Page 573 - Polymer-based Nanocomposites for Energy and Environmental Applications

526 Polymer-based Nanocomposites for Energy and Environmental Applications

such as chemical precipitation, ionexchange, membrane filtration, and adsorption.
Most of these techniques do not have a high efficiency for removing a low concen-
tration of metal pollutants. Among the materials employed for removing conta-
minants, activated carbon is widely used in applications due to the formation of
fine pores in the carbon particles. This particularity provides a large-area contact
with the media and therefore a high efficiency for chemical reactions. However,
the disadvantage of this material is the high cost of fabrication.
Adsorption is a suitable technique for solving the problems of heavy metal con-
tamination. Indeed, this technique is efficient for removing low concentration of pol-
lutants in wastewater and is cheap as compared with others. Another advantage of
the technique is the possibility to regenerate the absorbent for reuse by appropriate
desorption processes. It consists of transferring a substance contained in an aqueous
system to the surface of a solid, called absorbent. The substance will be bound to this
surface by a physical or chemical process and thus can be further eliminated. Many
materials have been employed as absorbents. They include nanostructured carbon
such as activated carbon [92], CNTs [93], graphene [94], metal oxides [95], and nat-
ural materials such as cellulose [96] and clay [97]. Polymers are also efficient absor-
bents for metal removal with a further advantage to exhibit a large surface that can
be shaped into porous structure in order to enhance the contact with surrounding
medium. Pristine PS has been used as absorbent for removal of organic pollutants
in wastewater [98]. However, the polymer surface should be modified to enable
interaction with metal contaminants [99]. Other functional groups can be also
attached to the polymer backbone to facilitate interactions with pollutants. Indeed,
the most efficient absorbents for heavy metal removal are hybrid polymer-inorganic
composites in which the active component is an inorganic absorbent. The polymer
provides a support on which will be incorporated into nanosized inorganic compo-
nents (metal oxide and graphene) to enable interactions with metal ions. The char-
acteristics of such hybrid composites can be controlled to provide desired porosity
and surface morphology for obtaining high-efficiency devices. For instance, com-
posites of graphene and PPy [100] have been used for removing Hg 2+ ions from
water with a high efficiency and a high capacity, reaching 980 mg/g. GO has also
been associated with chitosan for adsorption of metal ions such as Cu 2+ and Pb 2+
[101]. High-adsorption capacity of graphene-based composites is partially due to
the large contact area between adsorbent and metal ions. In composites made of
nanoparticles of metal ions, the adsorption capacity is lower as obtained in iron
oxyhydroxide/cellulose composite [102]. Among composites for metal removal
use, polymer-clay absorbents have been intensively studied because of its advantages
such as low cost, high efficiency, and high-specific surface area. These characteris-
tics are due to clays, which are hydrous aluminosilicates and are abundant present in
nature. Natural or raw clays like montmorillonites are not good absorbents, but
with treatments, they can show a very high efficiency for removal of heavy metals
from aqueous solutions [103]. Another approach to improve their performance is to
use polymers as a support to form polymer/clay composites like chitosan/clay
[104,105]. The large surface of the polymer will enhance the adsorption capacity
by increasing the accessibility of the metal binding sites.