Environmental Engineering Reference
In-Depth Information
is the most versatile and environmentally benign among all the methods of preparing
polymer-clay nanocomposites (PCNs). Nanocomposite synthesis via this method involves
compounding and annealing (usually under shear) of a mixture of polymer and clay above
the melting point of the polymer. During compounding and blending, the polymer melt
diffuses into the cavities of the clay. This method therefore allows the processing of PCNs
to be articulated directly from the precursors without using any solvent but ordinary com-
pounding devices such as mixers and/or extruders.
Some polymers, for example, ethylene vinyl alcohol,
23
thermoplastic polyurethane,
24
and
polycaprolactone (PCL),
25
are thermoplastic polymers that have been used in the study of
nanocomposite synthesis by melt intercalation. Technically, the melt-blending method is
much simpler and more straightforward than the methods discussed in the previous sec-
tions. In addition, although relatively new, this method has more appealing advantages
that promise to greatly expand the commercial opportunities for PCN technology.
26,27
One
such advantage is that this approach does not use any organic solvent, and it is compat-
ible with existing industrial polymer extrusion and blending processes.
9
The compatibility
with existing thermoplastic polymer-processing techniques minimizes capital costs, and
the nonuse of organic solvents eliminates environmental concerns.
14,15
On the negative aspects, this method forms microcomposites or tactoids at higher clay
loading as a result of clay agglomeration. Furthermore, this method employs thermoplastic
polymers that are generally hydrophobic, and this limits the application of the nanocom-
posites in water treatment. Finally, the compounding and extrusion processes are likely to
amplify the range of particle shapes and sizes, principally when the clay is not uniformly
exfoliated.
20.2.2 PCNs in Heavy Metal Removal from Water
PCNs have been used for heavy metal adsorption because of the suitable clay proper-
ties such as speciic surface area, chemical and mechanical stability, layered structure,
and high cation-exchange capacity. In addition, clays can be regarded as possessing both
Brönsted and Lewis types of acidity,
2
and this acidity makes them suitable for the adsorp-
tion of heavy metals. The Brönsted acidity arises from two situations, irst through the
formation of H
+
ions on the surface, resulting from the dissociation of water molecules of
hydrated exchangeable metal cations on the surface, as follows:
n
+
)
(
n
−+ +
1
)
[(
MHO
)]
→
[(
OH
)(
HO
+
H
2
x
2
x
−
1
Second, it can be as a result of a net negative charge on the surface due to the sub-
stitution of Si
4+
by Al
3+
in some of the tetrahedral positions, and the resultant charge is
counterbalanced by H
3
O
+
cations. On the other hand, Lewis acidity can emanate from
three scenarios: (i) through dehydroxylation of some Brönsted acid sites; (ii) exposed Al
3+
ions at the edges; and (iii) Al
3+
arising from the breaking of Si-O-Al bonds. The resulting
negative net charge is counterbalanced by exchangeable cations such as K
+
, Ca
2+
, and Mg
2+
,
adsorbed between the unit layers and around the edges, making it possible for clay to
remove heavy metals from water through ion exchange, chemisorption, or physisorption.
20.2.3 Ion-Imprinted Polymers for Selective Metals Adsorption
Ion-imprinted polymers (IIPs) are chemically and physically stable materials synthesized
with recognition cavities, and thus highly selective for the adsorption of targeted inorganic
