Stability of pyrochlores in alkaline matrices: Solubility of calcium antimonate

Geert Cornelis; Tom Van Gerven; Ruben Snellings; Bram Verbinnen; Jan Elsen; Carlo Vandecasteele

doi:10.1016/j.apgeochem.2011.02.002

Stability of pyrochlores in alkaline matrices: Solubility of calcium antimonate

Geert Cornelis
, Tom Van Gerven
, Ruben Snellings
, Bram Verbinnen
, Jan Elsen
, Carlo Vandecasteele

Publication: Contribution to journal › Journal article › peer-review

Abstract

The stability of pyrochlores as a function of solution composition is relevant in the context of hazardous waste immobilisation as this mineral family comprises minerals that can control actinide solubility in ceramic formulations as well as romeites or calcium antimonates that possibly control the solubility of Sb in cement pastes. However, to date, no thermodynamic model exists that can be used for long-term risk assessment of pyrochlores as stabilising minerals. In this work synthetic romeites were precipitated at pH 6.5 and pH 12 and at varying molar Ca:Sb ratio and analysed by XRD and Rietveld refinement which showed that, like naturally occurring romeites, synthesized romeites have a Ca1+xSb2O6OH2-2x structure with less vacancies and a higher Ca-content as pH and Ca-availability were higher during synthesis. These romeites dissolve incongruently with preferential Ca leaching and antimonate is less soluble from romeites synthesized in alkaline conditions. Batch extractions of romeite in equilibrium with increasing Ca(NO3)(2) additions up to 1 mol L-1 showed a minimum equilibrium Sb concentration at [Ca] = 0.01 mol L-1 that could only be explained by assuming the formation of a CaSb(OH)(6)(+) complex. Increasing the Ca concentration in romeite suspensions also resulted in a pH decline in the equilibrium solution, which may be explained by the increasing concomitant uptake of Ca2+ and OH- in the romeite structure as a function of increasing Ca(NO3)(2) additions. Thermodynamic modelling supported these assumptions that may explain the incongruent dissolution behaviour of romeite, but the fitting of two equilibrium constants was required. Although the validity of these constants needs more experimental confirmation, this study suggests that despite possible CaSb(OH)(6)(+) formation, the solubility of Ca antimonate in alkaline conditions can be responsible for lower equilibrium Sb concentrations than previously assumed. (C) 2011 Elsevier Ltd. All rights reserved.

Original language	English
Pages (from-to)	809-817
Number of pages	9
Journal	Applied Geochemistry
Volume	26
Issue number	5
DOIs	https://doi.org/10.1016/j.apgeochem.2011.02.002
Publication status	Published - 2011
Externally published	Yes

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10.1016/j.apgeochem.2011.02.002

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title = "Stability of pyrochlores in alkaline matrices: Solubility of calcium antimonate",

abstract = "The stability of pyrochlores as a function of solution composition is relevant in the context of hazardous waste immobilisation as this mineral family comprises minerals that can control actinide solubility in ceramic formulations as well as romeites or calcium antimonates that possibly control the solubility of Sb in cement pastes. However, to date, no thermodynamic model exists that can be used for long-term risk assessment of pyrochlores as stabilising minerals. In this work synthetic romeites were precipitated at pH 6.5 and pH 12 and at varying molar Ca:Sb ratio and analysed by XRD and Rietveld refinement which showed that, like naturally occurring romeites, synthesized romeites have a Ca1+xSb2O6OH2-2x structure with less vacancies and a higher Ca-content as pH and Ca-availability were higher during synthesis. These romeites dissolve incongruently with preferential Ca leaching and antimonate is less soluble from romeites synthesized in alkaline conditions. Batch extractions of romeite in equilibrium with increasing Ca(NO3)(2) additions up to 1 mol L-1 showed a minimum equilibrium Sb concentration at [Ca] = 0.01 mol L-1 that could only be explained by assuming the formation of a CaSb(OH)(6)(+) complex. Increasing the Ca concentration in romeite suspensions also resulted in a pH decline in the equilibrium solution, which may be explained by the increasing concomitant uptake of Ca2+ and OH- in the romeite structure as a function of increasing Ca(NO3)(2) additions. Thermodynamic modelling supported these assumptions that may explain the incongruent dissolution behaviour of romeite, but the fitting of two equilibrium constants was required. Although the validity of these constants needs more experimental confirmation, this study suggests that despite possible CaSb(OH)(6)(+) formation, the solubility of Ca antimonate in alkaline conditions can be responsible for lower equilibrium Sb concentrations than previously assumed. (C) 2011 Elsevier Ltd. All rights reserved.",

author = "Geert Cornelis and \{Van Gerven\}, Tom and Ruben Snellings and Bram Verbinnen and Jan Elsen and Carlo Vandecasteele",

year = "2011",

doi = "10.1016/j.apgeochem.2011.02.002",

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journal = "Applied Geochemistry",

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T1 - Stability of pyrochlores in alkaline matrices: Solubility of calcium antimonate

AU - Cornelis, Geert

AU - Van Gerven, Tom

AU - Snellings, Ruben

AU - Verbinnen, Bram

AU - Elsen, Jan

AU - Vandecasteele, Carlo

PY - 2011

Y1 - 2011

N2 - The stability of pyrochlores as a function of solution composition is relevant in the context of hazardous waste immobilisation as this mineral family comprises minerals that can control actinide solubility in ceramic formulations as well as romeites or calcium antimonates that possibly control the solubility of Sb in cement pastes. However, to date, no thermodynamic model exists that can be used for long-term risk assessment of pyrochlores as stabilising minerals. In this work synthetic romeites were precipitated at pH 6.5 and pH 12 and at varying molar Ca:Sb ratio and analysed by XRD and Rietveld refinement which showed that, like naturally occurring romeites, synthesized romeites have a Ca1+xSb2O6OH2-2x structure with less vacancies and a higher Ca-content as pH and Ca-availability were higher during synthesis. These romeites dissolve incongruently with preferential Ca leaching and antimonate is less soluble from romeites synthesized in alkaline conditions. Batch extractions of romeite in equilibrium with increasing Ca(NO3)(2) additions up to 1 mol L-1 showed a minimum equilibrium Sb concentration at [Ca] = 0.01 mol L-1 that could only be explained by assuming the formation of a CaSb(OH)(6)(+) complex. Increasing the Ca concentration in romeite suspensions also resulted in a pH decline in the equilibrium solution, which may be explained by the increasing concomitant uptake of Ca2+ and OH- in the romeite structure as a function of increasing Ca(NO3)(2) additions. Thermodynamic modelling supported these assumptions that may explain the incongruent dissolution behaviour of romeite, but the fitting of two equilibrium constants was required. Although the validity of these constants needs more experimental confirmation, this study suggests that despite possible CaSb(OH)(6)(+) formation, the solubility of Ca antimonate in alkaline conditions can be responsible for lower equilibrium Sb concentrations than previously assumed. (C) 2011 Elsevier Ltd. All rights reserved.

AB - The stability of pyrochlores as a function of solution composition is relevant in the context of hazardous waste immobilisation as this mineral family comprises minerals that can control actinide solubility in ceramic formulations as well as romeites or calcium antimonates that possibly control the solubility of Sb in cement pastes. However, to date, no thermodynamic model exists that can be used for long-term risk assessment of pyrochlores as stabilising minerals. In this work synthetic romeites were precipitated at pH 6.5 and pH 12 and at varying molar Ca:Sb ratio and analysed by XRD and Rietveld refinement which showed that, like naturally occurring romeites, synthesized romeites have a Ca1+xSb2O6OH2-2x structure with less vacancies and a higher Ca-content as pH and Ca-availability were higher during synthesis. These romeites dissolve incongruently with preferential Ca leaching and antimonate is less soluble from romeites synthesized in alkaline conditions. Batch extractions of romeite in equilibrium with increasing Ca(NO3)(2) additions up to 1 mol L-1 showed a minimum equilibrium Sb concentration at [Ca] = 0.01 mol L-1 that could only be explained by assuming the formation of a CaSb(OH)(6)(+) complex. Increasing the Ca concentration in romeite suspensions also resulted in a pH decline in the equilibrium solution, which may be explained by the increasing concomitant uptake of Ca2+ and OH- in the romeite structure as a function of increasing Ca(NO3)(2) additions. Thermodynamic modelling supported these assumptions that may explain the incongruent dissolution behaviour of romeite, but the fitting of two equilibrium constants was required. Although the validity of these constants needs more experimental confirmation, this study suggests that despite possible CaSb(OH)(6)(+) formation, the solubility of Ca antimonate in alkaline conditions can be responsible for lower equilibrium Sb concentrations than previously assumed. (C) 2011 Elsevier Ltd. All rights reserved.

UR - https://res.slu.se/id/publ/80977

U2 - 10.1016/j.apgeochem.2011.02.002

DO - 10.1016/j.apgeochem.2011.02.002

M3 - Journal article

SN - 0883-2927

VL - 26

SP - 809

EP - 817

JO - Applied Geochemistry

JF - Applied Geochemistry

IS - 5

ER -

Stability of pyrochlores in alkaline matrices: Solubility of calcium antimonate

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