Production of Bacteria for Structural Concrete - Biotechnologies and Biomimetics for Civil Engineering - page 306

1 mol of carbamate (Eq. 14.1 ), which spontaneously hydrolyses to form an
additional 1 mol of ammonia and carbonic acid (Eq. 14.2 ) (Burne and Chen 2000 ).
These products subsequently equilibrate in water to form bicarbonate and 2 mol of
ammonium and hydroxide ions (Eqs. 14.3 and 14.4 ) that give rise to an increase in
pH and ultimately shift the bicarbonate equilibrium, resulting in the formation of
carbonate ions (Eq. 14.5 ). High pH condition favors the formation of CO 2

3 from
HCO 3 (Knoll 2003 ). Finally, the carbonate concentration will increase, inducing
an increase in supersaturation level leading to CaCO 3 precipitation around the cell
in the presence of soluble calcium ions (Eqs. 14.6 and 14.7 ).

CO(NH 2 Þ 2 +H 2 O ! NH 2 COOH + NH 3 ð 14 : 1 Þ

NH 2 COOH + H 2 O ! NH 3 +H 2 CO 3 ð 14 : 2 Þ

H 2 CO 3 $ HCO 3 +H þ

ð 14 : 3 Þ

2NH 3 +2H 2 O $ 2NH 4 + 2OH

ð 14 : 4 Þ

HCO 3 +H þ + 2NH 4 + 2OH $ CO 2 + 2NH 4 +2H 2 O ð 14 : 5 Þ

3

Ca 2 þ + Cell ! Cell - Ca 2 þ

ð 14 : 6 Þ

Cell - Ca 2 þ +CO 2 3 ! Cell - CaCO 3

ð 14 : 7 Þ

Based on the above equations, it may be said that calcium carbonate precipi-
tation is a biochemical process governed mainly by five key factors: (1) the urease,
(2) the calcium concentration, (3) the concentration of dissolved inorganic carbon
(DIC), (4) the pH, and (5) the availability of nucleation sites. The availability of
nucleation site is very important for continuous and stable calcium carbonate
formation. It is isolated from the environment by a delimiting geometry by limiting
the diffusion in and out of the system (Sarayu et al. 2014 ). The ion movement is
enabled by active pumping with organelles or passive diffusion to enable the
microorganisms to use a great variety of anatomical arrangements (Perry 2003 ), as
shown in Fig. 14.1 . The production of CO 2
3

from bicarbonate (HCO 3 ) in water is

strongly pH dependent, an increase in CO 2

3 concentration occurs under alkaline
conditions that lead to deprotonation of the functional groups like carboxyl,
hydroxyl, and phosphate of the bacterial cell wall and creates a strong electrostatic
affinity to attract cations and enables the accumulation of calcium ions on the
surface of the cell wall. On the other hand, the calcium ions in the solution are
attracted to the bacterial cell wall due to the negative charge of the later. Upon
addition of urea to the bacteria, dissolved inorganic carbon and ammonium are
released in the microenvironment of the bacteria. In the presence of calcium ions,
this can result in a local supersaturation and hence heterogeneous precipitation
of calcium carbonate occurs on the bacterial cell wall (Reddy et al. 2012 ).