179
intermediate product of reduced graphene oxide synthesis (Fig. 7.2c) prepared by
oxidative exfoliation of graphite (Loh et al. 2010; Dreyer et al. 2010).
All the synthesis methods involve the oxidation of graphite. The synthesis
method by Hummers uses a broad variation of oxidizing agents to exfoliate graphite
flakes. The Brodie and Staudenmaier methods combine a mixing of potassium chlorate (KClO 3 ) with nitric acid (HNO 3 ) to induce oxidation of graphite. In contrast,
Hummers method uses potassium permanganate (KMnO 4 ) and sulfuric acid (H 2 SO 4 )
(Hummers and Offeman 1958; Tang et al. 2013; Shahriary and Athawale 2014;
Nanda et al. 2015; Liu et al. 2016).
According to the synthesis methods for obtaining reduced graphene oxide from
graphene oxide, sodium citrate is used as a reducing agent. Sodium citrate is mixed
with an aqueous dispersion of graphene oxide and maintained for 24 h under reflux,
until the dispersion color changes from light brown to homogeneous black color.
The excess of sodium citrate is removed by centrifugation, and the resulting precipitate can be redispersed in water (Zhang et al. 2011).
7.2.4 Hydroxyapatite Nanoparticles
Hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 ) is a natural bioceramic material which presents
potential applications in areas ranging from fertilizers, drug delivery systems, pharmaceuticals, water treatment, and biomedicine (Geng et al. 2017; Haider et al. 2017;
Pereira et al. 2017; Wijesinghe et al. 2017). There are several methods by which
nanoparticles can be obtained, including hydroxyapatite (Haider et al. 2017). For
instance, hydroxyapatite nanoparticles can be obtained by several methods, and a
selection of them is displayed below.
Wet Synthesis
Conventional homogeneous wet-chemical precipitation: this method is widely used
due to its simplicity. The hydroxyapatite nanoparticles are synthesized by mixing
the aqueous solutions containing the orthophosphate and calcium, which are then
submitted to specific temperature and pressure in order to obtain the stoichiometry,
crystalline phase, and grain size desired. Results available in the literature show that
the reaction speed is slow, the crystallinity is poor, and the control of the growth is
hard (Jarcho et al. 1976; Ahn et al. 2001; De Lima Souza et al. 2008).
Hydrothermal treatment: this method consists of the reaction of an aqueous solution of calcium and phosphorous precursors at ambient pressure and temperature.
When compared with others methods, hydrothermal treatment is a suitable strategy
to obtain hydroxyapatite nanoparticles with high crystallinity and dispersibility
(Okada and Furuzono 2012; Qi et al. 2017), but may present difficulties for controlling the size distribution and morphology (Sadat-Shojai et al. 2013).
7 Toxicity of Engineered Nanostructures in Aquatic Environments
intermediate product of reduced graphene oxide synthesis (Fig. 7.2c) prepared by
oxidative exfoliation of graphite (Loh et al. 2010; Dreyer et al. 2010).
All the synthesis methods involve the oxidation of graphite. The synthesis
method by Hummers uses a broad variation of oxidizing agents to exfoliate graphite
flakes. The Brodie and Staudenmaier methods combine a mixing of potassium chlorate (KClO 3 ) with nitric acid (HNO 3 ) to induce oxidation of graphite. In contrast,
Hummers method uses potassium permanganate (KMnO 4 ) and sulfuric acid (H 2 SO 4 )
(Hummers and Offeman 1958; Tang et al. 2013; Shahriary and Athawale 2014;
Nanda et al. 2015; Liu et al. 2016).
According to the synthesis methods for obtaining reduced graphene oxide from
graphene oxide, sodium citrate is used as a reducing agent. Sodium citrate is mixed
with an aqueous dispersion of graphene oxide and maintained for 24 h under reflux,
until the dispersion color changes from light brown to homogeneous black color.
The excess of sodium citrate is removed by centrifugation, and the resulting precipitate can be redispersed in water (Zhang et al. 2011).
7.2.4 Hydroxyapatite Nanoparticles
Hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 ) is a natural bioceramic material which presents
potential applications in areas ranging from fertilizers, drug delivery systems, pharmaceuticals, water treatment, and biomedicine (Geng et al. 2017; Haider et al. 2017;
Pereira et al. 2017; Wijesinghe et al. 2017). There are several methods by which
nanoparticles can be obtained, including hydroxyapatite (Haider et al. 2017). For
instance, hydroxyapatite nanoparticles can be obtained by several methods, and a
selection of them is displayed below.
Wet Synthesis
Conventional homogeneous wet-chemical precipitation: this method is widely used
due to its simplicity. The hydroxyapatite nanoparticles are synthesized by mixing
the aqueous solutions containing the orthophosphate and calcium, which are then
submitted to specific temperature and pressure in order to obtain the stoichiometry,
crystalline phase, and grain size desired. Results available in the literature show that
the reaction speed is slow, the crystallinity is poor, and the control of the growth is
hard (Jarcho et al. 1976; Ahn et al. 2001; De Lima Souza et al. 2008).
Hydrothermal treatment: this method consists of the reaction of an aqueous solution of calcium and phosphorous precursors at ambient pressure and temperature.
When compared with others methods, hydrothermal treatment is a suitable strategy
to obtain hydroxyapatite nanoparticles with high crystallinity and dispersibility
(Okada and Furuzono 2012; Qi et al. 2017), but may present difficulties for controlling the size distribution and morphology (Sadat-Shojai et al. 2013).
7 Toxicity of Engineered Nanostructures in Aquatic Environments