Stability of pyrochlores in alkaline matrices: Solubility of calcium antimonate

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.

Research highlights► Calcium antimonate or romeite may control antimonate solubility in alkaline matrices. ► The effect of vacancies in the romeite structure has been poorly examined. ► The CaSb(OH)6+ association affects antimonate solubility. ► Vacancies in romeite fill up at high pH and high calcium availability. ► Romeite causes lower antimonate solubility than previously assumed.