much lower retention time [32]. Whereas plants significantly affect the removal of
pollutants in HSSF with long hydraulic retention times, their role is minor in
pollutant removal in periodically loaded VF [76].
Photodegradation is only a relevant process in SF, but not for subsurface systems
(HSSF and VF), where water is kept below the surface of the substrate. Moreover,
photodegradation in SF will be dependent on the depth of the bed and type/density of
vegetation used. It should be noted that SF can have plants rooted in the bottom that
can be emergent, submerged, or floating plants, as well as free-floating plants. In
addition, the biofilm system responsible for the transformation of the pollutants is
distinct between SF and subsurface systems. In SF, the substrate layer is reduced,
and water flows on top of it, thus the majority of the biodegradation processes occur
in the water column by action of the biofilm and periphyton [78] attached to the plant
surfaces (e.g. leaves, stems, roots in case of free-floating plants) [79]. For the
subsurface systems (HSSF and VF), biofilm is attached to the substrate, usually
sand or gravel as in classical sand filters and other type of biofilters, but in CWs the
plant have a critical role shaping the microbial community of the biofilm [80].
Removal processes are also dependent on physicochemical properties of the
pollutants, most notably evaporation, sorption and consequently sedimentation
(when compounds are sorbed to particles/solids that are trapped by sedimentation).
Different organic chemicals can show specific characteristic for a wide range of
physicochemical properties such as water solubility, vapour pressure, octanol-water
partition coefficient (Log K ow ), organic carbon partition coefficient (K oc ), Henry’s
law constant [73], acid dissociation constant (pKa) and the partition coefficient (Log
K d ) [27]. The relationship between these physicochemical characteristics and the fate
of pharmaceuticals in CWs is integrated in the next mechanisms description.
From the main CWs mechanisms, volatilization and phytovolatilization of pharmaceuticals can be excluded due to their low Henry’s law constant values; pharmaceuticals are not volatile at the normal environmental temperatures. Therefore,
pharmaceuticals are removed in CWs by the combination of (1) sorption and
sedimentation; (2) plant uptake combined with translocation and further
phytodegradation and microbial degradation within the plant; and (3) microbial
degradation in the water-substrate interface (biofilm and/or periphyton) (Fig. 2). In
the following paragraphs, each of the relevant processes are detailed. It should be
denoted that sorption and sedimentation, as well as plant uptake and
phytoaccumulation are non-destructive processes. They contribute for reducing the
concentration of the contaminants simply by relocating them, which might imply
that the contaminant can become available in the future (e.g. plants die off) or pose as
hazardous (e.g. disposable of substrate by the end-life of the treatment system)
[73]. The other processes imply the degradation of the compounds, also the
photodegradation in the SF systems. Pharmaceuticals degradation rarely occurs by
mineralization, thus a high variety of transformation products are formed, not only
by photodegradation [27] but also in biological systems [65].
Constructed Wetlands and Phytoremediation as a Tool for Pharmaceutical Removal
397
pollutants in HSSF with long hydraulic retention times, their role is minor in
pollutant removal in periodically loaded VF [76].
Photodegradation is only a relevant process in SF, but not for subsurface systems
(HSSF and VF), where water is kept below the surface of the substrate. Moreover,
photodegradation in SF will be dependent on the depth of the bed and type/density of
vegetation used. It should be noted that SF can have plants rooted in the bottom that
can be emergent, submerged, or floating plants, as well as free-floating plants. In
addition, the biofilm system responsible for the transformation of the pollutants is
distinct between SF and subsurface systems. In SF, the substrate layer is reduced,
and water flows on top of it, thus the majority of the biodegradation processes occur
in the water column by action of the biofilm and periphyton [78] attached to the plant
surfaces (e.g. leaves, stems, roots in case of free-floating plants) [79]. For the
subsurface systems (HSSF and VF), biofilm is attached to the substrate, usually
sand or gravel as in classical sand filters and other type of biofilters, but in CWs the
plant have a critical role shaping the microbial community of the biofilm [80].
Removal processes are also dependent on physicochemical properties of the
pollutants, most notably evaporation, sorption and consequently sedimentation
(when compounds are sorbed to particles/solids that are trapped by sedimentation).
Different organic chemicals can show specific characteristic for a wide range of
physicochemical properties such as water solubility, vapour pressure, octanol-water
partition coefficient (Log K ow ), organic carbon partition coefficient (K oc ), Henry’s
law constant [73], acid dissociation constant (pKa) and the partition coefficient (Log
K d ) [27]. The relationship between these physicochemical characteristics and the fate
of pharmaceuticals in CWs is integrated in the next mechanisms description.
From the main CWs mechanisms, volatilization and phytovolatilization of pharmaceuticals can be excluded due to their low Henry’s law constant values; pharmaceuticals are not volatile at the normal environmental temperatures. Therefore,
pharmaceuticals are removed in CWs by the combination of (1) sorption and
sedimentation; (2) plant uptake combined with translocation and further
phytodegradation and microbial degradation within the plant; and (3) microbial
degradation in the water-substrate interface (biofilm and/or periphyton) (Fig. 2). In
the following paragraphs, each of the relevant processes are detailed. It should be
denoted that sorption and sedimentation, as well as plant uptake and
phytoaccumulation are non-destructive processes. They contribute for reducing the
concentration of the contaminants simply by relocating them, which might imply
that the contaminant can become available in the future (e.g. plants die off) or pose as
hazardous (e.g. disposable of substrate by the end-life of the treatment system)
[73]. The other processes imply the degradation of the compounds, also the
photodegradation in the SF systems. Pharmaceuticals degradation rarely occurs by
mineralization, thus a high variety of transformation products are formed, not only
by photodegradation [27] but also in biological systems [65].
Constructed Wetlands and Phytoremediation as a Tool for Pharmaceutical Removal
397