72
S. Mohapatra et al.
behavior is more complex. Such pharmaceuticals can be either in neutral or ionic
form depending upon pH of the water-sediment system. In such cases, the sorption
capacity may be evaluated based on the pH-dependent octanol–water partition coefficient (Corada-Fernández et al. 2015). Hydrophilic interactions and ion exchange
between ionic pharmaceuticals and sediments may also result in differences in sorption. Pharmaceuticals with at least one amine group (e.g., metoprolol, fluoxetine,
and tamoxifen) are positively charged (pK a > 8) under neural pH commonly encountered in water-sediment systems. In spite of low log K ow values (1.88–3.16), positively charged metoprolol and clarithromycin were positively correlated with the
clay content in soil indicating the importance of cation-exchange processes between
negatively charged surfaces of clays and/or organic matter present in the sediment
with the ionic pharmaceuticals (Lara-Martín et al. 2015). Similar ion exchange phenomenon is also reported for several psychoactive drugs and antibiotics (Lara-Martín
et al. 2015; Schaffer et al. 2012). A strong interaction is expected between aromatic
moieties of the compounds and aromatic fractions of natural organic matter. Thus, the
sorption of pharmaceuticals on sediments cannot be determined solely based on log
K ow . Even comparatively more polar compounds (log K d < 1), such as propranolol,
hydrochlorothiazide, and trimethoprim have been found to remain associated with
sediments. Compounds that tend to sorb strongly usually accumulate on the top layers of sediments, while leaching into groundwater is more prevalent for compounds
that do not sorb strongly or are mobile.
3.2.3 Biotransformation
Biodegradation/biotransformation is an essential process for the elimination of pharmaceuticals in the water-sediment system. Such elimination is usually carried out
by microorganisms through their metabolic and co-metabolic pathways only if the
toxicity of the compound does not inhibit the growth of the microbes. Along a river
stretch in a typical water-sediment system, there exist aerobic, anaerobic, and anoxic
zones, and each zone is characterized by a unique microbial community that can participate in the biotransformation of such compounds. The rate of biotransformation
may vary depending upon the ambient temperature, hydrogeology of the river, and
physicochemical properties of the pharmaceuticals.
In general, the concentration of pharmaceuticals downstream is expected to be
low in comparison with that upstream, due to dilution or in-stream biotic and abiotic
removal processes, such as biodegradation or sorption onto sediment. Biodegradation is an important process that aids in attenuation of micropollutants including
pharmaceuticals. The rate of biodegradation has been reported to be higher during
the summer season compared to the winter season, based on a study conducted in
Yamuna river in India (Mutiyar et al. 2018). However, it is very difficult to find
any specific trend in any river system from upstream to downstream, particularly
when the stream is continually being polluted with additional point or non-point
sources. Therefore, a higher concentration of some specific pharmaceuticals, such as
S. Mohapatra et al.
behavior is more complex. Such pharmaceuticals can be either in neutral or ionic
form depending upon pH of the water-sediment system. In such cases, the sorption
capacity may be evaluated based on the pH-dependent octanol–water partition coefficient (Corada-Fernández et al. 2015). Hydrophilic interactions and ion exchange
between ionic pharmaceuticals and sediments may also result in differences in sorption. Pharmaceuticals with at least one amine group (e.g., metoprolol, fluoxetine,
and tamoxifen) are positively charged (pK a > 8) under neural pH commonly encountered in water-sediment systems. In spite of low log K ow values (1.88–3.16), positively charged metoprolol and clarithromycin were positively correlated with the
clay content in soil indicating the importance of cation-exchange processes between
negatively charged surfaces of clays and/or organic matter present in the sediment
with the ionic pharmaceuticals (Lara-Martín et al. 2015). Similar ion exchange phenomenon is also reported for several psychoactive drugs and antibiotics (Lara-Martín
et al. 2015; Schaffer et al. 2012). A strong interaction is expected between aromatic
moieties of the compounds and aromatic fractions of natural organic matter. Thus, the
sorption of pharmaceuticals on sediments cannot be determined solely based on log
K ow . Even comparatively more polar compounds (log K d < 1), such as propranolol,
hydrochlorothiazide, and trimethoprim have been found to remain associated with
sediments. Compounds that tend to sorb strongly usually accumulate on the top layers of sediments, while leaching into groundwater is more prevalent for compounds
that do not sorb strongly or are mobile.
3.2.3 Biotransformation
Biodegradation/biotransformation is an essential process for the elimination of pharmaceuticals in the water-sediment system. Such elimination is usually carried out
by microorganisms through their metabolic and co-metabolic pathways only if the
toxicity of the compound does not inhibit the growth of the microbes. Along a river
stretch in a typical water-sediment system, there exist aerobic, anaerobic, and anoxic
zones, and each zone is characterized by a unique microbial community that can participate in the biotransformation of such compounds. The rate of biotransformation
may vary depending upon the ambient temperature, hydrogeology of the river, and
physicochemical properties of the pharmaceuticals.
In general, the concentration of pharmaceuticals downstream is expected to be
low in comparison with that upstream, due to dilution or in-stream biotic and abiotic
removal processes, such as biodegradation or sorption onto sediment. Biodegradation is an important process that aids in attenuation of micropollutants including
pharmaceuticals. The rate of biodegradation has been reported to be higher during
the summer season compared to the winter season, based on a study conducted in
Yamuna river in India (Mutiyar et al. 2018). However, it is very difficult to find
any specific trend in any river system from upstream to downstream, particularly
when the stream is continually being polluted with additional point or non-point
sources. Therefore, a higher concentration of some specific pharmaceuticals, such as