Tuning Polymorphs and Morphology of Microbially Induced Calcium Carbonate: Controlling Factors and Underlying Mechanisms

Microbially precipitated calcium carbonate (CaCO₃) has drawn broad attention due to its potential applications in various areas, for example, biocementation, medicine, and soil reinforcement. Sporosarcina pasteurii (S. pasteurii), formerly known as Bacillus pasteurii, has been investigated for CaCO₃ biomineralization due to its high ureolytic activity. A high degree of supersaturation with respect to the presence of bacterial cell wall, extracellular polymeric substances, and organic byproducts of bacterial activity plays an important role in the formation and stabilization of CaCO₃ polymorphs. Although microbially induced CaCO₃ and its polymorphs have been investigated broadly, the mechanisms of polymorph selection and morphological evolution are not well understood. This study employs ex situ approaches to address the complication of biomineralization in the presence of living organisms and to elucidate how solution chemistry, bacterial activity, and precipitation kinetics alter the polymorphism and morphology of CaCO₃ induced by S. pasteurii. The results indicate that in the presence of enough calcium ions and urea (as a carbonate source), the bacterial activity favors the formation and stabilization of vaterite. The morphological observations also provide valuable information on the particles' microstructure. The morphology of calcite evolves from single crystal to polycrystalline structures, and the morphology of vaterite evolved from spherical to oval-shaped structures on increasing the organic material concentration. Specific functional groups also exert morphological control on CaCO₃ polymorphs. However, the sensitivity of the calcite polymorph to the composition and orientation of these functional groups is higher compared to that of the vaterite polymorph. These findings offer important insights that can be used to constrain a set of experimental conditions for synthesizing a certain polymorph ratio for vaterite/calcite or a particular morphology of each polymorph and shed light on the crystallization and phase transformation mechanisms in such complicated bioenvironments.