Effects of Bacterial Density on Growth Rate and Characteristics of Microbial-Induced CaCO3 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 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 0.6x10(8) and 5.2x10(8) cells/mL. Bacterial density also affects both the size and number of CaCO3 crystals. A low bacterial density of 0.6x10(8) cells/mL produced 1.1x10(6) crystals/mL with an average crystal volume of 8,000 mu m(3), whereas a high bacterial density of 5.2x10(8) cells/mL resulted in more crystals (2.0x10(7) crystals/mL), but with a smaller average crystal volume of 450 mu m(3). The produced CaCO3 crystals were stable when the bacterial density was 0.6x10(8) 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. (C) 2021 American Society of Civil Engineers.

Automated summary

Bacterial density has a significant impact on both the precipitation rate and characteristics of CaCO3 crystals in MICP-treated soils, with low density leading to larger crystals and high density resulting in more but smaller crystals. Adjusting bacterial density should be considered in the design of MICP protocols for optimal performance.