113 In addition, increase in oxidation temperature lead H
2
O
2
to further undergo self- accelerating decomposition. For comparison, De et al. [146] reported an estimated
hydroxyl radical rate constant of 1 x 10
10
g mol
-1
s
-1
for the degradation of 2- and 4- chlorophenol using UVH
2
O
2
in aqueous solution at room temperature. During the mineralization
of monoethanolamine MEA using Fenton’s reagent at room temperature, the reported hydroxyl radical rate constant was 2.9 x 10
6
M
-1
min
-1
4.8 x 10
4
M
-1
s
-1
[6]. The present estimated hydroxyl radical rate constants for the mineralization of MDEA by using UVH
2
O
2
are smaller compared to the reported rate constants for aromatic compounds such as phenol [146]. Nevertheless, the present
estimated values are higher compared to those obtained using Fenton treatment for the oxidation of same group of compound i.e. monoethanolamine. Table 4.8 presents the
comparison of the reported hydroxyl radical rate constants for different pollutants using various methods of oxidation process. The present estimated values are
satisfactorily comparable with those reported in the literature.
4.3.2 Temperature Dependence of MDEA Mineralization
In this study, the experiments on the mineralization of MDEA by UVH
2
O
2
were conducted at four different temperatures ranging from 20 ºC to 50 ºC, since the
optimum oxidation temperature was found within this range. The activation energy of any reaction can be calculated using Arrhenius’ law Equation 4.1λ [147, 15γ – 154]:
RT Ea
e A
k
4.19
where k = hydroxyl radical reaction rate constant; A
o
= preexponential factor; Ea = activation energy; R = ideal gas constant 8.314 Joule K
-1
mol
-1
; T = temperature ºK. The logarithmic form of Arrhenius’ law can be written asμ
T R
Ea A
k
o
1 ln
ln
4.20
114
Table 4. 8 Literature values for hydroxyl radical rate constants for different
compounds.
Process Method
Rate constants References
Degradation of: -
Phenol -
2-chlorophenols -
4-chlorophenols UVH
2
O
2
1.41±0.6 x 10
10
g mol
-1
s
-1
9.10±2.1 x 10
10
g mol
-1
s
-1
1.07±0.4 x 10
10
g mol
-1
s
-1
De et al., 1999 [146]
Degradation of methyl tert-butyl ether MTBE
UVH
2
O
2
3.9 ±0.73 x 10
9
M
-1
s
-1
Chang et al., 2000 [152]
Degradation of 4-chloro-3,5-di nitrobenzoic acid
UVH
2
O
2
3.5 x 10
9
M
-1
s
-1
Lopez et al., 2000 [63]
Degradation of carbendazim UVH
2
O
2
2.2±0.3 x 10
9
M
-1
s
-1
Mazellier et al., 2003 [69]
Degradation of sulphamethoxazole
UVH
2
O
2
3.5 x 10
9
M
-1
s
-1
to 6.8 x 10
9
M
-1
s
-1
Lester et al., 2010 [129]
Mineralization of monoethanolamine
UVH
2
O
2
4.7 x 10
10
M
-3
s
-1
to 15 x 10
10
M
-3
s
-1
Ariff, 2010 [10] Mineralization of
monoethanolamine Fenton’s
reagent 4.8 x 10
4
M
-1
s
-1
Harimurti et al., 2010 [6]
Mineralization of di- isopropanolamine
Fenton’s reagent
2.38 x 10
5
M
-1
s
-1
Omar et al., 2010 [8]
Mineralization of methyldiethanolamine
- at 20
o
C -
at 30
o
C -
at 40
o
C -
at 50
o
C UVH
2
O
2
9.50 x 10
9
M
-1
s
-1
9.58 x 10
9
M
-1
s
-1
10.73 x 10
9
M
-1
s
-1
13.64 x 10
9
M
-1
s
-1
Present work
Using the present calculated rate constant values Table 4.7, a plot of ln k
3
vs 1T is made Figure 4.30 which shows a linear correlation with R
2
= 0.8580. From the slope, the activation energy was estimated as 10.20 kJ mol
-1
. Table 4.9 compares the reported activation energy values for the hydroxyl radical oxidation process obtained
using different pollutants. The present estimated activation energy 10.20 kJ mol
-1
obtained for the mineralization of MDEA is in similar range and comparable with the reported activation energies for degradation of formaldehyde [157] and less than that
115 reported for simple phenolic compounds such as ortho, meta, and para form of cresol
[155], complex phenolic compounds such as 2,4,6-trichlorophenol [136], p- hydroxybenzoic acid [148], and 2,4-dichlorophenoxyacetic acid [156], which might
also be attributed to the type of oxidation process involved.
Figure 4. 30 Plot of ln k
3
vs 1T.
Table 4. 9 Activation energies of hydroxyl radical oxidation for different pollutants.
Process Method
Activation energy References
Degradation of p- hydroxybenzoic acid
UVFenton’s reagent
32.8 kJ mol
-1
Beltran et al., 2001 [148] Degradation of 2,4-
dichlorophenoxyacetic acid
Anodic Fenton 26.1 ±0.9 kJ mol
-1
Wang and Lemley, 2001 [156]
Destruction of: -
o-cresol -
m-cresol -
p-cresol Fen
ton’s reagent
16.25 kJ mol
-1
12.90 kJ mol
-1
14.95 kJ mol
-1
Kavitha and Palanivelu, 2005 [155]
Photocatalitic oxidation of 2,4,6-trichlorophenol
UVO
2
TiO
2
19.98 kJ mol
-1
Ochuma et al., 2007 [136] Degradation of
formaldehyde UV-Fenton
9.85 kJ mol
-1
Liu et al., 2011 [157] Mineralization of
methyldiethanolamine UVH
2
O
2
10.20 kJ mol
-1
Present work
116
4.4 Effect of Bicarbonate on MDEA Mineralization by UVH
