碳酸盐δ238U数据的整理(数据来源于文献[13-14,16,51,63,67-68])

Fig.8 Collation of δ238U data of modern marine carbonate. Data from [13-14,16,51,63,67-68].

图9 生物吸收U及同位素分馏示意图(据文献[67]修改) a—菌核珊瑚; b—绿藻。

Fig.9 Biological absorption of U and isotope fractionation diagram. Modified after [67].

图10 巴哈马地区的碳酸盐岩δ238U值对比(据文献[85]修改) a—表生碳酸盐岩; b—同沉积碳酸盐岩; c—沉积后碳酸盐岩。

Fig.10 Comparison of δ238U values of carbonate rocks in the Bahamas. Modified after [85].

表4 U同位素在地球环境科学领域中的应用

Table 4 Application of U isotopes in the field of earth environmental science

U同位素的地质应用		应用结果	后续研究方向	典型案例来源
示踪陆地表生环境中U的分布、迁移和扩散	示踪地下水	地下水还原导致δ²³⁸U比值降低;氧化导致地下水δ²³⁸U比值升高;吸附与解析过程δ²³⁸U比值不变	后续还需要更加可靠且持久的地下水U污染处理手段	李高军等^[20];Jemison等^[97];Bopp等^[105];Jemison等^[106];Shiel等^[107];钟娟等^[108]
	示踪沉积物	U同位素是示踪沉积物中U污染源和驱动力的有力工具	未来可以开展更为系统的、区域范围更大的表生沉积物的U同位素测试与研究	黄钊等^[3];Wang等^[109]
	示踪大气	U同位素示踪大气污染技术具有灵敏度高、准确性好、定性识别污染来源和定量计算污染程度的优势	定量源计算解析模型有待进一步建立和完善	欧阳洁等^[27];Aciego等^[110];Bellis等^[111];Shinonaga等^[112]
重建地质历史时期中环境与生命协同演化过程	碳酸盐岩	海相碳酸盐岩的U同位素已成为定性确定古海洋氧化状态的重要工具,并可作为定量示踪全球氧化还原状态转化时间、持续时间和程度的指标	需要考虑成岩等作用影响因素,成岩作用分馏效应矫正需进一步研究	Lau等^[101];邱晨等^[113];Brennecka 等^[114];Dahl等^[115];Gilleaudeau等^[116]; Rooney等^[117];Cheng等^[118];Wei等^[119]
重建地质历史时期中环境与生命协同演化过程	黑色页岩	黑色页岩的U同位素极好地保存了原始U同位素信号	利用黑色页岩U同位素时需要准确扣除局部分馏信号;U同位素可以与其他氧化还原系统相关的示踪剂联合使用,从而提高准确性和可靠性	李聪颖等^[120];Kendall等^[121]; Kendall等^[122];Yang等^[123];Wang等^[124]; Wang等^[125]; Kendall等^[126];Lau等^[127]

表4 U同位素在地球环境科学领域中的应用

Table 4 Application of U isotopes in the field of earth environmental science

U同位素的地质应用		应用结果	后续研究方向	典型案例来源
示踪陆地表生环境中U的分布、迁移和扩散	示踪地下水	地下水还原导致δ²³⁸U比值降低;氧化导致地下水δ²³⁸U比值升高;吸附与解析过程δ²³⁸U比值不变	后续还需要更加可靠且持久的地下水U污染处理手段	李高军等^[20];Jemison等^[97];Bopp等^[105];Jemison等^[106];Shiel等^[107];钟娟等^[108]
	示踪沉积物	U同位素是示踪沉积物中U污染源和驱动力的有力工具	未来可以开展更为系统的、区域范围更大的表生沉积物的U同位素测试与研究	黄钊等^[3];Wang等^[109]
	示踪大气	U同位素示踪大气污染技术具有灵敏度高、准确性好、定性识别污染来源和定量计算污染程度的优势	定量源计算解析模型有待进一步建立和完善	欧阳洁等^[27];Aciego等^[110];Bellis等^[111];Shinonaga等^[112]
重建地质历史时期中环境与生命协同演化过程	碳酸盐岩	海相碳酸盐岩的U同位素已成为定性确定古海洋氧化状态的重要工具,并可作为定量示踪全球氧化还原状态转化时间、持续时间和程度的指标	需要考虑成岩等作用影响因素,成岩作用分馏效应矫正需进一步研究	Lau等^[101];邱晨等^[113];Brennecka 等^[114];Dahl等^[115];Gilleaudeau等^[116]; Rooney等^[117];Cheng等^[118];Wei等^[119]
重建地质历史时期中环境与生命协同演化过程	黑色页岩	黑色页岩的U同位素极好地保存了原始U同位素信号	利用黑色页岩U同位素时需要准确扣除局部分馏信号;U同位素可以与其他氧化还原系统相关的示踪剂联合使用,从而提高准确性和可靠性	李聪颖等^[120];Kendall等^[121]; Kendall等^[122];Yang等^[123];Wang等^[124]; Wang等^[125]; Kendall等^[126];Lau等^[127]

图11 U同位素示踪地下水U迁移和转化方式示意图(据文献[105⇓⇓-108]修改)

Fig.11 Diagram of U isotope tracing groundwater U migration and transformation. Modified after [105⇓⇓-108].

图12 不同样品(地下水、沉积物和铀矿)δ238U的比较(据文献[109]修改)

Fig.12 Comparison of δ238U in different samples(water, sediment and uranium ore). Modified after [109].

图13 显生宙生物大灭绝事件与海洋缺氧事件相关(据文献[132]修改)

Fig.13 Phanerozoic biological extinction and marine hypoxia. Modified after [132].

图14 碳酸盐岩、黑色页岩和铁锰结核指示地质历史时期中氧化还原状态原理模型 a—碳酸盐岩指示地质历史时期中氧化还原状态原理模型;b—黑色页岩指示地质历史时期中氧化还原状态原理模型;c—铁锰结核指示地质历史时期中氧化还原状态原理模型。

Fig.14 Principle model of carbonate rocks, black shale and ferromanganese nodules indicating redox state in geological history

图15 碳酸盐岩δ238U和δ13C垂向变化趋势示踪北美早密西西比期氧化还原事件(据文献[118]修改)

Fig.15 Vertical variation trend of δ238U and δ13C of carbonate rocks to trace redox events of Early Mississippi in North America. Modified after [118].

图16 南非Kaapvaal克拉通盆地陆缘海U地球化学循环(据文献[125]修改)

Fig.16 U geochemical cycle of continental margin sea in Kaapvaal craton basin, South Africa. Modified after [125].

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