Combined effects of microorganisms and inorganic templates on the nucleation and precipitation of magnesium-calcium minerals: Experimental evidences and theoretical calculations

In order to examine the combined effects of microorganisms and inorganic templates on the nucleation and precipitation of magnesium-bearing minerals, microbially induced carbonate precipitation (MICP) experiments were carried out in the presence of dolomite template. The results showed that the dolomite template provided nucleation sites for aqueous magnesium ions, and the organic components weakened the hydration layer of magnesium ions. Under the synergistic actions of organic components and dolomite template, the precipitation of magnesium-bearing minerals is readily promoted as compared with the direct precipitation from solution. Molecular dynamics (MD) was used to distinguish the precipitation pathways of calcium and magnesium ions. Calcium ions were demonstrated to bind directly with carbonate in solution, thus forming calcium carbonate clusters prior to precipitation. However, Magnesium ions tended to combine with organic components and undergone a series of dehydration reactions to form MgCO3 assisted by dolomite templates. The results of density functional theory (DFT) calculations indicated that the presence of organic components greatly affected the electron distribution profile of dolomite templates and reduced the Gibbs free energy required for dehydration reaction of Mg[6(H2O)]2+ at the interface. In addition, the template provided a large amount of carbonate to share with magnesium ions, which results in spontaneous, continuous nucleation and growth of magnesium ions at the dolomite template interface. MICP experiment coupled with MD-DFT calculations revealed the mechanistic influences of microorganisms and dolomite templates on the nucleation and precipitation of magnesium-bearing minerals. These findings advance our understanding of the reaction mechanisms and pathways of biogenic precipitation of calcium-magnesium minerals, and provide insights into the recovery of metal ions with strong hydration capacity.