# 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  
Dentirification occurs when bacteria uses nitrate as an electron acceptor to oxidize organic matter under anoxic conditions. Therefore, it has the benefit that precipitation can occur in oxygen-limited environments, such as the inner part of a concrete crack. Application of the MICP technique via denitrification has recently been reported to provide crack healing by inducing CaCO\({}_{3}\) precipitation within the crack [34; 35; 58; 59; 60]. [PERSON] et al. [58] was the first to use axenic denitrifying strains of _Pseudomonas aeruginosa_ and _Diaphorobacter nitroroducens_ protected within either granular activated carbon particles or expanded clay particles as microbial healing agents, in order to test the feasibility of repairing concrete cracks using the MICP through denitrification. Their results showed that these denitrifying axenic strains induced the healing of concrete cracks up to 400 \(\upmu\)m crack width upon four weeks (Figure 3a,b), and cracks up to 470 \(\upmu\)m crack width upon seven weeks (Figure 3c,d), making their use as effective as commonly proposed microbial methods (e.g., MICP through lactate oxidation or ureolysis), but more environment-friendly [58]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Author and Ref** & **Electron Donor** & **NO\({}_{3}\)\({}^{\text{-}}\)Consumed** & **Initial Ca\({}^{2\text{+}}\)** & **CaCO\({}_{3}\)by** & **Time** \\  & **(Concentration)** & **(mM)** & **Concentrate** & **(mM)** & **Reported** & **(Days)** \\ \hline \multirow{4}{*}{Karatas [24]} & NB\({}^{*}\) (16.7 g/L) & 8 & & \\  & Acetate (75 mM) & 12 & & \\  & L-glutamic acid (50/75 mM) & 15 & 20–100 & ND\({}^{*}\) & 2–40 \\  & Acetate (24 mM) & 16 & 20 & Less than 1\% at the & \\  & Acetate (60 mM) & 40 & 50 & bottom to 10\% at & \\  & Acetate (120 mM) & 80 & 100 & the top of the sand & \\  & Acetate (240 mM) & 160 & 200 & column & \\  & Acetate (45 mM) & 50 & 100 & 6\% at the top to & \\  & & NB\({}^{*}\) (20 g/L) & 17.6 & 20 & \(\approx\)0.01\% & 7 \\  & L-glutamic acid (75 mM) & & 130 (first three & & \\  & Acetate (160 mM, first three & 100 (first three & & \\  & flushes)–120 (fourth to & flushes)–120 & & Average 1.1\% & 65 \\  & Acetate (130 mM, fourth to & ninth flush) & ninth flush) & flush & & \\  & Acetate (120 mM, three flushes) & 100 (three flushes) & 110 (three & 0.28\% by weight & \\  & Acetate (24 mM, 15 flushes) & 20 (15 flushes) & 22 (15 flushes) & 0.65\% by weight & \\ \hline \hline \end{tabular} ND: Not Determined, NB: Nutrient Broth  
\end{table} 
Table 3: Overview of key findings reported in the literature for soil improvement by denitrification-based MICP. [PERSON] et al. [30] also replaced the protected axenic cultures with self-protected non-axenic biogranules called activated compact denittifying core (ACDC). ACDC biogranules were compatible with concrete up to 3% _w/w_ cement incorporation dose [61] and they could survive in mortar [38], inhibit steel corrosion [34], and induce complete healing of 500 \(\upmu\)m-wide cracks upon four weeks of tap water treatment [60]. Additionally, it was recently reported that even under wet/dry cycles, this nitrate reducing biogranule containing bioconcrete can self-heal cracks as wide as 400 \(\upmu\)m [36]. They also tested the crack healing performance in biogranule (ACDC) containing aged concrete and found out that the developed bioconcrete was effectively self-healing the cracks in mature specimens [60]. Further investigations via X-ray \(\upmu\)CT scanning of healed and unheated mortar specimens with a 400 \(\upmu\)m wide initial crack width revealed that after four weeks of healing period, the healed crack volume was 6% and the thickness of the calcite layer was 10 mm which decreased the water permeability of cracked concrete by 83% [35]. A significant added benefit of using denittifying cultures in self-healing concrete was reported as the simultaneous corrosion inhibition of steel rebar during autonomous crack healing process.