Inhibition of urea hydrolysis by free Cu concentration of soil solution in microbially induced calcium carbonate precipitation.

Urea hydrolysis is an initiating step of microbially induced calcium carbonate precipitation (MICP) which can be used as a stabilization technology in heavy metals contaminated soil. In this study, inhibition of urea hydrolysis was investigated in Cu-contaminated soil. At soil Cu concentration from 0 to 1000 mg/kg, the amount of urea hydrolyzed (i.e., initial urea 450 mM) ranged from 449.3 ± 1.4 to 10.5 ± 0.8 mM. Correspondingly, decrease in calcium carbonate precipitation was commensurate with the inhibition of urea hydrolysis. Interestingly, 2.75 times more urea were hydrolyzed in 350 days-aged soil than in freshly spiked soil even at the same soil Cu concentration of 250 mg/kg, suggesting the inhibitory effect of Cu in soil solution. Indeed, the concentrations of Cu in soil solution were 4.9 ± 0.1 and 21.0 ± 0.3 mg/L, respectively. Since MICP application involved an increase in Ca2+ concentration in soil, its effect was also determined. In the freshly spiked soil with 250 mg-Cu/kg, the Cu concentration in the soil solution increased from 7.6 ± 0.1 to 21.0 ± 0.3 mg/L as the calcium concentration increased from 0 to 450 mM. Accordingly, urea hydrolysis was significantly reduced from 217.5 ± 59.0 to 11.9 ± 0.2 mM. The effect of pH was also determined, showing that 32.8 ± 3.4 and 205.9 ± 32.5 mM of urea was hydrolyzed at soil pH of 4.5 and 7.8, respectively. The reason was attributed to the great difference in free Cu concentration in soil solution (i.e., 3.3 and 0.3 mg/L at pH 4.5 and 7.8, respectively). The relationship between amounts of urea hydrolyzed and free Cu concentrations was established and half-maximal inhibition concentration (IC50) of free Cu concentration in soil solution was predicted to be 0.39 mg/L.