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 (CaCO 3 ) remains incompletely understood. In this study, a droplet microfluidic chip was developed to observe the growth process of calcium carbonate and bacterial behaviour during the MICP process under various bacterial density conditions. Scanning electron microscope was then utilised to analyse the calcium carbonate morphology, and Raman spectroscopy was employed to identify calcium carbonate 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 on the bacterial surface. However, this precipitation pattern differed from the formation of monocrystalline calcium carbonate. The 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 debatable. This study provides new insights into the bio-induced calcium carbonate formation mechanism.