Biomedical Engineering Reference
In-Depth Information
parameters for the two initial steps of denitrification at pH 8. The data suggest that strain KC
has a competitive advantage over native microflora in reducing nitrate to nitrite (higher specific
growth rate), while native microflora have a competitive advantage over strain KC in reducing
nitrite to nitrogen gas. The expected ecological niche occupied by strain KC at the Schoolcraft
site would thus be the first step in denitrification, while native denitrifiers would occupy
subsequent steps.
9.7 pH ADJUSTMENT
Adjusting the pH of CT-contaminated soil and groundwater to a value of ~8 creates
conditions that enhance the competitiveness of strain KC and promotes CT degradation.
Specifically, iron solubility is decreased to low levels, the
fur
gene is expressed, PDTC is
produced and strain KC becomes more competitive. A challenge in the execution of this step
in the field is the potential for clogging well screens due to the formation of calcium carbonate
(CaCO
3
) scale, especially given that groundwater is often at or near saturation with respect to
calcium carbonate. To prevent CaCO
3
formation, care must be taken to ensure well develop-
ment, minimize supersaturation and prevent excessively high pH levels.
Figure
9.9
illustrates a pC-pH diagram for a closed system using data for Schoolcraft
groundwater (Warnick,
1998
), and thermodynamic data that is adjusted to 15
C. The plot
provides understanding of solution chemistry prior to pH adjustment.
As shown in Figure
9.9
, for Schoolcraft groundwater at a pH of 7.25 and temperature of
15
C, the equilibrium concentration of Ca
2+
is 10
2.6
molar (M) (where the line “
Groundwater
pH
” intersects the line labeled “Ca”). Because this equilibrium value is close to the measured
1.0
15°C
H
2
CO
3
*
HCO
3
-
CO
3
2-
Ca
2+
, sat
H
+
OH
-
1.5
2.0
Groundwater [Ca
2+
]
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
p[H]
Figure 9.9. pC-pH diagram for Schoolcraft groundwater at 15
C in a closed system. Ca
2+
¼ 10
-2.6
M.
Line A represents the base addition needed to increase pH from the initial value of 7.25 to a final
value of 8.0.
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