Pesticide Residues in the Environment - Pesticides: Evaluation of Environmental Pollution - page 162

Theanalyticalresultsofp,pʹ-DDTresiduesinthesoilcolumn,obtainedbyNoegrohati
etal.(2008),ontheonehand,showedthepresenceofp,pʹ-DDEandp,pʹ-DDD,whileonthe
other hand, showed no degradation products in the soil column treated with p,pʹ-DDE.
Thesedataindicatethatp,pʹ-DDDdetectedinthesoilcolumntreatedwithp,pʹ-DDTwas
nottheconversionproductofp,pʹ-DDE.Therefore,itisreasonabletoassumethatinthis
typeofsoilenvironment,aparallelmodeofreactiongovernstheconversionofp,pʹ-DDT
intop,pʹ-DDEandp,pʹ-DDD.Thefollowingmodel(Equation6.3)wasappliedforthereac-
tions(Noegrohatietal.2008).

k conversion,1

p,p'-DDE

p,p'-DDT

(6.3)

k conversion,2

p,p'-DDD

Thechangeintheconcentrationofp,pʹ-DDTisgivenbyNoegrohatietal.(2008)(Equation6.4).

[ ] =

d p,p DDT
dt

′ −

k p,p DDT k p,p DDT k p ,p DDT

′ − + ′ − = ′ − 0 (6.4)

conv,1 conv,2

0 0

wherekisthedegradationrateconstantofp,pʹ-DDTandk=k conv,1 +k conv,2 .
Thechangeinconcentrationforp,pʹ-DDEorp,pʹ-DDDcanbedescribedinEquations6.5
and6.6(Noegrohatietal.2008):

d p,o DDE
dt

′ − =

(6.5)

k p,p DDT e

− kt

′ −

conv,1

0

d p,p DDD
dt

′ − =

k p,p DDT e

− kt

′ −

(6.6)

conv,2

0

TheresultsobtainedbyNoegrohatietal.(2008)showedthatinwetconditions,thecon-
versionfromp,pʹ-DDTtop,pʹ-DDEfollowsairst-orderkineticsmodel,withanr 2 valueof
0.7039,givingarateconstant,k conv,1 ,of0.0140/week.Similarly,theconversionfromp,pʹ-DDT
top,pʹ-DDDalsofollowsairst-orderkineticsmodelwithanr 2 valueof0.5024,givingarate
constant,k conv,2 ,of0.0128/week.Thetotalofthetwoconstants,k conv,1 +k conv,2 =0.0268,that
is,thedegradationrateconstantforp,pʹ-DDTduringwetconditionsdemonstratesthat,
inwetconditions,theconversionfromp,pʹ-DDTintop,pʹ-DDEandp,pʹ-DDDisgovernedby
aparallelreaction(Noegrohatietal.2008).
Indryconditions,theconversionfromp,pʹ-DDTtop,pʹ-DDEfollowsairst-orderkinetics
model,withanr 2 valueof0.7170,givingarateconstant,k conv,1 ,of0.0016/week.Thecalcu-
latedrateconstantfortheconversionfromp,pʹ-DDTtop,pʹ-DDDwasnegative,indicating
that no conversion from p,pʹ-DDT to p,pʹ-DDD occurred in dry conditions (Noegrohati
et al. 2008). The data obtained by Noegrohati et al. (2008) support the observations of
Tolosa et al. (1995) and Lee et al. (2001). The main metabolite of DDT in dry conditions,
whicharemoreaerobicthanwetconditions,isDDE,whereasthemainmetaboliteunder