# Microbially Induced Desaturation and Carbonate Precipitation through Denitrification: A Review
## 4 Potential Applications of Denitrification-Based MICP Biotechnology
### MICP-Driven Applications
#### 4.1.2 Remediation of Cracks and Inhibition of Steel Corrosion in Reinforced Concrete
In their study, [PERSON] et al. [34] benefit from the production and accumulation of NO\({}_{2}^{-}\), a well-known anodic corrosion inhibitor, as a metabolite during MICP through denitrification in a microbial self-healing concrete. In the construction sector, chemical compounds of NO\({}_{2}^{-}\), such as Ca(NO\({}_{2}\))\({}_{2}\), are used as corrosion inhibiting  
Figure 3: Representative photomicrographs of crack healing in microbial nitrate reduction based self-healing concrete containing (**a**) _Diaphorobacter nitroreducens_ and (**b**) _Pseudomonas aeruginosa_ loaded expanded clay particles; (**c**) _Diaphorobacter nitroreducens_ and (**d**) _Pseudomonas aeruginosa_ loaded granular activated carbon particles (redrafted after [PERSON] et al. [58]). admixtures, as NO\({}_{2}^{-}\) rapidly stabilizes the mobile ferrous compounds formed in anodic reactions in the form of ferric compounds (Equation (8)), which hinders the migration of the Fe\({}^{2+}\) from the anodic zones of the rebar, and thus, inhibit steel rebar corrosion. In this process, the precipitation of ferric oxides locally creates a new passive layer which prevents material loss and inhibits corrosion. Microbially produced NO\({}_{2}^{-}\) inside a concrete crack was found to be as effective as commercially available NO\({}_{2}^{-}\) containing chemical admixtures in terms of rebar corrosion inhibition [34]. \[2\text{Fe}^{2+}+2\text{OH}^{-}+2\text{ NO}_{2}^{-}\ \rightarrow\ 2\text{NO}+\text{Fe}_{2}\text{O}_{3}+\text{H}_{2}\text{O} \tag{8}\]  
[PERSON] et al. [34] monitored the open circuit potentials of steel rebars either placed in ureolysis or denitrification based self-healing concrete which were immersed in a 0.5 M Cl-solution for 17 weeks upon crack formation (Figure 4a,b). It was reported that although both of the self-healing concretes completely healed cracks of 300 \(\upmu\)m crack width in four weeks, only in denitrification based self-healing concrete, the open circuit potential stayed above the critical value, and steel corrosion was successfully inhibited. It was emphasized that sole crack healing was not enough to protect the steel in aggressive environments which compromised the application of ureolysis based self-healing concrete (Figure 4c,d) in aggressive environments and promoted denitrification based self-healing concrete as a better alternative. Overall, MICP through denitrification is a promising process to develop a multifunctional microbial self-healing concrete that is suitable for structures exposed to marine conditions (wet/dry cycles, aggressive ions, etc.). Regular monitoring of cracks and the state of the reinforcement bars in immersed or underground concrete structures are grueling. Moreover, maintenance works on structures such as tunnels (underground or underwater), bridges and car parks cause significant pause in the provided service and thus people using these structures are also affected negatively. Additionally, maintenance works to stop corrosion and protect the healthy state of steel reinforcement bars are both labor and cost-intensive. Furthermore, in conventional concrete structures, crack and corrosion related durability issues also cause renewal of these structures long before their actual service life and thus almost double their carbon footprint. As studies on microbial self-healing of concrete cracks revealed that application of MICP through denitrification can avoid the need for external crack repair, similar to other internal self-healing mechanisms, it will avoid the disruptions in the service provided by concrete structures. Additionally, continuous self-healing of microcracks will avoid the need for regular monitoring of concrete structures for cracks. Lastly, recovery of water tightness and microbial corrosion inhibition mechanism will avoid most of the durability issues and prolong the service life of the structures. Overall, with these benefits, novel microbial self-healing concrete can find a solid place in the approaching low-carbon economy era.