Experimental and numerical modeling of bacterially induced pH increase and calcite precipitation in saline aquifers

As a part of an effort to investigate potential implications of microbial activity upon CO2 geological sequestration, both the alkalinization of a urea-containing artificial ground water and the subsequent calcium carbonate precipitation, induced by Bacillus pasteurii, have been studied in batch experiments. Four reproducible stages of this microbial process were identified and numerically modeled: (1) a rapid rise of pH values caused by bacterial ureolysis, (2) a pH plateau due to a dynamic equilibrium between CO2 transfer through the liquid/gas interface and the ureolysis process, (3) a decrease in pH due to CaCO3 precipitation (4) a slow long term evolution of pH depending on the presence of viable microorganisms which have survived to carbonate precipitation. Correlations between the durations and pH values of these four steps were also evidenced. To interpret quantitatively the observed trends, the geochemical code CHESS© was adapted for taking into account the enzymatically catalyzed ureolysis reaction as well as the kinetics of gas/solution exchanges and the rate of calcium carbonate precipitation. Finally, new original aspects of B. pasteurii biomineralization were evidenced, namely a cellular calcium phosphate precipitation preceding the formation of calcite and a negative impact of phosphate on ureolysis and calcite precipitation.