Uranium in Soil: Natural accumulation in organic-rich soil of an alpine region (Switzerland) Camille Roquier, Simona Regenspurg,

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1 Uranium in Soil: Natural accumulation in organic-rich soil of an alpine region (Switzerland) Camille Roquier, Simona Regenspurg, Messaoud Harfouche, Pascal Froidevaux, Philipp Steinmann, Pilar Junier, Rizlan Bernier-Latmani

2 N 100 m

3 N 100 m

4 Sampling area Core Dischma valley Porewater Very high concentrations of uranium (up to 7,000 ppm) Samples Collection from different places and different depth

5 Geological map of the Dischma valley and GPS coordinates of sampling points.

6 Uranium in soil Uranium (U) occurrence: Uranium concentration in Dischma valley soil : 20 to 7000 ppm In natural soils: 0 to 1.5 ppm (Zielinsky et al., 2006) Which processes are responsible for the U enrichment in the Dischma soil?

7 Uranium chemistry Uranium occurs mainly in two redox states Reducing condi:ons : U(IV) UO 2 (uraninite); solid (immobile) Oxidizing condi:ons : U(VI) UO 2 2+ (uranyl); more soluble Webmineral.com e.g. uranophane Wikipedia.org 2 axial O atoms

8 Uranium chemistry Why U(VI) in Organic Soil? Goal of the study - to determine the redox state - to trace nature of the binding of uranium to the soil - to assess the potential for uranium mobilization.

9 Foud chain

10 Porewater analysis Porewater sampling anaerobic Porewater condieons profiles with from increasing one locaeon at depth Dischma valley

11 U and Total Organic Carbon release at various ph EXAFS: U is octahedrally coordinated U and TOC release increases with increasing ph

12 Soil samples - methods 0-10 cm 1. Soil cores sampling, slicing, homogenization anoxic 2. U Extraction: (bicarbonate, ph variation,) sequential cm cm cm cm 3. X-ray absorption spectroscopy: XANES and EXAFS Redox state of U Binding form of U

13 Redox state of U X- ray absorp:on near edge structure (XANES) on one soil core (fresh and anoxic) SuperXAS beamline at SLS Linear combina,on fit results of the core profile % U(IV) depth (cm) C3-1(pH 9) From the XANES: Uranium is mostly hexavalent From LC fitting: Samples contain both U(IV) and U(VI) U(IV) concentration doesn t depend on the depht

14 Uranium binding to the soil Sequential extraction: U binding to different soil fractions (modified by Schulz et al., 1998) 1. Dissolution of exchangable fraction: 0.4 M MgCl 2 2. Dissolution of organic bound fraction: 6-7 % NaOCl 3. Dissolution of carbonate fraction: 1 M NaAc in 25 % Hac 4. Dissolution of iron- and manganese oxides: 1 M EDTA, 0.01 M ascorbic acid Total dissolution by microwave digestion Release of U with the organic bound fraceon

15 Associa:on U and TOC Uranium and TOC in all collected soil samples of the Dischma valley U and TOC in different depths of one core AssociaEon of U and TOC

16 U binding to the soil Extended X-ray absorption spectroscopy (EXAFS) Comparison of all slices of one core Experimental k 3 -weighed uranium L III EXAFS Fourier Transforms Splitting in the coordination shell showing that U(VI) is dominating

17 U binding to the soil Uranium L 3 -edge Fourier Transformed EXAFS data of one core slice sample (collected at 20 cm depth) compared to four references. UO 2 U(IV) humic acid U(VI) adsorbed U(VI) humic acid sample R+ ΔR (Å) Not UO 2 very similar to U(VI) humic acid standard

18 U binding environment EXAFS fitting - example: sample from 20 cm depth with fit of UO 3 K 3 -weighed Simplified structure of U binding in uranyl (UO 2 2+ ) Fourier transformed Wikipedia.org Structure 2 axial (1.77 Å) and 6 equatorial O atoms (2.36 Å) uranyl- like AddiEonal signal: 3.2 Å (in some spectra) U is also bound to a light element (e.g. C, Si, P)

19 U binding to the soil Presence of [1.6] axial (~1.77Å) and [~5] equatorial (~2.36 Å) oxygen. Uranium is bond to light elements (Si, P and can be also C). Hexavalent uranium is bound to the organic soil in a uranyl-like short range order. IXS

20 Summary and outlook Summary U occurs predominantly as U(VI)/ uranyl in the soil U is strongly associated with organic matter Probably direct binding of uranyl to the organic matter Need to use micro-xrf combined ot microxafs for more results

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