3 Natural Attenuation of Pharmaceuticals in the Aquatic …
83
the authors showed that both aerobic and anaerobic biodegradation were more or less
ineffective, and they were characterized by long lag phases of 42 days and 63 days,
respectively. On the other hand, the photolytic transformation of EE2 was extremely
rapid, and almost complete transformation of the parent compound was observed at
the end of two days during summer as compared to the dark controls kept over two
days. However, the estimation of residual estrogenicity was not in the scope of these
studies.
Generally, the photolytic removal of all pharmaceuticals is assumed to follow
pseudo-first-order kinetics, with the first-order rate constant being controlled by both
direct and indirect photolysis in the case of steroids. Additionally, the rate constant
is also indirectly influenced by the light intensity up to a point where mass transfer
becomes rate-limiting (Sornalingam et al., 2016). The latter case rarely occurs in
dilute surface waters. Typically, the degradation efficiency of steroid hormones was
reported to increase as the wavelength of the source moved from visible to UV owing
partly to increased photon flux and partly to strong characteristic UV absorbance of
hormones (Li Puma et al. 2010). Additionally, it is also reported that the effect of
light intensity varied with the nature of the steroid molecule. These inferences were
made based on studies conducted using bench-scale reactors employing artificial
light sources and concentrations of steroids comparable to that in surface water. For
instance, photolytic degradation efficiency under UVA irradiation followed the order
EE2 > E1 > E2 (Coleman et al. 2004). While most of the steroids were susceptible to direct photolysis, Whidbey et al. (2012) reported that the half-lives varied
from 40 min for E1 to about 8 h for E2 and EE2. Similarly, high pressure (HP)
Hg lamp (λ > 365 nm) did not lead to phototransformation of EE2, while a significant amount of removal occurred in the presence of a UV disinfection lamp (λ =
254 nm) (Sornalingam et al. 2016). Such variation in the direct photolysis rates of
these molecules stems from their different light absorbance characteristics (Mazellier et al. 2008). However, researchers have also shown experimentally that exposure
to medium pressure UV light (λ = 254 nm) for a period of four days was inadequate
for the elimination of estrogenicity of E2 and EE2 at an environmentally relevant
concentration (Sornalingam et al. 2016). This indicated that even after exposure to
UV light sources, the phenolic group of the steroids was unaffected. In addition, irrespective of the nature of the steroid and the type of irradiation (monochromatic or
polychromatic light source), the photoproducts were similar (Mazellier et al. 2008).
The absorption spectrum of E2 and EE2 is in the range 240–330 nm (Mazellier et al.
2008), where the overlap with natural sunlight is narrow. Hence, a long exposure
period may be required for degradation and elimination of estrogenic activity.
While direct photolysis is plausible in the case of steroids, such as E1, E2,
E3, and EE2, indirect photolysis plays an equally important role in expediting the
removal rates. However, the type and nature of constituents of DOM present are
also reported to play a role in the process. For instance, the presence of humic
acid and Suwannee River fulvic acid constituents of DOM led to an exponential
increase in the free radicals that oxidized the steroids (Sornalingam et al. 2016).
On the other hand, Whidbey et al. (2012) attributed the drop in E1 photolysis to
83
the authors showed that both aerobic and anaerobic biodegradation were more or less
ineffective, and they were characterized by long lag phases of 42 days and 63 days,
respectively. On the other hand, the photolytic transformation of EE2 was extremely
rapid, and almost complete transformation of the parent compound was observed at
the end of two days during summer as compared to the dark controls kept over two
days. However, the estimation of residual estrogenicity was not in the scope of these
studies.
Generally, the photolytic removal of all pharmaceuticals is assumed to follow
pseudo-first-order kinetics, with the first-order rate constant being controlled by both
direct and indirect photolysis in the case of steroids. Additionally, the rate constant
is also indirectly influenced by the light intensity up to a point where mass transfer
becomes rate-limiting (Sornalingam et al., 2016). The latter case rarely occurs in
dilute surface waters. Typically, the degradation efficiency of steroid hormones was
reported to increase as the wavelength of the source moved from visible to UV owing
partly to increased photon flux and partly to strong characteristic UV absorbance of
hormones (Li Puma et al. 2010). Additionally, it is also reported that the effect of
light intensity varied with the nature of the steroid molecule. These inferences were
made based on studies conducted using bench-scale reactors employing artificial
light sources and concentrations of steroids comparable to that in surface water. For
instance, photolytic degradation efficiency under UVA irradiation followed the order
EE2 > E1 > E2 (Coleman et al. 2004). While most of the steroids were susceptible to direct photolysis, Whidbey et al. (2012) reported that the half-lives varied
from 40 min for E1 to about 8 h for E2 and EE2. Similarly, high pressure (HP)
Hg lamp (λ > 365 nm) did not lead to phototransformation of EE2, while a significant amount of removal occurred in the presence of a UV disinfection lamp (λ =
254 nm) (Sornalingam et al. 2016). Such variation in the direct photolysis rates of
these molecules stems from their different light absorbance characteristics (Mazellier et al. 2008). However, researchers have also shown experimentally that exposure
to medium pressure UV light (λ = 254 nm) for a period of four days was inadequate
for the elimination of estrogenicity of E2 and EE2 at an environmentally relevant
concentration (Sornalingam et al. 2016). This indicated that even after exposure to
UV light sources, the phenolic group of the steroids was unaffected. In addition, irrespective of the nature of the steroid and the type of irradiation (monochromatic or
polychromatic light source), the photoproducts were similar (Mazellier et al. 2008).
The absorption spectrum of E2 and EE2 is in the range 240–330 nm (Mazellier et al.
2008), where the overlap with natural sunlight is narrow. Hence, a long exposure
period may be required for degradation and elimination of estrogenic activity.
While direct photolysis is plausible in the case of steroids, such as E1, E2,
E3, and EE2, indirect photolysis plays an equally important role in expediting the
removal rates. However, the type and nature of constituents of DOM present are
also reported to play a role in the process. For instance, the presence of humic
acid and Suwannee River fulvic acid constituents of DOM led to an exponential
increase in the free radicals that oxidized the steroids (Sornalingam et al. 2016).
On the other hand, Whidbey et al. (2012) attributed the drop in E1 photolysis to