187
Furthermore, there are no standards established for permissible levels of nanomaterials in the environment (Hund-Rinke et al. 2016). Among the problems with
the standard test guidelines was the difficulty in the maintenance of test solutions
and exposure concentration within ±20 % of nominal concentrations as recommended by OECD. OECD guidance suggests that variation in substance concentration larger than ±20% should be addressed using modified experimental procedures
as a little change in pH or monitoring strategies. To overcome this, some adaptations
in protocols were proposed (Petersen et al. 2015).
In some cases, however, modifying the medium may cause changes in the
obtained result. Thus, for example, graphene oxide fate can be affected by pH and
divalent cations as Ca
+2
, which presence in the medium promote less stability (Wu
et al. 2013; Lanphere et al. 2014; Chowdhury et al. 2015). Besides, release of silver
ions was shown to be pH dependent: in zebrafish embryo medium, silver toxicity
was decreased owing to the formation of silver chloro-complexes and silver nanocolloid toxicity was higher at pH 4.0 than at 7.0 or 9.0 (Shaw et al. 2016; Kennedy
et al. 2017; Kataoka et al. 2018).
The fate and behavior of metallic nanoparticles in aquatic waters are complex with
varied levels of variability and uncertainty. Silver may accumulate in the food chain,
and the widespread use of silver nanoparticle in several applications exposes organisms (Yu et al. 2013) and raises some health concerns related to the potential risks to
humans and to the environment (Zhang et al. 2013), while some studies have observed
adverse effects at different levels of biological integration as reactive oxygen species
(ROS) generation (Zhang et al. 2013; Gaillet and Rouanet 2015; McGillicuddy et al.
2017; Ale et al. 2018; Strużyński et al. 2014). Ellis et al. (2018) observed that silver
nanoparticle stability was influenced by the seasonal variations in natural water
chemistry by using a diffusion–sedimentation model to calculate silver nanoparticle
migration behavior in microcosm experiments. Then, transformation, fate, bioavailability, morphology, and toxicity of silver nanoparticle are critical factors and should
be considered for its environmental risk assessment (Zhang et al. 2018).
In addition, the role of intact nanoparticles along with dissolved metals is
required, when toxicity cannot always be explained solely by soluble metal ions
(Garcia-Reyero et al. 2014; Abramenko et al. 2018). In this way, Schiavo et al.
(2018) reported that zinc oxide nanoparticle toxicity was related to Zn ions and to
interactions of particle/aggregates with target organisms.
Titanium dioxide nanoparticles are widely used photocatalytic materials that
show stability in water, photo and chemical stability over a wide range of pH, activation by sunlight (Woan et al. 2009, Fekete-Kertész et al. 2017). The ecotoxicology of titanium dioxide nanoparticles has been extensively studied, but not all the
studies considered its photocatalytic properties, which can enhance toxicity to
aquatic biota.
Clemente et al. (2013) evaluated the effects on fish (Piaractus mesopotamicus)
exposed to different titanium dioxide nanoparticle concentrations and illumination,
under visible and ultraviolet (UV) light (22.47 J/cm
2
/h). Titanium dioxide nanoparticles caused no mortality under any of the conditions tested, but stimulated sublethal effects that were induced by illumination condition. Also, fish prolonged
exposures (21 days) to two different titanium dioxide nanoparticles crystal phases
7 Toxicity of Engineered Nanostructures in Aquatic Environments
Furthermore, there are no standards established for permissible levels of nanomaterials in the environment (Hund-Rinke et al. 2016). Among the problems with
the standard test guidelines was the difficulty in the maintenance of test solutions
and exposure concentration within ±20 % of nominal concentrations as recommended by OECD. OECD guidance suggests that variation in substance concentration larger than ±20% should be addressed using modified experimental procedures
as a little change in pH or monitoring strategies. To overcome this, some adaptations
in protocols were proposed (Petersen et al. 2015).
In some cases, however, modifying the medium may cause changes in the
obtained result. Thus, for example, graphene oxide fate can be affected by pH and
divalent cations as Ca
+2
, which presence in the medium promote less stability (Wu
et al. 2013; Lanphere et al. 2014; Chowdhury et al. 2015). Besides, release of silver
ions was shown to be pH dependent: in zebrafish embryo medium, silver toxicity
was decreased owing to the formation of silver chloro-complexes and silver nanocolloid toxicity was higher at pH 4.0 than at 7.0 or 9.0 (Shaw et al. 2016; Kennedy
et al. 2017; Kataoka et al. 2018).
The fate and behavior of metallic nanoparticles in aquatic waters are complex with
varied levels of variability and uncertainty. Silver may accumulate in the food chain,
and the widespread use of silver nanoparticle in several applications exposes organisms (Yu et al. 2013) and raises some health concerns related to the potential risks to
humans and to the environment (Zhang et al. 2013), while some studies have observed
adverse effects at different levels of biological integration as reactive oxygen species
(ROS) generation (Zhang et al. 2013; Gaillet and Rouanet 2015; McGillicuddy et al.
2017; Ale et al. 2018; Strużyński et al. 2014). Ellis et al. (2018) observed that silver
nanoparticle stability was influenced by the seasonal variations in natural water
chemistry by using a diffusion–sedimentation model to calculate silver nanoparticle
migration behavior in microcosm experiments. Then, transformation, fate, bioavailability, morphology, and toxicity of silver nanoparticle are critical factors and should
be considered for its environmental risk assessment (Zhang et al. 2018).
In addition, the role of intact nanoparticles along with dissolved metals is
required, when toxicity cannot always be explained solely by soluble metal ions
(Garcia-Reyero et al. 2014; Abramenko et al. 2018). In this way, Schiavo et al.
(2018) reported that zinc oxide nanoparticle toxicity was related to Zn ions and to
interactions of particle/aggregates with target organisms.
Titanium dioxide nanoparticles are widely used photocatalytic materials that
show stability in water, photo and chemical stability over a wide range of pH, activation by sunlight (Woan et al. 2009, Fekete-Kertész et al. 2017). The ecotoxicology of titanium dioxide nanoparticles has been extensively studied, but not all the
studies considered its photocatalytic properties, which can enhance toxicity to
aquatic biota.
Clemente et al. (2013) evaluated the effects on fish (Piaractus mesopotamicus)
exposed to different titanium dioxide nanoparticle concentrations and illumination,
under visible and ultraviolet (UV) light (22.47 J/cm
2
/h). Titanium dioxide nanoparticles caused no mortality under any of the conditions tested, but stimulated sublethal effects that were induced by illumination condition. Also, fish prolonged
exposures (21 days) to two different titanium dioxide nanoparticles crystal phases
7 Toxicity of Engineered Nanostructures in Aquatic Environments