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Philipp T, Huittinen N, Shams Aldin Azzam S, Stohr R, Stietz J, Reich T, Schmeide K. Effect of Ca(II) on U(VI) and Np(VI) retention on Ca-bentonite and clay minerals at hyperalkaline conditions - New insights from batch sorption experiments and luminescence spectroscopy. THE SCIENCE OF THE TOTAL ENVIRONMENT 2022; 842:156837. [PMID: 35750178 DOI: 10.1016/j.scitotenv.2022.156837] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/21/2022] [Revised: 05/24/2022] [Accepted: 06/16/2022] [Indexed: 06/15/2023]
Abstract
In deep geological repositories for radioactive waste, interactions of radionuclides with mineral surfaces occur under complex geochemical conditions involving complex solution compositions and high pH resulting from degradation of cementitious geo-engineered barriers. Ca2+ cations have been hypothesized to play an important role as mediators for the retention of U(VI) on Ca-bentonite at (hyper)alkaline conditions, despite the anionic character of both the mineral surface and the aqueous uranyl species. To gain deeper insight into this sorption process, the effect of Ca2+ on U(VI) and Np(VI) retention on alumosilicate minerals has been comprehensively evaluated, using batch sorption experiments and time-resolved laser-induced luminescence spectroscopy (TRLFS). Sorption experiments with Ca2+ or Sr2+ and zeta potential measurements showed that the alkaline earth metals sorb strongly onto Ca-bentonite at pH 8-13, leading to a partial compensation of the negative surface charge, thereby generating potential sorption sites for anionic actinyl species. U(VI) and Np(VI) sorption experiments in the absence and presence of Ca2+ or Sr2+ confirmed that these cations strongly enhance radionuclide retention on kaolinite and muscovite at pH ≥ 10. Concerning the underlying retention mechanisms, site-selective TRLFS provided spectroscopic proof for two dominating U(VI) species at the alumosilicate surfaces: (i) A ternary U(VI) complex, where U(VI) is bound to the surface via bridging Ca cations with the configuration surface ≡ Ca - OH - U(VI) and, (ii) U(VI) sorption into the interlayer space of calcium (aluminum) silicate hydrates (C-(A-)S-H), which form as secondary phases in the presence of Ca due to partial dissolution of alumosilicates under hyperalkaline conditions. Consequently, the present study confirms that alkaline earth elements, which are ubiquitous in geologic systems, enable strong retention of hexavalent actinides on clay minerals under hyperalkaline repository conditions.
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Affiliation(s)
- Thimo Philipp
- Helmholtz-Zentrum Dresden - Rossendorf e.V., Institute of Resource Ecology, Bautzner Landstraße 400, 01328 Dresden, Germany
| | - Nina Huittinen
- Helmholtz-Zentrum Dresden - Rossendorf e.V., Institute of Resource Ecology, Bautzner Landstraße 400, 01328 Dresden, Germany
| | - Salim Shams Aldin Azzam
- Helmholtz-Zentrum Dresden - Rossendorf e.V., Institute of Resource Ecology, Bautzner Landstraße 400, 01328 Dresden, Germany
| | - Robin Stohr
- Johannes Gutenberg-Universität Mainz, Department of Chemistry, Fritz Strassmann Weg 2, 55128 Mainz, Germany
| | - Janina Stietz
- Johannes Gutenberg-Universität Mainz, Department of Chemistry, Fritz Strassmann Weg 2, 55128 Mainz, Germany
| | - Tobias Reich
- Johannes Gutenberg-Universität Mainz, Department of Chemistry, Fritz Strassmann Weg 2, 55128 Mainz, Germany
| | - Katja Schmeide
- Helmholtz-Zentrum Dresden - Rossendorf e.V., Institute of Resource Ecology, Bautzner Landstraße 400, 01328 Dresden, Germany.
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Stockmann M, Fritsch K, Bok F, Fernandes MM, Baeyens B, Steudtner R, Müller K, Nebelung C, Brendler V, Stumpf T, Schmeide K. New insights into U(VI) sorption onto montmorillonite from batch sorption and spectroscopic studies at increased ionic strength. THE SCIENCE OF THE TOTAL ENVIRONMENT 2022; 806:150653. [PMID: 34597569 DOI: 10.1016/j.scitotenv.2021.150653] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/06/2021] [Revised: 09/06/2021] [Accepted: 09/24/2021] [Indexed: 06/13/2023]
Abstract
The influence of ionic strength up to 3 mol kg-1 (background electrolytes NaCl or CaCl2) on U(VI) sorption onto montmorillonite was investigated as function of pHc in absence and presence of CO2. A multi-method approach combined batch sorption experiments with spectroscopic methods (time-resolved laser-induced fluorescence spectroscopy (TRLFS) and in situ attenuated total reflection Fourier-transform infrared spectroscopy (ATR FT-IR)). In the absence of atmospheric carbonate, U(VI) sorption was nearly 99% above pHc 6 in both NaCl and CaCl2 and no significant effect of ionic strength was found. At lower pH, cation exchange was strongly reduced with increasing ionic strength. In the presence of carbonate, U(VI) sorption was reduced above pHc 7.5 in NaCl and pHc 6 in CaCl2 system due to formation of aqueous UO2(CO3)x(2-2x) and Ca2UO2(CO3)3 complexes, respectively, as verified by TRLFS. A significant ionic strength effect was observed due to the formation of Ca2UO2(CO3)3(aq), which strongly decreases U(VI) sorption with increasing ionic strength. The joint analysis of determined sorption data together with literature data (giving a total of 213 experimental data points) allowed to derive a consistent set of surface complexation reactions and constants based on the 2SPNE SC/CE approach, yielding log K°≡SSOUO2+ = 2.42 ± 0.04, log K°≡SSOUO2OH = -4.49 ± 0.7, and log K°≡SSOUO2(OH)32- = -20.5 ± 0.4. Ternary uranyl carbonate surface complexes were not required to describe the data. With this reduced set of surface complexes, an improved robust sorption model was obtained covering a broad variety of geochemical settings over wide ranges of ionic strengths and groundwater compositions, which subsequently was validated by an independent original dataset. This model improves the understanding of U(VI) retention by clay minerals and enables now predictive modeling of U(VI) sorption processes in complex clay rich natural environments.
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Affiliation(s)
- M Stockmann
- Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, Bautzner Landstr. 400, 01328 Dresden, Germany.
| | - K Fritsch
- Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, Bautzner Landstr. 400, 01328 Dresden, Germany
| | - F Bok
- Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, Bautzner Landstr. 400, 01328 Dresden, Germany
| | - M Marques Fernandes
- Paul Scherrer Institute, Laboratory for Waste Management, 5232 Villigen PSI, Switzerland
| | - B Baeyens
- Paul Scherrer Institute, Laboratory for Waste Management, 5232 Villigen PSI, Switzerland
| | - R Steudtner
- Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, Bautzner Landstr. 400, 01328 Dresden, Germany
| | - K Müller
- Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, Bautzner Landstr. 400, 01328 Dresden, Germany
| | - C Nebelung
- Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, Bautzner Landstr. 400, 01328 Dresden, Germany
| | - V Brendler
- Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, Bautzner Landstr. 400, 01328 Dresden, Germany
| | - T Stumpf
- Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, Bautzner Landstr. 400, 01328 Dresden, Germany
| | - K Schmeide
- Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, Bautzner Landstr. 400, 01328 Dresden, Germany.
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Wolter JM, Schmeide K, Huittinen N, Stumpf T. Cm(III) retention by calcium silicate hydrate (C-S-H) gel and secondary alteration phases in carbonate solutions with high ionic strength: A site-selective TRLFS study. Sci Rep 2019; 9:14255. [PMID: 31582765 PMCID: PMC6776662 DOI: 10.1038/s41598-019-50402-x] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/13/2019] [Accepted: 09/12/2019] [Indexed: 11/16/2022] Open
Abstract
We studied the Cm(III) retention by calcium silicate hydrate (C-S-H), portlandite (Ca(OH)2) and their alteration products calcite, vaterite, and aragonite in high ionic strength carbonate-containing solutions representing specific formation waters. For this, we synthesized C-S-H gels with calcium to silicon (C/S) ratios of 1.0 and 2.0 in the absence and presence of Cm(III), resulting in Cm(III)-free and Cm(III) doped C-S-H gel, respectively. For phase identification purposes we applied X-ray diffraction (XRD) while for the identification of the Cm(III)/C-S-H binding mode we applied site-selective time-resolved laser-induced luminescence spectroscopy (TRLFS). The stability of Cm(III) doped phases under repository-relevant conditions was evaluated by studying the time-dependent release of Cm(III) from the Cm(III) doped C-S-H gel into leaching solutions containing 0.02 M NaHCO3 or 2.5 M NaCl/0.02 M NaHCO3 over 60 d. Speciation changes of Cm(III) due to leaching were followed with TRLFS while C-S-H structure alterations and secondary phase formation were monitored with XRD. From the results it could be concluded that Cm(III) is not mobilized by aqueous carbonate but either remains incorporated in the C-S-H structure and portlandite or becomes partially re-immobilized into secondary CaCO3 phases. The presence of NaCl led to an accelerated conversion of metastable secondary CaCO3 phases into calcite.
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Affiliation(s)
- Jan-Martin Wolter
- Helmholtz-Zentrum Dresden - Rossendorf, Institute of Resource Ecology, Bautzner Landstraße 400, 01328, Dresden, Germany
| | - Katja Schmeide
- Helmholtz-Zentrum Dresden - Rossendorf, Institute of Resource Ecology, Bautzner Landstraße 400, 01328, Dresden, Germany.
| | - Nina Huittinen
- Helmholtz-Zentrum Dresden - Rossendorf, Institute of Resource Ecology, Bautzner Landstraße 400, 01328, Dresden, Germany
| | - Thorsten Stumpf
- Helmholtz-Zentrum Dresden - Rossendorf, Institute of Resource Ecology, Bautzner Landstraße 400, 01328, Dresden, Germany
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Philipp T, Shams Aldin Azzam S, Rossberg A, Huittinen N, Schmeide K, Stumpf T. U(VI) sorption on Ca-bentonite at (hyper)alkaline conditions - Spectroscopic investigations of retention mechanisms. THE SCIENCE OF THE TOTAL ENVIRONMENT 2019; 676:469-481. [PMID: 31048176 DOI: 10.1016/j.scitotenv.2019.04.274] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/07/2019] [Revised: 03/15/2019] [Accepted: 04/18/2019] [Indexed: 06/09/2023]
Abstract
Environmental conditions in deep geological repositories for radioactive waste may involve high pH values due to the degradation of concrete. However, the U(VI) sorption at such (hyper)alkaline conditions is still poorly understood. In this study, batch sorption experiments with Ca-bentonite in the pH range 8-13 at different carbonate concentrations were combined with spectroscopic investigations in order to gain insight into the underlying retention mechanisms. It was found that U(VI) sorption strongly correlates with the aqueous U(VI) speciation determined by time-resolved laser-induced luminescence spectroscopy (TRLFS). Increasing retention with increasing pH was accompanied by a change in aqueous speciation from uranyl carbonates to uranyl hydroxides. The occurrence of luminescence line-narrowing and a decreased frequency of the symmetric stretch vibration, deduced from site-selective TRLFS, indicate the presence of adsorbed U(VI) surface complexes. X-ray absorption fine structure (EXAFS) spectroscopy confirms that surface precipitation does not contribute significantly to the removal of U(VI) from solution but that retention occurs through the formation of two non-equivalent U(VI)-complexes on the bentonite surface. The present study demonstrates that in alkaline environments, where often only precipitation processes are considered, adsorption can provide effective retention of U(VI), despite the anionic character of prevailing aqueous species.
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Affiliation(s)
- Thimo Philipp
- Helmholtz-Zentrum Dresden - Rossendorf, Institute of Resource Ecology, Bautzner Landstraße 400, 01328 Dresden, Germany.
| | - Salim Shams Aldin Azzam
- Helmholtz-Zentrum Dresden - Rossendorf, Institute of Resource Ecology, Bautzner Landstraße 400, 01328 Dresden, Germany.
| | - André Rossberg
- Helmholtz-Zentrum Dresden - Rossendorf, Institute of Resource Ecology, Bautzner Landstraße 400, 01328 Dresden, Germany; The Rossendorf Beamline at ESRF, F-38043 Grenoble, France.
| | - Nina Huittinen
- Helmholtz-Zentrum Dresden - Rossendorf, Institute of Resource Ecology, Bautzner Landstraße 400, 01328 Dresden, Germany.
| | - Katja Schmeide
- Helmholtz-Zentrum Dresden - Rossendorf, Institute of Resource Ecology, Bautzner Landstraße 400, 01328 Dresden, Germany.
| | - Thorsten Stumpf
- Helmholtz-Zentrum Dresden - Rossendorf, Institute of Resource Ecology, Bautzner Landstraße 400, 01328 Dresden, Germany.
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