Role of bacteria on bio-induced calcium carbonate formation: insights from droplet microfluidic experiments

Microbially induced calcium carbonate precipitation (MICP) has emerged as a promising solution for geotechnical issues. However, the role of bacteria in the formation of calcium carbonate (CaCO3) remains incompletely comprehended. In this study, a droplet microfluidic chip was developed to observe the growth process of CaCO3 and bacterial behavior during the MICP process under various bacterial density conditions at the monocrystal-scale. Scanning electron microscope (SEM) was then utilized to analyze the CaCO3 morphology, and Raman spectroscopy was employed to identify CaCO3 polymorphs. Nucleation within microspaces showed a stochastic nature. Within the droplets where crystals formed, all crystals manifested as cubic calcite. Higher bacterial density led to the formation of larger and more irregularly shaped crystals, with crystal size showing a significant correlation with urease activity. In droplets where no crystals formed, higher bacterial density and urease activity resulted in the precipitation of amorphous calcium carbonate (ACC) on the bacterial surface. However, this precipitation pattern differed from the formation of monocrystalline CaCO3. Our results demonstrate that bacteria act primarily as urease secretors to regulate crystal growth during the MICP process, while their role as nucleation sites for crystals remains controversial. This study provides a novel insight into understanding the bio-induced CaCO3 formation mechanism.