Environmental Engineering Reference
In-Depth Information
The activity of N-doped TiO 2 will vary when the pesticides are present individually
and in a complex mixture. The three pesticides, viz., methyl parathion, dichlorvos, and
lindane, were completely degraded using N-doped TiO 2 under visible light when the com-
pounds were present individually in the water sample. Dichlorvos was degraded faster
compared with other two compounds (Senthilnathan and Philip, 2010c).
The same reactions were carried out under solar light in a batch reactor, in single and
mixed pesticides. The time required for degrading 33.33 μg/L of each pesticide was more
compared with the time required to degrade 100 μg/L of pesticides when used individ-
ually. This clearly shows that the time required to treat mixed pesticides will be more
(Senthilnathan and Philip, 2011). A thin ilm continuous reactor was used to study the pho-
tocatalytic activity of immobilized N-doped TiO 2 in degrading pesticides (Senthilnathan
and Philip, 2012). The total volume of the reactor was 5500 mL, whereas the effective vol-
ume of the reactor was 1350 mL. Methyl parathion (250 μg/L) was used to ix the optimum
low rate. As low rate increased, the time taken for degrading the pesticide also increased.
Hence, providing more residence time can increase the degradation of pesticides. Optimum
low rate was ixed as 12.5 mL/min, and the corresponding residence time was 108 min.
Different concentrations of pesticides were used for the study. No intermediate was found
at the outlet of the reactor showing that the pesticides were completely degraded within
the residence time (Senthilnathan and Philip, 2012). N-doped TiO 2 showed maximum efi-
ciency under solar radiation compared with UV and visible light. In the case of mixed pes-
ticides, also complete removal was obtained after treatment. Even after 24 h of operation,
complete removal of pesticide was achieved. Proposed pathways for methyl parathion,
and lindane are presented in Figures 25.7 and 25.8, respectively. The rate of degradation of
S
O
P
NO 2
H 3 CO
P
NO 2
H 3 CO
OC H 3
OCH 3
Methyl paraoxon
m/z: 245
Methyl parathion
m/z: 263
S
H 3 CO
P
OCH 3
HO
NO 2
OCH 3
O,O,O-trimethyl
phosphoric thiourate
m/z: 156
p-Nitrophenol
m/z: 139
NO - , PO 3 - , SO 2 - , CO 2
Complete mineralization
FIGURE 25.7
Possible degradation pathways of methyl parathion in immobilized TiO 2 (methyl parathion concentration =
10  mg/L; Degussa P-25 coated inner surface area = 169.56 cm 2 ; oxygen purging rate = 300 mL/min; stirring
rate = 150 rpm).
 
Search WWH ::




Custom Search