3 Natural Attenuation of Pharmaceuticals in the Aquatic …
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a selective reagent in breaking chemical bonds, subsequently forming simpler low
molecular weight compounds (Gmurek et al. 2017).
Some recent studies have indicated that photochemical or light-induced degradation and transformation of pharmaceuticals are the most important natural attenuation
process in surface waters (Baena-nogueras et al. 2017). It is also the simplest method
for the degradation of pharmaceuticals in the natural aquatic environment (Gmurek
et al. 2017). Although turbidity may partially block sunlight due to inner filter effect,
water in the top layer in streams is exposed to substantial amounts of direct sunlight,
especially in summer and during noon times (Zhang et al. 2008a, b). In general, the
degree and extent of photolysis of pharmaceuticals are influenced by the structure of
the pharmaceuticals and also several environmental factors, such as, the intensity of
solar radiation, depth of the water column, pH, temperature, salinity, organic matter
content, and eutrophic conditions (Baena-nogueras et al. 2017; Jennifer et al. 2017).
The intensity of solar radiation and irradiance, in turn, would depend on the existing
season and the latitude of the location (Jennifer et al. 2017). Photochemical reactions
can degrade pharmaceuticals via both direct and indirect pathways. In direct photolysis pathway, the pharmaceutical absorbs light, usually of a specific wavelength,
which triggers its degradation by cleavage of the light-absorbing function group. In
indirect pathways, photochemical reactions of oxygen, water, and other organic matter may yield reactive species, which may then attack the pharmaceutical molecule
(Avetta et al. 2016). In the indirect pathway, sunlight is absorbed by naturally occurring photosensitizers, such as nitrate, nitrite, and chromophoric dissolved organic
matter (cDOM) (Latch et al. 2003; Packer et al. 2003). Photosensitized nitrate and
nitrite, in turn, yield hydroxyl radicals (
• OH), while the irradiated cDOM helps in
generating triplet state DOM, which reacts with oxygen to form singlet state oxygen
(
3 cDOM
• and
1 O 2 , respectively) (Avetta et al. 2016). These processes have been
pictorially depicted in Fig. 3.1.
Since phototransformation plays such an important role in the removal of pharmaceutical pollutants from surface waters, it is important to understand the resulting
photoproducts that may be formed. It is also imperative to assess whether sunlight
assisted photochemical reactions are sufficient to bring about complete mineralization of the compounds. In the case of an indirect pathway, mainly due to the
non-selectivity of radicals, various photoproducts formed during the reaction may
persist and cause secondary toxic effects (Yin et al. 2017). Unfortunately, due to the
myriad of reactions that occur during photolysis, the trace levels of photoproducts
formed, and the lack of suitable standards and instrumentation to identify them, it is
difficult to quantify them and comment on their nature and toxicity. The formation
of such photoproducts is inevitable and thus they cannot be completely eliminated.
Hence, the impact of these photoproducts in surface water is a concern that needs to
be addressed.
75
a selective reagent in breaking chemical bonds, subsequently forming simpler low
molecular weight compounds (Gmurek et al. 2017).
Some recent studies have indicated that photochemical or light-induced degradation and transformation of pharmaceuticals are the most important natural attenuation
process in surface waters (Baena-nogueras et al. 2017). It is also the simplest method
for the degradation of pharmaceuticals in the natural aquatic environment (Gmurek
et al. 2017). Although turbidity may partially block sunlight due to inner filter effect,
water in the top layer in streams is exposed to substantial amounts of direct sunlight,
especially in summer and during noon times (Zhang et al. 2008a, b). In general, the
degree and extent of photolysis of pharmaceuticals are influenced by the structure of
the pharmaceuticals and also several environmental factors, such as, the intensity of
solar radiation, depth of the water column, pH, temperature, salinity, organic matter
content, and eutrophic conditions (Baena-nogueras et al. 2017; Jennifer et al. 2017).
The intensity of solar radiation and irradiance, in turn, would depend on the existing
season and the latitude of the location (Jennifer et al. 2017). Photochemical reactions
can degrade pharmaceuticals via both direct and indirect pathways. In direct photolysis pathway, the pharmaceutical absorbs light, usually of a specific wavelength,
which triggers its degradation by cleavage of the light-absorbing function group. In
indirect pathways, photochemical reactions of oxygen, water, and other organic matter may yield reactive species, which may then attack the pharmaceutical molecule
(Avetta et al. 2016). In the indirect pathway, sunlight is absorbed by naturally occurring photosensitizers, such as nitrate, nitrite, and chromophoric dissolved organic
matter (cDOM) (Latch et al. 2003; Packer et al. 2003). Photosensitized nitrate and
nitrite, in turn, yield hydroxyl radicals (
• OH), while the irradiated cDOM helps in
generating triplet state DOM, which reacts with oxygen to form singlet state oxygen
(
3 cDOM
• and
1 O 2 , respectively) (Avetta et al. 2016). These processes have been
pictorially depicted in Fig. 3.1.
Since phototransformation plays such an important role in the removal of pharmaceutical pollutants from surface waters, it is important to understand the resulting
photoproducts that may be formed. It is also imperative to assess whether sunlight
assisted photochemical reactions are sufficient to bring about complete mineralization of the compounds. In the case of an indirect pathway, mainly due to the
non-selectivity of radicals, various photoproducts formed during the reaction may
persist and cause secondary toxic effects (Yin et al. 2017). Unfortunately, due to the
myriad of reactions that occur during photolysis, the trace levels of photoproducts
formed, and the lack of suitable standards and instrumentation to identify them, it is
difficult to quantify them and comment on their nature and toxicity. The formation
of such photoproducts is inevitable and thus they cannot be completely eliminated.
Hence, the impact of these photoproducts in surface water is a concern that needs to
be addressed.