Environmental Engineering Reference
In-Depth Information
A pilot plant, working in recycle mode, is operated in the Volkswagen AG factory in
Wolfsburg (Germany) after getting promising results in laboratory-scale studies. The plant
consists of 12 doubleskin sheet photoreactors. It has a total irradiated area of 27.6 m 2 . The
inluent of the photoreactor is biologically pretreated. The reaction takes place for 8-11 h
during daytime. A suspended catalyst slurry is used for the treatment. The catalyst par-
ticles are sedimented and reused after the reaction (Bahnemann, 2004).
Solar photocatalysis was used to treat biologically pretreated textile wastewater and was
able to give a maximum degradation rate of 3 g COD/h/m 2 (COD: chemical oxygen demand)
(Freudenhammer et al., 1997). The studies were done using a thin ilm ixed-bed reactor.
In Spain, under the SOLARDETOX project (Solar Detoxiication Technology for the
Treatment of Industrial Non-Biodegradable Persistent Chlorinated Water Contaminants),
a commercial nonconcentrating solar detoxiication system is used to treat water. The plant
can treat 1 m 3 of contaminated water. It uses a CPC with an aperture area of 100 m 2 . In
this reactor, catalyst slurry is used for treatment. After treatment, catalyst is recovered by
sedimentation by adjusting the pH.
25.9 TiO 2 for Pesticide Degradation
Pesticides are widely used for the agricultural purposes in the world. These pesticides
reach nearby water bodies through agricultural runoff, thereby contaminating them.
Percolation of pesticides into the soil contaminates groundwater also. Many studies have
revealed the presence of pesticides like DDT, lindane, endosulfan, methyl parathion, etc.,
in the water. Most of the pesticides belong to the category of EDCs, making pesticide-
contaminated water a threat to human health. Hence, it is very important to remove these
contaminants from drinking water. Among all the pesticides, organochlorine pesticides
are the most dificult to degrade. Conventional methods fail to remove the pesticides com-
pletely and they are also dangerous since the intermediates of these compounds can also
be toxic. Advanced oxidation is a promising method in the removal of pesticides due to its
nonselectivity, high reaction rate, and the fact that it leads to the complete mineralization
of the target compound.
25.9.1 Photodegradation of Methyl Parathion Using Suspended
and Immobilized TiO 2 Systems under UV Light
Many researchers have studied the photocatalytic degradation of methyl parathion in aque-
ous TiO 2 suspensions. Pignatello and Sun (1995) have reported on the complete oxidation of
methyl parathion (initial concentration 0.08 mM) in aqueous solution using the UV photo-
assisted Fenton (Fe 3+ and H 2 O 2 ) reaction. Under these conditions, methyl parathion yielded
quantities corresponding to stoichiometric ratios of HNO 3 , H 2 SO 4 , and H 3 PO 4 , with oxalic
acid, 4-nitrophenol, and dimethylphosphoric acid being identiied as intermediate species.
Konstantinou et al. (2001) investigated the photocatalytic oxidation of several organophos-
phorus compounds like ethyl parathion, methyl parathion (1 mg/L), dichlorofenthion, and
ethyl bromophos in aqueous TiO 2 (100 mg/L) suspensions. All investigated pesticides
were suficiently degraded in aqueous TiO 2 suspensions, irradiated with simulated solar
light. The half-lives ranged from 10.2 to 35.5 min for the organophosphorus insecticides
used for the study. The rates of catalytic disappearance depend on various parameters
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