Geoscience Reference
In-Depth Information
high concentrations of ammonium (Kaltwasser
et al
., 1972; Friedrich and Magasanik,
1977) to promote CaCO
3
precipitation and ureolysis (Nemati and Voordouw, 2003).
In bacterium cells, calcium concentrations are high in extracellular compared to
intracellular (as a result of alkaline pH regimes). The combination of extracellular
alkaline pH and calcium ions presents an unavoidable stressful environment for bacte-
ria: because of Ca
2
+
/2H
+
electrochemical gradients caused by the process, the passive
calcium rush will lead to intracellular calcium build-up and excessive proton expul-
sion (Hammes and Verstraete, 2002). At the cellular level, this could be detrimental
due to (1) the disruption of intracellular calcium-regulated signal processes, (2) the
alkalization of intracellular pH and (3) the depletion of the proton pool required for
numerous other physiological processes (Norris
et al
., 1996). The microbiological
CaCO
3
precipitation began at pH 8 and was completed at pH 9.0, consolidating 98%
of the initial concentration of Ca
2
+
. Calcium carbonate precipitation appeared to be
correlated with the growth of
S. pasteurii
and was completed within 16 h following
inoculation (Stocks-Fischer
et al
., 1999).
Calcite is produced by
S. pasteurii
that have been studied in standard nutrient
broth (NB) and Corn steep liquor (CSL). 100ml of NB and CSL media were added
to 2% urea and 25mM CaCl
2
mixed with 1mM of overnight grown Bp M-3. The
bacteria were grown at 37
◦
C with continuous aeration at 120 rpm (Achal
et al
., 2010).
The urease activity was determined for bacterial isolates in NB media containing filter-
sterilized 2% urea and 25mM CaCl
2
by measuring the amount of ammonia released
from urea. One unit of urease is defined as the amount of enzyme hydrolyzing one
micro mole urea per minute (Achal and Pan, 2011).
7.6 BIOGROUTING AND ITS CHALLENGES
Biogrouting is intended as a ground improvement technique for sandy soils. The crys-
tals of calcium carbonate precipitate in the presence of dissolved calcium, which form
bridges between the sand grains and hence increase the stiffness and strength up to
12MPa (Van Paassen
et al
., 2010). Biogrouting significantly reduce the permeability
of the strengthened soil, which hinders ground water flow and limits long-distance
injection, making large scale treatment unfeasible. Biological techniques (biogrout-
ing) can provide the solution (Whiffin, 2004; De Jong
et al
., 2006; Ivanov and
Chu, 2008).
By injecting specific groups of microorganisms into the soil, in combination with
substrates, precipitation of inorganic minerals is induced at the desired location. These
minerals connect the existing sand grains, thereby increasing the strength of the mate-
rial. The product has similar properties to natural sandstone and it remains permeable,
thereby enabling large-scale applications (Van Paassen
et al
., 2010). Microbial grouting
decreased the permeability after two injections by about 98%. Enzymatic formation
of CaCO
3
in situ
is an effective method for reducing the permeability of porous media
(Nemati and Voordouw, 2003). The attachment of bacteria depends on many factors,
including grain size distribution, mineralogy, the properties of the pore fluid and the
properties of the bacteria themselves (Scholl
et al
., 1990; Torkzaban
et al
., 2008).
Transport of bacteria is limited in fine grained soils. As bacteria have a typical
size of 0.5 to 3
µ
m, they cannot be moved through silt or clay soils; nor can they
