74 In others words, for complete oxidation of 2 moles of MDEA require 14.5 moles of
H
2
O
2
. Further, for example when we want to conduct the experiment at 2000 ppm of initial concentration of MDEA, the estimation of theoretical amount of H
2
O
2
required for complete oxidation of MDEA is as much as 0.12 M.
4.1.1 Effect of UV, H
2
O
2
, and Combination of UVH
2
O
2
on MDEA Degradation
The objective of the study is to visualize the individual effect of UV and H
2
O
2
, and the combination of UVH
2
O
2
, on the degradation of MDEA. The capability of UV light to degrade the organic compound follows photolysis
mechanism. The organic compound absorbs UV spectrum and then results in an excited organic compound, which later decomposed to form a product [31
– 32, 129 – 130]. The results of the present experiments showed no degradation was found
when the UV alone was applied Figure 4.1.
Figure 4. 1 Individual effect of UV, H
2
O
2
, and the combination of UVH
2
O
2
on MDEA degradation [MDEA]
= 1000 ppm TOC; [H
2
O
2
] = 0.12 M; pH = 7;
Temperature = 30 ⁰C;UV intensity = 12.06 mWcm
2
75 UV spectrum at 254 nm that was used in this experiment was not capable to
degrade the MDEA and to remove the total organic carbon from the system Figure 4.2, and the reason could be attributed to the fact that MDEA did not absorb the UV
light at 254 nm. Based on the screening of UV spectrum Figure 4.3, the region of the spectrum absorbed by MDEA was at spectrum below than 254 nm region, and
therefore the direct photolysis did not occur.
Figure 4. 2 Individual effect of UV, H
2
O
2
, and the combination of UVH
2
O
2
on the MDEA mineralization [MDEA]
= 1000 ppm TOC; [H
2
O
2
] = 0.12 M; pH = 7;
Temperature = 30 ⁰C;UV intensity = 12.06 mWcm
2
.
Meanwhile, the capability of H
2
O
2
to degrade organic compound is mainly due to the high redox potential of H
2
O
2
i.e. + 1.8 V. This reduction potential indicates the high tendency of H
2
O
2
to act as an oxidant, which refers to direct electron-transfer reaction between organic compound and H
2
O
2
[131]. The results of the present experiments showed no degradation when the H
2
O
2
alone was used. H
2
O
2
alone was not capable to degrade the MDEA Figure 4.1 and to remove the total organic carbon in the system
Figure 4.2.
76 This might be due to the redox potential of H
2
O
2
that is not sufficient for the oxidation process. The photolysis resistance of MDEA toward UV light and H
2
O
2
was in agreement with the observation of Xua et al. [132], based on their studies on the
photolysis resistance of dimethyl phthalate against UV photolysis and H
2
O
2
.
Figure 4. 3 UV absorption spectra of MDEA.
However, the reduction of MDEA and total organic carbon was found when the UV and H
2
O
2
were applied in combination. Complete 100 degradation of MDEA was achieved at 40 minutes of reaction time Figure 4.1 and the total organic carbon was
reduced to a certain level i.e. 41.23 TOC removed Figure 4.2, which was due to hydroxyl radical generated from H
2
O
2
photolysis. It is well known that H
2
O
2
strongly absorbs UV spectrum at 254 nm [31]. Therefore the probability of H
2
O
2
photolysis to generate hydroxyl radicals is very high. Due to the photolysis of H
2
O
2
the concentration of H
2
O
2
in the system gets reduced during the UVH
2
O
2
process, and the profile of H
2
O
2
is shown in Figure 4.4. Hence, it can be concluded that the combination of UV and H
2
O
2
will generate hydroxyl radical, which plays an important role in the degradation of many recalcitrant organic contaminant [80, 129,
133 – 134].
77
Figure 4. 4 H
2
O
2
profile during the UVH
2
O
2
process.
4.1.2 Effect of Initial Concentration of MDEA
