1.7.1 Graphene/Semiconductor Composites as Adsorbents
Rapid population growth, industrial, and agricultural activities have given rise to a
large number of contaminants being released into the environment, which represent
a major public health and environmental concern (Lubchenco 1998). Heavy metal
pollutants from municipal waste, industrial waste, corrosion of pipes, and soldered
joints are common contaminants that can unpleasantly enter drinking water sources
and aquatic environments (USEPA 2014). Therefore, much effort has been made to
develop a technology, which can effectively remove pollutants from water. Among
them, adsorption is the most effective technology for water/wastewater treatment
owing to its inexpensive, low-cost, and efficacy for pollutant removal from aquatic
environments (Ali and Gupta 2006). Adsorption involves the capturing of pollutants
(adsorbate) by nanomaterials (adsorbents) through a physicochemical interaction
(Dąbrowski 2001). Ideally, graphene with a large surface area (2630 m
2 g
À1
) makes
it an attractive adsorbent for the decomposition of pollutants (Ramesha et al. 2011)
compared to the conventional adsorbents, such as mesoporous and activated carbons. Moreover, the large π–π conjugation on the surface of graphene could be used
for the adsorption of different reactants during the photocatalytic reaction. Moreover, the semiconductor has a large surface-to-volume ratio. Therefore, coupling a
graphene sheet with a semiconductor can effectively improve the adsorption capacity and surface area of the composite. Graphene/magnetic nanoparticle composites
exhibited enhanced adsorption activity and improved the number of binding sites for
heavy metal ions and the surface area of the nanoparticle (Zhu et al. 2011). This is
due to the combined effect of adsorption sites on the graphene layer and the metal
complexation on the nanoparticles (Zhu et al. 2011). Among the magnetic semiconductor, Fe 3 O 4 is the most used semiconductor for water purification owing to its
extraordinary biocompatibility (Chandra et al. 2010). Higher conductivity and
surface area influence the deionization activity of semiconductors (Wang et al.
2013a). Fortunately, graphene shows both properties, which makes it suitable for
the deionization of semiconductors (Zhang et al. 2012a). Wang et al. (2012a)
observed a reduction in conductivity from 86.9 to 10.2 μS cm
À1 after 120 min
when graphene was used as an electrode for the deionization of sodium chloride
solution. The magnetic graphene/Fe 3 O 4 nanocomposites, which was fabricated
using a facile one-pot technique, was utilized as an adsorbent to eliminate rhodamine
B (RhB) dye from wastewater (Lü et al. 2014). The as-fabricated nanocomposite
displays high ability to extract organic pollutants. Thus, the magnetic graphene/
Fe 3 O 4 adsorbent allows efficient separation of pollutants from wastewater. Zhu et al.
(2011) revealed that a complete removal of Cr(IV) by the magnetic graphene
composite in acidic pH ranges from 1 to 3. Similarly, a high acidic pH solution
can also inhibit the adsorption ability of metal ions by the composites (Alyüz and
Veli 2009). The Fe 3 O 4 /graphene composite with the covalent binding between
graphene and Fe 3 O 4 showed high adsorption capacity of 190.14 and
140.79 mg g
À1 for methylene blue (MB) and neutral red dyes, respectively (He et al.
2010). Through a chemical reduction process, a magnetic Fe 3 O 4 /graphene
14
F. Opoku et al.
Rapid population growth, industrial, and agricultural activities have given rise to a
large number of contaminants being released into the environment, which represent
a major public health and environmental concern (Lubchenco 1998). Heavy metal
pollutants from municipal waste, industrial waste, corrosion of pipes, and soldered
joints are common contaminants that can unpleasantly enter drinking water sources
and aquatic environments (USEPA 2014). Therefore, much effort has been made to
develop a technology, which can effectively remove pollutants from water. Among
them, adsorption is the most effective technology for water/wastewater treatment
owing to its inexpensive, low-cost, and efficacy for pollutant removal from aquatic
environments (Ali and Gupta 2006). Adsorption involves the capturing of pollutants
(adsorbate) by nanomaterials (adsorbents) through a physicochemical interaction
(Dąbrowski 2001). Ideally, graphene with a large surface area (2630 m
2 g
À1
) makes
it an attractive adsorbent for the decomposition of pollutants (Ramesha et al. 2011)
compared to the conventional adsorbents, such as mesoporous and activated carbons. Moreover, the large π–π conjugation on the surface of graphene could be used
for the adsorption of different reactants during the photocatalytic reaction. Moreover, the semiconductor has a large surface-to-volume ratio. Therefore, coupling a
graphene sheet with a semiconductor can effectively improve the adsorption capacity and surface area of the composite. Graphene/magnetic nanoparticle composites
exhibited enhanced adsorption activity and improved the number of binding sites for
heavy metal ions and the surface area of the nanoparticle (Zhu et al. 2011). This is
due to the combined effect of adsorption sites on the graphene layer and the metal
complexation on the nanoparticles (Zhu et al. 2011). Among the magnetic semiconductor, Fe 3 O 4 is the most used semiconductor for water purification owing to its
extraordinary biocompatibility (Chandra et al. 2010). Higher conductivity and
surface area influence the deionization activity of semiconductors (Wang et al.
2013a). Fortunately, graphene shows both properties, which makes it suitable for
the deionization of semiconductors (Zhang et al. 2012a). Wang et al. (2012a)
observed a reduction in conductivity from 86.9 to 10.2 μS cm
À1 after 120 min
when graphene was used as an electrode for the deionization of sodium chloride
solution. The magnetic graphene/Fe 3 O 4 nanocomposites, which was fabricated
using a facile one-pot technique, was utilized as an adsorbent to eliminate rhodamine
B (RhB) dye from wastewater (Lü et al. 2014). The as-fabricated nanocomposite
displays high ability to extract organic pollutants. Thus, the magnetic graphene/
Fe 3 O 4 adsorbent allows efficient separation of pollutants from wastewater. Zhu et al.
(2011) revealed that a complete removal of Cr(IV) by the magnetic graphene
composite in acidic pH ranges from 1 to 3. Similarly, a high acidic pH solution
can also inhibit the adsorption ability of metal ions by the composites (Alyüz and
Veli 2009). The Fe 3 O 4 /graphene composite with the covalent binding between
graphene and Fe 3 O 4 showed high adsorption capacity of 190.14 and
140.79 mg g
À1 for methylene blue (MB) and neutral red dyes, respectively (He et al.
2010). Through a chemical reduction process, a magnetic Fe 3 O 4 /graphene
14
F. Opoku et al.