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

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)${}_2$(cr) is the solid phase controlling the solubility in all evaluated systems. No transformation of α-Be(OH)${}_2$(cr) to β-Be(OH)${}_2$(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 pH${}_m$ ≈ 9.5 (with [Be(II)] ≈ 10${}^{-6.8}$M), regardless of the ionic strength. The combination of solubility data determined in acidic pH${}_m$ 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)${}_2$(cr), log *K°${}_{s,0}$ = (6.9 ± 0.4). Slope analysis of the solubility data in alkaline to hyperalkaline conditions and ${}^9$Be NMR support the predominance of the monomeric hydrolysis species Be(OH)${}_2$(aq), Be(OH)${}_3$– and Be(OH)${}_4$${}^{2-}$ within the strongly alkaline pH${}_m$-conditions relevant in cementitious systems. ... mehrThe 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 Be${}^{2+}$–Na${}^{+}$–K${}^{+}$–H${}^{+}$–Cl${}^{-}$–OH${}^{-}$–H${}_2$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.