Sustainable Ground Improvement Technique for Geo-Disaster Mitigation - Sustainable Practices in Geoenvironmental Engineering - page 453

OD = 1.0

120

OD = 0.75

100

OD = 0.59

80

OD = 0.37
OD = 0.25

60

OD = 0.1

40

20

0

0 12 24 36 48

Elapsed time (h)

FIGURE 12.9
Carbonate precipitation rates with different concentrations of microbes. OD is the optical density at a wavelength
of 600 nm. The 100% point on the vertical axis indicates 1 M CaCO 3 , i.e., 100 g/L.

are irreversible, the altered urease will never recover. Meanwhile, the increase in pH due
to ammonia production will counter the drop in pH resulting from the death of microbes.
If the pH becomes low, then hydrolysis will produce NH 3 and CO 2 . This means that no
carbonate will be produced. If the pH remains at a relatively high range, NH 4 and CO 3 ions
will be produced.

12.4.3.2 Carbonate Precipitation

The rate of microbial carbonate precipitation varies with species of microbes. The example
of precipitation rates with different concentrations of microbes shown in Figure 12.9 shows
that most of the data lie in the region of greater than 100% carbonate precipitation, mean-
ing that some crystalline water was produced. For an optical density (OD) value of 1.0,
the precipitation rate is approximately 6 mM/min. The greater the OD, the higher is the
precipitation rate. The precipitation rate can inluence the crystalline characteristics of cal-
cium carbonate. Calcite is the main calcium carbonate produced when the reaction rate is
rapid, whereas aragonite and vaterite are most likely produced when the rate is slow. It is
useful to note that the chemical compositions of calcite, aragonite, and vaterite are essen-
tially identical, i.e., CaCO 3 .

12.5 Definition and Measurement of Carbonate Content
A closed vessel device is used as a rapid method for measuring CO 2 gaseous pressure by
mixing soil and acid (Fukue et al., 2010), utilizing the following reactions.