# Effects of Bacterial Density on Growth Rate and Characteristics of Microbial-Induced CaCO3 Precipitates: Particle-Scale Experimental Study | Journal of Geotechnical and Geoenvironmental Engineering

Abstract

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

$CaCO3$

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

$CaCO3$

precipitates during MICP processes occurring at the sand particle scale. Results show that the precipitation rate of

$CaCO3$

increases with bacterial density in the range between

$0.6×108$

and

$5.2×108 cells/mL$

. Bacterial density also affects both the size and number of

$CaCO3$

crystals. A low bacterial density of

$0.6×108 cells/mL$

produced

$1.1×106$

crystals/mL with an average crystal volume of 8,000

$μm3$

, whereas a high bacterial density of

$5.2×108 cells/mL$

resulted in more crystals (

$2.0×107crystals/mL$

), but with a smaller average crystal volume of

$450 μm3$

. The produced

$CaCO3$

crystals were stable when the bacterial density was

$0.6×108 cells/mL$

. When the bacterial density was 4–10 times higher, the crystals were first unstable and then transformed into more stable

$CaCO3$

crystals. This suggests that bacterial density should be an important consideration in the design of MICP protocols.