Environmental Engineering Reference
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increasing particle concentration and the rate of aggregation and the stable size of the
individual aggregates formed. After 10 min hydrodynamic radius of RNIP was up to 4.0
μm. After 30 min, the linking of each of these aggregates into chains and gelation
resulted in aggregates of size range of 15-50 μm. The rapid aggregation of NZVI into
micrometer-sized fractal aggregates make it necessary to modify the NZVI surfaces
(e.g., by polymer coating) to inhibit aggregation and improve particle mobility.
4.4.4.2 Suggested Mechanisms of PCB Degradation by NZVI
It was first proposed that PCB molecules accept electrons from donor Fe, release
Cl
-
and take H from water molecules to form biphenyl. Chemical reactions of the
processes can be summarized as follows (Yak et al., 2000):
Fe
0
(s)
-2e
-
Æ
Fe
2+
(aq)
The electron acceptors that accept electrons released from Fe
0
can be H
2
O, dissolved O
2
or PCBs themselves. When PCBs accept electron directly, they release Cl
-
:
CB-Cl
(aq)
+ 2e
-
Æ
CB
-
(aq)
+ Cl
-
(aq)
CB
-
(aq)
+ H
2
O
(l)
Æ
CB
(aq)
+ OH
-
(aq)
PCBs can be indirectly reduced by fresh hydrogen gas generated from water:
2H
2
O
(l)
+ 2e
-
Æ
H
2(g)
+ 2OH
-
(aq)
CB
(aq)
+ H
2(g)
Æ
CB
(aq)
+ HCl
(aq)
The most probable pathway of PCB degradation would be the direct transfer of electrons
from iron surface to PCB rings (Yak et al., 2000).
The degradation of PCBs likely include stepwise processes; possible degradation
processes of 2,2',5,5'- and 3,3',4,4'- tetra CBs are illustrated in Figures 4.2 and 4.3.
By analyzing all possible chlorinated CBs during the degradation processes, Yak
et al. (2000) observed that the removal efficiency of chlorine atoms decreased from para
to meta to ortho positions. For BZ#52 instance, the mass ratio of BZ#18 to BZ# 26 was
approximate 20:1 at the 1
st
-generation product. Then at the 2
nd
-generation products,
BZ#4 was the major product identified while the other congeners were minimal.
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