Agriculture Reference
In-Depth Information
in the first 4 days, about 53 % of NO
3
was removed, of which 64 % was
transformed into NH
4
+
, indicating that during this period, chemical reduction
played a major role in NO
3
removal. However, evaluation of the proportion of
NO
3
that was not transformed to NH
4
+
might not be possible because of the
sorption of nano-iron or denitrification by the bacteria. During the last 4 days, the
residual NO
3
was removed completely without an increase in NH
4
+
concentration.
Meanwhile, a brief trend of NO
2
accumulation (up to 22 %) was observed, with
nano-iron playing a leading role in the denitrification process, and NO
2
was not
detected in the previous 4 days. These results indicated that the chemical reduction
of nano-iron was not obvious in the last 4 days and that biological denitrification
played a leading role in the NO
3
removal process. The reason for the transition in
the reaction mechanism could be the toxicity of nano-iron, which prevented the
normal growth of the denitrifying bacteria; however, with increasing iron corrosion,
the toxicity of nano-iron was decreased, the denitrifying bacteria gradually adapted
to the new environment, and the denitrification process proceeded normally.
Many reports have shown that coating the appropriate proportion of another
metal with a high reduction potential (e.g., Ni, Pd, Ag, Cu, etc.) to form a binary
metal system can increase the number of active adsorption sites on the iron surface
as well as the stability of the nanoscale zero-valent iron. In a previous study
(An et al.
2010
), nano-iron/Ni and nano-iron/Cu systems were used (Ni and Cu
loading of about 5 %) with denitrifying bacteria to examine the NO
3
removal rate
and reaction products.
Figure
12.3
shows the effect of different catalysts on the reactivity of nano-iron.
Under the same conditions, the reactivity of the nano-materials on NO
3
reduction
was as follows: nano-iron/Ni
nano-iron. This finding demon-
strated that the introduction of Ni, Cu, and other metals could significantly improve
the denitrification rate. Although no difference could be observed between Ni and
Cu catalysts based on the denitrification rate curve, the catalytic mechanisms of
these two metals were noted to be completely different with respect to NO
2
and
NH
4
+
concentration curves and products (An et al.
2010
), as shown in Fig.
12.4
.
From Fig.
12.4
, it can be observed that although the introduction of Ni and Cu
did not produce any effect on the NO
3
removal rate, taking only 6 and 7 days to
complete the degradation process, respectively, the NO
3
degradation products
were completely different. With regard to nano-iron/Ni bimetallic composite sys-
tem, NO
2
was not detectable throughout the denitrification process, but the
proportion of ammonia was as high as 69 % after the reaction. On the other hand,
with respect to nano-iron/Cu bimetallic composite system, the NO
2
concentration
increased throughout the reaction and remained at about 33 % even after the
complete degradation of NO
3
. At the end of the reaction, only 39 % of NO
3
was converted to NH
4
+
in the nano-iron/Cu bimetallic composite system.
The addition of Ni was found to catalyze simultaneous reduction of NO
2
and
NO
3
by nano-iron; therefore, NO
2
accumulation did not occur throughout the
denitrification process. Unlike the ammonium generation curve in nanoiron-bacteria
system, that increased monotonously when Ni added. It indicated that Ni catalyzed
>
nano-iron/Cu
>
