Environmental Engineering Reference
In-Depth Information
Gkika et al. (2004) have studied the photocatalytic degradation of lindane (3.7 × 10
−5
M)
using
PW O
− −
×( )
under UV light in aqueous solution. This study was carried out
in the presence and absence of dioxygen. In the absence of dioxygen, neither the regenera-
tion (reoxidation) of the catalyst nor the photodegradation of lindane took place. Under the
reported experimental conditions, decomposition of lindane was achieved within 60 min.
The complete photodegradation of lindane leads to its mineralization to CO
2
and Cl
−
as
inal products.
3
4
7 0
M
12
40
25.9.3 Photodegradation of Dichlorvos Using Suspended and
Immobilized TiO
2
Systems under UV Light
Many reports on the photocatalytic reduction of dichromate and the photocatalytic oxi-
dation of dichlorvos are available (Zhao et al., 1995; Chen and Cheng, 1999; Khalil et al.,
1998; Obee and Satyapal, 1998; Konstantinou et al., 2001; Ku and Jung, 2001; Schrank et al.,
2002). Dichromate (Cr
6+
) is a common inorganic pollutant, while dichlorvos, a widely used
organophosphorous pesticide, is also an organic pollutant. Shifu and Gengyu (2005) have
used dichromate and dichlorvos as inorganic and organic pollutants and TiO
2
/beads as a
photocatalyst. Initial concentrations of 3.8 × 10
−4
M and 1.0 × 10
−4
M of
Cr
2
−
and dichlor-
vos, respectively, were used for oxidation and reduction reactions. The optimum amount
of the photocatalyst used was 6.0 g/cm
3
. The addition of trace amounts of Fe
3+
or Cu
2+
accelerated both the reactions. On the other hand, addition of Zn
2+
and Na
+
did not inlu-
ence the reactions. Acidic solution was favorable for the photocatalytic reduction of dichro-
mate, and acidic and alkaline solutions were favorable for the photocatalytic oxidation of
dichlorvos. Addition of
SO
2−
accelerated the photocatalytic oxidation, whereas addition of
Cl
−
slowed down the reaction.
Evgenidou et al. (2006) have investigated the photocatalytic degradation of two organo-
phosphorous insecticides such as dichlorvos (initial concentration 20 mg/L) and dimethoate
(initial concentration 20 mg/L) using a suspension of TiO
2
(100 mg/L) under UV light. The
photocatalytic intermediates, end products, and reaction mechanisms were established in
this study. The intermediate by-products such as
O
,
O
-dimethyl phosphonic ester, 2,2-dichlo-
rovinyl
O
-methylphosphate, and
O
,
O
,
O
-trimethyl phosphoric ester have been identiied dur-
ing the degradation of dichlorvos. Harada et al. (1990) reported that the eficient degradation
of dichlorvos occurred by using a Pt/TiO
2
photocatalyst under a super-high-pressure mer-
cury lamp and produced mineralized nontoxic end products such as Cl
−
,
PO
3−
, H
+
, and CO
2
.
Rahman and Muneer (2005) have studied the degradation of dichlorvos (1 and 0.5 mM)
using three different photocatalysts (1 g/L), namely Degussa P-25, Hombikat UVI00
(SachtlebenChemie GmbH), and PC 500 (Millennium Inorganics). Among these three catalysts,
Degussa P-25 showed highest photocatalytic activity. Oancea and Oncescu (2008) reported that
the photocatalytic degradation of dichlorvos under UV irradiation was optimized with respect
to the low rate of O
2
gas. The photocatalyst concentration of dichlorvos used in the study was
1.66 × 10
−4
M, and pH 4 was maintained in the system. The rate of degradation depended on
the DO concentration. Decrease in reaction solution pH suggested the formation of organic
acids. The presence of organic intermediates was conirmed also by TOC measurements.
25.9.4 Degradation of Dichlorvos, Methyl Parathion, and
Lindane under Visible and Solar Light
Although photocatalytic degradation of pesticide using TiO
2
under UV light is possible, it
may not be a practical proposition for the treatment of drinking water sources due to the
