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
The effect of initial pH of the system was studied at five different pH values pH = 3 to 11 while maintaining the volume of liquid 400 ml; UV intensity
12.06 mWcm
2
, concentration of MDEA 2000 ppm; concentration of H
2
O
2
0.18 M; and same temperature 30
⁰C as constant. The influence of the initial pH on MDEA degradation using UVH
2
O
2
process is shown in Figure 4.11. The degradation rate of organic carbon increased with an increase in pH up to 9, and further increase in
pH reduced the degradation rate of organic carbon. At this optimum pH pH = 9, the removal of TOC was found to be 79.28. Figure 4.12 presents the percentage of TOC
removal at different initial pH conditions. Further increase in pH beyond 9, resulted in approximately a constant TOC reduction, which might be due to the decomposition of
hydrogen peroxide itself at higher pH levels. At high pH conditions, H
2
O
2
tend to ionize to form hydroperoxide anion HO
2 -
with pKa equal to 11.6 Equation 2.31 [132, 140]. Hydroperoxide anion is well known to be a strong scavenger to hydroxyl radical:
H
2
O
2
↔ HO
2 -
+ H
+
2.31 HO
• + HO
2 -
→ HO
2
• + HO
-
2.32
85
Figure 4. 11
The degradation profile of organic carbon for different initial pH conditions
Figure 4. 12 Effect of initial pH condition on TOC removal.
86 Reaction between hydroperoxide anion HO
2 -
and hydroxyl radical HO
•
generates a less reactive radical i.e. hydroperoxyl radical HO
2
• Equation 2.32 and
hence decrease in degradation rate. Similar trend was also reported for the degradation of dimethyl phthalate DMP and microcystin-LR MCLR using UV
H
2
O
2
by Xu et al. and Ren et al., respectively [132, 140]. They concluded that, the optimum pH for MCLR was at 7.2 and that for DMP was 4.0, for the degradation of
the respective compounds using UVH
2
O
2
process. The optimum pH condition for the degradation of different compounds using UVH
2
O
2
process depends mainly on the characteristic of the individual compounds.
4.1.6 Effect of Temperature
