Geology Reference
In-Depth Information
are displayed in this way and partition maps will likely provide a useful
tier for chemical risk assessment in the future.
Unlike the older 'legacy' POPs, such as DDT and PCBs, many current
chemicals used in agriculture, industry and for domestic purposes, are
generally more susceptible to degradation processes once present in the
environment (although, ironically, the persistent nature of DDT can be
attributed to its biological degradate p,p
0
-DDE). In some instances these
degradates may be more persistent and/or harmful to the environment
than the parent chemicals. The photolytic degradation of Triclosan and
certain BFRs are a case in point. Interestingly, a recent review on
pesticide-degradates highlighted their higher frequency of detection over
the parent compounds in surface water sampled across the US.
50
These
pesticide-degradates typically had longer half-lives and lower log K
oc
values than the parent pesticides, making them relatively long-lived and
more mobile in the environment. Future research efforts will be required
to understand the fate of these chemicals and the threat they pose to
both humans and the environment.
6.8 QUESTIONS
(i) A pesticide has an experimentally determined K
d
value of 5.6,
calculate its K
oc
value assuming the soil has a 2% organic carbon
content. What type of polymeric material comprises a large
proportion of the organic carbon fraction in soil? Describe how
you could operationally separate its base-soluble fraction?
(ii) The Henry's Law constant for a pesticide at 25
o
C has been
experimentally determined as 0.008 Pa m
3
mol
1
. Calculate K
aw
(dimensionless Henry's Law constant)? If this pesticide has a log
K
ow
2.9, which environmental compartment is the chemical likely
to reside in (use the partition map of Figure 3 to help with your
answer)?
(iii) The pesticide outlined in Question (i) and (ii) does not absorb
sunlight in aqueous solution, however upon dissociation its anion
strongly absorbs wavelengths 4300 nm. If this pesticide has a
pKa of 6 and is present in surface waters with a pH
B
8, describe
the probable fate of this chemical.
REFERENCES
1. D. Ashton, M. Hilton and K.V. Thomas, Sci. Tot. Environ., 2004,
333, 167-184.
