Carbonate facilitated mobilization of uranium from lacustrine

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Carbonate facilitated mobilization of uranium from lacustrine sediments under anoxic conditions Marina Seder-Colomina, Arnaud Mangeret, Lucie Stetten, Pauline Merrot, Olivier Diez, Anthony Julien, Evelyne Barker, Antoine Thouvenot, John Bargar, Charlotte Cazala, and Guillaume Morin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01255 • Publication Date (Web): 07 Jul 2018 Downloaded from http://pubs.acs.org on July 8, 2018

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Environmental Science & Technology

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Carbonate facilitated mobilization of uranium from lacustrine

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sediments under anoxic conditions

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Marina Seder-Colomina1, Arnaud Mangeret1*, Lucie Stetten1,2, Pauline Merrot2, Olivier

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Diez1, Anthony Julien1, Evelyne Barker1, Antoine Thouvenot3, John Bargar4, Charlotte

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Cazala1, Guillaume Morin2

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1 Institut de Radioprotection et de Sûreté Nucléaire (IRSN). PRP-DGE. BP 17. Fontenay-aux-

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Roses. 92262 France

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2 Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC). UMR

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CNRS 6023. Paris. 75005 France.

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3 Laboratoire Microorganismes Génome et Environnement (LMGE). UMR CNRS 6023.

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Aubière. 63177 France

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4 Stanford Synchrotron Radiation Lightsource (SSRL). Menlo Park, CA 94025, USA

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*Corresponding author:

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Tel +33 1 58 35 76 95

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Fax +33 1 46 57 62 58

[email protected]

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1 ACS Paragon Plus Environment


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ABSTRACT

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Sorbed U(IV) species can be major products of U(VI) reduction in natural reducing

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environments as sediments and waterlogged soils. These species are considered more labile

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than crystalline U(IV) minerals, which could potentially influence uranium migration in

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natural systems subjected to redox oscillations. In this study, we examined the role of oxygen

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and carbonate on the remobilization of uranium from contaminated lake sediments, in which

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∼70 % of the 150-300 ppm U is under the form of mononuclear U(IV) sorbed species. Our

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results show that both drying and oxic incubation only slightly increase the amount of

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remobilized U after 8 days, compared to anoxic drying and anoxic incubation. In contrast, the

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amount of remobilized U increases with the quantity of added bicarbonate even under anoxic

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conditions. Moreover, U LIII-edge XANES data shows that a significant amount of the solid

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U(IV) is mobilized in such conditions. Thermodynamic speciation calculations based on the

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supernatant composition indicates the predominance of aqueous UO2(CO3)34- and, to a lesser

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extent, CaUO2(CO3)32- complexes. These results suggest that monomeric U(IV) species could

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be oxidized into aqueous U(VI) carbonate complexes even under anoxic conditions via

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carbonate promoted oxidative dissolution, which emphasizes the need for considering such a

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process when modeling U dynamics in reducing environments.


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1. INTRODUCTION

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Uranium (U) pollution is a major concern to the environment and potentially to human

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health because of the chemical and radiological toxicity of this element (Brugge et al., 2005).

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Uranium has been released in terrestrial and aquatic ecosystems from natural sources and,

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during the last decades, from anthropogenic activities (WHO, 2011). The contribution of

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former uranium mining activities is unequivocal for uranium and other metals as evidenced by

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local contaminations of lake sediments (Morin et al., 2018; Laird et al., 2014), aquifers (Janot

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et al., 2016), surface waters (Wang et al., 2012), soils (Haribala et al., 2016) or wetlands

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(Wang et al., 2013, 2014). For instance, in France, sediments of several water bodies have

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been reported to be enriched in uranium as a consequence of former uranium mining activities

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from 1948 to 2001 (GEP, 2010). Appropriate management of these U-enriched materials are

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currently investigated and developed in order to minimize the risks of U remobilization and

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therefore exposure to living organisms. Among existing management strategies for the

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treatment of U-impacted natural materials, remediation options mainly focus on microbial

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reduction of U(VI) to U(IV) processes such as biostimulation operations (Yabusaki et al.,

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2007). Indeed, U(IV) compounds are mainly reported to be sparingly soluble in reducing

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conditions (McCleskey et al., 2001; Guillaumont et al., 2003; Bernier-Latmani et al., 2010).

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Another option consists in using former uranium mining sites for the storage of U-enriched

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fresh sediments after dredging operations, such as in France (GEP, 2010). As a result, these

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sediments might be exposed to variable physico-chemical conditions that could influence the

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ultimate fate of uranium. For instance, the disposal of lacustrine anoxic sediments into open-

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air surface-storage facilities could induce U(IV) reoxidation to U(VI) (Moon et al., 2007;

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2012), which might then potentially facilitate uranium mobilization in the form of aqueous

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complexes of the uranyl ion UO22+. Such unwanted reactions are expected to occur especially

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when uranium is present in the form of sorbed U(IV) species that are considered as being 4 ACS Paragon Plus Environment

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more labile than crystalline U(IV) minerals (Alessi et al., 2012, 2014). Such sorbed U(IV)

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species have been recently identified in wetland soils (Wang et al., 2013; Mikutta et al.,

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2016), alluvial aquifer sediments (Campbell et al., 2012), and lake sediments (Morin et al.,

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2016 ; Stetten et al., 2018).

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To date, most studies about U(IV) remobilization are motivated by the concern of the long-

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term stability of both uraninite (Bargar et al., 2008; Sharp et al.; 2011)and non-uraninite

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species (Wang et al., 2014), especially for in situ bioreduction strategies applied to

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remediation of subsurface uranium legacies (Bargar et al., 2008). In such a context, non-

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uraninite U(IV) species have been shown to be less stable than uraninite under both oxic and

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anoxic conditions (Alessi et al., 2012; Sharp et al., 2011; Cerrato et al., 2013; Latta et al.,

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2016; Newsome et al., 2015; Noël et al., 2017). Uranium mobility also largely depends on

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complexation of U(IV) and U(VI) with a variety of ligands. For instance, mobilization of

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U(IV) in bioreduced sediments has been explained by uranium complexation to aqueous

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organic ligands (Luo et al., 2008, 2011) and U(IV) may be mobilized in wetlands soils in the

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form of colloidal species (Wang et al., 2013). In addition, laboratory experiments have given

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evidences for carbonate promoted oxidative dissolution of UO2 under reducing conditions

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(Ulrich et al., 2008, 2009). Evaluating the role of this latter mechanism on the dissolution of

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non-uraninite species may further help in deciphering among the various possible processes

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that control the long term behavior of uranium in natural settings and repository sites.

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In this study we aimed at evaluating the influence of carbonates and redox conditions on

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the remobilization of uranium from lacustrine sediments in which 70% of U is present as

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mononuclear U(IV) (Morin et al., 2016). We have investigated the influence of oxygenation

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during drying and incubation of the sediments in order to mimic dredging operations and

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surface storage of U-enriched sediments. In addition, we have evaluated the ability of aqueous

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carbonate species to remobilize U(VI) and U(IV) under such conditions. Thanks to the 5 ACS Paragon Plus Environment


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monitoring of both aqueous chemistry and U speciation in the solid phase, we show that

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aqueous U(VI) carbonate species as well as oxygenation conditions play a major role on U

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remobilization from the sediments.

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2. MATERIALS AND METHODS 2.1.

Sediment samples

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Sediments cores up to ~1.80 meters length were collected in 2011, 2012 and 2015 in Lake

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Saint Clément, located at approximately 20 kilometers downstream from the former uranium

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mine of the Bois Noirs - Limouzat in the Massif Central (France). The lake is supplied by the

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Besbre River, in which treated mine waters from the U-tailings pond and the former open-pit

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mine are discharged (Figure S1). Coring operations were conducted from a boat using an

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Uwitec© gravity hand corer, at approximately 120 meters upstream from the dam. The

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sediments were conditioned in N2-filled glove bag on the field. Special care was given to the

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preservation of the uranium oxidation state during samples transport to the laboratory, using

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aluminized plastic films heat-sealed within the glove bag on the field and conditioned below

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4°C for transportation. A previous study of the Lake Saint Clément sediments reported an

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increase of U concentration with depth, from 40 mg.kg-1 at the top of the sedimentary column

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to 360 mg.kg-1 at 180-cm depth (Morin et al., 2016).

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For this study, sediment samples containing between 150 and 300 mg.kg-1 of U were

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selected within the 110-150 cm depth interval of the cores, which takes into account for U

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concentration gradient in the sedimentary columns of Lake Saint Clément (Morin et al.,

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2016). These sediments are characterized by a high organic content (12 wt% of total organic

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carbon) according to the previous chemical analyses (Morin et al., 2016). Within 48 h after

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field sampling, two types of drying conditions were applied to the fresh well-preserved 6 ACS Paragon Plus Environment

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sediment core samples, in order to yield two types of starting samples for the incubation

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experiments: i) oxidized sediments, referred to as O2-dried samples that were dried under

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surface storage-like open-air conditions, and ii) anoxic sediments referred to as Anox-dried

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samples that were vacuum-dried under N2 atmosphere in an anoxic glove box. Once dried,

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samples were conditioned in glass vials sealed by butyl rubber stopper and stored in the

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glovebox until the experiments. For each type of sample, O2 or Anox, three replicate samples

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were used, corresponding to cores respectively collected in March 2011, October 2012 and

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November 2015 at the same coring site, in order to integrate possible influence of seasonal

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conditions on U behavior. Extended X-ray Absorption Fine Structure (EXAFS) analysis by

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Morin et al. (2016) indicated that U was mainly present as mononuclear U(IV) in the Anox-

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dried sample. In the present study, EXAFS analysis showed that U was mainly present as

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mononuclear U(VI) in the O2-dried sample (Figure S2 and Table S1).

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2.2.

Experimental setup

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In order to investigate the effect of oxygen on uranium remobilization, O2-dried and Anox-

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dried samples were incubated under gentle stirring for 8 days, either under storage-like open-

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air conditions (referred to as O2/O2 and Anox/O2 experiments, respectively) or inside an N2-

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filled glove box (referred to as O2/Anox and Anox/Anox experiments respectively). A solid-

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to-liquid ratio of 1:100 was used for the experiments. For this purpose, 0.6 g of sediments

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were incubated for each condition with ultrapure water (O2-free or not). At each sampling

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time, 0.75 mL were collected, filtered through 0.2 µm PTFE syringe filters, and acidified and

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diluted for analyses if needed. Dissolved oxygen concentration was monitored during the

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experiments and values were 8.90 ± 0.6 mg.L-1 (Cell Ox® 325 O2 electrode with a WTW

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Multi 340i handheld meter) for O2 incubation conditions and 0.0009 ± 0.0003 mg.L-1 ,

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calculated using Henry’s law, based on PO2 measured in the anoxic glovebox atmosphere



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(JACOMEX GPT3 glove box, ± 1 ppm) for Anox incubation conditions, respectively. Three

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concentrations of bicarbonate (0, 0.01 and 0.05 M HCO3-) via initial addition of sodium

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bicarbonate were tested in order to assess their influence on uranium mobility. The first and

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latter values (0 and 0.05 M) correspond to negative and positive controls respectively and the

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0.01 M value is a proxy for alkalinity concentration found in the sediment porewaters (Stetten

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et al., 2018). Temperature (20 ± 2 °C) and relative humidity (28 ± 5%) (EL-USB-2,

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EasyLog®, Humidity and temperature logger) were constant through the experiments while

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pH varied freely from ~7 at the starting point to 6.2 ± 0.3, 8.5 ± 0.4 and 9.1 ± 0.4 for the three

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different HCO3- concentrations, respectively (Figure 1g-i).

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Liquid samples were aliquoted during the incubation (Time = 0, 1, 2, 4 and 8 days) and

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filtered through a 0.22 µm pore-size PTFE filter. Filtration was performed in an anoxic glove

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box for the anoxic incubation experiments. Analyses of alkalinity (Sarazin et al., 1999) and of

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dissolved iron speciation (Viollier et al., 2000)were performed by colorimetric methods

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immediately after filtration. Samples were acidified with concentrated ultrapure HNO3 for

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major and trace metals analyses by ICP-AES and ICP-MS. Non-acidified samples for

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TOC/TC/TIC measurements were stored at 4°C until analyses.

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In order to evaluate potential colloid-facilitated migration of uranium (Crançon et al.,

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2010), selected 0.22-µm filtrates of the 8-day incubation samples were refiltered at smaller

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cutoffs: 500 kDa and 10 kDa, using Poly-Ether-Sulfone ultrafiltration discs and an Amicon®

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stirred cell. Total U, Fe and C concentrations were measured after each filtration step.

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Uranium in solution was determined using an ICP-MS 8800 Triple Quadrupole (Agilent).

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Relative standard deviation was below 10% as estimated from 5 measurements of the same

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sample, and limit of quantification was 0.03 ppm. Dissolved cations were measured using an

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ICP-OES iCAP™ 7600 Duo (Thermo Fisher Scientific). Relative standard deviation was


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below 10% as estimated from 5 measurements of the same sample, and detection limits were

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10, 100, 40, 30, 100 and 10 ppb for Fe, Ca, K, Si, P and Al, respectively.

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Total Organic Carbon, Total Carbon and Total Inorganic Carbon (TOC/TC/TIC) were

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determined using a Vario TOC Cube analyzer (Elementar). Relative standard deviation was

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below 10% as estimated from 5 measurements of the same sample, and limit of quantification

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were 0.019 and 0.05 ppm for solid TC and TOC respectively and 0.5 ppm for dissolved

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organic carbon.

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2.3.

X-ray absorption spectroscopy

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Since chemical extractions have not been reported efficient enough to discern changes in

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the oxidation state of uranium in environmental samples (Newsome et al., 2015; Stoliker et

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al., 2013), X-ray Absorption Spectroscopy (XAS) was used here to determine uranium

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oxidation state in the sediments after drying and during the incubation experiments. For this

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purpose, ten samples were analyzed using XAS: i) the O2-dried and Anox-dried samples used

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as starting samples, and ii) O2/O2, O2/Anox, Anox/O2 and Anox/Anox samples collected after

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8 days of incubation with addition of 0.01 and 0.05 M HCO3-, samples with highest U

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extraction rates.

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Solids were harvested by centrifugation (13000 rpm, 10 min, 10°C) after 8 days of

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incubation and the wet sediments were completely dried either at open air for the O2

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incubation conditions or inside the anoxic glove box for the Anox incubation conditions.

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Samples were ground in an agate mortar, pressed as pellets and sealed between two layers of

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Kapton tape. They were resealed with Kapton and stored within aluminized foil sealed bags

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(Protpack®) under anoxic atmosphere preserved using GasPak™ during the transport to the

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synchrotron facility. Bags were opened in anoxic glove box next to the beamline and rapidly

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transferred inside the cryostat, where they were placed under N2 atmosphere.



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Uranium LIII-edge X-ray Absorption Near Edge Structure spectroscopy (XANES) data of

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the sediment samples were collected in fluorescence detection mode using a 100-element

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solid state Ge array fluorescence detector at the high-flux 11-2 wiggler beam line of the

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Stanford Synchrotron Radiation Lightsource (SSRL, USA). The XANES spectrum of the

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Anox/Anox 0.01 M sample was recorded in High Energy Resolution Fluorescence Detection

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mode using the (777) reflection of Ge spherical crystals analyzers on the FAME-UHD BM16

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beamline at the European Synchrotron Radiation Facility (ESRF, France). On both beamlines,

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the energy of the incident beam delivered by the Si(220) double-crystal monochromator, with

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saggital focusing of the second crystal on BM16, was calibrated by setting the first inflection

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point of Y at 17038 eV, using a double transmission setup. Data were collected at 80K (liquid

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N2 cryostat) on 11-2, and at 20K (liquid He cryostat) on BM16, in order to avoid

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photoreduction of U(VI) under the X-ray beam (Mercier-Bion et al., 2011). A minimum of 2

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scans were collected for each sample and Sixpack (Webb, 2005)was used to average data and

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apply the dead time correction.

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U LIII-edge XANES data of the sediment samples were fitted using Linear Combination

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Least Square (LC-LS) of the spectra of our purest U(IV) and U(VI) model compounds,

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namely U(IV) citrate and U(VI) pyrophosphate (Stetten et al., 2018). The Anox/Anox 0.01 M

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sample spectrum was fit using the HERFD-XANES spectra of a natural autunite

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“Ca(UVIO2)2(PO4)2, 10-12H2O” from the University Pierre and Marie Curie collection (Paris,

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France) and of biogenic uraninite “UIVO2” (Morin et al., 2016) that were recorded on the

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BM16 beamline. The quality of the LC-LS fits was estimated by a R-factor: Rf = Σ [ µexp-

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µcalc ]2 / Σ yexp2 where µ is the normalized absorbance. The uncertainty on XANES LC-LS

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fitting components was estimated by 3 × , where VAR(p) is the variance of

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component p for the lowest χ2R value; with χ2R = N / (N-Np) ∑ [µexp - µcalc]2, where Np is the

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number of fitting components and N is the number of independent parameters corresponding 10 ACS Paragon Plus Environment

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to the energy range divided by the natural width of the U LIII levels (Krause et al., 1979).

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2.4.

Aqueous speciation calculations

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The geochemical code JChess (van der Lee et al., 2002) was used for the calculation of

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aqueous U(VI) and U(IV) speciation for samples collected at the end of each experiment.

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Thermodynamic data were detailed in Table S6 (Guillaumont et al., 2003; Neck and Kim,

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2001; Grenthe et al., 1992; Warwick et al., 2005; Reiller et al., 2008; Sanding et al., 1992;

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Dong et al., 2006). Aqueous speciation of uranium was calculated on the basis of the

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following input data: total aqueous U, Ca, P, dissolved organic and inorganic carbon and pH.

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Total dissolved Fe was implemented as aqueous Fe2+ since Fe aqueous speciation was

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difficult to predict regarding the presence of organic and inorganic ligand complexation and

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significant colloidal fraction as shown hereafter. Dissolved oxygen concentration was fixed at

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8.9 and 0 mg/L for O2/O2 and Anox/Anox experiments respectively. For these latter

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experiments, equilibrium redox potential values returned by the JChess code were then

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directly related to the Fe2+ aqueous concentrations. The pH values were fixed to the input

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values.

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3. RESULTS 3.1.

Aqueous chemistry

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Dissolved concentrations of uranium and carbon were analysed in the liquid phase after

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filtration at 0.22 µm. The major elements Fe, Ca, K, Si, P and Al were also analysed and

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values are reported in Figures S3 and S4. The oxidation stage of dissolved Fe (< 0.2 µm) was

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measured for two of our experimental conditions (O2/O2 and Anox/Anox) using colorimetric

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methods (Fig. S5). Iron was mostly present as Fe3+ in the oxic experiments, with half of this 11 ACS Paragon Plus Environment


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pool in colloidal form as show thereafter. In contrast, as expected, aqueous Fe2+ was high in

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the anoxic experiments.

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Figure 1 shows the proportion of uranium released from the solid phase, together with the

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dissolved inorganic and organic carbon concentrations and the pH, as a function of incubation

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time for the various drying and incubation conditions investigated. Corresponding aqueous U

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concentrations are reported in Tables S2-S5. Less than 1% of total U was released from the

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solid phase (Figure 1a) when no HCO3- was added to the system (Figure 1d) and the pH was

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the lowest, ∼6 (Figures 1g). In contrast, ∼40% U was released (Figure 1b) when 0.01 M

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HCO3- was added (Figure 1e) and the resulting pH was 8.5 (Figure 1h). Likewise, ~60-90% of

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U was released from the solid phase (Figure 1c) when 0.05 M HCO3- was added (Figure 1f)

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and when the pH increased up to ∼9.5 (Figure 1i). In all cases, even for 0 M HCO3- (Figure

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1a, insert), the sediments initially dried under open-air conditions (O2-dried samples) yielded

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a higher U release than those dried under N2 atmosphere (Anox-dried samples), displayed in

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red and black symbols respectively in Figure 1.

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As shown in Figure 1j, 1k and 1l, organic carbon was preferentially released when

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sediments were incubated under anoxic conditions whatever the drying conditions, i.e.

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O2/Anox and Anox-Anox experiments. The proportion of released organic carbon represented

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∼60% of the initial carbon in the sediment for the three HCO3- concentrations tested when

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sediments were incubated under anoxic conditions. On the contrary, when incubated under

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oxic conditions, only small fraction (≤10%) was extracted. No correlation could then be found

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between U and organic carbon releases, which suggests that organic carbon was likely not

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responsible for U mobility. In contrast, the growth in U release with increasing inorganic

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carbon concentrations suggests that bicarbonate ions could have played a key role in the


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remobilization of U, whether the incubation conditions were oxidizing (due to O2 drying or O2

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incubation) or reducing (in Anox-drying, Anox-incubation systems).

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3.2.

Solid-phase analyses

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We used XANES spectroscopy at the U LIII-edge to evaluate the changes in U redox state

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that could have been caused by the sample drying and those that occurred during the

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incubations. In the O2-dried sediment samples that initially contained 215 ppm U (Figure

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2ab), U was almost fully oxidized before the incubation experiments (U(IV) < 10%). After 8

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days of incubation under oxic and anoxic conditions, U remained oxidized in all cases and the

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U concentrations decreased down to 130 and 20-30 ppm in the solid phase, with the addition

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of 0.01 M and 0.05 M of HCO3-, respectively (Figure 2b).

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In the Anox-dried samples that initially contained 310 ppm U (Figure 2cd), ∼80% of U was

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under its reduced form (U(IV)), in agreement with (Morin et al., 2016). After 8 days of

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incubation under oxic conditions, U got almost fully oxidized (≤ 10% of U(IV)) and the

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remaining U concentration in the solid decreased down to ∼200 and ∼80 ppm, with the

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addition of 0.01 and 0.05 M HCO3-, respectively (Figure 2d). When these Anox-dried

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sediments were incubated under anoxic conditions, the extraction rate decreased, resulting in

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solid total U concentration of ∼215 and ∼120 ppm (Figure 2d). At the same time, the

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proportion of U(IV) in the solid did not vary significantly after 8 days of incubation under

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Anoxic conditions (Table S5), which resulted in a net decrease in the absolute amount of

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U(IV) in the solid after the 0.01 and 0.05 M HCO3- experiments (Figure 2d). This result

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suggests that a significant fraction of the initial U(IV) solid pool was solubilized in these

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experiments. Further assessment of U speciation in the aqueous phase was then performed

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through size fractionation and thermodynamic modelling. 13 ACS Paragon Plus Environment


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3.3.

Size distribution of aqueous species

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In order to better understand the mechanism of U(VI) and U(IV) mobilization from the

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solid to the liquid phase, a sequential ultra-filtration procedure was applied to the aqueous

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samples obtained at the end of the experiments that yielded the lowest and the highest U

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release, i.e. those conducted without HCO3- and with the addition 0.05 M HCO3-. Aqueous

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concentrations of U, Fe, total organic carbon and total inorganic carbon were then measured

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for the

Carbonate facilitated mobilization of uranium from lacustrine

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