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

Abstract

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 crystallization 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.