Effects of Bacterial Density on Growth Rate and Characteristics of Microbial-Induced ${\mathrm{CaCO}}_3$ Precipitates: Particle-Scale Experimental Study

Microbial-induced carbonate precipitation (MICP) has been explored for more than a decade as a promising soil improvement technique. However, it is still challenging to predict and control the growth rate and characteristics of ${\mathrm{CaCO}}_3$ precipitates, which directly affect the engineering performance of MICP-treated soils. In this study, we employ a microfluidics-based pore-scale model to observe the effect of bacterial density on the growth rate and characteristics of ${\mathrm{CaCO}}_3$ precipitates during MICP processes occurring at the sand particle scale. Results show that the precipitation rate of ${\mathrm{CaCO}}_3$ increases with bacterial density in the range between $0.6{\times 10}^8$ and $5.2{\times 10}^8\mathrm{cells}/\mathrm{mL}$. Bacterial density also affects both the size and number of ${\mathrm{CaCO}}_3$ crystals. A low bacterial density of $0.6{\times 10}^8\mathrm{cells}/\mathrm{mL}$ produced $1.1{\times 10}^6$ crystals/mL with an average crystal volume of 8,000 ${\mu \mathrm{m}}^3$, whereas a high bacterial density of $5.2{\times 10}^8\mathrm{cells}/\mathrm{mL}$ resulted in more crystals ($2.0{\times 10}^7\mathrm{crystals}/\mathrm{mL}$), but with a smaller average crystal volume of $450\mu {\mathrm{m}}^3$. The produced ${\mathrm{CaCO}}_3$ crystals were stable when the bacterial density was $0.6{\times 10}^8\mathrm{cells}/\mathrm{mL}$. When the bacterial density was 4–10 times higher, the crystals were first unstable and then transformed into more stable ${\mathrm{CaCO}}_3$ crystals. This suggests that bacterial density should be an important consideration in the design of MICP protocols.