Environmental Engineering Reference
In-Depth Information
distribution,porosity,bandgap,andsurfacehydroxyldensity(Ahmedetal.2011).Atlow
substrateconcentrations,forexample,inthecaseofpesticides,thephotocatalyticdegra-
dationrateoforganiccompoundscanbeexplainedbyapseudoirst-orderpattern,with
Equation6.14demonstratingtherelationshipbetweenCandt
lnC C
0
=
k t
obs
(6.14)
where,k
obs
istheapparentreactionrateconstant;t,thereactiontime
C
0
,theinitialconcentrationoftargetpesticideinaqueoussolution
C,theresidualconcentrationofpesticideattime
t
Successfulapplicationofphotocatalyticoxidationsystemrequiresthedegradationrate
onthesubstrateconcentration(C
0
).
Table 6.5 summarizes a variety of pesticides studied under various initial concentra-
tions,andTable6.6showstheinluenceofcatalystloadingonthephotocatalyticdegrada-
tionofvariousgroupsofpesticides.
Electrostaticinteractionamongsemiconductorsurface,solventmolecules,substrate,and
chargedradicalsformedduringphotocatalyticoxidationstronglydependsonthesolution
pH.ThepHofthesolutioncanplayakeyroleinadsorptionandphotocatalyticoxidation
ofpesticides.TheinluenceofpHonthephotocatalyticdegradationofpesticidesissum-
marized in Table 6.7. The ionization state of the surface of the photocatalyst can also be
protonatedanddeprotonatedunderacidicandalkalineconditions,respectively,asshown
inthefollowingreactions(Ahmedetal.2011)(Equations6.15and6.16)
+
+
2
(6.15)
pH P
<
TiOH H
+ →
TiOH
zc
−
−
pH P
>
TiOH OH
+
→
TiO H TiO
+
(6.16)
zc
2
where,P
zc
isthepointofzerochargeoftheTiO
2
(Degussa25);TiO
2
(Degussa25)iswidely
investigatedandreportedatpH~6.25(Zhuetal.2005a).
Whileunderacidicconditions,thepositivechargeoftheTiO
2
surfaceincreasesasthe
pHdecreases(Equation(6.15)),abovepH6.25,thenegativechargeatthesurfaceofTiO
2
increases with increasing pH. Moreover, the pH of the solution affects the formation of
hydroxylradicalsbythereactionbetweenthehydroxideionsandthephoto-inducedholes
ontheTiO
2
surface(Ahmedetal.2011).Thepositiveholesareconsideredthemajoroxida-
tionstepsatlowpH,whereasthehydroxylradicalsareconsideredthepredominantspe-
ciesatneutralorhighpHlevels(ShifuandGengyu2005;Mathews1988).Thegeneration
of•OHishigherduetothepresenceofmoreavailablehydroxylionsontheTiO
2
surface.
ThedegradationeficiencyoftheprocessisenhancedathigherpH.
TiO
2
hasbeensuggestedtobeaneficientandviablephotocatalyticdegradationofcom-
poundsforvariouspesticides(Ahmedetal.2011).
6.5.1 Organochlorine Insecticides
Organochlorinepesticideshavebeenwidelyusedingrowthieldsinordertoeliminate
pestsandinthiswayincreasethecrops'outputs,improvethequalityoftheproducts,and
decrease the incidence of illness propagated by insects. Because of their toxic potential,
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