Environmental Engineering Reference
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etal.1996).TheresultsobtainedbyArshadetal.(2008)demonstratedthatbioaugmentation
ofthecontaminatedsoilswithα-andβ-endosulfandegradingbacteriumunderoptimized
conditionsprovidesaneffectivebioremediationstrategy.Zhangetal.(2007)determined
theresponseofantioxidativeenzymesofcucumber( Cucumis sativus L.)whencarbendazim
wasappliedassoildrenchat0,5,50,and100mg/kg.Onthebasisoftheresults,Zhang
etal.(2007)concludedthatincreasedsuperoxidedismutase,catalaseandglutathioneper-
oxidaseactivityprovidesplantswithincreasedcarbendazimstresstolerance.
6.5 Degradation of Pesticides by Sunlight
Sunlight at the Earth's surface consists of radiation in the so-called UV-B and UV-A
regions(295-400nm),inadditiontovisiblelight(~400-800nm)andinfrared(IR)radia-
tion.Approximately,4%ofthetotalenergyinsunlightoccursintheUVband,butthe
intensity varies greatly with latitude, season, time of day, and thickness of the atmo-
sphere and the ozone layer. The sun produces 0.2-0.3 mol photons/m 2 h in the range
of300-400nmwithatypicalUVluxof20-30W/m 2 .Naturalradiationfromthesun
hasalsobeenfoundtofadethecolorandreducetheconcentrationofdissolvedorganic
matter(DOM).
HeterogeneousphotocatalyticoxidationprocessemployingcatalystssuchasTiO 2 ,ZnO,
etc., and UV light has demonstrated promising results for the degradation of pesticides
andproducingmorebiologicallydegradableandlesstoxicsubstances(Garciaetal.2006,
2008; Vora et al. 2009). This process mainly relies on the in situ generation of hydroxyl
radicals under ambient conditions, which are capable of converting a wide spectrum of
toxicorganiccompoundsincludingthenonbiodegradableonesintorelativelyinnocuous
end-productssuchasCO 2 andH 2 O(Ahmedetal.2011).
Photocatalytic degradation of pesticides depends on the type and composition of the
photocatalyst and light intensity, initial substrate concentration, amount of catalyst, pH
ofthereactionmedium,ioniccompoundsofwastewater,solventtypes,oxidizingagents/
electroncatalyst,catalystapplicationmode,andcalcinationtemperature(Shaktiveletal.
2003).
In the photocatalytic oxidation process, organic pollutants are destroyed in the pres-
ence of semiconductor photocatalysts (e.g., TiO 2 , ZnO), an energetic light source, and an
oxidizingagentsuchasoxygenorair(Ahmedetal.2011). Figure6.9 illustratesthatonly
photonswithenergiesgreaterthantheband-gapenergy(ΔE)canresultintheexcitationof
valenceband(VB)electrons,whichthenpromotepossiblereactionswithorganicpollut-
ants(Ahmedetal.2011).TheabsorptionofphotonswithenergylowerthanΔEorlonger
wavelengths usually causes energy dissipation in the form of heat. The illumination of
thephotocatalyticsurfacewithsuficientenergyleadstotheformationofapositivehole
(h + )inthevalencebandandanelectron(e )intheconductionband(CB).Thepositivehole
oxidizeseitherthepollutantdirectlyorwatertoproducehydroxylradical•OH,whereas
the electron in the conduction band reduces the oxygen adsorbed on the photocatalyst
(TiO 2 )(Ahmedetal.2011).Ahmedetal.(2011)proposedthefollowingsteps(Equations6.9
through6.13)fortheactivationofTiO 2 byUVlight
(
) → +
+
TiO hv
2 +
λ
<
387 nm
e
h
(6.9)
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