Colloidal Size and Redox State of Uranium Species in the Porewater

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Characterization of Natural and Affected Environments

Colloidal size and redox state of uranium species in the porewater of a pristine mountain wetland Gabrielle Dublet, Isabelle A. M. Worms, Manon Frutschi, Ashley Brown, Giada Chantal Zünd, Barbora Bartova, Vera I Slaveykova, and Rizlan Bernier-Latmani Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b01417 • Publication Date (Web): 29 Jul 2019 Downloaded from pubs.acs.org on July 29, 2019

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Environmental Science & Technology

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Colloidal size and redox state of uranium species in the porewater

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of a pristine mountain wetland

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Gabrielle Dublet*1,2, Isabelle Worms3, Manon Frutschi1, Ashley Brown1, Giada C. Zünd1,

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Barbora Bartova1, Vera I. Slaveykova3, and Rizlan Bernier-Latmani1

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EPFL-ENAC-IIE-EML, Station 6, CH-1015 Lausanne, Switzerland

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Department of Chemistry, University of Oslo, P.O. Box 1033, NO-0315 Oslo, Norway  

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Environmental Biogeochemistry and Ecotoxicology, Department F.-A. Forel for environmental and

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aquatic sciences, School of Earth and Environmental Sciences, Faculty of Sciences, University of

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Geneva, Uni Carl Vogt, Bvd Carl-Vogt 66, CH-1211 Geneva 4, Switzerland

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* [email protected]

Environmental Microbiology Laboratory (EML), Ecole Polytechnique Federale de Lausanne (EPFL),

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To be submitted to Environmental Science and Technology

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Keywords: Gola di Lago, Asymmetrical Flow Field-Flow Fractionation, wetland, colloid,

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tetravalent uranium

ACS Paragon Plus Environment


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ABSTRACT

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Uranium (U) speciation was investigated in anoxically-preserved porewater samples of a

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natural mountain wetland in Gola di Lago, Ticino, Switzerland. U porewater concentrations

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ranged from less than one µg/L to tens of µg/L, challenging the available analytical

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approaches for U speciation in natural samples. Asymmetrical flow-field flow fractionation

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coupled with inductively coupled plasma mass spectrometry, allowed the characterization

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of colloid populations and the determination of the size distribution of U species in the

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porewater. Most of the U was associated with three fractions: < 0.3 kDa, likely including

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dissolved U and very small U colloids; a 1–3 kDa fraction containing humic-like organic

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compounds, dispersed Fe and, to a small extent, Fe nanoparticles; and a third fraction (5–50

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nm), containing a higher amount of Fe and a lower amount of organic matter and U relative

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to the 1–3 kDa fraction. The proportion of U associated with the 1–3 kDa colloids varied

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spatially and seasonally. Using anion exchange resins, we also found that a significant

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proportion of U occurs in its reduced form, U(IV). Tetravalent U was interpreted as occurring

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within the colloidal pool of U. This study suggests that U(IV) can occur as small (1–3 kDa),

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organic-rich, and thus potentially mobile colloidal species in naturally reducing wetland

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environments. Wetland

Porewater

“Dissolved” 10% higher or smaller than the expected total concentrations).

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Therefore, the proportion of reduced U could not be interpreted quantitatively for these

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samples.

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The occurrence of U(IV) colloids in the GdL wetland is noteworthy because U(IV) is generally

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considered immobile in remediation strategies. There are prior occurrence of U(IV) colloids

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in contaminated sites. Wang et al.14 suggested the occurrence of colloid-facilitated

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dispersion of U(IV) downstream from a mining-impacted peat wetland, involving OM- and

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Fe-rich colloids. U(IV)-colloids were also found previously in alkaline anoxic environments.

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Another example is reported by Bouby et al.42, who found humic U colloids of a few

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nanometers and inorganic U-colloids of tens of nanometers in alkaline groundwaters from

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the Gorleben borehole in Germany. Because the groundwater was anoxic at that site, the

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study suggested that U occurred as U(IV) in the colloids but no direct evidence was

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provided. In addition, using successive micro- and ultrafiltration, Kalmykov et al.43 found

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U(IV)-hydroxocolloids in alkaline groundwaters highly impacted by waste disposal at the

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Tomsk site, Russia. Nonetheless, this is the first time that U(IV), interpreted as U(IV)-

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colloids, is found in the porewater of an undisturbed natural wetland, harboring U

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concentrations naturally built up over thousands of years.

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Thus, the approach presented here, using AF4 coupled with ICP-MS and resin separation

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provided new information on the occurrence of U(IV) and small OM,Fe-bearing U-colloids in

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wetland porewaters under natural environmental conditions. This is the first time such an


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approach is developed for environmental samples with such low U concentrations and while

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preserving anoxic conditions. The variability of the results in U-bearing colloidal sizes

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reported in the literature emphasizes the need to study various wetland environments to

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understand the parameters controlling U colloid formation. In our case, because of the low

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U concentrations, we could not probe the specific association of U with organic or Fe phases

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in the colloids. However, with STEM-EDS, we did not observe any U hotspots colocalized

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with the few Fe nanoparticles observed in the 1–3 kDa colloidal pool, suggesting that U may

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not be associated directly with Fe, but rather with OM.

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Environmental implications

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In the wetland investigated, U(IV) and U(VI) are associated with OM micrometric particles in

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the solid phase (Mikutta et al. 13) and small, mostly 1–3 kDa colloids, in porewater (this

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study). Here, we suggest that small, U-bearing organic colloids can form in pristine wetland

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environments, even where porewater concentrations of U are very low (0.05–30 µg/L). In

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those colloids, U may occur as U(IV). Such environments are rather common and store

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significant amounts of total U in surface environments3. In the case of an environmental

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disturbance of such pristine wetlands, for example changes in hydraulic conditions or

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geochemistry due to climate change or to changes of land management, the U(IV) colloids

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and particles present in the wetland porewaters could be mobilized. Small, organic-rich

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colloids can be expected to be mobile and quite stable over significant distances16. Hence,

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their presence suggests that there is the potential for colloid-facilitated transport of U far

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afield, including downstream from reducing wetlands.

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Our results also provide new insights into the understanding of the formation mechanisms

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of organic U(IV) complexes in the wetland of GdL. Mikutta et al.13 suggested that the



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formation of U(IV) in the sediment of this site most likely involved the reduction of

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organically complexed U(VI). Similarly in the porewater, we could envisage that U(VI)

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derived from the streams may be reduced to U(IV) by colloidal OM.

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ACKNOWLEDGMENTS

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This project has received funding from the European Union's Horizon 2020 research and

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innovation programme under grant agreement No 701542, entitled “Association of Uranium

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with Organic Matter- and Iron-bearing Colloids in Wetland Environments (UMIC)”. The

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“Ufficio patriziale di Camignolo” is thanked for authorizing our work on the wetland of GdL.

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Laurent Gilliard, Anna M. Cissé and Shannon Dyer are thanked for their help with field

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sampling. Karine Vernez, Sylvain Coudret and Karen Vicava are thanked for their help with

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DOC and ICP-MS analyses. Ralf Kägi is thanked for providing the Al cones that were used for

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the preparation of TEM samples.

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Supporting information Available. Location of the site, the piezometers and the samples.

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Details on the analytical methods. Chemistry of the porewaters for three other sampling

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dates (July 2017, October 2017, April 2018). Correlations between U and DOC or Fe

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concentrations. Specific Ultraviolet Absorbance at 254 nm and Humification Indexes as a

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function of depth. Details on the AF4 analytical method and data interpretation.

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Complementary AF4 results (other sampling dates, intermittent stream, other elements,

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other cross-flow, recovery tests, comparison of results of size-distribution obtained with AF4

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vs. ultrafiltration, analysis of a U-SRFA reference). Complementary results of U(VI)/U(IV)


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separation with anion resin exchange. This information is available free of charge via the

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Internet at http://pubs.acs.org.

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dh (nm)

dh (nm)

Figure 2: U, Fe concentration and UV254nm absorbance in colloids as a function of retention time (and related to molecular weight, Mw, and hydrodynamic diameter, dh, via a PSS standard curve and an internal calibration with Bovine Serum Albumin) in samples from September 2018 ACS Paragon Plus Environment in piezometers M and B0, at selected depths. These fractograms were obtained at a cross-flow of 1.5 mL/min. XF0 indicates the time when the cross-flow was stopped (see text). Similar fractograms obtained in October 2017 and April 2018 are presented in SI.


Figure 3: Representative C-rich colloids of the sample at 152 cm depth in the piezometer M in April 2018, collected downstream of the AF4 and corresponding to the 0.3-3 kDa size fraction or the 3 kDa-0.2 µm size fraction, respectively for A or for B and C. The material observed is interpreted as A: aggregated humic-like organic matter with a nanometric hotspot of Fe; B: nanoparticulate Fe embedded in humic-like organic matter; C: particulate (fibrous) organic matter coated with Fe. HAADF: Images were obtained by High-angle annular dark-field scanning transmission electron microscopy ACS Paragon Plus Environment (HAADF-STEM). Images A were obtained with a FEI Titan Themis microscope, and images B and C were obtained with a FEI Technai Osiris microscope. Scale bars are 50 nm.

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0.15 0,15

0,05 0,10 0.10 0.05 U (µg/L)

0

1 000

2 000 Fe (µg/L)

0

5

10 15 U (µg/L)

0

500

3 000

1 000 1 500 Fe (µg/L)

Depth 45 cm

0,00 0.00

B0 piezometer

20

25

2 000

2 500

0,00 0.00

0

4 000

Depth 150 cm

Depth 158 cm

Depth 45 cm

M piezometer

0.0 0,0

0

0,02 0.02

0,04 0.06 0.04 0,06 U (µg/L)

1 000

0.2 0,2

2 000 Fe (µg/L)

0.4 0.6 0,4 0,6 U (µg/L)

0,08 0.08

0,10 0.10

3 000

0,8 0.8

2 000 4 000 Fe (µg/L)

< 300 Da 1-3 kDa 3-1375 kDa 1375 kDa

Colloidal Size and Redox State of Uranium Species in the Porewater

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