Bacteria for Concrete Surface Treatment - Biotechnologies and Biomimetics for Civil Engineering - page 327

CO NH 2 ð Þ 2 þ H 2 O ! NH 2 COOH þ NH 3 ð 5 : 13 Þ

NH 2 COOH þ H 2 O ! NH 3 þ H 2 CO 3 ð 5 : 14 Þ

These products subsequently equilibrate in water to form bicarbonate and 2 mol
of ammonium and hydroxide ions (Eqs. 15.15 and 15.16 ). The two reactions result
in a pH increase, which in turn shifts the bicarbonate equilibrium, resulting in the
formation of carbonate ions (Eq. 15.17 ). The pH increase takes place initially in
the local microenvironment around the bacterial cell and propagates in the bulk
solution of the bacterial cell suspension.

H 2 CO 3 $ HCO3 þ H þ ð 5 : 15 Þ

NH 3 þ 2H 2 O $ 2NH 4 þ 2OH ð 5 : 16 Þ

HCO 3 þ H þ þ 2NH 4 þ 2OH $ CO 2 3 þ 2NH 4 þ 2H 2 O

ð 5 : 17 Þ

Thus, the carbonate concentration will increase, inducing an increase in X
(according to Eq. 15.2 ) and resulting in CaCO 3 deposition around the cell in the
presence of soluble calcium ions as shown in Eqs. 15.18 and 15.19 .

Ca 2 þ þ Cell ! Cell Ca 2 þ ð 5 : 18 Þ

Cell Ca 2 þ þ CO 2 3 ! Cell CaCO 3 #

ð 5 : 19 Þ

The pH of solution occurs have strongly influence on the production of CO 3 2-
from bicarbonate (HCO 3 - ), an increase in CO 3 2- concentration with increase of
alkaline conditions. Therefore, calcium carbonate deposition readily occurs in
alkaline environments abundant of the calcium (Ca 2+ ) and carbonate (CO 3 2- ) ions
(Stocks-Fischer et al. 1999 ; Ramachandran et al. 2001 ; Qian et al. 2010a ). Urease
activity promotes deposition outside the cells. Calcium ions in the solution are
attracted to the bacterial cell wall with the negative charge. In the presence of
calcium ions, a local supersaturation result in heterogeneous deposition of calcium
carbonate occurs on the bacterial cell wall. Furthermore, it has been demonstrated
that specific bacterial outer structures (glycocalyx and parietal polymers), a variety
of organic polymers outside the cell wall (Lappin-Scott et al. 1988 ; MacLeod et al.
1988 ) or exopolymeric substances consisting of exopolysaccharides, proteins, and
amino acids play an essential role in the morphology and mineralogy of bacterially
induced carbonate deposition (Braissant et al. 2003 ; Ercole et al. 2007 ). The
carbonate deposition has subsequently led to the exploration of this process in the
field of bioremediation. The use of carbonate deposition has also been proposed
for the removal of heavy metals (Warren et al. 2001 ) and biodegradation of pol-
lutants (Simon et al. 2004; Chaturvedi et al. 2006). Urease is the key enzyme
involved in the process of calcite deposition induced by bacteria. Bacteria are
known to hydrolyze urea by urease for the purposes of (1) increasing the ambient