Geoscience Reference
In-Depth Information
Table 16.6 Half-lives (t
1/2
) and rate constants (k) of BDE-209 adsorbed on minerals and sedi-
ments, with the assumption of first-order reaction
Sorbents/solvents
UV
Sunlight
k (d
-1
)
k (d
-1
)
t
1/2
t
1/2
Montmorillonite
36 days
0.0192 ± 0.0067
261 days
0.0032 ± 0.0009
Kaolinite
44 days
0.0158 ± 0.0012
408 days
0.0017 ± 0.0003
Sediment
150 days
0.0046 ± 0.0003
990 days
0.0007 ± 0.0003
40-60 h
80 h
53 h
81 h
-
a
Al hydroxide
178 days
0.0039 ± 0.0007
-
a
Mn dioxide (birnessite)
1423 days
0.0005 ± 0.0002
-
a
-
a
Fe oxide (ferrihydrite)
Sand
12 h
37 h
12 h
37 h
533 h
Soil
150-200 h
185 days
Silica gel
\0.25 h
Toluene
\0.25 h
0.0011 (s
-1
)
Hexane
\0.2 h
Methanol/water 0.5 h
Methanol 0.85 h
Tetrahydrofuran 1-1.5 h
Reprinted with permission from Ahn et al. (
2006
). Copyright 2006 American Chemical Society
a
k & 0
following order of BDE-209 loss from the adsorbents: montmorillonite [ kao-
linite [ sediment [ aluminum hydroxide [ birnessite. Ahn et al. (
2006
) consider
that reductive debromination is the result of the electron-donating ability of clay
minerals during the irradiation process. The fact that montmorillonite and kaolinite
have different bonding affinities for BDE-209 but degrade it at relatively similar
rates suggests that bonding forces do not affect the photodegradation rate.
BDE-209 debromination during photolysis leads to the formation of less bro-
minated congeners. The appearance of these photoinduced products is faster under
UV irradiation than under sunlight. Figure
16.19
shows gas chromatograms over
time of BDE-209 adsorbed on montmorillonite, during 101 days of irradiation by
sunlight, and compared with a control held in darkness. Over time, the areas of
peaks for nona-, octa-, and heptabromodiphenyl ethers increased in the irradiated
samples and remained at ''trace'' values in the dark control. Ahn et al. (
2006
) note
a comparable but slower time trend for BDE-209 adsorbed on kaolinite.
These results indicate that the first step in BDE-209 photodegradation is the loss
of one bromine atom to form nonabromodiphenyl ether congeners, followed by the
subsequent formation of octabromodiphenyl ethers.
