Environmental Engineering Reference
In-Depth Information
nally was 0.44 g/g.
The influence of heating time on mercury release is shown in Fig. 5.39. The Hg
released (%) represented the percentage of mercury released from the original
Hg-laden AC. There was much more mercury released from the AC at longer
heating times for both kinds of mercury. The higher the temperature, the more the
mercury released. Mercury was not stable at longer heating times and at higher
temperatures. Consequently, it is disadvantageous to place mercury-contaminated
AC under the sun for long periods of time, most especially in the summer.
32
60
o
C Hg
0
60
o
C Hg
2+
90
o
C Hg
0
90
o
C Hg
2+
28
24
20
16
12
8
4
0
4
1
7
Time (h)
Fig. 5.39
Mercury released as a function of heating time at 60 and 90 °C
It is interesting to note that the Hg
2+
originally adsorbed was released more
easily from AC than the Hg
0
originally
adsorbed under the same conditions. How-
ever, the action of Hg sorption, especially of Hg
0
, is very complicated. Previous
research shows that some original Hg
0
reacts with the surface functional group on
the AC surface, which results in the formation of Hg-anion (e.g., Hg-O, Hg-S,
Hg-Cl, etc.) chemical bonds. Some original Hg
0
was oxidized first in the flue gas
and then condensed heterogeneously on the sorbent. In other words, there were
almost no differences between original Hg
2+
and original Hg
0
for AC after adsorp-
tion. However, the strength of the bond of the original Hg
0
on AC may be stronger
than that of original Hg
2+
under the same thermal conditions (Fig. 5.39). Further
work is in progress to elucidate the differences between original Hg
0
and original
Hg
2+
in terms of bond strength on AC.
At the same heating time of 0.5 h, the stabilities of Hg
0
and Hg
2+
on AC(MZ)
were compared over a broad temperature range (from 150 to 900 °C). The quantity
of the original Hg
0
adsorbed on AC(MZ) is 0.41 g/g, and the quantity of the
original Hg
2+
adsorbed is 0.44 g/g. The result is shown in Fig. 5.40. As the tem-
perature increased from 150 to 900 °C, more and more mercury (Hg
0
and Hg
2+
) was
released from AC(MZ). Thus, there was a rapid increase in mercury release be-
tween the temperatures of 250 and 500 °C. Furthermore, almost all mercury had
already escaped before the temperature reached 500 °C. However, after 500 °C, the
increase in mercury release changed slowly and even stopped completely after 700
°C. The experiment indicates that AC burns in air at ca. 350 - 400 °C. The burning
completely destroyed the texture of AC (MZ), which then induced mercury to lose
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