Improving the strength of sandy soils via ureolytic CaCO₃ solidification by <i>Sporosarcina ureae</i>

Abstract

Microbially induced carbonate precipitation (MICP) is a biogeochemical process that can be applied to strengthen materials. The hydrolysis of urea by microbial catalysis to form carbonate is a commonly studied example of MICP. In this study, Sporosarcina ureae, a ureolytic organism, was compared to other ureolytic and non-ureolytic organisms of Bacillus and Sporosarcina genera in the assessment of its ability to produce carbonates by ureolytic MICP for ground reinforcement. It was found that S. ureae grew optimally in alkaline (pH  ∼  9.0) conditions which favoured MICP and could degrade urea (units U mL−1 represent µmol min−1 mL OD600) at levels (30.28 U mL−1) similar to S. pasteurii (32.76 U mL−1), the model ureolytic MICP organism. When cells of S. ureae were concentrated (OD600 ∼  15–20) and mixed with cementation medium containing 0.5 M calcium chloride (CaCl2) and urea into a model sand, repeated treatments (3  ×  24 h) were able to improve the confined direct shear strength of samples from 15.77 kPa to as much as 135.80 kPa. This was more than any other organism observed in the study. Imaging of the reinforced samples with scanning electron microscopy and energy-dispersive spectroscopy confirmed the successful precipitation of calcium carbonate (CaCO3) across sand particles by S. ureae. Treated samples were also tested experimentally according to model North American climatic conditions to understand the environmental durability of MICP. No statistically significant (p < 0.05, n =  3) difference in strength was observed for samples that underwent freeze–thaw cycling or flood-like simulations. However, shear strength of samples following acid rain simulations fell to 29.2 % of control MICP samples. Overall, the species S. ureae was found to be an excellent organism for MICP by ureolysis to achieve ground strengthening. However, the feasibility of MICP as a durable reinforcement technique is limited by specific climate conditions (i.e. acid rain).