Bio-inspired Soil Improvement using Silicatein-α Enzyme

Silicatein‑α enzyme, which is naturally found in sea sponges, was produced in the laboratory and applied in biomineralization to control the CaCO ₃ crystal morphology. In this study, silicatein-α enzyme was engineered and expressed in modified Escherichia coli bacteria and then extracted and purified for lab use. Syringe tests were conducted to characterize the morphology and the structure of the CaCO ₃ precipitates with the guide of silicatein-α enzyme in soils. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) were used to characterize the CaCO ₃ at the microscale. Spicule (needle-like structures) morphology of CaCO ₃ precipitates were observed from the syringe tests compared to the cubic morphology of the CaCO ₃ precipitates treated by Microbial Induced Carbonate Precipitation (MICP) and Enzyme (urease enzyme) Induced Carbonate Precipitation (EICP). The optimized silicatein‑α enzyme treatment protocol including enzyme medium (0.1 M CaCl ₂ , 0.1 M MgCl ₂ , and active silicatein-α enzyme (concentration from 2×10 ^-6 to 6×10 ^-6 M)) and cementation medium, containing 0.2 M (NH ₄ ) ₂ CO ₃ at pH=11 and saturated with CO ₂ gas, was determined based on the conditions tested in this study. Confined compression tests were also used to investigate the mechanical property of the bio-cemented soil using the silicatein‑α enzyme. The shear modulus of soil was improved after silicatein-α treatment through shear wave velocity measurement, and the silicatein-α enzyme-treated soil (pH value of the cementation medium of 11) was less compressible than the untreated soil but more compressible than MICP and EICP-treated soils. Overall, spicule morphology of CaCO ₃ precipitates can be created and the mechanical property of the soil can be improved using silicatein‑α enzyme treatment.