Predicting Calcium and Magnesium Precipitation in Saline Solutions following Evaporation

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Abstract
Salt precipitation associated with water evaporation from saline water is a fundamental process occurring in irrigation and drainage waters and aquaculture ponds. This study examined five different methods of predicting Ca and Mg precipitation in saline solutions following evaporation: (i) Ca precipitation as calcite; (ii) Ca precipitation as metastable CaCO3, such as vaterite; (iii) CaCO3 plus Mg precipitation as magnesite or nesquehonite; (iv) formation of homogeneous magnesian calcite and resulting changes in solubility; and (v) formation of heterogeneous magnesian calcite particles having various levels of Mg substitution and solubility. Computed results were then compared with laboratory data in which 10 saline solutions simulating irrigation waters were concentrated up to 30‐fold through evaporation at 25 °C. Precipitated salts from selected cases were also analyzed by x‐ray diffraction and an electron probe. Results indicated that Ca precipitation is extensive, and that Mg precipitation or substitution is minimal, except when HCO3 concentrations greatly exceed Ca concentrations. The presence of Mg reduced the rate of precipitation in evaporating solutions, and provided quasi‐stable dissolved Ca concentrations following evaporation, which were significantly higher than calcite solubility. X‐ray diffraction analyses indicated the formation of magnesian calcite with a Mg molar content up to 7% with some aragonite. Electron probe microanalyses supported a theoretical prediction (based on Method v) that magnesian calcite particles are heterogeneous, having higher Mg contents toward the particle surface. Calcium and Mg concentrations in the saline solutions following evaporation were best estimated by Method v, which incorporated the coevolution of the composition of solution and solid phases.

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