Agriculture Reference
In-Depth Information
acetate, and other organic substrates, which do not exist in groundwater. In addition,
the fast growth of heterotrophic bacteria is more likely to cause clogging of ground-
water aquifers, thus increasing residual bacteria and organic contaminants in the
effluent.
On the other hand, autotrophic biological denitrification process can utilize
gaseous hydrogen or sulfide, rather than organic carbon sources, to transform
carbon dioxide, bicarbonate, and other compounds to cellular components. To
date, there have been many reports on the sulfur and hydrogen autotrophic denitri-
fication process. In the sulfur/limestone biological denitrification process, sulfur
acts as an electron donor, while limestone provides alkalinity as well as acts as a
biocarrier, which can effectively remove NO
3
from groundwater. This process is
economical and simple because it does not require organic substrates and uses less
sulfur and limestone. However, the major disadvantage of the sulfur/limestone
biological denitrification process is the production of sulfate; 7.54 mg of SO
4
2
is
produced per mg of NO
3
removed. Therefore, this method is suitable for removing
NO
3
from groundwater with low sulfate concentrations. Comparatively, hydrogen
is an ideal electron donor because hydrogen itself or its oxidation product is not
toxic, and autotrophic denitrification with hydrogen as the electron donor is a clean
denitrification process. However, the application of this process is limited by low
solubility, low efficiency, and explosibility of hydrogen.
12.3 Zero-Valent Iron Enhances Hydrogen Availability
for Autotrophic Microorganisms Involved
in Denitrification of Groundwater
In recent years, it has been proposed that hydrogen produced during corrosion of
zero-valent iron in water could be utilized by denitrifying bacteria during the
reduction of NO
3
to nitrogen.
The iron-denitrifying bacteria composite system contains a variety of molecules,
ions, and enzymes, which make the reaction more complex. From the theoretical
point of view, the reaction mechanism of the system can be divided into main
reactions and secondary reactions. Among them, the main reactions include ammo-
nium (NH
4
+
) generation, hydrogen evolution, and denitrification, as shown in
Eqs. (
12.1
)-(
12.3
):
4Fe
0
NO
3
þ
4Fe
2
þ
þ
NH
4
þ
þ
10OH
þ
7H
2
O
!
ð
12
:
1
Þ
Fe
0
Fe
2
þ
þ
2OH
þ
2H
2
O
!
H
2
þ
ð
12
:
2
Þ
33NO
3
þ
34H
þ
0
:
H
2
þ
0
:
08CO
2
þ
0
:
!
0
:
015C
5
H
7
O
2
N
þ
0
:
16N
2
þ
1
:
11H
2
O
ð
12
:
3
Þ
As can be noted in the equations, NH
4
+
is generated during the reaction between Fe
0
and NO
3
, thus indicating that the amount of NH
4
+
should be decreased and the
