U(VI) behaviour in hyperalkaline calcite systems

Kurt F. Smith, Nicholas D. Bryan, Adam N. Swinburne, Pieter Bots, Samuel Shaw, Louise S. Natrajan, J. Frederick W. Mosselmans, Francis R. Livens, Katherine Morris

Research output: Contribution to journalArticle

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Abstract

The behaviour of U(VI) in hyperalkaline fluid/calcite systems was studied over a range of U(VI) concentrations (5.27×10-5μM to 42.0μM) and in two high pH systems, young and old synthetic cement leachate in batch sorption experiments. These systems were selected to be representative of young- (pH 13.3) and old-stage (pH 10.5) leachate evolution within a cementitious geological disposal facility. Batch sorption experiments, modelling, extended X-ray absorption fine structure spectroscopy, electron microscopy, small angle X-ray scattering and luminescence spectroscopy were used to define the speciation of U(VI) across the systems of study. At the lowest concentrations (5.27×10-5μM 232U(VI)) significant U removal was observed for both old and young cement leachates, and this was successfully modelled using a first order kinetic adsorption modelling approach. At higher concentrations (>4.20μM) in the young cement leachate, U(VI) showed no interaction with the calcite surface over an 18month period. Small angle X-ray scattering techniques indicated that at high U concentrations (42.0μM) and after 18months, the U(VI) was present in a colloidal form which had little interaction with the calcite surface and consisted of both primary and aggregated particles with a radius of 7.6±1.1 and 217±24Å, respectively. In the old cement leachate, luminescence spectroscopy identified two surface binding sites for U(VI) on calcite: in the system with 0.21μM U(VI), a liebigite-like Ca2UO2(CO3)3 surface complex was identified; at higher U(VI) concentrations (0.42μM), a second binding site of undetermined coordination was identified. At elevated U(VI) concentrations (>2.10μM) in old cement leachate, both geochemical data and luminescence spectroscopy suggested that surface mediated precipitation was controlling U(VI) behaviour. A focused ion beam mill was used to create a section across the U(VI) precipitate-calcite interface. Transmission electron microscope images of the section revealed that the calcite surface was coated with a nano crystalline, U containing phase. Selected area electron diffraction images of the U precipitate which was formed at a U(VI) concentration of 4.20μM were consistent with the formation of calcium uranate. XAS spectroscopy at higher concentrations (≥21.0μM) suggested the formation of a second U(VI) phase, possibly a uranyl oxyhydroxide phase. These results indicated that in the young cement leachate, U(VI) did not react with the calcite surface unless U(VI) concentrations were very low (5.27×10-5μM). At higher concentrations, speciation calculations suggested that U(VI) was significantly oversaturated and experimental observations confirmed it existed in a colloidal form that interacted with the mineral surface only weakly. In the old cement leachate systems at low concentrations batch sorption and luminescence data suggested that U(VI) removal was being driven by a surface complexation mechanism. However, at higher concentrations, spectroscopic methods suggest a combination of both surface complexation and surface mediated precipitation was responsible for the observed removal. Overall, U(VI) behaviour in hyperalkaline calcite systems is distinct from that at circumneutral pH conditions: at high pH and anything but low U(VI) concentrations, a surface mediated precipitation mechanism occurs; this is in contrast to circumneutral pH conditions where U(VI) surface complexation reactions tend to dominate. © 2014 The Authors.
LanguageEnglish
Pages343-359
Number of pages17
JournalGeochimica et Cosmochimica Acta
Volume148
Early online date13 Oct 2014
DOIs
Publication statusPublished - 1 Jan 2015

Fingerprint

Calcium Carbonate
calcite
leachate
Cements
cement
luminescence
spectroscopy
Luminescence
Complexation
complexation
Spectroscopy
Sorption
sorption
X ray scattering
Precipitates
Binding Sites
scattering
Extended X ray absorption fine structure spectroscopy
electron
Focused ion beams

Keywords

  • calcite
  • geochemical method
  • leachate
  • sorption
  • uranium
  • waste disposal
  • waste facility

Cite this

Smith, K. F., Bryan, N. D., Swinburne, A. N., Bots, P., Shaw, S., Natrajan, L. S., ... Morris, K. (2015). U(VI) behaviour in hyperalkaline calcite systems. Geochimica et Cosmochimica Acta, 148, 343-359. https://doi.org/10.1016/j.gca.2014.09.043
Smith, Kurt F. ; Bryan, Nicholas D. ; Swinburne, Adam N. ; Bots, Pieter ; Shaw, Samuel ; Natrajan, Louise S. ; Mosselmans, J. Frederick W. ; Livens, Francis R. ; Morris, Katherine. / U(VI) behaviour in hyperalkaline calcite systems. In: Geochimica et Cosmochimica Acta. 2015 ; Vol. 148. pp. 343-359.
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abstract = "The behaviour of U(VI) in hyperalkaline fluid/calcite systems was studied over a range of U(VI) concentrations (5.27×10-5μM to 42.0μM) and in two high pH systems, young and old synthetic cement leachate in batch sorption experiments. These systems were selected to be representative of young- (pH 13.3) and old-stage (pH 10.5) leachate evolution within a cementitious geological disposal facility. Batch sorption experiments, modelling, extended X-ray absorption fine structure spectroscopy, electron microscopy, small angle X-ray scattering and luminescence spectroscopy were used to define the speciation of U(VI) across the systems of study. At the lowest concentrations (5.27×10-5μM 232U(VI)) significant U removal was observed for both old and young cement leachates, and this was successfully modelled using a first order kinetic adsorption modelling approach. At higher concentrations (>4.20μM) in the young cement leachate, U(VI) showed no interaction with the calcite surface over an 18month period. Small angle X-ray scattering techniques indicated that at high U concentrations (42.0μM) and after 18months, the U(VI) was present in a colloidal form which had little interaction with the calcite surface and consisted of both primary and aggregated particles with a radius of 7.6±1.1 and 217±24{\AA}, respectively. In the old cement leachate, luminescence spectroscopy identified two surface binding sites for U(VI) on calcite: in the system with 0.21μM U(VI), a liebigite-like Ca2UO2(CO3)3 surface complex was identified; at higher U(VI) concentrations (0.42μM), a second binding site of undetermined coordination was identified. At elevated U(VI) concentrations (>2.10μM) in old cement leachate, both geochemical data and luminescence spectroscopy suggested that surface mediated precipitation was controlling U(VI) behaviour. A focused ion beam mill was used to create a section across the U(VI) precipitate-calcite interface. Transmission electron microscope images of the section revealed that the calcite surface was coated with a nano crystalline, U containing phase. Selected area electron diffraction images of the U precipitate which was formed at a U(VI) concentration of 4.20μM were consistent with the formation of calcium uranate. XAS spectroscopy at higher concentrations (≥21.0μM) suggested the formation of a second U(VI) phase, possibly a uranyl oxyhydroxide phase. These results indicated that in the young cement leachate, U(VI) did not react with the calcite surface unless U(VI) concentrations were very low (5.27×10-5μM). At higher concentrations, speciation calculations suggested that U(VI) was significantly oversaturated and experimental observations confirmed it existed in a colloidal form that interacted with the mineral surface only weakly. In the old cement leachate systems at low concentrations batch sorption and luminescence data suggested that U(VI) removal was being driven by a surface complexation mechanism. However, at higher concentrations, spectroscopic methods suggest a combination of both surface complexation and surface mediated precipitation was responsible for the observed removal. Overall, U(VI) behaviour in hyperalkaline calcite systems is distinct from that at circumneutral pH conditions: at high pH and anything but low U(VI) concentrations, a surface mediated precipitation mechanism occurs; this is in contrast to circumneutral pH conditions where U(VI) surface complexation reactions tend to dominate. {\circledC} 2014 The Authors.",
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Smith, KF, Bryan, ND, Swinburne, AN, Bots, P, Shaw, S, Natrajan, LS, Mosselmans, JFW, Livens, FR & Morris, K 2015, 'U(VI) behaviour in hyperalkaline calcite systems' Geochimica et Cosmochimica Acta, vol. 148, pp. 343-359. https://doi.org/10.1016/j.gca.2014.09.043

U(VI) behaviour in hyperalkaline calcite systems. / Smith, Kurt F.; Bryan, Nicholas D.; Swinburne, Adam N.; Bots, Pieter; Shaw, Samuel; Natrajan, Louise S.; Mosselmans, J. Frederick W.; Livens, Francis R.; Morris, Katherine.

In: Geochimica et Cosmochimica Acta, Vol. 148, 01.01.2015, p. 343-359.

Research output: Contribution to journalArticle

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T1 - U(VI) behaviour in hyperalkaline calcite systems

AU - Smith, Kurt F.

AU - Bryan, Nicholas D.

AU - Swinburne, Adam N.

AU - Bots, Pieter

AU - Shaw, Samuel

AU - Natrajan, Louise S.

AU - Mosselmans, J. Frederick W.

AU - Livens, Francis R.

AU - Morris, Katherine

PY - 2015/1/1

Y1 - 2015/1/1

N2 - The behaviour of U(VI) in hyperalkaline fluid/calcite systems was studied over a range of U(VI) concentrations (5.27×10-5μM to 42.0μM) and in two high pH systems, young and old synthetic cement leachate in batch sorption experiments. These systems were selected to be representative of young- (pH 13.3) and old-stage (pH 10.5) leachate evolution within a cementitious geological disposal facility. Batch sorption experiments, modelling, extended X-ray absorption fine structure spectroscopy, electron microscopy, small angle X-ray scattering and luminescence spectroscopy were used to define the speciation of U(VI) across the systems of study. At the lowest concentrations (5.27×10-5μM 232U(VI)) significant U removal was observed for both old and young cement leachates, and this was successfully modelled using a first order kinetic adsorption modelling approach. At higher concentrations (>4.20μM) in the young cement leachate, U(VI) showed no interaction with the calcite surface over an 18month period. Small angle X-ray scattering techniques indicated that at high U concentrations (42.0μM) and after 18months, the U(VI) was present in a colloidal form which had little interaction with the calcite surface and consisted of both primary and aggregated particles with a radius of 7.6±1.1 and 217±24Å, respectively. In the old cement leachate, luminescence spectroscopy identified two surface binding sites for U(VI) on calcite: in the system with 0.21μM U(VI), a liebigite-like Ca2UO2(CO3)3 surface complex was identified; at higher U(VI) concentrations (0.42μM), a second binding site of undetermined coordination was identified. At elevated U(VI) concentrations (>2.10μM) in old cement leachate, both geochemical data and luminescence spectroscopy suggested that surface mediated precipitation was controlling U(VI) behaviour. A focused ion beam mill was used to create a section across the U(VI) precipitate-calcite interface. Transmission electron microscope images of the section revealed that the calcite surface was coated with a nano crystalline, U containing phase. Selected area electron diffraction images of the U precipitate which was formed at a U(VI) concentration of 4.20μM were consistent with the formation of calcium uranate. XAS spectroscopy at higher concentrations (≥21.0μM) suggested the formation of a second U(VI) phase, possibly a uranyl oxyhydroxide phase. These results indicated that in the young cement leachate, U(VI) did not react with the calcite surface unless U(VI) concentrations were very low (5.27×10-5μM). At higher concentrations, speciation calculations suggested that U(VI) was significantly oversaturated and experimental observations confirmed it existed in a colloidal form that interacted with the mineral surface only weakly. In the old cement leachate systems at low concentrations batch sorption and luminescence data suggested that U(VI) removal was being driven by a surface complexation mechanism. However, at higher concentrations, spectroscopic methods suggest a combination of both surface complexation and surface mediated precipitation was responsible for the observed removal. Overall, U(VI) behaviour in hyperalkaline calcite systems is distinct from that at circumneutral pH conditions: at high pH and anything but low U(VI) concentrations, a surface mediated precipitation mechanism occurs; this is in contrast to circumneutral pH conditions where U(VI) surface complexation reactions tend to dominate. © 2014 The Authors.

AB - The behaviour of U(VI) in hyperalkaline fluid/calcite systems was studied over a range of U(VI) concentrations (5.27×10-5μM to 42.0μM) and in two high pH systems, young and old synthetic cement leachate in batch sorption experiments. These systems were selected to be representative of young- (pH 13.3) and old-stage (pH 10.5) leachate evolution within a cementitious geological disposal facility. Batch sorption experiments, modelling, extended X-ray absorption fine structure spectroscopy, electron microscopy, small angle X-ray scattering and luminescence spectroscopy were used to define the speciation of U(VI) across the systems of study. At the lowest concentrations (5.27×10-5μM 232U(VI)) significant U removal was observed for both old and young cement leachates, and this was successfully modelled using a first order kinetic adsorption modelling approach. At higher concentrations (>4.20μM) in the young cement leachate, U(VI) showed no interaction with the calcite surface over an 18month period. Small angle X-ray scattering techniques indicated that at high U concentrations (42.0μM) and after 18months, the U(VI) was present in a colloidal form which had little interaction with the calcite surface and consisted of both primary and aggregated particles with a radius of 7.6±1.1 and 217±24Å, respectively. In the old cement leachate, luminescence spectroscopy identified two surface binding sites for U(VI) on calcite: in the system with 0.21μM U(VI), a liebigite-like Ca2UO2(CO3)3 surface complex was identified; at higher U(VI) concentrations (0.42μM), a second binding site of undetermined coordination was identified. At elevated U(VI) concentrations (>2.10μM) in old cement leachate, both geochemical data and luminescence spectroscopy suggested that surface mediated precipitation was controlling U(VI) behaviour. A focused ion beam mill was used to create a section across the U(VI) precipitate-calcite interface. Transmission electron microscope images of the section revealed that the calcite surface was coated with a nano crystalline, U containing phase. Selected area electron diffraction images of the U precipitate which was formed at a U(VI) concentration of 4.20μM were consistent with the formation of calcium uranate. XAS spectroscopy at higher concentrations (≥21.0μM) suggested the formation of a second U(VI) phase, possibly a uranyl oxyhydroxide phase. These results indicated that in the young cement leachate, U(VI) did not react with the calcite surface unless U(VI) concentrations were very low (5.27×10-5μM). At higher concentrations, speciation calculations suggested that U(VI) was significantly oversaturated and experimental observations confirmed it existed in a colloidal form that interacted with the mineral surface only weakly. In the old cement leachate systems at low concentrations batch sorption and luminescence data suggested that U(VI) removal was being driven by a surface complexation mechanism. However, at higher concentrations, spectroscopic methods suggest a combination of both surface complexation and surface mediated precipitation was responsible for the observed removal. Overall, U(VI) behaviour in hyperalkaline calcite systems is distinct from that at circumneutral pH conditions: at high pH and anything but low U(VI) concentrations, a surface mediated precipitation mechanism occurs; this is in contrast to circumneutral pH conditions where U(VI) surface complexation reactions tend to dominate. © 2014 The Authors.

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KW - geochemical method

KW - leachate

KW - sorption

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KW - waste disposal

KW - waste facility

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Smith KF, Bryan ND, Swinburne AN, Bots P, Shaw S, Natrajan LS et al. U(VI) behaviour in hyperalkaline calcite systems. Geochimica et Cosmochimica Acta. 2015 Jan 1;148:343-359. https://doi.org/10.1016/j.gca.2014.09.043