# Microbially Induced Desaturation and Carbonate Precipitation through Denitrification: A Review
## 1 Introduction
In MICP through denitrification, the denitrifying bacteria are introduced into the soil together with a carbon source as an electron donor, nitrate (NO\({}_{3}^{-}\)) as a terminal electron acceptor, calcium as a precursor, and the general nutrients for bacterial growth and reproduction. In complete nitrate reduction process, so called denitrification, nitrate is reduced to nitrogen gas and the carbon source is oxidized to carbon dioxide. Since nitrate  
Figure 1: Number of research papers conducted on MICP through different microbial metabolic pathways based on the database of Web of Science from 2000 to 2021. The first column represents the search on the topic of “calcium carbonate” OR “calcite precipitation” OR “carbonate precipitation”, and the keyword of photosynthesis”, ureoly*, “aerobic respiration,”, ammonification, “sulfate reduction,” “iron reduction,” “methane oxidation,” and “denitrification”, respectively. The second column represents the number of articles searched on two additional keywords “technology” OR “biotechnology”. reduction process inherently removes H+ from the environment, the denitrification process leads to the production of alkalinity which further converts part of the produced CO2 gas into CO32- (Table 1). Consequently, nitrogen (N2) and CO2 gases are generated, while CaCO3 is precipitated out of the solution in the presence of free Ca2+ ions (Table 1). If the aforementioned process occurs in a saturated porous environment, biogas production (N2 and CO2) and mineral (CaCO3) precipitation result in partial desaturation of the porous media and thus changes in its hydromechanical behavior. The process of simultaneous desaturation and CaCO3 precipitation is specific to denitrification pathway and thus MICP through denitrification becomes prominent among the other commonly investigated MICP pathways. In fact, this new process of simultaneous desaturation and CaCO3 precipitation occurring in porous environments upgraded MICP to a new level named as microbially induced desaturation and precipitation (MIDP) [17; 20]. Currently, the potential of the denitrification pathway for MICP applications is overlooked (Figure 1). Both technical studies and review studies focus on MICP via ureolysis and the use of ureolytic pure cultures. Critical reviews on alternative MICP pathways are necessary to create a ground for detailed evaluation of advantages and disadvantages of various MICP pathways in different applications which will pave the way for tailored solutions. Therefore, this review study reveals the potential of denitrification pathway as an alternative to commonly proposed, less sustainable MICP pathways by covering the added benefits (i.e., corrosion inhibition, MIDP) offered by the stepwise occurrence of denitrification based MICP. This review study consists of four major parts (i) denitrification mechanism and, the activities of denitrifying bacteria related to desaturation and CaCO3 precipitation, (ii) applications of MICP and MIDP through denitrification, (iii) the challenges involved in the practical applications and (iv) suggestions of future research to overcome those challenges and enable process upscaling and optimization of the novel applications.