Chemistry Reference
In-Depth Information
Fig. 11.46
Phenol oxidative
degradation scheme.
A variety of industries produce phenolic or similar
aromatic wastestreams, mainly as by-products of
specific chemical manufacture but also associated
with coking, refinery, tar distillery and foundry
operations. Use of phenolic disinfectants ultimately
leads to these compounds appearing in effluents.
Essentially, the hydroxyl radical produced by
Fenton's reagent [295] or by H
2
O
2
/UV systems (or
O
3
/H
2
O
2
, as described below) is one of very few
chemical species capable of attacking refractory halo-
organic compounds. The treatment of halo-organics
before release from current processes, and the retro-
spective treatment of previous accumulations of
industrial waste containing them, is an important
issue because of their environmental persistence and
both the acute and chronic toxicity. Compounds
involved vary from volatile aliphatics such as chlo-
roform or 1,1,1-trichloroethane to heavy aromatics
such as polychlorinated biphenyls (PCBs) and
dioxins (e.g. 2,3,6,7-tetrachloro-dibenzo-
p
-dioxan).
•H
2
O
2
•O
3
• Adsorption on active carbon or other solids, etc.
Within these permutations, pairs of H
2
O
2
, ozone and
UV are in common use and can be used effectively
on concentrated or dilute effluents. The combination
of ozone and H
2
O
2
is covered to some extent in a
recent ozone review [296]. Ozone/H
2
O
2
[297] is
becoming a standard treatment in the USA: there is
evidence that H
2
O
2
also enhances the degradation of
organics by ozone [298]. Although hydroxyl radical
and H
2
O
2
itself are both generated to an extent in
aqueous ozone reactions, the two oxidants do tend
to sustain different, and often complementary, reac-
tion pathways. For example, O
3
and HO
•
(from
H
2
O
2
/UV) were compared in glycine oxidation [299].
Molecular ozone attacks the nitrogen centre first and
gives nitrites/nitrates, but hydroxyl radical only gives
ammonia nitrogen. In the presence of oxygen,
hydroxyl radical products include oxalic acid, along
with some formic and oxamic. Trace organochlorine
and pesticide compounds are removed using O
3
/H
2
O
2
in California [300]. The scope and utility of the
H
2
O
2
/UV treatment system has been reviewed [301]:
this has benefited greatly from improvements in
lamp efficiency and reactor design over the last two
decades.
A more unusual technique is the use of H
2
O
2
with
5.2 Advanced oxidation processes (AOPs)
This title embraces a large number of treatment com-
binations, of which the individual elements include:
• Sonolysis
• Photolysis (including UV)
• Electrolysis
