94
Y. S. Dzyazko et al.
(0.768 nm, this distance corresponds to graphene) [5]. Graphene-like materials,
which consist of several graphite layers, are also known: a thickness of their flakes
is several nanometers [6]. They show a diffuse peak between 11.3° and 26.4°. No
electrical conductivity is attributed to GO. At the same time, rGO is an electric
conductor. This gives a possibility to use it for the manufacture of electrodes for
different electrochemical devices.
GO is obtained from graphite by means of Brodie [7], Staudenmaier [8], or
Hummer methods [9]. These techniques provide oxidation of graphite down to
various levels. Brodie and Staudenmaier used KClO3 and HNO3. KMnO4 and
H2SO4 are applied to oxidation by means of Hummer’s method. Other initial materials for graphene obtaining are carbon nanotubes [10, 11]. Oxidation causes the
formation of phenolic, carboxyl, and epoxy groups, which are attached to the graphite
layers. Carboxyl groups and a part of phenolic groups are located along the perimeter
of GO flakes. Epoxy groups and other parts of hydroxyl groups are located on their
basal plane [12]. rGO is synthesized by means of GO reduction.
Carboxylic groups of GO provide its adsorption properties toward inorganic
cations, for instance, UO 2
2+ [13], Cu
2+ [14], Pb
2+ [15], and other toxic ionic components of aqueous solutions. However, GO demonstrates practically no anion exchange
properties [15]. At the same time, the adsorption of anionic azo dyes has been
reported [16]. Adsorption of anionic species is evidently due to their interaction with
hydrophobic regions of GO (benzene rings). These regions of GO flakes are responsible for the adsorption of organic molecules, such as aromatic [17] and nitroaromatic compounds [18]. In the case of deoxyribonucleic acid [19] or hormones [20],
hydrophobic regions are adsorption centers in neutral media, when isoelectric point
of proteins is achieved.
It should be stressed that GO is more attractive than rGO, since it can be used for
adsorption of both inorganic ions and organic molecules. Thus, GO can be considered
as a multifunctional adsorbent. However, it is impossible to fill adsorption columns
with this material due to small size of its particles, and good dispersibility in water
or other polar solvents.
The solution to the problem is to develop GO-containing composites, which
contain the component possessing adsorption ability. In this chapter, the mutual
effect of GO and inorganic particles on their structure on nano- and microlevels
is considered. Hydrophilicity–hydrophobicity of different types of graphene (especially GO) and their composites are also in a focus of attention. These properties
determine the adsorption ability of the composite materials toward both inorganic
and organic species. Adsorption properties of the composites are also reviewed.
2 Morphology of Composites
Among inorganic adsorbents, materials based on hydrated oxides of multivalent
metals are attractive as a support of GO. These adsorbents are amphoteric: depending
on the solution pH, they are able to sorb either cations or anions [21]. It should be
Y. S. Dzyazko et al.
(0.768 nm, this distance corresponds to graphene) [5]. Graphene-like materials,
which consist of several graphite layers, are also known: a thickness of their flakes
is several nanometers [6]. They show a diffuse peak between 11.3° and 26.4°. No
electrical conductivity is attributed to GO. At the same time, rGO is an electric
conductor. This gives a possibility to use it for the manufacture of electrodes for
different electrochemical devices.
GO is obtained from graphite by means of Brodie [7], Staudenmaier [8], or
Hummer methods [9]. These techniques provide oxidation of graphite down to
various levels. Brodie and Staudenmaier used KClO3 and HNO3. KMnO4 and
H2SO4 are applied to oxidation by means of Hummer’s method. Other initial materials for graphene obtaining are carbon nanotubes [10, 11]. Oxidation causes the
formation of phenolic, carboxyl, and epoxy groups, which are attached to the graphite
layers. Carboxyl groups and a part of phenolic groups are located along the perimeter
of GO flakes. Epoxy groups and other parts of hydroxyl groups are located on their
basal plane [12]. rGO is synthesized by means of GO reduction.
Carboxylic groups of GO provide its adsorption properties toward inorganic
cations, for instance, UO 2
2+ [13], Cu
2+ [14], Pb
2+ [15], and other toxic ionic components of aqueous solutions. However, GO demonstrates practically no anion exchange
properties [15]. At the same time, the adsorption of anionic azo dyes has been
reported [16]. Adsorption of anionic species is evidently due to their interaction with
hydrophobic regions of GO (benzene rings). These regions of GO flakes are responsible for the adsorption of organic molecules, such as aromatic [17] and nitroaromatic compounds [18]. In the case of deoxyribonucleic acid [19] or hormones [20],
hydrophobic regions are adsorption centers in neutral media, when isoelectric point
of proteins is achieved.
It should be stressed that GO is more attractive than rGO, since it can be used for
adsorption of both inorganic ions and organic molecules. Thus, GO can be considered
as a multifunctional adsorbent. However, it is impossible to fill adsorption columns
with this material due to small size of its particles, and good dispersibility in water
or other polar solvents.
The solution to the problem is to develop GO-containing composites, which
contain the component possessing adsorption ability. In this chapter, the mutual
effect of GO and inorganic particles on their structure on nano- and microlevels
is considered. Hydrophilicity–hydrophobicity of different types of graphene (especially GO) and their composites are also in a focus of attention. These properties
determine the adsorption ability of the composite materials toward both inorganic
and organic species. Adsorption properties of the composites are also reviewed.
2 Morphology of Composites
Among inorganic adsorbents, materials based on hydrated oxides of multivalent
metals are attractive as a support of GO. These adsorbents are amphoteric: depending
on the solution pH, they are able to sorb either cations or anions [21]. It should be