Microbe-Mediated Degradation of Synthetic Dyes in Wastewater - Microbial Degradation of Synthetic Dyes in Wastewaters - page 217

2.3 Non-biological Color Removal

While advanced oxidation processes (AOPs) have been studied extensively both for
recalcitrant wastewater in general and dye wastewater in particular, their com-
mercialization has yet not been realized because of certain barriers (Gogate and
Pandit 2004 ). These processes are costly and complex also at the present level of
their development (Papic 2004). Additional impediment exists in the treatment of
dye wastewater with higher concentration of dyes, as AOPs are only effective for
wastewater with very low concentrations of organic dyes. Thus, signi

cant dilution
is necessary for the wastewater treatment. For the AOPs, the basic reaction
mechanism is the generation of free radicals and their subsequent attack on the
organic pollutant species. Hence, it is strongly believed that their combination will
result in more free radicals, thereby increasing the rates of reactions (Gogate and
Pandit 2004 ). Moreover, some of the drawbacks of the individual AOPs may be
overcome by the characteristics of other AOPs. The cost/energy ef ciency, how-

ever, will be dependent on the operating conditions and the type of the wastewater.

2.3.1 Different Photochemical Processes

The photo-activated chemical reactions are characterized by a free radical mecha-
nism initiated by the interaction of photons of a proper energy level with the
chemical species present in the solution. Generation of radicals through UV radi-
ation by the homogenous photochemical degradation of oxidizing compounds like
hydrogen peroxide (Aleboyeh et al. 2003 ), ozone (Chen et al. 2004 ) or Fenton

s
reagent (Neamtu et al. 2002 ) has been frequently reported to be superior to sole UV
radiation or sole utilization of such oxidants. The photocatalytic process can be
carried out by simply using slurry of the

'

ne catalyst particles dispersed in the
liquid phase in a reactor or by using supported/immobilized catalysts. Fenton
reagent (a mixture of H 2 O 2 and Fe 2+ ) and its modications, such as thermal Fenton
process (Suty et al. 2004 ) or photo-Fenton reaction using Fe(II)/Fe(III) oxalate ion,
H 2 O 2 and UV light, have received a great attention as means for decolorization of
synthetic dyes (Shah et al. 2003 ; Swaminathan et al. 2003 ). In the case of photo-
Fenton technique, H 2 O 2 is utilized more rapidly by three simultaneous reactions,
namely direct Fenton action, photo-reduction of Fe (III) ions to Fe (II) and H 2 O 2
photolysis. Thus, this process produces more hydroxyl radicals in comparison to the
conventional Fenton method or the photolysis (Bandara et al. 1996 ; Gogate and
Pandit 2004 ). Among the AOPs, the photo-Fenton reaction (Torrades et al. 2004 )
and TiO 2 -mediated heterogeneous photocatalytic treatment (Chen et al. 2002 )
processes are capable of absorbing near-UV spectral region to initiate radical
reactions. Their application would practically eliminate major operating costs when
solar radiation is employed instead of arti cial UV light. The ferrioxalate solution,

which has long been used as chemical actinometer, may be used in photo-Fenton
process to derive further bene t by replacing UV with solar radiation (Arslan et al.