Geoscience Reference
In-Depth Information
exchange complex. The main polyvalent cations responsible for the binding of humus
to soil clays are Ca
2
+
,Fe
3
+
and Al
3
+
. The divalent Ca
2
+
ion does not form strong
coordination complexes with organic molecules and would be effective only to the
extent that a bridge linkage could be formed (Stevenson, 1994). In contrast, Fe
3
+
and
Al
3
+
form coordination complexes with organic compounds and strong bonding of
humus with clay could be possible through this mechanism.
Since the CEC of peat is high, and since the cations Na
+
,K
+
,Ca
2
+
,Mg
2
+
and
silica are removed from top soil due to leaching, there is a probable higher presence
of the trivalent ions (e.g. Fe
3
+
and Al
3
+
) at the surface in peat in tropical regions.
Therefore, using sodium silicate systems, electromigration and injection of trivalent
cations could be a winning combination in electro-strengthening of peat at the cathode
area. Cationic surfactants are expected to flocculate peat through a combination of
charge neutralization and bridging (Asadi
et al
., 2011a-g).
7.5 BIOCEMENTING STABILIZATION
Biocementing is a new method to improve soils based on microbiologically induced
precipitation of carbonate calcium. In particular, methods are being developed using
microorganisms which are able to increase the strength and stiffness of granular soils by
inducing the precipitation of calcium carbonate (Whiffin 2004; De Jong
et al
., 2006;
Whiffin
et al
., 2007; Ivanov and Chu, 2008). Most studies on biological grouting
(biogrouting) report the use of microorganisms containing the enzyme urease and,
in particular, the bacterium
Sporosarcina pasteurii
(DSM 33, renamed from
Bacillus
pasteurii
) (Whiffin 2004; De Jong
et al
. 2006, Whiffin
et al
., 2007). This process
induces carbonate calcium precipitation, increasing the strength of soils.
2NH
4
CO
2
3
CO(NH
2
)
2
+
2H
2
O
→
+
(7.6)
Ca
2
+
+
CO
2
3
→
CaCO
3
↓
(7.7)
The enzyme hydrolyzes urea to CO
2
and ammonia, resulting in an increase in the
pH and carbonate concentration. Microbial activity-induced CaCO
3
precipitation on
concrete has also been indicated (Ramachandran
et al
., 2001).
S. pasteurii
is a rod-
shaped, gram-positive soil bacterium that secretes numerous enzymes to degrade a
variety of substrates, enabling the bacterium to survive in a continuously changing
environment (Westers
et al
., 2004). In addition, the bacterial net cell surface charge
is negative and absorbs cations (for example Ca
2
+
) from the environment to deposit
on the cell surface (Achal
et al
., 2010). Because the bacterial cell surface has many
negative charges, if Ca
2
+
is added first without urea there will be enough time for
Ca
2
+
to be attached to the bacterial surface and the bacteria activity will be greatly
influenced and retarded.
The bacteria grow at 37
◦
C and are facultative alkaliphiles which grow optimally
at pH 9.2 in relatively high amounts of NH
4
+
(Wiley and Stokes, 1962, 1963) or
urea (Larson and Kallio, 1954; Bornside and Kallio, 1956). The bacteria should have
high negative zeta-potential, ureolytic ability, be alkalophilic (optimum growth rate
occurs at pH around 9, and no growth at all around pH 6.5 (Dick
et al
., 2006; de
Muynck
et al
., 2007) to increase adhesion), and produce urease in the presence of
