Control of Water Chemistry in Alkaline Lakes: Solubility of Monohydrocalcite and Amorphous Magnesium Carbonate in CaCl2-MgCl2-Na2CO3 Solutions

Alkaline lakes play an important role in the global carbon cycle. Nevertheless, little is known about the chemical processes related to sequestration of CO2 in these lakes. Our earlier study demonstrated that the formation of monohydrocalcite (MHC), a hydrous carbonate mineral frequently found in alkaline lakes, requires coexistence of amorphous Mg carbonate (AMC). It is therefore possible that MHC and AMC control the water chemistries of alkaline lakes. To test this hypothesis, we measured the solubilities of MHC and AMC in Na2CO3-MgCl2-CaCl2 solutions and compared them with the water chemistries of alkaline lakes. Results showed that the solubility of MHC is independent of the Mg contents of the system. The solubility product of AMC was almost 2 orders of magnitude higher than that of MHC. The water chemistries obtained from the alkaline saline lakes with pH > 9 around the world closely approximated saturation with respect to both MHC and AMC. MHC, which is a metastable phase, transforms quickly to aragonite or calcite. Under conditions in which MHC controls the solution chemistries, dissolved inorganic carbon is irreversibly and spontaneously isolated via formation of aragonite or calcite in the lakes. Results of this study suggest that alkaline lakes are natural devices for effective CO2 sequestration.