Geology Reference
In-Depth Information
Triclosan
Cl
OH
O
Cl
Cl
Cl
OH
pKa = 7.9
2,4-DCP
Cl
O
-
Cl
hv
O
hv
Cl
O
Cl
Cl
Cl
Triclosan-anion
hv
O
2,8-DCDD
DOC
Coupling reaction
with humic material
Figure 7 Direct photochemical reaction pathway of Triclosan (5-chloro-2-(2,4-di-
chlorophenoxy)phenol) in natural water (3-100 mM of Triclosan in pH-
buffered solutions), irradiated with a Hg-vapour lamp. Only the triclosan
phenolate anion (present in water at pH 48) is susceptible to significant
photochemical transformation and will also react through indirect photolysis
(not shown) with singlet oxygen (
1
O
2
) and OH radicals to form 2,4-dichloro-
phenol (2,4-DCP). The photoproduct 2,8-dichlorodibenzo-p-dioxin (2,8-
DCDD) belongs to a notoriously persistent group of chemicals commonly
referred to as 'dioxins'
From Latch et al.
42
courtesy of SETAC press
into the environment and occurrence in wildlife and human blood has
resulted in growing concern about these groups of chemical.
44,45
BFRs function by decomposing under heat to give rise to Br
d
radicals
that react with hydrocarbons (flammable gases), given off by the burning
substance, to form HBr molecules. These, in turn, 'mop-up' high energy
OH
d
and H
d
radicals formed as part of the combustion chain reaction,
thereby effectively retarding or slowing the combustion process. The
effectiveness of a BFR is therefore dependent on the quantity of Br
d
radicals that can be released and the control of their release. Ideally, for
the latter, the BFR should decompose at a lower temperature than that
of the polymer in which it is incorporated and this will determine,
among other things, the type of BFR used in a particular application.
45
Two important BFRs are the polybrominated diphenyl ethers
(PBDEs) and tetrabromobisphenol A (TBBPA) and their respective
structures are presented in Figure 8. PBDEs are utilised as additives for
treating plastics and polymers used within electronic components and
