Organic compounds in soils 23
2.2.2 Carboxylic acids
Strobel et al [106] studied the distribution of oxalate in soils under rhubarb. Solid
samples were collected at depths of 0–2.5 and 2.5–5 cm from 10 cm sections along
100 cm transects from rhubarb plants at four locations in Denmark, and from seven
layers in a soil profile to 80 cm depth at one location. Oxalate was extracted from the
soil with 0.2 M phosphate at pH 2 by reciprocal shaking for 23 h and then determined
by fast capillary zone electrophoresis method with 300 mM KH 2 PO 4 and 0.30 mM
TTAB electrolyte adjusted to PH 7, developed and tested to analyse high-ionic-strength.
Rhubarb increases the oxalate content in soil under the leaves slightly. The average
content of oxalate in the upper 0–5 cm soil was 444 µmol/kg at the Kaldred site,
and 111–333 µmol/kg at the three other locations. In the soil profile, the content of
oxalate decreased from 500 µmol/kg in 0–5 cm depth to 110 µmol/kg at 75–80 cm
depth. No significant seasonal changes in oxalate contents were observed, while an
annual variation of 100 µmol/kg could be observed at 0–24 cm depth. During plant
decay in autumn, a slight increase in oxalate content was observed at 30 cm soil depth.
In conclusion, the role of oxalate in weathering and metal transport appears to be
limited in soils under rhubarb.
2.2.3 Phenols
Phenols in soil come from natural sources (e.g. degradation of lignin and humic acids
present in soils) and anthropogenic sources (e.g. degradation of pesticides, herbicides
and fungicides containing phenols; direct emissions from chemical factories). Phenols
in the environment (especially in soils) are usually present as pollutants. Because of
their toxicological potential, some have been included on lists of priority pollutants
from the European Community (EC) and the US Environmental Protection Agency
(EPA) [107].
Traditional Soxhlet extraction is gradually being superseded by new alternative
approaches, including supercritical carbon dioxide [108–110] and supercritical water
extraction [111] accelerated solvent extraction [112, 113] microwave-assisted extraction [91, 112], and solid-liquid extraction [115, 116]. Supercritical fluid extraction is
increasingly being used to extract organic pollutants from environmental solids. However, obtaining adequate supercritical fluid recoveries of phenols entails using a high
temperature [114], an organic modifier to increase the polarity of the carbon dioxide,
or supercritical water as extractant.
To overcome some of the difficulties, Crespin et al [117] developed a semiautomatic module for the direct continuous extraction and preconcentration of phenols
from soils. The extraction fluid flows were controlled by multiport rotary valves and
were driven by peristaltic pumps and by compressed nitrogen. The interface plays a
crucial role in changing the pH and homogenising the sample before preconcentration;
in addition the sample volume was controlled at the interface. Spiked uncontaminated
soils were prepared two months before treatment in order to simulate weathering
and allow for the occurrence of analyte-matrix interactions. Soil extractions were
done with alkaline aqueous solutions and solid-sorbent preconcentration in an acid
medium, using XAD-2 as sorbent. Soil samples (0.1–10 g) containing 50–5000 ng/g
phenols were analysed by gas chromatography with a high precision (4–7%). Average
2.2.2 Carboxylic acids
Strobel et al [106] studied the distribution of oxalate in soils under rhubarb. Solid
samples were collected at depths of 0–2.5 and 2.5–5 cm from 10 cm sections along
100 cm transects from rhubarb plants at four locations in Denmark, and from seven
layers in a soil profile to 80 cm depth at one location. Oxalate was extracted from the
soil with 0.2 M phosphate at pH 2 by reciprocal shaking for 23 h and then determined
by fast capillary zone electrophoresis method with 300 mM KH 2 PO 4 and 0.30 mM
TTAB electrolyte adjusted to PH 7, developed and tested to analyse high-ionic-strength.
Rhubarb increases the oxalate content in soil under the leaves slightly. The average
content of oxalate in the upper 0–5 cm soil was 444 µmol/kg at the Kaldred site,
and 111–333 µmol/kg at the three other locations. In the soil profile, the content of
oxalate decreased from 500 µmol/kg in 0–5 cm depth to 110 µmol/kg at 75–80 cm
depth. No significant seasonal changes in oxalate contents were observed, while an
annual variation of 100 µmol/kg could be observed at 0–24 cm depth. During plant
decay in autumn, a slight increase in oxalate content was observed at 30 cm soil depth.
In conclusion, the role of oxalate in weathering and metal transport appears to be
limited in soils under rhubarb.
2.2.3 Phenols
Phenols in soil come from natural sources (e.g. degradation of lignin and humic acids
present in soils) and anthropogenic sources (e.g. degradation of pesticides, herbicides
and fungicides containing phenols; direct emissions from chemical factories). Phenols
in the environment (especially in soils) are usually present as pollutants. Because of
their toxicological potential, some have been included on lists of priority pollutants
from the European Community (EC) and the US Environmental Protection Agency
(EPA) [107].
Traditional Soxhlet extraction is gradually being superseded by new alternative
approaches, including supercritical carbon dioxide [108–110] and supercritical water
extraction [111] accelerated solvent extraction [112, 113] microwave-assisted extraction [91, 112], and solid-liquid extraction [115, 116]. Supercritical fluid extraction is
increasingly being used to extract organic pollutants from environmental solids. However, obtaining adequate supercritical fluid recoveries of phenols entails using a high
temperature [114], an organic modifier to increase the polarity of the carbon dioxide,
or supercritical water as extractant.
To overcome some of the difficulties, Crespin et al [117] developed a semiautomatic module for the direct continuous extraction and preconcentration of phenols
from soils. The extraction fluid flows were controlled by multiport rotary valves and
were driven by peristaltic pumps and by compressed nitrogen. The interface plays a
crucial role in changing the pH and homogenising the sample before preconcentration;
in addition the sample volume was controlled at the interface. Spiked uncontaminated
soils were prepared two months before treatment in order to simulate weathering
and allow for the occurrence of analyte-matrix interactions. Soil extractions were
done with alkaline aqueous solutions and solid-sorbent preconcentration in an acid
medium, using XAD-2 as sorbent. Soil samples (0.1–10 g) containing 50–5000 ng/g
phenols were analysed by gas chromatography with a high precision (4–7%). Average