at acid conditions and neutral at pH ! 7. The highest sorption affinity for this
antibiotic was obtained in the range of pH 4 to 6. These results were similar to
those reported in Bekçi et al. [67]. The pH can also influence the pH-dependent
charge on organic matter and clay minerals which can have an effect on pharmaceutical sorption [68].
2.2.5 Cation Exchange Capacity (CEC)
Clay cation exchange charge (CEC) depends on the type of clay. The sorption of
nonionic (neutral) PhACs is mainly driven by hydrophobic partitioning to the soil
organic matter via van der Waals and electron donor-acceptor interactions and by
hydrogen bonding with hydroxyl groups on the solid surfaces. Sorption of neutral
compounds is therefore highly dependent on the soil organic matter content. In
contrast, the mechanisms and effects of ionization on the behavior of PhACs in soil
are less known and remain inconclusive. The sorption of cationic molecules is
mainly governed by the attraction to negative charges of the solid surface (e.g., a
clay mineral surface or organic matter). In an attempt to predict persistence and
mobility of ionizable PhACs, the determination of the cation exchange capacity
(CEC) of soils, which is a measure of the negatively charged site on the soil surface,
has been suggested. Clay may exhibit an anion exchange capacity (AEC) due to the
protonation of the surface hydroxyl groups. The ratio AEC/CEC gives an indication
about the difference between the soil pore water pH and the pH of the net zero charge
of the clay, as reported by Hyun and Lee [69]. The cation exchange charge of the
clay has been reported to be important for the sorption of some antibiotics such as
fluoroquinolones [65].
The larger the CEC value, the more important the sorption of the positively
charged PhACs can be [70]. However, sorption of organic compounds and CEC
evaluated on disintegrated soils (i.e., in soil slurry) could be greater than that
measured on soil aggregates. The main reason is that a part of the sorption capacity
of the soil components is not available due to their interactions with aggregated soil
and acidic environments.
2.2.6 Soil Organic Matter
Soil organic matter is made up of particulate organic matter (POM) and, in pore
water, colloidal dissolved organic matter (CDOM). In soils, organic matter (OM) is
the most important sorbent for hydrophobic organic pollutants due to their colloidal
properties that can increase the solubility and contribute to reduce their sorption to
solid matrixes in soil and sediments. The mobility and the bioavailability of organic
compounds may be influenced by the interactions with dissolved organic matter
[71]. Leenheer [72] reported that DOM can be fractionated based on the
hydrophobic-hydrophilic characteristics of its materials, and it was demonstrated
that this fractionation scheme has provided important information on the interaction
Soil Sorption and Degradation Studies of Pharmaceutical Compounds Present in. . .
159
antibiotic was obtained in the range of pH 4 to 6. These results were similar to
those reported in Bekçi et al. [67]. The pH can also influence the pH-dependent
charge on organic matter and clay minerals which can have an effect on pharmaceutical sorption [68].
2.2.5 Cation Exchange Capacity (CEC)
Clay cation exchange charge (CEC) depends on the type of clay. The sorption of
nonionic (neutral) PhACs is mainly driven by hydrophobic partitioning to the soil
organic matter via van der Waals and electron donor-acceptor interactions and by
hydrogen bonding with hydroxyl groups on the solid surfaces. Sorption of neutral
compounds is therefore highly dependent on the soil organic matter content. In
contrast, the mechanisms and effects of ionization on the behavior of PhACs in soil
are less known and remain inconclusive. The sorption of cationic molecules is
mainly governed by the attraction to negative charges of the solid surface (e.g., a
clay mineral surface or organic matter). In an attempt to predict persistence and
mobility of ionizable PhACs, the determination of the cation exchange capacity
(CEC) of soils, which is a measure of the negatively charged site on the soil surface,
has been suggested. Clay may exhibit an anion exchange capacity (AEC) due to the
protonation of the surface hydroxyl groups. The ratio AEC/CEC gives an indication
about the difference between the soil pore water pH and the pH of the net zero charge
of the clay, as reported by Hyun and Lee [69]. The cation exchange charge of the
clay has been reported to be important for the sorption of some antibiotics such as
fluoroquinolones [65].
The larger the CEC value, the more important the sorption of the positively
charged PhACs can be [70]. However, sorption of organic compounds and CEC
evaluated on disintegrated soils (i.e., in soil slurry) could be greater than that
measured on soil aggregates. The main reason is that a part of the sorption capacity
of the soil components is not available due to their interactions with aggregated soil
and acidic environments.
2.2.6 Soil Organic Matter
Soil organic matter is made up of particulate organic matter (POM) and, in pore
water, colloidal dissolved organic matter (CDOM). In soils, organic matter (OM) is
the most important sorbent for hydrophobic organic pollutants due to their colloidal
properties that can increase the solubility and contribute to reduce their sorption to
solid matrixes in soil and sediments. The mobility and the bioavailability of organic
compounds may be influenced by the interactions with dissolved organic matter
[71]. Leenheer [72] reported that DOM can be fractionated based on the
hydrophobic-hydrophilic characteristics of its materials, and it was demonstrated
that this fractionation scheme has provided important information on the interaction
Soil Sorption and Degradation Studies of Pharmaceutical Compounds Present in. . .
159