Pesticide Residues in the Environment - Pesticides: Evaluation of Environmental Pollution - page 186

theirpersistence,andtheirtendencytobioconcentrate,thewidespreaduseofpesticidesin
theenvironmentgeneratesincreasingriskstohumanhealth(e.g.,canceranddisruptionof
hormonalfunctions).Therefore,controloverthesecompoundsinwater,plants,foodstuffs,
andsoilsisnecessary.
The results obtained by Kulovaara et al. (1995) show that the sunlight-induced pho-
tochemical degradation rate of DDT is higher in water with high DOM contents, which
means that the effects on the bioavailability will be further accentuated. Natural DOM
isacomplexmixtureandthelight-initiatedreactionsarealsocomplexinhumicsurface
waters.DDTwasdegradedbothbytheUV 254 radiationandtheampliiedartiicialsolar
radiation(Kulovaaraetal.1995).DDTwasdegradedfasterduringUVirradiations(min-
utes)thanduringthesimulatedsunlightirradiations(hours).Differentreactionpathways
areinvolvedinthetwoexperiments(Kulovaaraetal.1995).
6.5.2 Organophosphate Insecticides
Organophosphorus(OP)pesticideshavebeenwidelyusedasanalternativetoorganochlo-
rinecompoundsforthecontrolofinsectsinawiderangeoffruits,vegetables,andgrains
allovertheworld.OrganophosphoruspesticidesinwaterareeasilydegradedbyUVlight
and immobilized by the TiO 2 system. The photocatalyticdegradation of the three pesti-
cides,acephate(phosphoramidothioateinsecticide),dimethoate(aliphaticamideorgano-
thiophosphate insecticide), and glyphosate (organophosphorus herbicide), in water has
beeninvestigatedusingUVlightandTiO 2 immobilizedonsilicagelasphotocatalystand
describedbyEchaviaetal.(2009).TheresultsobtainedbyEchaviaetal.(2009)showthat
pesticides can be eficiently degraded by the UV-TiO 2 system. Complete (100%) decom-
position of dimethoate and glyphosatewas attained within 60 min of irradiation, while
totaldegradationofacephateoccurredafter105minofphotocatalytictreatment(Echavia
etal.2009).Decompositionofacephateanddimethoatewasfoundtobephotocatalyticin
nature, while glyphosate degradation appeared to be governed by both adsorption and
photocatalyticreactions.AstheresultsofthestudybyEchaviaetal.(2009)demonstrate,
adsorption of organic substrates on the surface of TiO 2 is not a precondition for effec-
tive photocatalysis to proceed since both weakly and strongly adsorbed pesticides were
eficientlydegradedbythesystem.Resultsobtainedbytheauthors(Echaviaetal.2009)
indicatethatthe120mintreatmenttimeemployedwasnotsuficienttocompletelyminer-
alizetheparentpesticidemoleculesandtoxicintermediatessuchasmethamidophosand
methoatewerenotdetected,revealinggoodadvantageoftheUVandimmobilizedTiO 2
systemoverothertreatmentprocesses.Thephotocatalyticdegradationpathwayforeach
pesticidewastentativelyproposedbyEchaviaetal.(2009),basedonthereductionofTOC
andtheformationofmajorby-productsduringthecourseofUV-TiO 2 photocatalysis,as
shownin Figures6.10 through6.12.
Thephotocatalyticdegradationofasmallconcentrationofanorganophosphorusinsec-
ticide,phosphamidon,inwateronZnOandTiO 2 wasdescribedbyRabindranathanetal.
(2003).InapaperpublishedbyShifuandGengyu(2005),theloatingTiO 2- SiO 2 photocata-
lystbeadsbythedip-coatingmethodwasapplied;hollowglassmicrobeadswereusedas
thecarrierandtitaniumtetraisopropoxide[Ti(iso-OC 3 H 7 ) 4 ]andethylsilicatewereusedas
therawmaterials.Theresultsobtainedbytheauthors(ShifuandGengyu2005)indicate
that the best heat treatment condition for TiO 2- SiO 2 beads is at 650°C for 5 h. After 420
min illumination by sunlight, 0.65×10 −4 mol/dm 3 of four organophosphorus pesticides
of three structures can be completely photocatalytically degraded into PO 3− . The addi-
tionofasmallamountofCu 2+ (upto0.05mmol/dm 3 )increasesthephotodegradationof