Thermodynamic description of Be(II) solubility and hydrolysis in acidic to hyperalkaline NaCl and KCl solutions

Abstract

The solubility of Be(II) is investigated in carbonate-free dilute to concentrated HCl–NaCl–NaOH, KCl–KOH, NaOH and KOH solutions. Undersaturation experiments were performed under Ar atmosphere at T= (22 ± 2) °C. XRD, XPS, SEM and quantitative chemical analysis confirm that α-Be(OH)2_{2}(cr) is the solid phase controlling the solubility in all evaluated systems. No transformation of α-Be(OH)2_{2}(cr) to β-Be(OH)2_{2}(cr) or ternary solid phases Na/K–Be(II)–OH(s) is observed in the investigated systems within the timeframe of this study (t ≤ 600 days). An amphoteric solubility behaviour of Be(II) is observed with a solubility minimum at pHm_{m} ≈ 9.5 (with [Be(II)] ≈ 106.8^{-6.8}M), regardless of the ionic strength. The combination of solubility data determined in acidic pHm_{m} conditions and the hydrolysis scheme reported in the literature for cationic hydrolysis species of Be(II) is used for the determination of the solubility constant of α-Be(OH)2_{2}(cr), log *K°s,0_{s,0} = (6.9 ± 0.4). Slope analysis of the solubility data in alkaline to hyperalkaline conditions and 9^{9}Be NMR support the predominance of the monomeric hydrolysis species Be(OH)2_{2}(aq), Be(OH)3_{3}– and Be(OH)4_{4}2^{2-} within the strongly alkaline pHm_{m}-conditions relevant in cementitious systems. The comprehensive solubility dataset collected within this study in combination with extensive solid and aqueous phase characterization allow the development of a complete chemical, thermodynamic and (SIT) activity model for the system Be2+^{2+}–Na+^{+}–K+^{+}–H+^{+}–Cl^{-}–OH^{-}–H2_{2}O(l). This model provides an accurate and robust tool for the evaluation of Be(II) solubility and speciation in a diversity of geochemical conditions, including source term calculations of beryllium in the context of the nuclear waste disposal Safety Case

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