Multivariate experimental study on soybean urease induced calcium carbonate precipitation

The plant-derived enzyme induced calcium carbonate precipitation (EICP) can significantly improve the engineering mechanical properties of sand. However, there is no specification for choosing the parameter value in the specific operation, and the solidification effect needs to be improved. Based on soybean urease, the effects of temperature, urease concentration, urea concentration, calcium concentration, pH value, calcium source type and other variables on urease activity and calcium carbonate precipitation were studied, and the SEM and XRD tests on precipitated calcium carbonate crystals were carried out. On this basis, the unconfined compressive strength and solidification effect of soybean urease-solidified sand were tested. The results showed that the urease activity increased linearly with the increase of urease concentration, but there was a temperature threshold. When the temperature exceeded the threshold, urease activity was completely inactivated, and the threshold decreased with the increase of urease concentration. Urea concentration and pH value affected urease activity together, and there was an optimum combination, i.e. the optimum pH value is 7 when urea concentration is 0.1–1 mol/L, and 8 when urea concentration is 1.0–1.5 mol/L. Urease is the catalyst of a precipitation reaction. The higher the urease concentration was, the more complete the reaction was, and the higher the precipitation rate of calcium carbonate was. For urea and calcium solutions, the dosage mainly affected the precipitation of calcium carbonate, and the dosage ratio should be 1:1. The concentration and pH value of urea and calcium solutions can affect the precipitation of calcium carbonate by affecting urease activity. Different calcium sources had little influence on the amount of precipitated calcium carbonate. The composition and density of precipitated calcium carbonate crystals from different calcium sources were basically the same, but the crystal structure was different. The calcium carbonate crystals with calcium chloride as calcium source are mainly massive, with spherical and spheroidal crystals on the surface and large cementation surface, which can be used as an ideal calcium source in EICP technology. The unconfined compressive strength of the sand solidified with urease from soybean and calcium chloride as calcium source was about 6 times that of the sand mixed with coal fly ash. SEM images show that the precipitated calcium carbonate crystals wrap and bond the sand into a whole, and the solidification effect is ideal.