Chemistry Reference
In-Depth Information
NH
2
N
N
H
2
O
2
SNH
SH
+
S
Fig. 11.44
Production of sulfenamides
by S-N coupling.
S
CN
CN
H
2
O
2
/ HBr
hν or initiator
Fig. 11.45
Aromatic side-chain
bromination with H
2
O
2
/HBr.
Br
5 Developments in Catalysed
Oxidations for Effluent Treatment
leading to precipitation of a sludge containing
organic residues that itself requires disposal.
Although the removal of some products by floccula-
tion into the sludge helps to reduce the H
2
O
2
con-
sumption, it may not be possible always to dispose
of this waste. In such cases, the Fe
III
can be reduced
to Fe
II
, using SO
2
or electrolysis, and recycled [287].
In an anaerobic/aerobic sequence, ferric can be
reduced to ferrous by some microorganisms [288].
Also recently, a 'photo-Fenton's' system has been
used successfully, where the waste is treated with
H
2
O
2
/ferrous salt (e.g. oxalate) in a holding lagoon
under the action of sunlight [289].
An extensive study has been made of the effects
of complexing agents on the Fenton system [290].
Although relatively high Fe/H
2
O
2
ratios were
employed (so re-reduction of Fe
III
was less of an
issue), a general conclusion was that uncomplexed
Fe only works at pH 3-4, whereas complexed forms
can work at pH 2-10.
A number of proprietary solid catalysts are offered
now in order to obtain Fenton-like chemistry
without adding metals to the wastewater [291,292].
Hydrogen peroxide/goethite (a-Fe(O)OH) was
investigated as a catalyst for organics destruction in
wastewater [293], using 1-chlorobutane as model.
The oxidation rate was found, surprisingly, to be
independent of solution pH/alkalinity. For this com-
pound, the system compared favourably with O
3
/UV,
H
2
O
2
/O
3
, H
2
O
2
/UV and H
2
O
2
/Fe
2+
.
The HO
•
active species produced by the Fenton
system can be generated also using H
2
O
2
and UV
radiation [294]. This is particularly suitable for very
dilute effluents ('polishing') where UV absorption by
other constituents is small.
Although the thrust of this topic is towards waste
minimisation, it is clear that end-of-pipe treatment
of effluents will remain important for many years as
one means used by industry to maintain and
improve environmental quality. The inherent advan-
tage of H
2
O
2
itself, in generating no significant waste
during use, is especially valuable in effluent treat-
ment. Increasingly, chemical techniques will be
applied not to the bulk of waste streams, but either
as a pretreatment—to 'soften' refractory effluents
before natural purification—or as a post-treatment
('polishing') stage—to make low residual pollutant
levels even lower and to remove environmentally
persistent compounds such as AOX compounds
(adsorbable organic halogens) and some pesticides.
A great amount of R&D has been devoted already
to combinations of treatment elements, known
as 'advanced oxidation processes' (AOPs)—many
chemists would think of them as 'hyphenated tech-
niques'—and these now are becoming applied
widely to real effluents. As will be seen, H
2
O
2
is a key
element in many such treatment combinations.
5.1 Catalysed H
2
O
2
systems
The Fenton system described in Section 2 has been
used over many decades for the treatment of refrac-
tory organics in wastewater. It is ideal, for example,
for the degradation of phenolic compounds to non-
toxic products [285], as summarised in Fig. 11.46.
This has been applied, for example, to aminophenols
in photographic effluent [286]. A characteristic of
this system is that Fe
III
salts are produced, often
