Bacteria for Concrete Surface Treatment - Biotechnologies and Biomimetics for Civil Engineering - page 324

laboratory test, acting as heterogeneous crystallization nuclei, with an applicative
potential in the reinforcement of monumental calcareous stones (Barabesi et al.
2003 ). Functional groups on the B. subtilis cell wall were considered able to bind
dissolved Ca 2+ in a calcite dissolution study with dead cells (Friis et al. 2003 ). Like
other deposition processes, calcium carbonate (CaCO 3 ) deposition can occur by
two different mechanisms: biologically controlled or induced. In biologically
controlled mechanisms, the organism controls the process, i.e., nucleation and
growth of the mineral particles, to a high degree. The organism synthesizes
crystals in a form that is unique to that species, independently of environmental
conditions. Examples of controlled mineralization are magnetite formation in
magnetotactic bacteria and silica deposition in the unicellular algae coccolitho-
phores and diatoms, respectively (Bazylinski et al. 2007 ; Barabesi et al. 2007 ).
Different types of bacteria, as well as abiotic factors (salinity and composition of
the synthetic medium), seem to contribute in a variety of ways to calcium car-
bonate deposition in a wide range of different environments (Von Knorre and
Krumbein 2000 ; Rivadeneyra et al. 2004 ). The type of calcium carbonate depo-
sition is largely dependent on the environmental conditions; accordingly, biode-
position has been generally regarded as induced mechanisms (Rivadeneyra et al.
1994 ; Perito and Mastromei 2011 ).

15.2.2 Role of Bacteria on Calcium Carbonate Deposition

According to calcium carbonate deposition governed by four key factors (Hammes
and Verstraete 2002 ), CaCO 3 deposition requires sufficient calcium and carbonate
ions so that the ion activity product (IAP) exceeds the solubility constant (Ksp)
(Eqs. ( 15.1 ) and ( 15.1 )). From the comparison of the IAP with the Ksp, the sat-
uration state (X) of the system can be defined; if X [ 1 the system is oversaturated
and deposition is likely (Morse 1983 ):
Ca 2 þ þ CO 2 3 $ CaCO 3

ð 15 : 1 Þ

CO 2
3

= K sp with K spcalcite ; 25 ¼ 4 : 8 10 9

X ¼ Ca 2 þ

ð 15 : 2 Þ

The concentration of carbonate ions is related to the concentration of DIC and
the pH of a given aquatic system. In addition, the concentration of DIC depends on
several environmental parameters such as temperature and the partial pressure of
carbon dioxide (for systems exposed to the atmosphere). The equilibrium reactions
and constants governing the dissolution of CO 2 in aqueous media (25 C and
1 atm) are given in Eqs. ( 15.3 )-( 15.6 ) (Stumm and Morgan 1981):

CO 2 ð g Þ $ CO 2 ð aq Þ ð pK h ¼ 1 : 468 Þ ð 15 : 3 Þ