Environmental Engineering Reference
In-Depth Information
reduced to lower chlorines-containing ones, which were almost completely reduced to
biphenyl within the timeframe of the experiment.
Treatment performance of PCBs with NZVI was compared with that of regular
ZVI. NZVI with size ranging from 1 to 100 nm was synthesized (Wang and Zhang
1997). Tests were setup under room temperature and the results showed that the
reactivity of NZVI was higher than that of regular ZVI. While little degradation of PCBs
was discovered when commercial ZVI powders were applied, up to 25% of aqueous
PCBs (Aroclor 1254) were degraded within 17 hours when samples were treated with
fresh NZVI. Biphenyl was detected as the end product of the reductive processes,
indicating that complete dechlorination of PCBs was achieved.
Results of PCB degradation are also found with respect to the use of catalyst in
addition to iron. Kim and his coworkers (Kim et al., 2004) determined the rates of
dechlorination of a variety of mono- and di-CBs with microscale ZVI and palladized Fe
(Fe/Pd) particles under room temperature. Less than 1% of dechlorinated PCBs were
observed over 40 days of treatment with plain ZVI while Fe/Pd showed a much higher
efficiency.
The dechlorination of six PCBs congeners including 2,2'-, 3,4'-, 2,3,4-, 2,2',3,5'-
, 2,2',4,5'- and 3,3',4,4'- CBs were compared between NZVI (35-50 nm in diameter)
and microscale Fe/Pd (Lowry and Johnson 2004). The Pd content of the Fe/Pd system
was 0.05% wt. Tests were setup at ambient room temperature for 45 days. Experimental
data showed biphenyl was detected, which implied that the mineralization in both
systems was incomplete. Microscale Fe/Pd was found to exhibit a significantly higher
reaction rate than that of NZVI. However, such a high rate on Fe/Pd was not sustainable
as it was only achieved in the first 2 days of the test. The degradation rates of PCBs by
NZVI and microscale Fe/Pd follow pseudo first order kinetics and depend on the surface
area of iron particles (Lowry and Johnson 2004). At room temperature, the degradation
rates of PCBs by NZVI and Fe/PdM were reported to be 1 x 10
-6
-5.5 x 10
-4
L•yr
-1
m
-2
and 3.8 x 10
-2
-1.7 x 10
-1
L•yr
-1
m
-2
, respectively. Fang and Al-Abed (Fang and Al-Abed
2008) observed a higher dechlorination rate of 2-CB with a rate constant k of 0.13 ±
0.03 L•h
-1
m
-2
by microscale Fe/Pd for the first 10 minutes at temperature ranging from 4
to 60
o
C.
Therefore, NZVI has been suggested as a very effective reducing agent to
degrade PCBs. However, because NZVI particles have an extremely high surface-to-
volume ratio, they are easily agglomerate together to form larger ones and their activities
may be quickly diminished. The aggregation rate of NZVI from single nanoparticles to
micrometer size aggregates and the size of the aggregates formed were determined
recently (Phenrat et al., 2007). In this study, after only 3.5 min rapid aggregation of
primary RNIP particles was observed. A positive correlation exists between the
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