Environmental Engineering Reference
In-Depth Information
such as initial concentration, radiant lux, wavelength, mass, type of photocatalyst, and
type of photoreactor. Sanjuan et al. (2000) described the use of 2,4,6-triphenylpyrylium
ion encapsulated within Y zeolite as a photocatalyst for the photodegradation of methyl
parathion (3 × 10
−4
M) in aqueous suspensions. Encapsulation stabilizes the pyrylium ion
leading to degradation of methyl parathion, and it was suggested that radical intermedi-
ates are involved in the degradation process.
Moctezuma et al. (2007) reported that the photocatalytic degradation of methyl para-
thion (initial concentration 50 mg/L) was monitored using different complementary ana-
lytical techniques to identify the organic intermediates. The results of their study clearly
indicated that the irst step of the photocatalytic degradation of methyl parathion was its
oxidation to methyl paraoxon, and this reaction was not affected by the pH of the aqueous
medium. On the other hand, the photocatalytic oxidation of methyl paraoxon was strongly
inluenced by the pH of the reaction solution. Hence, the pH of the solution is an important
parameter for the methyl parathion degradation. Under alkaline conditions, only 60% of
the total organic carbon (TOC) was converted to CO
2
after 6 h of reaction. In acidic pH, 90%
of TOC was converted to CO
2
in similar condition.
Evgenidou et al. (2007) studied the photocatalytic oxidation of methyl parathion using
TiO
2
and ZnO as catalysts. In the presence of TiO
2
, complete disappearance of the methyl
parathion was observed after 4 h, while dissolved organic carbon (DOC) was only slightly
reduced in the irst 2 h, indicating the formation of intermediates. At the end of treat-
ment, >90% of the DOC was reduced. Stoichiometric release of nitrate and sulfate ions
was achieved after 2 and 3 h, respectively, while phosphates were formed more slowly
indicating the formation of phosphate-containing intermediates. On the other hand, in the
presence of ZnO, mineralization proceeded in a slower rate since only 50% of DOC was
reduced after 6 h of irradiation. Nitrate and sulfate ions contributed only 50% and 70% of
the expected amount, respectively, while no formation of phosphates was observed even
at the end of treatment. This indicated that TiO
2
proved to be a more eficient photocata-
lyst than ZnO and complete mineralization was achieved only in the presence of TiO
2
.
Xiaodan et al. (2006) studied the degradation of methyl parathion (50 mg/L) using anatase
TiO
2
, Degussa P-25, and ZnS/TiO
2
, which was prepared with different ratios of ZnS and
TiO
2
(ZnS/TiO
2
= 1, ZnS/TiO
2
= 2, ZnS/TiO
2
= 3). The combination with ZnS/TiO
2
ratio = 3
exhibited the highest photocatalytic activity among the other photocatalytic materials
under visible light. The photocatalytic activity increased with increase of ZnS doping.
Kim et al. (2006) studied the photocatalytic degradation of methyl parathion using a
circulating TiO
2
/UV reactor. The experimental results showed that parathion was more
effectively degraded in the photocatalytic condition than the photolysis or TiO
2
catalytic
condition. With photocatalysis, 10 mg/L methyl parathion was completely degraded
within 60 min with a TOC decrease exceeding 90%, after 150 min. Organic intermediates
like 4-nitrophenol and paraoxon were also identiied during the degradation of methyl
parathion and these were further degraded.
25.9.2 Photodegradation of Lindane Using Suspended and
Immobilized TiO
2
Systems under UV Light
Zaleska et al. (2000) studied the degradation of lindane (5 mg/L) with suspended and
immobilized TiO
2
under UV light. In the presence of TiO
2
supported on glass micro-
spheres, a rapid decrease of lindane concentration was observed at the beginning of the
reaction and 30 min of irradiation eliminated 68% of lindane. A much slower concentra-
tion drop was observed during subsequent 120 min of irradiation. Lindane degradation
