Biomimetic mineralization and characterization of hierarchically structured hydrated magnesium carbonates: The effects of sodium alginate

Recently, CO2 mineralization involving magnesium salt provides a feasible way for carbon sequestration. The produced hydrated magnesium carbonates (HMCs) can be used as functional materials and will bring huge economic benefits. Inspired by the mineralization in biological organisms, the indirect CO2 mineralization process involving NH4HCO3-MgCl2 in the presence of sodium alginate (SA) was systematically investigated. XRD, SEM, FTIR, TG and XPS were used to characterize the crystal phase, morphology, structure and surface of HMCs. By the regulation of temperature, HMCs with a complex hierarchical structure were successfully synthesized, i.e., sea urchin-like nesquehonite assembled by nanorods and 3D spiderweb-like hydromagnesite assembled by nanosheets. The presence of SA inhibits the phase transition from nesquehonite to hydromagnesite, and the inhibitory effect increases as the concentration of SA increases. By decreasing the molar ratio of Mg2+/HCO3−, the shape of nesquehonite particle changes from a sphere to an ellipsoid or a gourd. The spherical structure of nesquehonite is destroyed in a weak alkaline solution and the crystalline product is flower-like hydromagnesite in a strong alkaline solution. The decomposition process of HMCs is composed of dehydration (or dehydration and dehydroxylation) stage and decarbonation stage. XPS results for nesquehonite and hydromagnesite are quite different due to their different surface structure. This study will provide insights into the preparation of hierarchically structured HMCs by indirect CO2 mineralization and promote added values of HMCs by biomimetic mineralization method.