Research paperEmergent thallium exposure from uranium mill tailings
Graphical Abstract
Introduction
The utilization of uranium (U) ores, especially those in the low grade, can inevitably generate large amounts of undesirable uranium mill tailings (UMT). For example, U deposits in China are mainly found in middle to low grade, and the grade of 0.1–0.3% accounts for 60% of the total reserves (Zhang et al., 2008). Undoubtedly, the vast and continuous production of UMT can lead to environmental pollution and pose a severe and long-term threat to human beings, due to the fact that U has both radioactivity and chemical toxicity (Sun et al., 2014, Pan et al., 2016, Ren et al., 2020, Singhal et al., 2020; Wang et al., 2020a). It is important to note that the UMT is not only the source of U pollution, but also the cause of toxic metal contamination. The release characteristics of radionuclides and other conventional heavy metals (such as Pb, Zn and Cu) from UMT (Déjeant et al., 2016, Liu, et al., 2017a, Liu et al., 2017b, Liu et al., 2020), as well as their geochemical behaviors, environmental impact assessment and pollution control in the environment have been extensively investigated and explored, disclosing important environmental implications and perspective in remediation strategy (Liu et al., 2015, Déjeant et al., 2016, Hu et al., 2016, Wang et al., 2018). However, hitherto, to our best knowledge, few study has been carried out to focus on the thallium (Tl) exposure from UMT.
In the past decades, Tl pollution has been overall neglected in the process of mineral deposits exploitation and utilization because Tl is highly dispersed and with a low content in the environment (Liu, et al., 2017a, Liu et al., 2017b, Liu et al., 2018b; Wang et al., 2020b). However, it has extremely severe biological toxicity which is higher than some well-known toxic metal(loid)s, such as Hg, Pb, As and Cr whose toxicity are well recognized (Beiyuan et al., 2017, Beiyuan et al., 2020; Sun et al., 2018; Zhang et al., 2018; Liu et al., 2019a; Rinklebe et al., 2020; Wang et al., 2020c). The high biological toxicity caused by Tl is mainly attributed to its monovalent form, viz., Tl+, which has similar ionic radii with K+ (an essential element for many metabolism activities) (Liu et al., 2020). Consequently, Tl+ can act as a substitute for K+ and concentrated in plants, animals and human bodies via food chains, posing considerable health risk (Karbowska, 2016, Cruz-Hernandez et al., 2018; Liu et al., 2021).
In recent years, more research interest has been given to Tl geochemical behaviors, which can shed novel light on the mechanisms of Tl migration and transformation in a wide range of environmental media like water, sediments, soils and plants (Li et al., 2019; Li et al., 2020; Liu et al., 2019, Liu et al., 2019, Zhou et al., 2020; Wang et al., 2021). Notably, Tl is a lithophile and sulphophile element, which makes it easily accumulated in K-feldspar, mica and sulphides (such as galena and pyrite) (Liu et al., 2016 and references therein). These associated minerals in U ores can be still observed in the UMT after hydrometallurgical process, thereby the occurrence of environmental issue of Tl can arise from the disposal of UMT. It was reported that Tl concentration up to 8700 μg/L was occasionally observed in the wastewater of a U mine area, which significantly exceeded the discharge standard (5 μg/L) (GPEPA-GBQTS, 2017) in China (Chen et al., 2017). The high Tl concentration in the wastewater verifies the possibility of Tl leaching from UMT. However, the large knowledge gap between Tl release driven by geochemical behavior and the mineralogical impacts combined with influencing factors like grain size and pH needs to be bridged.
Therefore, great effort should be strengthened on the Tl leaching, mobilization, and transportation from the UMT, which is conducive to comprehensive prevention and control of the toxic metal pollution originating from UMT. Herein, a series of semi-dynamic leaching experiments were performed with UMT. The aims of this study are to (i) ascertain Tl release behavior and capacity under varied conditions and (ii) reveal the mechanisms controlling Tl release from UMT. The study would benefit the establishment of preventive strategies for Tl immobilization of the UMT.
Section snippets
Site and sample information
The studied UMT was obtained from a granitic U deposit in South China (Luo et al., 2017, Liu et al., 2018). After extraction of U, UMT was stacked on the land surface and subjected to weathering and leaching, which led to the release of radionuclides and heavy metals (Liu et al., 2015, Wang et al., 2016). The medium-grained biotite granite, Early Jurassic porphyritic and Late Jurassic muscovite microgranite are dominant constitutes of the studied UMT, while pitchblende, pyrite, hydrogoethite,
Concentration and cumulative amount of Tl release
Long-term release behaviors of Tl from UMT in different scenarios were investigated elaborately. As shown in Fig. 1a, the variation of Tl concentration in leachates from UMT with different particle sizes under pH 6.0 condition showed different trends. Specifically, the release of Tl from UMT<0.45 mm, which is the smallest particle size, was at a low level in the first seven weeks, and significantly elevated in the later leaching period with a concentration range of 0.16–0.38 μg/L. The release
Conclusions
In the scenarios of UMT with varied particle sizes and leachant with different pH, elevated U release capacity was found in UMT with the smallest particle size (UMT<0.45 mm) and leachant with the highest pH (UMTpH=3.0), respectively. Moreover, the Tl cumulative release amount of the former was higher than that of the latter, while the content of labile Tl in the UMT<0.45 mm was not the highest. The high specific surface area associated with the small particle size is the predominant factor
CRediT authorship contribution statement
Meiling Yin: Writing - original draft, Formal analysis. Yuting Zhou: Writing - review & editing. Daniel C.W. Tsang: Writing - review & editing. Jingzi Beiyuan: Writing - review & editing. Lan Song: Editing. Jingye She: Editing. Jin Wang: Conceptualization, Supervision, Project administration, Writing - review & editing. Li Zhu: Writing - review & editing. Fa Fang: Editing. Lulu Wang: Editing. Juan Liu: Project administration, Writing - review & editing. Yanyi Liu: Editing. Gang Song: Review.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Natural Science Foundation of China (Nos. 41773011, 41873015, 41830753 and 41877290), the Guangdong Provincial Natural Science Foundation (2017A030313247 and 2014A030313527), the Scientific Research Projects in Colleges and Universities of Guangzhou Education Bureau, Guangzhou, China (201831803), the Guangzhou University's 2021 training program for young top-notch personnels, and the “Challenge Cup” Undergraduate Program.
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