Abstract:
The kinetics of calcite precipitation induced in response to the hydrolysis of urea by Bacillus pasteurii at different temperatures in artificial groundwater (AGW) was investigated. The hydrolysis of urea by B. pasteurii exhibited a temperature dependence with first order rate constants of 0.91 d-1 at 20°C, 0.18 d-1 at 15°C, and 0.09 d-1 at 10°C. At all temperatures, the pH of the AGW increased from 6.5 to 9.3 in less than 1 d. Dissolved Ca2+ concentrations decreased in an asymptotic fashion after 1 d at 20°C and 15°C, and 2 d at 10°C. The loss of Ca2+ from solution was accompanied by the development of solid phase precipitates that were identified as calcite by X-ray diffraction. The onset of calcite precipitation at each temperature occurred after similar amounts of urea were hydrolyzed, corresponding to 8.0 mM NH4+. Specific rate constants for calcite precipitation and critical saturation state were derived from time course data following a second-order chemical affinity-based rate law. The calcite precipitation rate constants and critical saturation states varied by less than 10% between the temperatures with mean values of 0.16 ± 0.01 μmoles L-1 d-1 and 73 ±3, respectively. The highest calcite precipitation rates (ca. 0.8 mmol L-1 d-1) occurred near the point of critical saturation. While unique time course trajectories of dissolved Ca2+ concentrations and saturation state values were observed at different temperatures, calcite precipitation rates all followed the same asymptotic profile decreasing with saturation state regardless of temperature. This emphasizes the fundamental kinetic dependence of calcite precipitation on saturation state, which connects the otherwise dissimilar temporal patterns of calcite precipitation that evolved under the different temperature and biogeochemical regimes of the experiments.