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increased from 7.55 to 9.14 with the addition of FeS
2
, whereas the pH only
increased from 7.55 to 7.73 without FeS
2
addition, and the NO
3
removal rate
was also enhanced.
12.4 Nano-iron Enhances Hydrogen Availability
for Autotrophic Microorganisms Involved
in the Denitrification of Groundwater
With the development of nanotechnology, nano-iron has been widely used in the
field of environmental remediation because of its high specific surface area and
reactivity. Instead of zero-valent iron, Shin and Cha (
2008
) used nano-iron together
with denitrifying bacteria to remove NO
3
. They found that the reaction rate was
substantially improved, and the denitrification process took only 3 days to com-
plete. Their experiments proved that the composite system exhibited more adapt-
ability to different environment
temperatures than the single-phase nano-iron
system.
To observe the denitrification capacity and products of the composite system
consisting of nano-iron and denitrifying bacteria, nanoscale zero-valent iron pre-
pared by liquid-phase reduction and
Alcaligenes eutrophus
(a kind of hydrogen
autotrophic denitrifying bacterium) were mixed with added NO
3
solution, and the
denitrification process was carried out in an anaerobic environment (An et al.
2009
).
As a control, only nano-iron or denitrifying bacteria were used.
As shown in Fig.
12.1
, with time, the NO
3
concentrations in the system with
denitrifying bacteria decreased by about only 15 % with no NH
4
+
generation within
8 days. As no additional electron donor was added to the system, the denitrification
process did not occur, and the slight decrease in the concentration of NO
3
may be
attributed to the adsorption of certain polysaccharides secreted by the bacteria.
Furthermore, the addition of denitrifying bacteria into the system significantly
affected NO
3
removal. It only took 2 days to complete NO
3
degradation by the
system with nano-iron alone, with about 95 % of NO
3
being converted to NH
4
+
.
On the other hand, the reaction time taken to remove NO
3
from the system with
added denitrifying bacteria increased from 2 to 8 days, with about only 33 % of
NO
3
being converted to NH
4
+
, thus indicating that the denitrifying bacteria
significantly decreased the proportion of NH
4
+
and nitrogen.
To study the denitrification process in the composite system, NO
3
,NO
2
, and
NH
4
+
concentrations were determined (An et al.
2009
), as shown in Fig.
12.2
.
From Fig.
12.2
, it can be observed that in the first 4 days, the NH
4
+
content of the
system increased from 0 to 35 % and subsequently remained stable between 33 and
36 %. As described earlier, NH
4
+
was produced only during the chemical reaction
between iron and NO
3
, whereas the biological denitrification process converted
NO
3
into nitrogen gas rather than NH
4
+
. Thus, according to the varying patterns,
the denitrification process in the composite system can be divided into two stages:
