Microbially induced calcite precipitation in calcareous soils by endogenous Bacillus cereus, at high pH and harsh...

Microbially induced calcite precipitation (MICP) improves the physical properties of soils by increasing the solid content, decreasing the pore sizes and improving the rigidity of the particle-to-particle contact ending with a better mechanical and geotechnical performances of the soils. First, the physical characteristics of soils in Qatar showed similar grain size distributions with most falling in the category of fine particles (30 μm-1 μm), which is appropriate for MICP processes. MICP is ensured by the ureolytic activity of urease producing bacteria. However, Qatari soils are characterized with high carbonate contents exceeding 20%, high alkalinity and fluctuations of temperature and aeration. Although such properties can cause calcite dissolution as reported in literature, two Qatari endogenous B. cereus strains, QBB4 and QBB5, were shown able to adapt to the harsh conditions and induce mineral formations by MICP. Their optimal potentials was at wide ranges of temperature from 30 °C to 42 °C and pH from 7.0 to 8.0. Using Qatari bacteria in liquid cultures and at syringe level at laboratory conditions, as well as in plots at field conditions, 16% CaCO₃ increase in minerals formation was obtained, leading to 7% increase of soils stability. Increase of carbonate contents was also shown by an increase in the stability of aggregates to disintegration when incubated in water using a 0.25 mm sieve. MICP in Qatari soils is feasible using B. cereus QBB4, as this is an endogenous bacterium that tolerates harsh conditions, high alkalinity and calcium contents. The performance of these bacterial strains was achieved with clear formation of aragonite, feldspars and quartz in the calcareous soils.