Denitrification processes, inhibitors, and their implications in ground improvement

Ureolysis and denitrification are the two major microbial metabolic pathways commonly used in Microbially induced calcite precipitation (MICP) for geoengineering applications. Although ureolysis is generally the more efficient pathway, the denitrification pathway has gained more attention recently because a diverse group of bacteria can precipitate calcite via denitrification, and no harmful byproduct is generated provided that the reduction of nitrate to nitrogen gas is complete. There are, however, many environmental factors that could inhibit or reduce the efficiency of the denitrification process in soil. Some examples of these factors include salinity, pH, temperature, biodiversity (abundance and species of denitrifiers and competitors), water stress (extreme wet-dry conditions), degree of saturation (anaerobic vs. aerobic conditions), high heavy metal content (e.g., mine tailings), and shortage of dissolved carbon sources. In this paper, the denitrification process, the denitrification inhibitors, and the mechanisms involved in their inhibition of the denitrification process are discussed in detail. This investigation indicates that although general optimum conditions can be formulated for MICP through denitrification, significant adjustments may be necessary if inhibitory conditions are anticipated. It was also shown that when inhibitors are expected, it is crucial to investigate not only the amount of precipitated calcium carbonate but also the N 2 O / N 2 gase ratio to ensure the complete reduction of nitrate to nitrogen gas and prevent the release of byproducts (especially N 2 O ) into the environment. Finally, the implications of the inhibitory factors on the field application of denitrification MICP treatment for different geotechnical projects are discussed.