3 Natural Attenuation of Pharmaceuticals in the Aquatic …
77
of plants (allochthonous) and microbially produced (autochthonous) DOM (Cottrell
et al. 2013) exists in surface water bodies. The absorption of actinic radiation (electromagnetic radiation that can produce photochemical reactions) by DOM results in
numerous chemical reactions involving nitrates, nitrites, carbonates and dissolved
oxygen, and various reactive species, such as, superoxide/hydroperoxyl radicals
(O
−•
2 /HO
•
2 ), singlet oxygen (
1 O 2 ), and the hydroxyl radical (OH
• ) are formed (Cottrell et al. 2013). Additionally, photosensitization of DOM due to the absorption of
actinic radiation can also generate singlet excited state DOM. Subsequently, this singlet DOM undergoes deactivation, generating triplet excited state DOM (
3 DOM
• ),
which is also known to play a significant role in the photolysis of pharmaceuticals.
However, owing to the complexity of DOM itself, reliable techniques for quantification of
3 DOM
• are not available (Cottrell et al. 2013). Therefore, the effect of
photosensitized excited states of DOM is studied by employing quenchers or by
employing DOM proxies, such as benzoquinone (Cottrell et al. 2013). The influence of DOM on indirect phototransformation of pharmaceuticals is dependent on
its concentration and chemical components. The combined techniques of excitationemission matrix spectroscopy (EEMs) and parallel factor analysis (PARAFAC) have
successfully been evaluated to understand the composition, source, and fate of DOM
in the aquatic environment (Bai et al. 2018). Other than DOM, particulate organic
matter (POM) can also cause some, yet not significant, effects on the photolysis of
pharmaceuticals in surface waters (Cottrell et al. 2013). Light scattering and partitioning of contaminants on POM influences photochemical degradation of the contaminants by reducing the intensity of actinic radiation available for photolysis of
the target compounds.
The effects of
3 DOM
• are extremely important as they contribute to almost 75%
of the aquatic photochemistry of natural waters (Cottrell et al. 2013).
3 DOM
• may
react with pharmaceutical contaminants directly through electron and energy transfer. Moreover, it may also generate other reactive oxygen species, such as singlet
oxygen and hydroxyl radicals, leading to other degradation reactions (Bai et al.
2018). Hydroxyl radical has an important effect on the phototransformation of pharmaceuticals in natural waters since it can react with a series of organic contaminants
because of its reactivity and non-selectivity (Bai et al. 2018). In contrast, due to the
high selectivity of
1 O 2 , the generation of these reactive species is possibly crucial
for phototransformation of only a limited number of contaminants. Examples of
such compounds include polycyclic aromatic hydrocarbons, furans, dialkyl sulfides,
phenolate anions. and pyrroles (Bai et al. 2018).
Light Intensity
Many organic compounds, including pharmaceuticals, can absorb light of wavelength less than 250 nm. This process in surface waters may not play a crucial role as
the percentage of sunlight falling within this wavelength range is very small. Therefore, these processes are usually characterized by low quantum yields (Gmurek et al.
77
of plants (allochthonous) and microbially produced (autochthonous) DOM (Cottrell
et al. 2013) exists in surface water bodies. The absorption of actinic radiation (electromagnetic radiation that can produce photochemical reactions) by DOM results in
numerous chemical reactions involving nitrates, nitrites, carbonates and dissolved
oxygen, and various reactive species, such as, superoxide/hydroperoxyl radicals
(O
−•
2 /HO
•
2 ), singlet oxygen (
1 O 2 ), and the hydroxyl radical (OH
• ) are formed (Cottrell et al. 2013). Additionally, photosensitization of DOM due to the absorption of
actinic radiation can also generate singlet excited state DOM. Subsequently, this singlet DOM undergoes deactivation, generating triplet excited state DOM (
3 DOM
• ),
which is also known to play a significant role in the photolysis of pharmaceuticals.
However, owing to the complexity of DOM itself, reliable techniques for quantification of
3 DOM
• are not available (Cottrell et al. 2013). Therefore, the effect of
photosensitized excited states of DOM is studied by employing quenchers or by
employing DOM proxies, such as benzoquinone (Cottrell et al. 2013). The influence of DOM on indirect phototransformation of pharmaceuticals is dependent on
its concentration and chemical components. The combined techniques of excitationemission matrix spectroscopy (EEMs) and parallel factor analysis (PARAFAC) have
successfully been evaluated to understand the composition, source, and fate of DOM
in the aquatic environment (Bai et al. 2018). Other than DOM, particulate organic
matter (POM) can also cause some, yet not significant, effects on the photolysis of
pharmaceuticals in surface waters (Cottrell et al. 2013). Light scattering and partitioning of contaminants on POM influences photochemical degradation of the contaminants by reducing the intensity of actinic radiation available for photolysis of
the target compounds.
The effects of
3 DOM
• are extremely important as they contribute to almost 75%
of the aquatic photochemistry of natural waters (Cottrell et al. 2013).
3 DOM
• may
react with pharmaceutical contaminants directly through electron and energy transfer. Moreover, it may also generate other reactive oxygen species, such as singlet
oxygen and hydroxyl radicals, leading to other degradation reactions (Bai et al.
2018). Hydroxyl radical has an important effect on the phototransformation of pharmaceuticals in natural waters since it can react with a series of organic contaminants
because of its reactivity and non-selectivity (Bai et al. 2018). In contrast, due to the
high selectivity of
1 O 2 , the generation of these reactive species is possibly crucial
for phototransformation of only a limited number of contaminants. Examples of
such compounds include polycyclic aromatic hydrocarbons, furans, dialkyl sulfides,
phenolate anions. and pyrroles (Bai et al. 2018).
Light Intensity
Many organic compounds, including pharmaceuticals, can absorb light of wavelength less than 250 nm. This process in surface waters may not play a crucial role as
the percentage of sunlight falling within this wavelength range is very small. Therefore, these processes are usually characterized by low quantum yields (Gmurek et al.