Environmental Engineering Reference
In-Depth Information
is different for the WAO and CWAO reactions, being that the intermediates
formed during the catalytic tests were less refractory to oxidation than those
obtained with WAO [110]. In particular, azo dyes are readily converted into
hazardous aromatic amines under anoxic conditions [111,112].
The catalysts included may be homogenous or heterogeneous types. The
key problems that need to be solved are recycling of homogenous cata-
lysts and better stability of heterogeneous catalysts [106]. Although the
homogenous catalysts such as copper and iron salts, and the heterogeneous
catalysts such as noble metals like (Pt, Pd, Ru, Rh) and transition metal
oxide catalysts (oxides of Cu, Ni, Mn, Fe, Co) are effective in CWAO, they
have some disadvantages like the need for an additional separation step to
remove or recover the metal ions from the treated effluent and deactivation
of the catalyst [113,114].
Recently, various carbon materials like activated carbon have been used
in catalytic wet oxidation processes effectively due to the stability, high
porosity and surface areas of the carbon materials [82].
The wet air oxidation mechanism is divided into two steps:
1. A physical step, which involves the transfer of oxygen from
the gas phase to the liquid phase, usually considers that oxy-
gen diffuses rapidly within the gas phase, and the transfer of
carbon dioxide to the gas from the liquid.
2. A chemical step, which involves the reaction between the
organic matter and dissolved oxygen in the liquid phase,
producing carbon dioxide.
Usually the mechanism includes initiation, propagation and termina-
tion reactions. Numerous reactions have been proposed, and the main
reactions in the presence of organic compounds (RH) are listed [104].
RH + O
2
HO
2
+ R
(3.7)
H
2
O + O
2
HO
2
+ OH
(3.8)
R + O
2
ROO (3.9)
ROO + RH ROOH + R (3.10)
OH + RH R + H
2
O (3.11)
HO
2
+ OH H
2
O + O
2
(3.12)
This method has been successfully applied to the removal of many
organic contaminants, including azo dyes [113,115-120].
Studies on the WAO of some model dyes are listed in Table 3.5.
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