Civil Engineering Reference
In-Depth Information
first published in 1917 (Chen et al., 2012). In this context biotechnology
seems to be able to provide a solution to concrete durability enhancement
by means of biomineralization, a phenomenon by which organisms form
minerals and first used for crack repair by Gollapuddi et al. (1995).
21.2 Bacteria mineralization mechanisms
bacteria are relatively simple, unicellular organisms. There are typically
40 million bacterial cells in a gram of soil and a million bacterial cells in a
milliliter of fresh water; in all, there are approximately five nonillion (5 ¥
10
30
) bacteria on earth, forming much of the world's biomass. Under optimal
conditions, bacteria can grow and divide extremely rapidly and bacterial
populations can double as quickly as every 9.8 min (Siddique and chhal 2011).
Biomineralization is defined as a biologically induced precipitation in which
an organism creates a local micro-environment, with conditions that allow
optimal extracellular chemical precipitation of mineral phases, like calcium
carbonate (caco
3
) (hamilton, 2003). Decomposition of urea by ureolytic
bacteria is one of the most common pathways to precipitate caco
3.
The
microbial urease enzyme hydrolyzes urea to produce dissolved ammonium,
dissolved inorganic carbon and co
2
. Furthermore, the ammonia released
in the surroundings subsequently increases ph, leading to accumulation
of insoluble caco
3
in a calcium-rich environment. Figure 21.1. shows a
DIC
AMM
Urea
Ca
Ca
Ca
Ca
Ca
Ca
Ca
Ca
Ca
Ca
Ca
Ca
Ca
(a)
(b)
(c)
21.1
Simplified representation of the events occurring during the
ureolytic-induced carbonate precipitation. Calcium ions in the
solution are attracted to the bacterial cell wall due to the negative
charge of the latter. Upon addition of urea to the bacteria, dissolved
inorganic carbon (DIC) and ammonium (AMM) are released in
the microenvironment of the bacteria (a). In the presence of
calcium ions, this can result in a local supersaturation and hence
heterogeneous precipitation of calcium carbonate on the bacterial
cell wall (b). After a while, the whole cell becomes encapsulated (c)
(De Muynck et al., 2010).
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