88
6 Applications of Carbon Nanostructures Produced in Molten Salts
this, adsorption is distinctive, due to its unique combination of advantages such as
simplicity, adaptability, cost-effectiveness, ease of design and insensitivity toward
highly toxic substances.
A large number of different adsorbents have been examined for the removal of
dyestuffs, from which carbon materials, particularly activated carbon (AC), are considered as effective and low-cost adsorbents [80]. Other carbon nanostructures, such
as carbon nanotubes (CNTs) [81], magnetic nanocarbons [82] and graphene oxide
(GO) materials [83–85], have also shown attractive adsorbent performances, because
of their favorable physical and chemical properties, such as high specific surface area
and also the presence of chemically or physically active functional groups on their
surfaces, which promote the adsorption of various pollutants on their surface [86–88].
During the adsorption process, the target pollutants are accumulated on the surface
of the adsorbent, and therefore, the adsorption capacity of adsorbent is progressively
reduced until it is saturated and finally exhausted. For the case of AC which is the
most common adsorbent, the exhausted adsorbent loaded with hazardous substances
is burnt or disposed in landfills, causing environmental and economic problems.
Therefore, the capability of the adsorbent to be regenerated after being used in the
absorption process, in an environmentally friendly way, is very important. The AC
and GO adsorbents can be regenerated electrochemically [89, 90], biologically [91]
and chemically [92]. However, these carbon materials are not commonly regenerated
by thermal methods since their resistance against oxidation in air and/or their thermal
stability is rather low; typically lower than 300 °C [93, 94]. In contrast to carbon
materials, ceramic absorbents based on sodium silicates [95] and zinc ferrites [96]
can effectively be thermally regenerated in air. The thermal regeneration of AC has
only been studied under protective atmospheres [97], limiting the viability of the
thermal process for regeneration of AC at larger scales.
3D graphene nanosheets produced in NaCl + LiCl molten salt were evaluated
as the adsorbent for the removal of dyes from aqueous solutions. Furthermore, the
thermal regeneration of the graphene nanosheets in air after being used in the process
was studied. The molten salt-produced graphene nanosheets are curved and contain a
high density of edge sites, as it can be observed from the SEM micrographs shown in
Fig. 6.7a–c. This morphology provides not only porosity, but also structural stability
by preventing the re-stacking of the exfoliated graphene.
The specific surface area and the pore diameter of the molten salt-produced 3D
graphene were calculated to be 186 m
2 g
−1 and 3.63 nm, respectively. A high fraction of single-, double- and few-layer graphene nanosheets together with graphitic
nanolayers with a thickness of less than 10 nm could be detected in the graphene
material, such as shown in the TEM micrograph of Fig. 6.7j. Furthermore, the termination of (002) planes of the graphene nanosheets can clearly be observed in this
micrograph. As mentioned, the presence of dangling bonds located at the edge sides
of the graphene nanosheets can enhance the adsorption performance of the material. The FFT patterns shown in Fig. 6.7 exhibit spots which can be attributed to the
interlayer spacing in graphite (0.34 nm).
6 Applications of Carbon Nanostructures Produced in Molten Salts
this, adsorption is distinctive, due to its unique combination of advantages such as
simplicity, adaptability, cost-effectiveness, ease of design and insensitivity toward
highly toxic substances.
A large number of different adsorbents have been examined for the removal of
dyestuffs, from which carbon materials, particularly activated carbon (AC), are considered as effective and low-cost adsorbents [80]. Other carbon nanostructures, such
as carbon nanotubes (CNTs) [81], magnetic nanocarbons [82] and graphene oxide
(GO) materials [83–85], have also shown attractive adsorbent performances, because
of their favorable physical and chemical properties, such as high specific surface area
and also the presence of chemically or physically active functional groups on their
surfaces, which promote the adsorption of various pollutants on their surface [86–88].
During the adsorption process, the target pollutants are accumulated on the surface
of the adsorbent, and therefore, the adsorption capacity of adsorbent is progressively
reduced until it is saturated and finally exhausted. For the case of AC which is the
most common adsorbent, the exhausted adsorbent loaded with hazardous substances
is burnt or disposed in landfills, causing environmental and economic problems.
Therefore, the capability of the adsorbent to be regenerated after being used in the
absorption process, in an environmentally friendly way, is very important. The AC
and GO adsorbents can be regenerated electrochemically [89, 90], biologically [91]
and chemically [92]. However, these carbon materials are not commonly regenerated
by thermal methods since their resistance against oxidation in air and/or their thermal
stability is rather low; typically lower than 300 °C [93, 94]. In contrast to carbon
materials, ceramic absorbents based on sodium silicates [95] and zinc ferrites [96]
can effectively be thermally regenerated in air. The thermal regeneration of AC has
only been studied under protective atmospheres [97], limiting the viability of the
thermal process for regeneration of AC at larger scales.
3D graphene nanosheets produced in NaCl + LiCl molten salt were evaluated
as the adsorbent for the removal of dyes from aqueous solutions. Furthermore, the
thermal regeneration of the graphene nanosheets in air after being used in the process
was studied. The molten salt-produced graphene nanosheets are curved and contain a
high density of edge sites, as it can be observed from the SEM micrographs shown in
Fig. 6.7a–c. This morphology provides not only porosity, but also structural stability
by preventing the re-stacking of the exfoliated graphene.
The specific surface area and the pore diameter of the molten salt-produced 3D
graphene were calculated to be 186 m
2 g
−1 and 3.63 nm, respectively. A high fraction of single-, double- and few-layer graphene nanosheets together with graphitic
nanolayers with a thickness of less than 10 nm could be detected in the graphene
material, such as shown in the TEM micrograph of Fig. 6.7j. Furthermore, the termination of (002) planes of the graphene nanosheets can clearly be observed in this
micrograph. As mentioned, the presence of dangling bonds located at the edge sides
of the graphene nanosheets can enhance the adsorption performance of the material. The FFT patterns shown in Fig. 6.7 exhibit spots which can be attributed to the
interlayer spacing in graphite (0.34 nm).