and the availability of nucleation sites is important for calcite precipitations
[230]. Comparative study on induced calcite precipitation at alkaline condition
(pH 9, buffer and medium), in the absence or presence of cells has shown that
98% of the Ca
2+ precipitated in the presence of microbial cells, but only 35 and 54%
of the Ca
2+ was precipitated in the buffered water and medium, respectively. This
difference is due to the bacterial cells which provide the nucleation sites for CaCO 3
precipitation and its ability to maintain an alkaline environment for induction and
further growth of calcite crystals [93]. Hence, it is possible to extrapolate that any
factor that influences the amount and composition of the exopolysaccharide produced by the microbe has effect on the efficiency of calcite precipitation. Culture
conditions (such as media composition, degree of aeration, pH of media, and
cultivation temperature) are among such factors. The density of negative charges
in microbial cell wall/EPS matrix which serves as the nucleation sites by attracting
Ca
2+ could also play a role in the efficiency of calcite precipitation. Alkaliphiles as a
way of adaptation to the high pH environment evolved cell wall/EPS matrices with
high density of anions, and this can be considered one benefit in using alkaliphiles in
concrete application. It is not only the ureolytic process that is affected by the type
and amount of cells, but the efficiency of the processes oxidizing organic compounds
and denitrifying nitrates is also affected by it. The microbes should be at least
tolerant to high pH and able to generate enough calcite under the
circumstances [175].
4.2 pH
Calcite precipitation is influenced by pH [243]. This is mainly due to better solubility
of CO 2 and high stability of carbonate at elevated pH. Moreover, in the case of
urease-driven precipitation, the optimum pH for urease activity is in the alkaline
range. Most of the microbial ureases studied are optimally active around pH 8
[133, 236]. Degradation of urea by urease generates NH 4
+ which raises the pH of
the medium. The increase in pH or maintenance of the alkaline condition is ideal for
calcite precipitation, because at low pH, the carbonate tends to dissolve rather than
precipitate [244]. The CO 3
2À that ends up in the calcite precipitation is CO 2 driven.
Since the solubility of CO 2 increases with pH, the CO 3
2À concentration rises
concomitantly, and in the presence of Ca
2+ , this facilitates the formation of
CaCO 3 . The CO 2 can be atmospheric or generated by cellular activities [230].
Most microbial-based calcite precipitation occurs under alkaline conditions from
pH 8.7 to 9.5 [133, 245, 246]. However, it is not clear if this is due to the activity of
urease or influenced by another factor. Salt concentration and temperature have a
clear effect on the activity of urease. From this point of view, urease-producing
alkaliphilic and halophilic microorganisms could be of interest for concrete applications. Recently, Stabnikov et al. [247] investigated whether haloalkaliphilic
ureolytic bacteria are active at high concentrations of salt and high pH, conditions
that are suitable for manufacturing biocement.
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
325
[230]. Comparative study on induced calcite precipitation at alkaline condition
(pH 9, buffer and medium), in the absence or presence of cells has shown that
98% of the Ca
2+ precipitated in the presence of microbial cells, but only 35 and 54%
of the Ca
2+ was precipitated in the buffered water and medium, respectively. This
difference is due to the bacterial cells which provide the nucleation sites for CaCO 3
precipitation and its ability to maintain an alkaline environment for induction and
further growth of calcite crystals [93]. Hence, it is possible to extrapolate that any
factor that influences the amount and composition of the exopolysaccharide produced by the microbe has effect on the efficiency of calcite precipitation. Culture
conditions (such as media composition, degree of aeration, pH of media, and
cultivation temperature) are among such factors. The density of negative charges
in microbial cell wall/EPS matrix which serves as the nucleation sites by attracting
Ca
2+ could also play a role in the efficiency of calcite precipitation. Alkaliphiles as a
way of adaptation to the high pH environment evolved cell wall/EPS matrices with
high density of anions, and this can be considered one benefit in using alkaliphiles in
concrete application. It is not only the ureolytic process that is affected by the type
and amount of cells, but the efficiency of the processes oxidizing organic compounds
and denitrifying nitrates is also affected by it. The microbes should be at least
tolerant to high pH and able to generate enough calcite under the
circumstances [175].
4.2 pH
Calcite precipitation is influenced by pH [243]. This is mainly due to better solubility
of CO 2 and high stability of carbonate at elevated pH. Moreover, in the case of
urease-driven precipitation, the optimum pH for urease activity is in the alkaline
range. Most of the microbial ureases studied are optimally active around pH 8
[133, 236]. Degradation of urea by urease generates NH 4
+ which raises the pH of
the medium. The increase in pH or maintenance of the alkaline condition is ideal for
calcite precipitation, because at low pH, the carbonate tends to dissolve rather than
precipitate [244]. The CO 3
2À that ends up in the calcite precipitation is CO 2 driven.
Since the solubility of CO 2 increases with pH, the CO 3
2À concentration rises
concomitantly, and in the presence of Ca
2+ , this facilitates the formation of
CaCO 3 . The CO 2 can be atmospheric or generated by cellular activities [230].
Most microbial-based calcite precipitation occurs under alkaline conditions from
pH 8.7 to 9.5 [133, 245, 246]. However, it is not clear if this is due to the activity of
urease or influenced by another factor. Salt concentration and temperature have a
clear effect on the activity of urease. From this point of view, urease-producing
alkaliphilic and halophilic microorganisms could be of interest for concrete applications. Recently, Stabnikov et al. [247] investigated whether haloalkaliphilic
ureolytic bacteria are active at high concentrations of salt and high pH, conditions
that are suitable for manufacturing biocement.
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
325