Bacteria for Concrete Surface Treatment - Biotechnologies and Biomimetics for Civil Engineering - page 325

CO 2 ð aq Þ þ H 2 O $ H 2 CO 3 ð pK ¼ 2 : 84 Þ ð 15 : 4 Þ

H 2 CO 3 $ H þ þ HCO 3

ð

pK1 ¼ 6 : 352

Þ ð 15 : 5 Þ

HCO 3 $ CO 2 3 þ H þ

ð pK 2 ¼ 10 : 329 Þ ð 15 : 6 Þ

Several authors recognized specific metabolic pathways to be involved in
carbonate deposition, with the pH change of the medium as the main mechanism
favoring deposition. In general, metabolic pathways able to shift the environmental
pH toward alkalinity can, in the presence of calcium ions, foster carbonate
deposition when a state of oversaturation develops (Fortin et al. 1997 ). Bacteria
can induce deposition by altering almost any of the deposition parameters
described above, either separately or in various combinations with one another.
Both autotrophic and heterotrophic pathways create an alkaline environment.
While the environmental conditions of heterotrophic pathways are diverse (aero-
biosis, anaerobiosis, and microaerophily), carbonate deposition always appears to
be a response of the heterotrophic bacterial communities to an enrichment of the
environment in organic matter (Castanier et al. 1999 ). Besides changes induced in
the macro-environment, bacteria have also been reported to influence calcium
carbonate deposition by acting as sites of nucleation or calcium enrichment
(Morita 1980 ). Due to the presence of several negatively charged groups on the
cell wall, at a neutral pH, positively charged metal ions can be bound on bacterial
surfaces (Douglas and Beveridge 1998 ). Such bound metal ions (e.g., calcium)
may subsequently react with anions (e.g., carbonate) to form an insoluble salt (e.g.,
calcium carbonate). In the case of a sufficient excess of the required cations and
anions, the metal salt on the cell surface initiates mineral formation by acting as a
nucleation site. The anion (e.g., carbonate) in this reaction may be a product of the
bacterial metabolism, or it may have an abiotic origin (Ehrlich 1998 ). Furthermore,
it has been demonstrated that specific bacterial outer structures (glycocalyx and
parietal polymers) consisting of exopolysaccharides 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 ).
An alkaline barrier formed as a result of bacterial metabolic activities,
responsible for carbonate deposition. The alkaline barrier is primarily due to
decomposition of anions rather than to production of alkali. Organisms that
increase the medium pH by the elimination of anions leave the scene ready for
calcium deposition. Rather than activities of single species or groups, activities of
microbial communities should be considered in natural environments (Zavarzin
2002 ). Metabolic pathways involved in bicarbonate deposition include autotrophic
as well as heterotrophic pathways (both in aerobiosis and in anaerobiosis) with a
different contribute. Three main kinds of bacteria are involved in autotrophic
production methanogenic archaebacteria, sulfurous or non-sulfurous, purple and
green bacteria, and cyanobacteria. All obtain carbon from gaseous or dissolved
CO 2 , the origin of which is complex and use it as carbon source to produce organic