Chemistry Reference
In-Depth Information
low-frequency-power ultrasound, which can also
produce hydroxyl radicals, along with very high local
temperatures and pressures in the cavitation bubbles
formed. A patent [302] describes its use in simple or
complex cyanide removal, including complexes with
Au, Ag, Ni, Fe, Cu and Zn.
Some processes produce concentrated organic
effluents (high chemical oxygen demand) that
cannot be discharged directly and are difficult to treat
on-site using conventional biological methods. In
addition to the peroxygen methods noted above,
there is a technique involving the use of H
2
O
2
and
air under moderately elevated temperature and pres-
sure—typically 150°C and 10 bar. This is known as
wet peroxide oxidation (WPO) [303-304] and oper-
ates under much milder conditions than the now-
established wet air oxidation (WAO)—the latter
involves temperatures over 200°C and pressures up
to 200 bar and can result in severe corrosion of the
plant. In general, running costs are somewhat higher
with WPO but capital costs are lower than WAO.
Both achieve a similar performance, removing
65-90% total organic carbon, depending on the
nature of the effluent [305].
in monitoring of water quality has led to a corre-
sponding demand for colour removal treatments.
Absorbents such as clays or active carbon can
perform such a function for some types of dyes but
not all are strongly absorbed. The use of oxidants is
a good alternative because this disrupts the chro-
mophoric groups (usually electron-rich) in the dye
molecules, leading to smaller fragments that are not
as highly coloured and often also are more amenable
to biological treatment. As discussed above, the gen-
eration of oxygen radicals, via the Fenton system or
UV/O
3
/H
2
O
2
combinations, provides an efficient way
of degrading coloured compounds in effluents and of
reducing the total oxygen demand of the effluent
[306].
Explosives are predominantly poly-nitro com-
pounds that are highly electron-deficient and chemi-
cally quite unreactive. A system like Fenton's
reagent, however, is powerful enough to destroy
intramine explosive compounds such as TNT, RDX
and HMX in effluent [307,308].
The origins of effluent chemicals and toxicity in
pulp and paper manufacture are considered [309] in
terms of recent research and future directions, one
conclusion being that there is not yet enough infor-
mation of the impact of changing bleach technology
(although this is generally positive).
The chemical oxidation of pesticide-containing
effluents [310] and the removal of pesticides from
drinking water [311] have been reviewed very
recently. Hydroxyl radical again gives different
oxidation pathways to, for example, ozone, direct
electron abstraction (such as is achieved by photo-
activated TiO
2
), etc. Photo-Fenton systems turned
out to be among the best choices, along with TiO
2
.
The technique also has been evaluated specifically
for pesticides [312].
Judging by the number of papers addressing it,
methyl
t
-butyl ether has become a very important
issue in groundwater contamination, judging by the
number of recent papers addressing it. Solutions
include the use of the H
2
O
2
/UV system [313] and
H
2
O
2
/solid catalysts [292]. This topic is somewhat
related to soil remediation, for which a number of
solutions involving peroxygens have been described
(see Section 5.5).
Landfill leachate can contain phenolics and other
organics, along with harmful inorganic pollutants,
especially sulfide. The system H
2
O
2
/UV has been
applied successfully [314].
5.3 Treatment of refractory effluents
Phenols and halo-organics have been mentioned
already as types of compound that are often impor-
tant in effluent treatment. Further to this, refractory
effluent types of common concern include:
• Dyes
• Explosives (TNT and other polynitro compounds)
• Pulp and paper (although some of the industry is
moving towards totally effluent free technology,
working in a closed loop)
• Pesticides
• Methyl
t
-butyl ether (and related gasoline addi-
tives)
• Chemical warfare agents (usually waste disposal
rather than effluent as such, although some pesti-
cides have similar structures)
• Landfill leachate
The manufacture of dyestuffs and, more particularly,
their use by very many different textile dyers and
finishers leads to the production of effluents that,
although often very dilute, are highly coloured, to
the extent of being noticeable even when further
diluted by discharge into watercourses. The increase
