Agriculture Reference
In-Depth Information
reaction probability between Fe
0
and NO
3
should be inhibited, whereas the
reaction between Fe
0
and H
2
O should be promoted.
On the other hand, secondary reactions may include five secondary reactions as
shown in Eqs. (
12.4
)-(
12.8
):
Fe
0
NO
3
þ
2H
þ
!
Fe
2
þ
þ
NO
2
þ
H
2
O
þ
ð
12
:
4
Þ
3Fe
0
NO
2
þ
8H
þ
!
3Fe
2
þ
þ
NH
4
þ
þ
þ
2H
2
O
ð
12
:
5
Þ
NO
3
þ
82Fe
0
75Fe
2
þ
þ
:
þ
:
:
2
0
2
25H
2
O
NH
4
þ
þ
50OH
!
:
19Fe
3
O
4
þ
:
ð
:
Þ
1
0
12
6
2NO
3
þ
2NO
2
þ
2H
2
!
2H
2
O
ð
12
:
7
Þ
2NO
2
þ
3H
2
!
N
2
þ
2H
2
O
þ
2OH
ð
12
:
8
Þ
Till et al. (
1998
) conducted experiments using a homemade bottleneck and verified
that hydrogen denitrifying bacteria could use the hydrogen generated during iron
corrosion for denitrification, during which NO
3
is degraded into harmless nitro-
gen. However, they also observed dynamic competition between zero-valent iron
and denitrifying bacteria, and the higher reactivity of iron was not conducive for the
denitrification process. In addition, the results of column experiments showed that
hydraulic retention time (HRT) was directly and inversely proportional to the NO
3
removal rate and ammonia production, respectively. Subsequently, Kielemoes
et al. (
2000
) studied the effect of NO
2
on the composite system, and the results
clearly showed that NO
2
remarkably inhibited the corrosion process and increased
the probability of the toxic byproduct, NO. Moreover, the addition of micro-
organisms was found to improve the production of hydrogen, which indicated
that a substance in the microorganisms (possibly a certain bioenzyme) could
catalyze iron corrosion.
Furthermore, Biswas and Bose (
2005
) simulated in situ remediation of NO
3
contamination by permeable reactive barrier (PRB) composed of zero-valent iron
and denitrifying bacteria and achieved improved denitrification rate as well as a
decrease in NH
4
+
production by optimizing the iron type, concentration, HRT, and
other parameters. Their results showed that more than 13 days of HRT were
required for the denitrification process using a PRB barrier composed of 0.5 g of
steel wool and 125 cm
3
of mixed sand. In addition, Jha and Bose (
2005
) considered
that continuous corrosion of zero-valent iron increased the pH value, which
inhibited the production of hydrogen and subsequently limited the denitrification
efficiency of the composite system. Therefore, they introduced FeS
2
into the
composite system, which reacted with OH
in the alkaline solution to produce Fe
(OH)
2
precipitate that was separated from the aqueous phase to maintain the pH of
the solution, as shown in Eq. (
12.9
):
18OH
!
2SO
4
2
þ
FeS
2
þ
ð
2
þ
þ
ð
:
Þ
Fe OH
14e
8H
2
O
12
9
The results showed that during the denitrification reaction,
the solution pH
