Biomineralization of soil with crude soybean urease using different calcium salts

Calcium salt is an important contributing factor for calcium-based biomineralization. To study the effect of calcium salt on soil biomineralization using crude soybean urease, the calcium salts, including the calcium chloride (CaCl2), calcium acetate ((CH3COO)2Ca) and calcium nitrate (Ca(NO3)2), were used to prepare the biotreatment solution to carry out the biomineralization tests in this paper. Two series of biomineralization tests in solution and sand column, respectively, were conducted. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were performed to determine the microscopic characteristics of the precipitated calcium carbonate (CaCO3) crystals. The experimental results indicate that the biomineralization effect is the best for the CaCl2 case, followed by (CH3COO)2Ca, and worst for Ca(NO3)2 under the test conditions of this study (i.e. 1 mol/L of calcium salt-urea). The mechanism for the effect of the calcium salt on the biomineralization of crude soybean urease mainly involves: (1) inhibition of urease activity, and (2) influence on the crystal size and morphology of CaCO3. Besides Ca2+, the anions in solution can inhibit the activity of crude soybean urease, and NO3− has a stronger inhibitory effect on the urease activity compared with both CH3COO− and Cl−. The co-inhibition of Ca2+ and NO3− on the activity of urease is the key reason for the worst biomineralization of the Ca(NO3)2 case in this study. The difference in biomineralization between the CaCl2 and (CH3COO)2Ca cases is strongly correlated with the crystal morphology of the precipitated CaCO3.