enzymes associated with co-metabolism of L-arginine, L-phenylalanine and putrescine were found to be potentially linked to ibuprofen transformations [104].
Overall, there are clear evidences of the importance of microbial biodegradation
for pharmaceuticals removal in CWs. However, there is a big knowledge gap in
relation to the microbial communities in CWs biofilms and their role and genetic
information for the removal/biotransformation. Mostly, there is a need to disassemble all the experimental systems [50] after testing removal and further sequence the
microbial communities. Furthermore, increased sequencing effort is also needed for
full-scale systems.
From what is known, microbial degradation is in general favoured by warmer
conditions. Higher removal in the summer of different pharmaceuticals, e.g. salicylic
acid and caffeine, has been consistently associated with higher activity of the
microbial communities (also when using biodegradation constants corrected for
biomass) [57]. Moreover, we can cycle back to CW design and concepts of potentially favouring specific reactions types. For example, anaerobic environments
favour the biodegradation of naproxen and diclofenac, while aerobic environments
favour the removal of ibuprofen or carbamazepine [28, 57]. In addition, it should be
stressed that the nature and extent of microbial degradation of organic chemicals
within a CW is also expected to strongly depend on the physicochemical properties
of the contaminant. Indeed, the biological degradability or recalcitrance of organic
compounds may often be explained by its chemical structure, for instance, the
presence of secondary, tertiary or quaternary carbon atoms as well as functional
groups [73]. Nevertheless, pharmaceuticals cannot be treated as groups with respect
to chemical behaviour. Small changes in chemical structure can have significant
impact on solubility and polarity [50]. Matamoros and Bayona [105] reported that in
HSSF, the removal of diclofenac was below 45%, while ibuprofen (80%) and
ketoprofen (69%) showed greater removal. These compounds are all in the therapeutic class of non-steroidal anti-inflammatory, are aryl derivatives of propionic acid
and are deprotonated and negatively charged (pH 5.3); only differing in the aryl
group. However, biodegradation involves enzymatic reactions specific to chemical
structures, which may result in varying rates of biodegradation [50]. Thus, the efforts
placed in trying to use Log K ow also for the prediction of biodegradation have failed
[27]. During biodegradation, pharmaceuticals may undergo (1) mineralization;
(2) transformation to more hydrophobic compounds, which partition onto the solid
phase; and (3) transformation to more hydrophilic compounds, which remain in the
liquid phase. To date, few published studies on microbiological degradation of
pharmaceuticals in CWs are available, and these studies only use indirect methods
to identify possible biodegradation pathways for pharmaceutical removal [50]. Since
only a few compounds and studies have reported on transformation products [28],
information on transformation pathways is severely lacking.
One last point to be mentioned is enantiomeric processes. Several pharmaceutical
compounds are chiral [58]. Thus, enantiomeric fractionation of chiral compounds
can be used as an indicator of enzyme-compound interactions and, together with
mass balance approaches, an indicator of biodegradation [106], including for
phytoremediation mechanisms [107]. Very little literature data was (and still is)
Constructed Wetlands and Phytoremediation as a Tool for Pharmaceutical Removal
405
Overall, there are clear evidences of the importance of microbial biodegradation
for pharmaceuticals removal in CWs. However, there is a big knowledge gap in
relation to the microbial communities in CWs biofilms and their role and genetic
information for the removal/biotransformation. Mostly, there is a need to disassemble all the experimental systems [50] after testing removal and further sequence the
microbial communities. Furthermore, increased sequencing effort is also needed for
full-scale systems.
From what is known, microbial degradation is in general favoured by warmer
conditions. Higher removal in the summer of different pharmaceuticals, e.g. salicylic
acid and caffeine, has been consistently associated with higher activity of the
microbial communities (also when using biodegradation constants corrected for
biomass) [57]. Moreover, we can cycle back to CW design and concepts of potentially favouring specific reactions types. For example, anaerobic environments
favour the biodegradation of naproxen and diclofenac, while aerobic environments
favour the removal of ibuprofen or carbamazepine [28, 57]. In addition, it should be
stressed that the nature and extent of microbial degradation of organic chemicals
within a CW is also expected to strongly depend on the physicochemical properties
of the contaminant. Indeed, the biological degradability or recalcitrance of organic
compounds may often be explained by its chemical structure, for instance, the
presence of secondary, tertiary or quaternary carbon atoms as well as functional
groups [73]. Nevertheless, pharmaceuticals cannot be treated as groups with respect
to chemical behaviour. Small changes in chemical structure can have significant
impact on solubility and polarity [50]. Matamoros and Bayona [105] reported that in
HSSF, the removal of diclofenac was below 45%, while ibuprofen (80%) and
ketoprofen (69%) showed greater removal. These compounds are all in the therapeutic class of non-steroidal anti-inflammatory, are aryl derivatives of propionic acid
and are deprotonated and negatively charged (pH 5.3); only differing in the aryl
group. However, biodegradation involves enzymatic reactions specific to chemical
structures, which may result in varying rates of biodegradation [50]. Thus, the efforts
placed in trying to use Log K ow also for the prediction of biodegradation have failed
[27]. During biodegradation, pharmaceuticals may undergo (1) mineralization;
(2) transformation to more hydrophobic compounds, which partition onto the solid
phase; and (3) transformation to more hydrophilic compounds, which remain in the
liquid phase. To date, few published studies on microbiological degradation of
pharmaceuticals in CWs are available, and these studies only use indirect methods
to identify possible biodegradation pathways for pharmaceutical removal [50]. Since
only a few compounds and studies have reported on transformation products [28],
information on transformation pathways is severely lacking.
One last point to be mentioned is enantiomeric processes. Several pharmaceutical
compounds are chiral [58]. Thus, enantiomeric fractionation of chiral compounds
can be used as an indicator of enzyme-compound interactions and, together with
mass balance approaches, an indicator of biodegradation [106], including for
phytoremediation mechanisms [107]. Very little literature data was (and still is)
Constructed Wetlands and Phytoremediation as a Tool for Pharmaceutical Removal
405