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degree of bromination, whereby the lower brominated congeners gen-
erally degraded at a much slower rate than that of the higher brominated
congeners. For example, the observed first order rate constant (k) for the
photodecomposition of BDE-47 (a tetrabrominated diphenyl ether) in
MeOH/H 2 O solution was 0.07 10 5 s 1 compared to 40 10 5 s 1 for
BDE-209 (the decabrominated diphenyl ether). The relatively slow rate
of reaction of BDE-47 (approximately 600-fold less than BDE-209) can
be largely attributed to its lower absorbance of light at longer wave-
lengths, which in turn may help to explain the prevalence of PBD-47 in
environmental samples. Interestingly, Eriksson et al. 46 observed that the
decaBDE decomposed to lower brominated diphenyl ethers and also
tentatively identified additional photodegradation by-products includ-
ing methoxylated polybrominated dibenzofurans through interpretation
of mass spectra following analysis of the photolysis solution by GC-MS.
A similar aqueous photochemistry study on TBBPA 47 has found
much faster rates of transformation for this chemical relative to the
PBDEs. However, the rate was dependent on the pH of the solution,
with phototransformation favouring the anion of TBBPA, present at a
significant fraction when the pH of the test solution was 48 (the pKaof
TBBA is
7.5-8.5). At pH 10, l max and e max were 310 nm and 9170
cm 1 M 1 respectively, compared to 290 nm and
B
2000 cm 1 M 1 at
pH 5.5, illustrating that at the higher pH the TBBA-anion is absorbing
strongly at a higher wavelength compared to TBBPA and therefore
more likely to be photodegraded under natural sunlight. The value of k
for the photodecomposition of TBBPA in an aqueous solution at pH 10
was measured as 0.7 10 3 s 1 , compared to 0.033 10 3 s 1 at pH 5.5,
and the photodegradates included a number of bromophenols and
alkylated bromophenols.
B
6.7 CONCLUSIONS
Knowledge of the partitioning properties of organic contaminants is
fundamental to understanding their environmental distribution and
hence fate. For many organic chemicals that pose a threat to the
environment, there is a lack of robust temperature-dependent partition
coefficients, and in many instances these have to be estimated either
from other partitioning descriptors or modelled from the molecular
properties of the chemical. Partition 'maps' provide a useful way to
visualise the environmental distribution of a wide range of chemical
classes, and to identify those relevant media where a chemical may
reside. Increasingly the results from multi-media chemical fate models
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