24
2.3.3 Ozone-based Processes
Naturally, ozone O
3
is present in the atmospheric layer around the earth, and it is formed by the recombination of atomic radical oxygen and diatomic oxygen. The
atomic radical oxygen O
• is commonly generated by the photolysis of diatomic
oxygen O
2
and further reacts with the diatomic oxygen O
2
to form ozone O
3
[30 – 32]. The reaction of ozone formation can be expressed in Equation 2.11 – 2.12.
O
2
+ hυ → βO
•
2.11 O
• + O
2
→ O
3
2.12
Ozone has a redox potential of 2.07 V, therefore ozone is very reactive either in the liquid or in gas. Reaction of ozone with organic contaminant can be considered
either on direct or indirect reaction. Equation 2.13 show the direct mechanism which involves organic compound degradation by molecular ozone and occurs in acidic pH
range.
O
3
+ 2H
+
+ 2e
-
→ O
2
+ H
2
O 2.13
Hydroxyl radical is generated from the reaction of ozone and hydroxyl ions present in water indirect ozone mechanism at basic pH conditions Equation 2.14. Further, the
hydroxyl radical reacts with an organic compound present in water.
O
3
+ H
2
O + OH
-
→ HO • + O
2
+ HO
2
•
2.14
Based on the Equation 2.13 and 2.14, indirect mechanism of ozonization can also
be classified as AOP’s. Meanwhile, direct mechanism of ozonization can be classified as classical chemical treatment method. Combination of O
3
H
2
O
2
has been widely investigated. By this combination, the hydroxyl radical is generated, and this
25 combination is well known as peroxone or perozonation [30, 46
– 47]. Overall mechanism of reaction can be expressed in Equation 2.15.
H
2
O
2
+ O
3
→ β HO
• + 3O
2
2.15
The O
3
UV process and O
3
H
2
O
2
process have been studied by Andreozzi et al. [48]
for the degradation of the mineral oil-contaminated wastewater, and they concluded that O
3
UV process was more effective to reduce the COD values compared to the O
3
H
2
O
2
process. Within 30 minutes of reaction time, around 80 – 90 of COD
removal was achieved. The UVO
3
process was also used for acetone removal [49] and improvement of drinking water quality in France [50]. Degradation of acetone
using UVO
3
was the most effective compared to the other processes i.e. H
2
O
2
O
3
and UVH
2
O
2
. Almost complete degradation was achieved by this process within a short duration 30 minutes of oxidation time. This process was used for disinfection,
oxidation of micro-pollutant, and minimization of bromate concentration in the drinking water for the improvement of the quality. Application of UV in drinking
water production was also capable of reducing the ozone consumption. Ma and Graham [51] reported an enhancement in the degradation of atrazine by ozone, which
was catalyzed by small amount of manganese Mn
2+
. This enhancement was due to the oxidation of Mn
2+
to Mn
4+
, that enhanced the generation of hydroxyl radical HO
•, which has a very high oxidation potential toward atrazine. Safarzadeh-Amiri
[52] used the O
3
H
2
O
2
process to degrade methyl-ter-butyl ether MTBE and proved that the operating cost of O
3
H
2
O
2
process was less compared to the UVH
2
O
2
process for reducing the same amount of MTBE and resulted in the same amount of removal
efficiency. Biodegradability of carbaryl a pesticide was enhanced using photocatalytic ozonization in the presence of TiO
2
as reported by Rajeswari and Kanmani [53]. It was also reported that the ratio of BOD
5
COD increased up to 0.38. Other than the report on the enhancement of biodegradability, they also reported a
reduction of COD and TOC up to 92 and 76.5, respectively, at the experimental concentration of carbaryl, ozon, and TiO
2
are 40 mgL, 0.28 gh, and 1 gL, respectively at pH = 6. In addition, Katsoyiannis et al. [54] reported that the energy
requirements between O
3
H
2
O
2
and UVH
2
O
2
for transformation of micropollutants
26 i.e. atrazine ATR, sulfamethoxazole SMX, and N-nitrodimethylamine NDMA
were quite similar, even though the NDMA transformation was more effective using direct photolysis. Park et al. [55] used the combination of microwaveUV and ozone
to degrade bromothymol blue in water solution, and reported a complete degradation of bromothymol blue within 10 minutes of reaction time. During ozone based process,
phosphate and carbonate were found to be the scavenger of the degradation reaction for sulfamethoxazole using UVTiO
2
O
3
[56]. The available literature on the ozone based oxidation process for different pollutants are summarized in Table 2.5.
2.3.4 High Voltage Electrical Discharge Processes
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