Environmental Engineering Reference
In-Depth Information
Hydroxyapatatite precipitation:
5Ca H
2
PO
4
ð
Þ
2
þ
8CO NH
2
ð
Þ
2
þ
8H
2
O
þ
acid urease
!
Ca
5
PO
ð Þ
3
O
ðÞ#þ
2NH
4
HCO
3
þ
6CO
2
þ
7NH
4
ð
Þ
2
HPO
4
:
ð
2
:
21
Þ
2.5.6 Calcium Bicarbonate Biocementation
Important biocementation technology could be precipitation of calcite using
removal of CO
2
from solution of calcium bicarbonate (Ehrlich
1999
) because it
releases low quantity of ammonia and can be performed without increase of pH to
8.5-9.0 as conventional MICP:
Ca HCO
3
ð
Þ
2
þ
CO NH
2
ð
Þ
2
þ
H
2
O
þ
acid urease
!
CaCO
3
#
þ
CO
2
þð
NH
4
Þ
2
CO
3
:
ð
2
:
22
Þ
Solubility of calcium bicarbonate is relatively high, about 1 M, to perform prac-
tically feasible biocementation. This method is a model of the naturally occuring
dissolution-precipitation of calcium carbonate
CaCO
3
þ
CO
2
þ
H
2
O
$
Ca HCO
3
ð
Þ
2
:
ð
2
:
23
Þ
The difference is that biocementation has to be performed at high concentration
of calcium bicarbonate and with significantly higher rate than in nature. The rate of
precipitation of calcium carbonate from calcium bicarbonate in nature is deter-
mined by the removal rate of CO
2
from the reaction. The rate of biocementation
can be accelerated due to increase of pH during hydrolysis of urea by enzyme
urease. The problem of bicarbonate biocement is its instability, so the solution
must be produced and stored at elevated partial pressure of CO
2
.
2.5.7 Iron-Based Bioclogging and Biocementation
Iron-based biocementation could be suitable for geotechnical applications if to
combine three bioprocesses shown below.
• Acidogenic fermentation of cellulose-containing agricultural or food-process-
ing residuals producing mainly acetic acid (see Eq.
2.1
);
• bioreduction of cheap commodity, iron ore, using products of acidogenic fer-
mentation or many organic electron donors (Ivanov et al.
2009
; Guo et al.
2010
), see equation for the reduction of ferric ions by iron-reducing bacteria
using acetate:
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