Experimental determination of solubilities of tri-calcium di-citrate tetrahydrate [Ca3[C3H5O(COO)3]2•4H2O], earlandite, in NaCl and MgCl2 solutions to high ionic strengths
2021
Xiong, Yongliang
In this study, solubility measurements on tri-calcium di-citrate tetrahydrate [Ca₃[C₃H₅O(COO)₃]₂•4H₂O, abbreviated as Ca₃[Citrate]₂•4H₂O, earlandite] as a function of ionic strength are conducted in NaCl solutions up to I = 5.0 mol kg⁻¹ and in MgCl₂ solutions up to I = 7.5 mol kg⁻¹, at room temperature (22.5 ± 0.5 °C). The solubility constant (logKsp0) for Ca₃[Citrate]₂•4H₂O and formation constant (logβ10) for Ca[C₃H₅O(COO)₃]–,(1)Ca₃[C₃H₅O(COO)₃]₂•4H₂O (earlandite) = 3Ca²⁺ + 2[C₃H₅O(COO)₃]³– + 4H₂O(2)Ca²⁺ + [C₃H₅O(COO)₃]³– = Ca[C₃H₅O(COO)₃]–are determined as −18.11 ± 0.15 and 4.97 ± 0.15, respectively, with a set of Pitzer parameters describing the specific interactions in NaCl-bearing and MgCl₂-bearing media.The solubility measurements and thermodynamic modeling indicate that earlandite could become a solubility-controlling phase for citrate in the near field of geological repositories for nuclear waste when the inventories of citrate reach the saturation concentrations for Ca₃[Citrate]₂•4H₂O, and it is a solubility-limiting phase for calcium/citrate in other low temperature environments such as agro-food industrial waste-waters. The Pitzer model presented in this work will enable researchers to assess the role of citrate in low temperature environments over a wide range of ionic strengths, and is expected to find applications in many fields such as food and pharmaceutical sciences, cement and concrete industries as well as nuclear waste management.
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