Geology Reference
In-Depth Information
should not comprise more than 1% by volume of the test solution.
Furthermore, the co-solvent must not solvolyse the test chemical, must
not absorb in the 290-800 nm region, and must not be a photosensitiser
(that is promote chemical change through indirect photolysis due to
exchange of molecular energy with the test chemical). To this end, aceto-
nitrile and methanol (MeOH) are the co-solvents of choice (e.g.aqueous
solutions comprising 99.5% H
2
O/0.5% CH
3
CN), but not acetone because
of its strong photosensitizing effect.
Natural waters contain dissolved organic carbon (DOC) and also vary
in pH (typically between pH 5 and 8). Both DOC and pH can have a
marked effect on the photochemical degradation of organic pollutants
and for realistic photochemical studies to be undertaken both factors need
to be taken into account, particularly for ionisable chemicals that may
dissociate in water depending on their acid dissociation constant (pKa)
and the pH of the water. DOC comprises the principal light absorbing
component in surface waters (comprising humic and fulvic acids) that
provide the natural colouration of waters arising in organic-rich environ-
ments. Direct photochemical reactions can be slowed by the shielding
effect of DOC, but additionally sensitised reactions may occur by indirect
light-induced reactions of the dissolved material. One example of this type
of light-induced reaction with DOC, is the possible coupling reactions of
triclosan with humic matter, to form insoluble particles.
42
Triclosan is a
relatively common anti-microbial agent added to hand-washes, tooth-
paste and other personal care products and is frequently detected in
surface waters throughout Europe and North America. Direct photo-
transformation is responsible for the majority of triclosan loss in surface
waters
43
and photobyproducts include 2,4-dichlorophenol and 2,8-
dichlorodibenzo-p-dioxin.
42
Only the anion of triclosan is susceptible to
significant phototransformation and as its pKa is 7.9 (weak acid), then the
anion will predominate in water with a pH of 48. The photochemical
reaction pathway is illustrated in Figure 7 and provides a useful example
of photochemistry that is pH dependent and where DOC appears to
influence the reaction by-products.
6.6.3 Photochemistry of Brominated Flame Retardants (BFRs)
Brominated flame retardants (BFRs) are an interesting and important
group of chemicals that are widely incorporated into plastics, foams,
electrical items and furnishings to reduce the risk of fire. Indeed, it is
likely that the polyurethane foam used in an upholstered chair in which
you maybe sitting, will have been treated with a BFR in order to
conform to fire-safety standards. However, their widespread use, release
