Insecticides and herbicides in soils 89
sample, which could be considered for the use in clay barriers aimed at protection of
soil and water pollution by hydrophobic pesticides.
3.9 MISCELLANEOUS INSECTICIDES AND HERBICIDES
Sorption-desorption and degradation studies
Understanding the sorption equilibria of microcontaminants in soils required the determination of the adsorbed analytes as well as those remaining in solution. On-line
filtration-plus-LC-type microextraction offers an efficient alternative to the timeconsuming classical procedures. Ramos et al [263] investigated the feasibility of
a simultaneous filtration-plus-liquid-chromatographic microextraction system with
subsequent GC-MS to study the partition equilibrium of pesticides in the interface
soil-water. The method allows the determination of the amount of pesticide adsorbed
in the soil and that remaining in solution by a single injection of the total slurry. As
an example, the adsorption equilibria of selected pesticides, ranging from relatively
polar triazines to nonpolar compounds such as hexachlorobenzene or Bromophosethyl, in an organic soil were studied. Once separated and preconcentrated, the filter
and the solid-phase cartridge fractions were independently dried by a stream of nitrogen, extracted with methyl acetate, and analysed by GC-MS. The standard deviations
for the total procedure were lower than 6.2% (soil) and 10% (solution). The soil-water
partition coefficients calculated for the selected compounds showed a good correlation with published octanol-water partition coefficient (r
2
= 0.973). This demonstrates
clearly the practicality of the proposed methodology for adsorption equilibrium
studies.
Syversen and Hearstad et al [264] carried out a laboratory study to examine the
retention processes of pesticides through vegetated buffer zones compared to bare soil.
Soil columns with low biological activity and vegetation columns with normal biological activity were tested. Pesticides frequently used in vegetable production (namely
Aclonifen, Azinphos-methyl, Chloropropham, Diazinon, Dimethoate, Fluazinam,
Iprodione, Linuron, Metalaxyl, Metamitron, Metribuzin and Propachlor) equal to
1/50 to 1/5 part of recommended doses, and nutrients equal to 1, 5 and 20 mg nitrogen L
−1 and 0.2 mg phosphorus L
−1 , were added. The pesticide retention was more
than 60% for all pesticides, except Dimethoate, with a retention of about 30% in
columns with low microbial activity. Biological transformation and plant uptake were
important for removal of nitrogen and organic matter. Nitrogen retention was high
(over 90%) in vegetation columns. Plant uptake and phosphorus content in soil were
important for phosphorus retention.
Ali et al [266, 267] have carried out detailed studies of the determination of
pyrethroids in soil. In one study [266] ultrasonic extraction was used to develop a
suitable solvent system for the analysis of synthetic pyrethroid pesticides and Mirex
on soil. The analysis was carried out by gas chromatography with negative ion chemical ionisation mass spectrometry. In the initial experiments, accurately weighed soil
samples were spiked with a mixture of standard solution pyrethroids and Mirex and
shaken for 24 hours to ensure homogeneity, then extracted with solvent. The extracts
were evaporated to dryness before the volumetric internal standard was added.
sample, which could be considered for the use in clay barriers aimed at protection of
soil and water pollution by hydrophobic pesticides.
3.9 MISCELLANEOUS INSECTICIDES AND HERBICIDES
Sorption-desorption and degradation studies
Understanding the sorption equilibria of microcontaminants in soils required the determination of the adsorbed analytes as well as those remaining in solution. On-line
filtration-plus-LC-type microextraction offers an efficient alternative to the timeconsuming classical procedures. Ramos et al [263] investigated the feasibility of
a simultaneous filtration-plus-liquid-chromatographic microextraction system with
subsequent GC-MS to study the partition equilibrium of pesticides in the interface
soil-water. The method allows the determination of the amount of pesticide adsorbed
in the soil and that remaining in solution by a single injection of the total slurry. As
an example, the adsorption equilibria of selected pesticides, ranging from relatively
polar triazines to nonpolar compounds such as hexachlorobenzene or Bromophosethyl, in an organic soil were studied. Once separated and preconcentrated, the filter
and the solid-phase cartridge fractions were independently dried by a stream of nitrogen, extracted with methyl acetate, and analysed by GC-MS. The standard deviations
for the total procedure were lower than 6.2% (soil) and 10% (solution). The soil-water
partition coefficients calculated for the selected compounds showed a good correlation with published octanol-water partition coefficient (r
2
= 0.973). This demonstrates
clearly the practicality of the proposed methodology for adsorption equilibrium
studies.
Syversen and Hearstad et al [264] carried out a laboratory study to examine the
retention processes of pesticides through vegetated buffer zones compared to bare soil.
Soil columns with low biological activity and vegetation columns with normal biological activity were tested. Pesticides frequently used in vegetable production (namely
Aclonifen, Azinphos-methyl, Chloropropham, Diazinon, Dimethoate, Fluazinam,
Iprodione, Linuron, Metalaxyl, Metamitron, Metribuzin and Propachlor) equal to
1/50 to 1/5 part of recommended doses, and nutrients equal to 1, 5 and 20 mg nitrogen L
−1 and 0.2 mg phosphorus L
−1 , were added. The pesticide retention was more
than 60% for all pesticides, except Dimethoate, with a retention of about 30% in
columns with low microbial activity. Biological transformation and plant uptake were
important for removal of nitrogen and organic matter. Nitrogen retention was high
(over 90%) in vegetation columns. Plant uptake and phosphorus content in soil were
important for phosphorus retention.
Ali et al [266, 267] have carried out detailed studies of the determination of
pyrethroids in soil. In one study [266] ultrasonic extraction was used to develop a
suitable solvent system for the analysis of synthetic pyrethroid pesticides and Mirex
on soil. The analysis was carried out by gas chromatography with negative ion chemical ionisation mass spectrometry. In the initial experiments, accurately weighed soil
samples were spiked with a mixture of standard solution pyrethroids and Mirex and
shaken for 24 hours to ensure homogeneity, then extracted with solvent. The extracts
were evaporated to dryness before the volumetric internal standard was added.