Insecticides and herbicides in soils 85
Tetrachlorovinphos (trans,-2-chloro-1-(2,4,5 trichlorophenyl) vinyl (chlorophenyl-Omethylphenyl phosphorothioate) and Methiadathion. Supercritical fluid extraction
with methanol modified carbon dioxide has been applied to the determination of
organophosphorus insecticides in soil.
Islam et al [308] studied the adsorption and desorption of Bromophos methyl
[O,O-dimethyl-O-2,5-dichloro-4 bromophenyl) phosphorothioate] and Quinalphos
[O,O-diethyl O-2-quiunoxalinyl phosphorothioate] on five soils from difference Greek
locations with varying physical and chemical properties. They used a batch equilibration method. Adsorption isotherms fitted well to the Freundlich equation. The
adsorption capacity of the soils for Bromophos methyl was higher than Quinalphos.
Freundlich adsorption coefficient (K fads ) showed significant correlation with soil
organic matter, (see Figure 3.7) indicating that soil organic matter content was the
main controlling factor for the adsorption of these pesticides on the soils. Desorption
isotherms also conformed well to the Freundlich equation and there was a hysteresis
effect in all the soils. The Freundlich K fdes values were also higher for the soils with
the highest organic matter per cent.
Adsorption and desorption studies of Bromophos methyl and Quinalphos indicate that selected soils adsorb high amount of these organophosphorus pesticides. The
isotherms fitted to the Freundlich equation well. Soul organic matter content is a major
parameter that affects mainly the adsorption behaviour of these studied organophosphorus pesticides. Bromophos methyl showed maximum adsorption and minimum
desorption. This suggests that Bromophos methyl is a relatively safer pesticide than
Quinalphos. According to the above results, in the natural environment, soils with significant organic matter content are expected to immobilise these pesticides, preventing
their movement to the water table.
Gas chromatography
Kjolholt et al [167] determined trace amounts of organophosphorus pesticides and
related compounds in soil using capillary gas chromatography and a nitrogen specific
detector. Homogenised samples were subjected to Soxhlet extraction with acetone-nhexane. The extract was partitioned between methylene chloride and water, subjected
to adsorption chromatography and analysed by gas chromatography. The influences
of freeze drying and of the pH on extraction efficiency were studied. Interference
by elemental sulphur was examined. Recoveries at the 10 µg kg
−1 level were 54.6–
82.4% and detection limits 95–220 g kg
−1 depending on the type of organophosphorus
compound.
Trichlorophon has been determined [168] in acid soil by solvent extraction followed by gas chromatography on a glass column (180 cm × 9 mm) packed with 16%
of XF-1150 on Chromosorb W-AW operated at 125
◦ C with a carrier gas flow of
60 ml m
−1 and a flame photometric detector operated in the phosphorus mode. Average
recoveries were 96% and down to 50 µg kg
−1 of Trichlorophon could be determined
in soil.
Fenophos (O-ethyl-S-phenylethyl-phosphorodithioate) insecticide has been determined in soil by gas chromatography. Fenophos is known to degrade to its oxygen
analogue (O-ethyl-S-phenylethyl phosphonothioate) in soil but none was found in the
soil samples examined [169].
Tetrachlorovinphos (trans,-2-chloro-1-(2,4,5 trichlorophenyl) vinyl (chlorophenyl-Omethylphenyl phosphorothioate) and Methiadathion. Supercritical fluid extraction
with methanol modified carbon dioxide has been applied to the determination of
organophosphorus insecticides in soil.
Islam et al [308] studied the adsorption and desorption of Bromophos methyl
[O,O-dimethyl-O-2,5-dichloro-4 bromophenyl) phosphorothioate] and Quinalphos
[O,O-diethyl O-2-quiunoxalinyl phosphorothioate] on five soils from difference Greek
locations with varying physical and chemical properties. They used a batch equilibration method. Adsorption isotherms fitted well to the Freundlich equation. The
adsorption capacity of the soils for Bromophos methyl was higher than Quinalphos.
Freundlich adsorption coefficient (K fads ) showed significant correlation with soil
organic matter, (see Figure 3.7) indicating that soil organic matter content was the
main controlling factor for the adsorption of these pesticides on the soils. Desorption
isotherms also conformed well to the Freundlich equation and there was a hysteresis
effect in all the soils. The Freundlich K fdes values were also higher for the soils with
the highest organic matter per cent.
Adsorption and desorption studies of Bromophos methyl and Quinalphos indicate that selected soils adsorb high amount of these organophosphorus pesticides. The
isotherms fitted to the Freundlich equation well. Soul organic matter content is a major
parameter that affects mainly the adsorption behaviour of these studied organophosphorus pesticides. Bromophos methyl showed maximum adsorption and minimum
desorption. This suggests that Bromophos methyl is a relatively safer pesticide than
Quinalphos. According to the above results, in the natural environment, soils with significant organic matter content are expected to immobilise these pesticides, preventing
their movement to the water table.
Gas chromatography
Kjolholt et al [167] determined trace amounts of organophosphorus pesticides and
related compounds in soil using capillary gas chromatography and a nitrogen specific
detector. Homogenised samples were subjected to Soxhlet extraction with acetone-nhexane. The extract was partitioned between methylene chloride and water, subjected
to adsorption chromatography and analysed by gas chromatography. The influences
of freeze drying and of the pH on extraction efficiency were studied. Interference
by elemental sulphur was examined. Recoveries at the 10 µg kg
−1 level were 54.6–
82.4% and detection limits 95–220 g kg
−1 depending on the type of organophosphorus
compound.
Trichlorophon has been determined [168] in acid soil by solvent extraction followed by gas chromatography on a glass column (180 cm × 9 mm) packed with 16%
of XF-1150 on Chromosorb W-AW operated at 125
◦ C with a carrier gas flow of
60 ml m
−1 and a flame photometric detector operated in the phosphorus mode. Average
recoveries were 96% and down to 50 µg kg
−1 of Trichlorophon could be determined
in soil.
Fenophos (O-ethyl-S-phenylethyl-phosphorodithioate) insecticide has been determined in soil by gas chromatography. Fenophos is known to degrade to its oxygen
analogue (O-ethyl-S-phenylethyl phosphonothioate) in soil but none was found in the
soil samples examined [169].