81 LR by using UVH
2
O
2
also enhanced by increasing the UV intensity as reported by Ren et al. [140].
Based on the results of these experiments and also based on our laboratory limitation, the UV intensity at 12.06 mWcm
2
was the highest intensity that gave the highest degradation efficiency i.e. 58.95 TOC removal Figure 4.8, and hence an
intensity of 12.06 mWcm
2
was used for further degradation experiments.
Figure 4. 8 Effect of different UV intensity on TOC removal.
4.1.4 Effect of Initial Concentration of H
2
O
2
in Combination with UV
In order to investigate the effect of initial concentration of H
2
O
2
, six different H
2
O
2
concentrations 0.06 M, 0.09 M, 0.12 M, 0.18 M, and 0.24M were used. The volume of liquid 400 ml, UV intensity 12.06 mWcm
2
, MDEA concentration 2000 ppm, pH 7, and temperature 30
⁰C were maintained at constant values, while the H
2
O
2
concentration was varied from 0 – 0.24 M. Degradation profile of organic carbon at
82 various initial concentration of H
2
O
2
is shown in Figure 4.9 and the TOC removal profile is presented in Figure 4.10.
Figure 4. 9 The degradation profile of organic carbon at different
initial concentration of H
2
O
2
.
Based on the initial experiments section 4.1.1, it was found that the hydroxyl radical is the main species to cause the degradation of organic matter. Organic matter
was degraded into simpler compounds by oxidation reaction. The hydroxyl radicals were generated from UV photolysis of H
2
O
2
.
H
2
O
2
+ hυ → β HO
•
2.28
A higher concentration of H
2
O
2
is expected to generate more hydroxyl radicals, which enhanced the TOC removal. Based on our preliminary study, the maximum
TOC removal i.e. 59.02 was achieved at 0.18 M of H
2
O
2
, and further increase in
83 hydrogen peroxide concentration decreased the TOC removal, which might be due to
the scavenging effect of H
2
O
2
to the hydroxyl radical.
Figure 4. 10 Effect of initial concentration of H
2
O
2
on TOC removal after 180 min.
The reactions of scavenging mechanism of H
2
O
2
toward the hydroxyl radical are expressed in Equation 4.29
– 4.30.
O H
HO HO
O H
2 2
2 2
2.29
2 2
2
2.30
Hydroperoxyl radicals HO
2
• are generated from reaction between hydrogen
peroxide H
2
O
2
and hydroxyl radical HO
• Equation 2.29. Even though the
hydroperoxyl radical is also well known to oxidize the organic matter, but the reactivity of hydroperoxyl is less compared to the hydroxyl radical. In addition, the
reaction between hydroperoxyl radical HO
2
• and hydroxyl radical HO •
Equation 2.30 also probably reduce the concentration of hydroxyl radical in the system, which plays as an important species in the degradation process. Hence, the
TOC removal was less at higher H
2
O
2
concentrations. Similar reports on the optimum
84 concentration of hydrogen peroxide for UVH
2
O
2
process have also been made by many researchers. Muruganandhan and Swaminathan [70] reported that the optimum
hydrogen peroxide concentration was at 20 mmol for the decolorization of azo dye Reactive Orange 4, and further increase in the initial concentration of H
2
O
2
decreased the percentage decolorization. Similarly Malik and Sanyal [79], concluded that the decolorization of azo dye was optimum at 5.88 x 10
-3
M, and further increase in the H
2
O
2
concentration reduced the percentage of decolorization. An optimum concentration of H
2
O
2
was also reported for the decolorization of azo dye by UVH
2
O
2
process by Chang et al. in 2010 [141]. Increasing H
2
O
2
concentration in the system lead to the decomposition rate of azo dye to certain level and further increase
in H
2
O
2
concentration decreased the decomposition rate.
4.1.5 Effect of pH
