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Experimental Determination of Solubilities of Brucite [Mg(OH)₂(cr)] in Na₂SO₄ Solutions with Borate to High Ionic Strengths: Interactions of MgB(OH)₄⁺ with Na₂SO₄

Xiong, Yongliang X.; Kirkes, Leslie D.; Westfall, Terry W.

In this paper, a solubility study on brucite [Mg(OH)₂(cr)] in Na₂SO₄ solutions ranging from 0.01 to 1.8 mol•kg^–1, with 0.001 mol•kg^–1 borate, has been conducted at 22.5°C. Based on the solubility data, the Pitzer interaction parameters for MgB(OH)₄⁺—SO₄^2– and MgB(OH)₄⁺—Na+ along with the formation constant for MgSO₄(aq) are evaluated using the Pitzer model. The formation constant (log₁₀β₁⁰= 2.38 ± 0.08) for MgSO₄(aq) at 25°C and infinite dilution obtained in this study is in excellent agreement with the literature values. The experimental data on the solubility of gypsum (CaSO₄•2H₂O), at 25°C, in aqueous solutions of MgSO₄ with ionic strengths up to ~11 mol•kg^–1 were analyzed using models with and without considering the MgSO₄(aq) species. The model incorporating MgSO₄(aq) fits better to the experimental data than the model without MgSO₄(aq), especially in the ionic strength range beyond ~4 mol•kg^–1, demonstrating the need for incorporation of MgSO₄(aq) into the model to improve the accuracy.