The role of Mg2+ in inhibiting CaCO3 precipitation from seawater

The precipitation and dissolution processes of CaCO3 are essential to the marine carbon cycle and are relevant to climate research. Nevertheless, one of the nagging questions is why the precipitation of CaCO3 rarely occurs without the help of biological activity even in the surface seawater, where the calcite saturation state (Omega, expressed as the ratio of the calcium and carbonate ion concentration product to the stoichiometric solubility constant of calcite) is up to 5, but commonly takes place in hard-water lakes, springs and the low-salinity zone (practical salinity <5) of estuaries. Studies have found that Mg2+ plays a significant role in inhibiting all aspects of CaCO3 precipitation, namely crystal formation, solubility, and precipitation rate. However, whether the presence of Mg2+ in seawater is sufficient to inhibit CaCO3 precipitation has not been clearly stated, and the specific inhibitory role of Mg2+ in CaCO3 precipitation still requires clarification. In this study, we used four different seed materials to induce CaCO3 precipitation in artificial seawater of four Mg:Ca ratios and examined the crystal morphologies of the resulting precipitates using scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). We found that Mg2+ inhibits the precipitation at the first stage of nucleation, which drastically increases the difficulty of calcite nucleation. At a seawater Mg:Ca ratio of 5.2, aragonite preferentially nucleates instead of calcite. Among the tested materials, only calcite and aragonite seeds exhibit marked catalysis of CaCO3 precipitation, indicating that not all the particles in natural seawater can catalyze CaCO3 precipitation. Our results demonstrate while the precipitation of CaCO3 is inhibited by Mg2+ at all stages, the inhibition at the nucleation stage is the main reason massive spontaneous CaCO3 precipitation does not occur under the surface seawater conditions, supporting the notion that the incorporation of foreign ions such as Mg2+ increases the surface energy and the solubility of the overgrowth or cluster, and thus inhibits CaCO3 precipitation.