Adsorption of U(VI) on the natural soil around a very low-level waste repository

https://doi.org/10.1016/j.jenvrad.2021.106619Get rights and content

Highlights

  • Interactions of U(VI) with natural soil were studied.

  • Soil organic matters strongly complexed with U(VI).

  • The reduction of U(VI) by Fe(II) was found in natural soil.

  • The strong complexation and reduction made the adsorption irreversible.

Abstract

The behaviors of U(VI) in environmental media around radioactive waste disposal site are important for safety assessment of geological repositories. However, the estimation of environmental behaviors of U(VI) in natural media was insufficient. This work aimed to determine the adsorption of U(VI) on natural soil surrounding a candidate very low-level radioactive waste (VLLW) disposal site in southwest China. Results showed that the adsorption process of U(VI) on soils could be well supported by pseudo-second-order kinetic and Freundlich model. The adsorption of U(VI) was pH-dependent but temperature-independent. High ionic strength (NaCl) strongly affected the adsorption process at low pH (2.0–5.5). CO32− remarkably inhibited the U(VI) adsorption, while the adsorption of U(VI) was promoted by PO43− and SO42−. Naturally occurred soil organic matters (SOMs) showed high affinity for U(VI), while the presence of additional humic acid (HA) strongly inhibited U(VI) adsorption. The occurrence of ferrous iron could result in the reduction of U(VI) at low pH values (pH < 4), leading to the promotion of immobilization of U(VI). These findings would provide some guidance for the safety assessments of the VLLW disposal as well as the remediation of contaminated soil.

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.

References (70)

  • J.G. He et al.

    Migration of 75Se(IV) in crushed Beishan granite: effects of the iron content

    J. Hazard Mater.

    (2017)
  • Y.S. Ho et al.

    The kinetics of sorption of divalent metal ions onto sphagnum moss peat

    Water Res.

    (2000)
  • A.S. Kar et al.

    U(VI) sorption by silica: effect of complexing anions

    Colloids Surf., A

    (2012)
  • D.E. Latta et al.

    Abiotic reduction of uranium by Fe(II) in soil

    Appl. Geochem.

    (2012)
  • P. Li et al.

    Photoconversion of U(VI) by TiO2: an efficient strategy for seawater uranium extraction

    Chem. Eng. J.

    (2019)
  • J.F. Liu et al.

    Soil organic matter and silt contents determine soil particle surface electrochemical properties across a long-term natural restoration grassland

    Catena

    (2020)
  • Y.H. Liu et al.

    Removal of uranium(VI) from aqueous solutions by CMK-3 and its polymer composite

    Appl. Surf. Sci.

    (2013)
  • S. Manoj et al.

    Determination of distribution coefficient of uranium from physical and chemical properties of soil

    Chemosphere

    (2020)
  • T. Missana et al.

    Kinetics and irreversibility of cesium and uranium sorption onto bentonite colloids in a deep granitic environment

    Appl. Clay Sci.

    (2004)
  • Z.W. Niu et al.

    Effect of pH, ionic strength and humic acid on the sorption of uranium(VI) to attapulgite

    Appl. Radiat. Isot.

    (2009)
  • Z.W. Niu et al.

    Spectroscopic studies on U(VI) incorporation into CaCO3: effects of aging time and U(VI) concentration

    Chemosphere

    (2019)
  • C. Richter et al.

    Macroscopic and spectroscopic characterization of uranium(VI) sorption onto orthoclase and muscovite and the influence of competing Ca2+

    Geochem. Cosmochim. Acta

    (2016)
  • S. Sachs et al.

    Sorption of U(VI) onto an artificial humic substance-kaolinite-associate

    Chemosphere

    (2008)
  • R.K. Sani et al.

    Reduction of uranium(VI) under sulfate-reducing conditions in the presence of Fe(III)-(hydr)oxides

    Geochem. Cosmochim. Acta

    (2004)
  • P. Sharma et al.

    Sorption behaviour of nanocrystalline MOR type zeolite for Th(IV) and Eu(III) removal from aqueous waste by batch treatment

    J. Colloid Interface Sci.

    (2011)
  • G. Sheng et al.

    Interaction of uranium(VI) with titanate nanotubes by macroscopic and spectroscopic investigation

    J. Mol. Liq.

    (2015)
  • Y.L. Shi et al.

    Sorption of U(VI) onto natural soils and different mineral compositions: the batch method and spectroscopy analysis

    J. Environ. Radioact.

    (2019)
  • N.K. Soliman et al.

    Industrial solid waste for heavy metals adsorption features and challenges; a review

    J. Mater. Res. Technol.

    (2020)
  • T.E.M. Ten Hulscher et al.

    Effect of temperature on sorption equilibrium and sorption kinetics of organic micropollutants - a review

    Chemosphere

    (1996)
  • E. Tertre et al.

    Europium retention onto clay minerals from 25 to 150 °C: experimental measurements, spectroscopic features and sorption modelling

    Geochem. Cosmochim. Acta

    (2006)
  • C. Tournassat et al.

    Modeling uranium(VI) adsorption onto montmorillonite under varying carbonate concentrations: a surface complexation model accounting for the spillover effect on surface potential

    Geochem. Cosmochim. Acta

    (2018)
  • L.D. Troyer et al.

    Effect of phosphate on U(VI) sorption to montmorillonite: ternary complexation and precipitation barriers

    Geochem. Cosmochim. Acta

    (2016)
  • M.K. Uddin

    A review on the adsorption of heavy metals by clay minerals, with special focus on the past decade

    Chem. Eng. J.

    (2017)
  • R.D. Van Der Weijden et al.

    Sorption and sorption reversibility of cadmium on calcite in the presence of phosphate and sulfate

    Mar. Chem.

    (1997)
  • Y. Vijaya et al.

    Modified chitosan and calcium alginate biopolymer sorbents for removal of nickel(II) through adsorption

    Carbohydr. Polym.

    (2008)
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