Adsorption of U(VI) on the natural soil around a very low-level waste repository
Graphical abstract
Introduction
The rapid development of nuclear power industry has caused an increasing attention to the treatment of radioactive wastes (Zhang et al., 2015). For a very low-level radioactive waste (VLLW) disposal site, shallow land geological repositories are designed as a multi-barrier system to ensure that radionuclides are not released from the site. However, over an extended period of time, the repository may be damaged by some unpredictable factors (geological activity or climate variations, such as earthquakes and landslides). Moreover, groundwater may interact with engineered barrier materials, resulting in the invalidation of barrier materials. Thus, the environmental behaviors of potentially released radionuclides in the surrounding natural ecosystems have emerged as an important concern in recent years.
As the dominant radionuclide by mass in radioactive wastes, uranium was widely concerned due to its long half-life, high solubility, and high chemotoxicity (Xie et al., 2019). In the environment, uranium typically occurs as soluble and mobile hexavalent uranium (U(VI)) species (Cao et al., 2020). The environmental behaviors of U(VI) are mainly controlled by geochemical processes such as adsorption/desorption, precipitation/diffusion, and redox reactions (Zhou et al., 2020). Adsorption plays a particularly important role in the retention and transport of U(VI) in natural systems. In recent years, the adsorption behaviors of U(VI) have been widely explored on pure minerals (muscovite, montmorillonite, calcite, attapulgite, etc.) (Niu et al., 2009, 2019; Richter et al., 2016; Troyer et al., 2016; Zhou et al., 2020; Dong et al., 2014; Kar et al., 2012) and (hydr)oxides (especially on Fe oxy(hydr)oxides) (Boland et al., 2014; Roberts et al., 2017; Sani et al., 2004). However, less attention has been paid to the reactions between U(VI) and natural soils. Compared with simple pure minerals and metal oxides, the adsorption behavior of cations on natural soils are generally much more complicated but more meaningful. For complicated natural soils, the adsorption behaviors of metals are usually controlled by many factors such as concentration of these ions between phases, soil components, organic matters, clay minerals, metal (hydr)oxides), and soil properties (the cation exchange capacity (CEC), grain size distribution, soil wettability, etc.) (Manojet al., 2020; Liu et al., 2020). For instance, naturally occurring organic matters in soils, such as fulvic substances (FA) and humic substances (HS), could strongly complexes with soluble U(VI), subsequently governing the adsorption/desorption process of U(VI) (Barger and Koretsky, 2011; Bordelet et al., 2018). Bednar et al. (2007) also found that the sorption of U(VI) in soils collected from Vicksburg and Yuma was strongly affected by the content of soil organic matters (SOMs). In cases for minerals containing reductive regents such as Fe2+, the reduction of U(VI) may become one of the main factors controlling the environmental behaviors of U(VI) (Roberts et al., 2017). For instance, in a Fe-rich natural soil taken from a hillside spring in Iowa, the abiotic reduction and immobilization of U(VI) was observed (Lattaet al., 2012). As a consequence, a better elucidation for the behaviors of uranium in the local geological environment, especially in natural soil around disposal site is urgently needed.
To evaluate the risk assessment of potentially released U(VI), it is necessary to illustrate the behavior of U(VI) in surrounding environment. Unfortunately, to the best of our knowledge, there is a lack of systematic investigation on U(VI) interaction with natural soil around repository site, and the corresponding mechanisms remain unknown. In this study, forest soil around the VLLW repository sites in China was selected as the adsorbent to estimate the adsorption behavior of U(VI) in natural media. Various environmental factors on U(VI) adsorption were evaluated. The reaction mechanisms between U(VI) and soil were also addressed combining batch experiments and spectroscopic methods. The objectives of this study are to shed light on the pre-safety performance estimation of the VLLW repository and environmental protection.
Section snippets
Materials
The stock solution of U(VI) with a concentration of 100 mg L−1 was prepared by dissolving UO2(NO3)2•6H2O solids into deionized water, the pH value was adjusted to below 3.0. Soil humic acid (C9H9NO6) was extracted from Gannan soil (Fan et al., 2009). All chemicals used in this study were of analytical grade.
Surface mineral soil (0–15 cm depth) excluding O horizon was collected from a forest near the candidate VLLW disposal site in southwest China. The type of the soil was the sandy soil. The
Characterization
The forest soil mainly consists of 51.92% SiO2, 15.73% Al2O3, 11.79% Fe2O3, and CaO, P2O5, MgO, Na2O, TiO2, K2O as minor components (Table S1 in the supplementary materials). The content of soil organic matter is 4.25%. The soil organic matters exhibited abundant functional groups, including carboxyl, amino, and hydroxyl (Fig. S1 in the supplementary materials), which could strongly influence the adsorption process of U(VI) on soil. The mineralogical compositions determined by XRD (Fig. S2)
Conclusions
Adsorption behavior and interface interaction mechanisms of U(VI) on natural soil near VLLW disposal site were investigated comprehensively by batch experiments and spectroscopic techniques. The adsorption process of U(VI) on soil fitted the pseudo-second-order kinetic and Freundlich model well. The occurrence of SOMs in soil caused a strong complexation with U(VI). At relatively low pH values (<4.0), U(VI) could be reductively immobilized by Fe(II) in soil. Both strong complexation and
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.
Acknowledgement
Financial supports from National Natural Science Foundation of China (21876172), the “Youth Innovation Promotion Association CAS”, Gansu Talent and Intelligence Center for Remediation of Closed and Old Deposits, and the Key Laboratory Project of Gansu Province (1309RTSA041) are acknowledged.
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