Environmental Engineering Reference
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carbaryl-degradingcultureshowedthatthebacteriawerecapableofdegradingonlythe
available(i.e.,water-extractable)fractionofthepesticide.Whenthesoilwaspulverizedin
aballmilltoenhancethereleaseoftheresidue,anadditional19%ofthecarbarylbecame
bioavailable. A signiicant proportion of the residue (~33%) remained unavailable. The
long(>12years)contacttimebetweenthepesticideandthesoil(i.e.,aging),allowingpos-
siblesequestrationintothesoilnanoporesandtheorganicmattermatrices,issuggested
tohaverenderedthepesticideunavailableformicrobialdegradation(Ahmadetal.2004).
Results obtained by the authors (Ahmad et al. 2004) demonstrated that even a weakly
sorbed and easily degradable pesticide, carbaryl, is effectively sequestrated in soil with
time,renderingitpartlyinaccessibletomicroorganismsandaffectingthebioavailability
ofthecompound.Dissipationof
14
Ccarbarylandquinalphosinthesoilunderaground-
nutcrop(
Arachis hypogaea
L.)wasalsodescribed(MenonandGopal2003).Themovement
ofcarbarylwaslimitedtoa15cmdepthintheloamysandofJaipur(India)andwasnot
detectedtill120days(DT
50
of14.93days)afterapplication(MenonandGopal2003).
6.4.4 Herbicides
6.4.4.1 Chlorotriazine Herbicides
Triazinesaretheoldestandthemostcommonlyusedherbicides,representingaround30%
ofthepesticidemarketintheworld.Thesecompoundshaveanappreciablepersistencein
soil(
Table6.4
).Thehalf-lifeinsoilindaysistheshortestforcyanazine(ca.14days)andthe
longest for simazine (27-102 days). Atrazine, a chlorotriazine herbicide, is widely used to
controlbroadleafandgrassyweedsincorn,sorghum,sugarcane,andrangeland;ithasbeen
detectedingroundwaterandsoil.Thereportedlevelsofatrazineinsoilsatpesticidemix-
loadsitescanvarybetween7.9×10
−5
mMand1.9mM(mmole/kgsoil)(Krapacetal.1993).
Griggetal.(1997)reportedonamixedmicrobialculture,capableofdegradingconcentra-
tionsofatrazineinexcessof1.9mM.Atinitialconcentrationsof0.046and0.23M,themixed
populationdegraded78%and21%ofatrazineinsoil(100days),respectively.Atthesame
initialconcentrationsinliquidcultures,90%and56%ofatrazinewasdegraded(80days),
respectively.Decreaseddegradationinsoilsamplesmayhaveresultedfromatrazinesorp-
tiontosoilsurfacesordecreasedcontactbetweenthepopulationandtheherbicide(Grigg
etal.1997).Inthe0.23Msystem,Griggetal.(1997)attributeincompletedegradationtophos-
phorusdepletion.Dataobtainedbytheseauthorsforcarbondioxideevolutionwasittedto
athree-half-orderregressionmodel,buttheyfeelthattherearelimitationsofapplicationof
thismodeltoatrazinedegradation.Thepopulationusestheherbicideasanitrogensource
andlittlecarbonisincorporatedintothebiomass,astheenergystatusofcarbonsinthering
leadstotheirdirectevolutionas[
14
C]carbondioxide.Griggetal.(1997)suggestthatthemixed
culturecouldbeusedforbioremediationofatrazineatconcentrationsuptoandexceeding
thosecurrentlyreportedforagrochemicalmixing-loadingfacilities.Microoganisms,includ-
ing
Pseudomonas
and
Actinomycetes
species,areknowntodegradeatrazineandotherpesti-
cidesinsoil.TheobjectiveoftheresearchbyGuptaandBaummerIII(1996)wastostudythe
degradationofatrazine(2or3ppm)insoilusingpoultrylitter,whichhasalargenumberof
microorganismsalongwithmanynutrients.Atrazineinthesoilwasextractedwithwater
andmethanolandanalyzedbypesticideimmunoassay(ELISA)1,5,10,30,and60daysafter
poultry litter treatment. Atrazine was signiicantly (86%) degraded in soil with untreated
poultrylitterwithin30days.Degradationwasvirtuallycompletewithin60days.Therate
ofatrazinebiodegradationwithpoultrylitterwasalmosttwotimesfasterthanthatwithout
thelitter(GuptaandBaummerIII1996).Thetoxicityofthesoil+atrazinemixturetreated
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