The influence of polymorphism and dispersity of zirconium dioxide on solubility in acidic and alkaline media

The thermodynamic characterization of the true solubility at 25°C of amorphous and crystalline nanoscale modifications of ZrO₂ depending on the pH of the aqueous medium in the range of 0–14.7, taking into account the formation of mono- and polynuclear hydroxocomplexes of Zr(IV) and the dispersity of the solid phase, has been given for the first time. The average effective particle radius (r ≤ 25nm for αZrO₂) should be taken into account as a thermodynamic parameter in determining the solubility of zirconium dioxide in the nanostate. The true solubility of polymorphic modifications of ZrO₂ in aqueous slightly acidic and slightly alkaline media in the absence of fluoride and carbonate ions is very low (7×10^–10 – 10^–7 mol/L), decreases with calcination and is determined by strong neutral complexes Zr(OH)₄, prevailing at pH 2-13. Solubility in aqueous media depends markedly on the crystal structure (α-ZrO₂ < β-ZrO₂ < γ–ZrO₂), increases significantly with decreasing particle size of crystalline nanopowders and strongly (by about 4 orders of magnitude) in the transition from stable crystalline baddeleyite α-ZrO₂ to dry amorphous modification. In strongly acidic medium (1 M HClO₄) the solubility of crystalline modifications of ZrO₂ is 3-4 orders of magnitude higher than in water and is determined mainly by the complex Zr(OH)³⁺, and for amorphous powder ZrO₂ - by the complexes Zr₄(OH)₈⁸⁺ (pH 0), Zr₅(OH)₁₂⁸⁺ (pH 1) and Zr(OH)₂²⁺ (pH 2). The Zr(OH)₃⁺complex makes a minor contribution to the solubility of ZrO₂ at pH 0-2. Zr⁴⁺ aquacations determine the solubility of α- ZrO₂ at pH<0. At pH=1–15 polynuclear Zr(IV) hydroxocomplexes have no appreciable effect on the solubility of macrocrystalline ZrO₂ modifications. In strongly alkaline aqueous media (pH =13,5–14,7) the solubility of ZrO₂ is 2-3 orders of magnitude higher than in water and is determined mainly by the complex Zr(OH)₆^2–.

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