3.3 Sorption and Sedimentation
Sorption of a chemical to soil or sediment results from physical and/or chemical
adhesion of molecules to the surfaces of solids, or from dissolved molecules
partitioning between the aqueous phase and soil organic matter [73]. Since
suspended particles from wastewater are retained in a CW bed, sorption of dissolved
organic contaminants on soil, organic carbon, mineral surfaces and biofilms coating
the substrates can be a significant mechanism for their removal [50].
Modelling sorption of organic compounds in sediments, soils and biosolids
typically relies on the sorption coefficient (K d ). According to [81], a pharmaceutical
with K d < 500 L kg
À1 or Log K d < 2.7 implies poor capacity for sorption onto
solids. Moreover, the organic carbon partition coefficient (K oc ), i.e. the ratio of
contaminant mass adsorbed per unit weight of organic carbon in the soil to the
concentration in solution, is also a commonly used proxy to estimate sorption of
pharmaceuticals [50]. Nevertheless, pharmaceuticals can also sorb to the inorganic
component of the substrate, convoluting predictions of the total sorption in CWs.
The octanol-water partition coefficient (Log K ow ) has been adopted as the standard
measure of hydrophobicity of a chemical compound [50]. Sorption may occur due to
hydrophobic interactions of the aliphatic and aromatic groups of an organic compound with the lipophilic cell membrane of the microorganisms, or the lipid fractions
of the suspended solids [82]. Hydrophobic compounds can be easily adsorbed onto
the organic matter coming with the wastewater and built up within the CW substrate
due to filtration and sedimentation. However, pharmaceuticals can also sorb due to
electrostatic interactions with the organic matter located on the surface of soil and/or
sediment that exhibit a negative charge because of the functional groups present
(i.e. carboxylic and phenolic groups). Thus, the electrostatic interaction with positively charged pharmaceuticals provides a suitable removal mechanism for the latest
[62]. While neutral molecules partition to solid phases via relatively weak van der
Waals and electron donor-acceptor interactions, charged species can interact with
charged sorbents (e.g. organic matter, clays, metal oxides and oxyhydroxides)
through stronger electrostatic mechanisms, such as cation-exchange, cation-bridging
and complexation [50]. The most common example are antibiotics, moderately water
soluble and ionizable, that exist as either neutral or charged species depending on pH
conditions, for which ionic interactions are possible sorption mechanisms. This
stresses that not only hydrophobicity but other compound’s properties
(e.g. chemical structure, water solubility, acid/base properties, etc.) and substrate
characteristics (e.g. composition of organic matter, redox potential, temperature, pH,
ionic strength, cations, anions, etc.) govern pharmaceuticals removal by sorption. It
should be stressed that many of the methodologies and relationships suggested for
determining sorption capacity have been derived from studies with neutral or
hydrophobic compounds, namely, classical organic pollutants such as PAHs or
PCBs. The fact that pharmaceuticals are often ionized at typical wastewater pH
adds to the complexity of predicting their behaviour.
Constructed Wetlands and Phytoremediation as a Tool for Pharmaceutical Removal
399
Sorption of a chemical to soil or sediment results from physical and/or chemical
adhesion of molecules to the surfaces of solids, or from dissolved molecules
partitioning between the aqueous phase and soil organic matter [73]. Since
suspended particles from wastewater are retained in a CW bed, sorption of dissolved
organic contaminants on soil, organic carbon, mineral surfaces and biofilms coating
the substrates can be a significant mechanism for their removal [50].
Modelling sorption of organic compounds in sediments, soils and biosolids
typically relies on the sorption coefficient (K d ). According to [81], a pharmaceutical
with K d < 500 L kg
À1 or Log K d < 2.7 implies poor capacity for sorption onto
solids. Moreover, the organic carbon partition coefficient (K oc ), i.e. the ratio of
contaminant mass adsorbed per unit weight of organic carbon in the soil to the
concentration in solution, is also a commonly used proxy to estimate sorption of
pharmaceuticals [50]. Nevertheless, pharmaceuticals can also sorb to the inorganic
component of the substrate, convoluting predictions of the total sorption in CWs.
The octanol-water partition coefficient (Log K ow ) has been adopted as the standard
measure of hydrophobicity of a chemical compound [50]. Sorption may occur due to
hydrophobic interactions of the aliphatic and aromatic groups of an organic compound with the lipophilic cell membrane of the microorganisms, or the lipid fractions
of the suspended solids [82]. Hydrophobic compounds can be easily adsorbed onto
the organic matter coming with the wastewater and built up within the CW substrate
due to filtration and sedimentation. However, pharmaceuticals can also sorb due to
electrostatic interactions with the organic matter located on the surface of soil and/or
sediment that exhibit a negative charge because of the functional groups present
(i.e. carboxylic and phenolic groups). Thus, the electrostatic interaction with positively charged pharmaceuticals provides a suitable removal mechanism for the latest
[62]. While neutral molecules partition to solid phases via relatively weak van der
Waals and electron donor-acceptor interactions, charged species can interact with
charged sorbents (e.g. organic matter, clays, metal oxides and oxyhydroxides)
through stronger electrostatic mechanisms, such as cation-exchange, cation-bridging
and complexation [50]. The most common example are antibiotics, moderately water
soluble and ionizable, that exist as either neutral or charged species depending on pH
conditions, for which ionic interactions are possible sorption mechanisms. This
stresses that not only hydrophobicity but other compound’s properties
(e.g. chemical structure, water solubility, acid/base properties, etc.) and substrate
characteristics (e.g. composition of organic matter, redox potential, temperature, pH,
ionic strength, cations, anions, etc.) govern pharmaceuticals removal by sorption. It
should be stressed that many of the methodologies and relationships suggested for
determining sorption capacity have been derived from studies with neutral or
hydrophobic compounds, namely, classical organic pollutants such as PAHs or
PCBs. The fact that pharmaceuticals are often ionized at typical wastewater pH
adds to the complexity of predicting their behaviour.
Constructed Wetlands and Phytoremediation as a Tool for Pharmaceutical Removal
399