Biocementation of coral sand under seawater environment and an improved three-stage biogrouting approach

The Sporosarcina pasteurii (S. pasteurii)-DSM 33 was acclimated to grow and secrete urease under different salinity level conditions via direct evolution method, and ultimately the new acclimated species, S. pasteurii-DSM 33-ASW100, was of higher cell growth rate and ureolysis activity under seawater environment. Meanwhile, this study further revealed a fact that high concentration of calcium chloride (>0.25 mol/L) would inhibit the existing urease activity but hardly affect the urease secretion function of S. pasteurii-DSM 33-ASW100. The rising of urea concentration would not mitigate the inhibitory effect of calcium chloride on urease activity; by contrast, it could stimulate S. pasteurii-DSM 33-ASW100 to secrete more urease, ultimately conducting the microbially induced carbonate precipitation (MICP) reaction under a higher pH level (-8.0). In the seawater environment, the optimal coral sand particle size for MICP reaction was about 0.5-1.0 mm. Moreover, the new designed three -stage biogrouting method significantly mitigate the issue of non-uniform distribution of cells and crystals in the column, thereby greatly increasing the unconfined compressive strength of MICP-treated coral sand column (>9.0 MPa) with a relatively high CaCO3 content (-32 % by weight) as well as calcium ion utilization rate (-85 %). Results also indicated that excessive input of CaCl2 (>0.5 mol/L) may result in less uniformity of crystal-lization and thus lead to a relative lower unconfined compressive strength (-4.0 MPa). Excessive addition of urea (>1.0 mol/L) may lead to partial clogging issues due to too fast reaction of MICP, consequently column strength dropping down to around 2.0 MPa.

Automated summary

This study successfully acclimated a new species, S. pasteurii-DSM 33-ASW100, with higher cell growth rate and ureolysis activity in seawater environment. The inhibitory effect of high concentration of calcium chloride on urease activity was observed, while urea concentration stimulated the secretion of urease by S. pasteurii-DSM 33-ASW100. The optimal coral sand particle size for microbially induced carbonate precipitation (MICP) reaction was found to be about 0.5-1.0 mm. Additionally, a newly designed three-stage biogrouting method greatly improved the compressive strength of MICP-treated coral sand columns.