Experimental determination of solubility constant of hydromagnesite (5424) in NaCl solutions up to 4.4 m at room temperature

This study reports the solubility constants of both synthetic and natural hydromagnesite (5424) determined in NaCl solutions with a wide range of ionic strength regarding the following reaction:Mg5(CO3)4(OH)2·4H2O (cr) + 10H+ ⇆ 5Mg2+ + 4CO2(g) + 10H2O(l)Solubility experiments were conducted from undersaturation in deionized water and 0.10–4.4 m NaCl solutions at PCO2 of 10− 3.4 atm and 22.5 °C, and lasting up to 1870 days. Based on the specific interaction theory, the weighted average solubility constant at infinite dilution calculated from the experimental results in 0.10–3.2 m NaCl solutions using the natural hydromagnesite (5424) from Staten Island, New York, is 58.39 ± 0.40 (2σ) in logarithmic units at 22.5 °C with a corresponding value of 57.93 ± 0.40 (2σ) at 25 °C. Similarly, the weighted average solubility constant using the natural hydromagnesite (5424) from Gabbs, Nevada, is 59.54 ± 0.72 (2σ) in logarithmic units at 22.5 °C with a corresponding value of 59.07 ± 0.72 (2σ) at 25 °C. The weighted average solubility constant of synthetic hydromagnesite (5424) determined from experiments in 0.10–4.4 m NaCl solutions is 61.53 ± 0.59 (2σ) in logarithmic units at 22.5 °C with a corresponding value of 61.04 ± 0.59 (2σ) at 25 °C. The natural hydromagnesite has lower solubilities because of its higher crystallinity related to their origins than synthetic hydromagnesite. The solubility constant of synthetic hydromagnesite is about one order of magnitude lower than the literature values. The Gibbs free energies of formation at the reference state (25 °C, 1 bar) are − 5896 ± 2 kJ mol− 1, –5889 ± 4 kJ mol− 1, and − 5,878 ± 3 kJ mol− 1 for the natural hydromagnesite from Staten Island, New York, from Gabbs, Nevada, and for the synthetic hydromagnesite, respectively.

Research highlights
► Solubility constants of hydromagnesite have been determined in an NaCl medium.
► Solubility constant of the synthetic hydromagnesite is 61.04 ± 0.59 (2σ) at 25 °C.
► Hydromagnesite from Staten Island, NY, has a value of 57.93 ± 0.40 (2σ) at 25 °C.
► Hydromagnesite from Gabbs, NV, has a value of 59.07 ± 0.72 (2σ) at 25 °C.
► Lower solubility of the natural hydromagnesites is attributed to their origins.