Nanoparticles as Sorbents for Removal of Heavy Metal Ions from Aqueous Solutions - Nanotechnologies for Water Environment Applications - page 151

between the carboxylate group and the positively charged lead species. Kanazawa et
al . (2004) improvised the idea of copolymerization further. By copolymerization
with appropriate comonomers, the resulting nanoparticles would display dual
functions: the ability to chelating heavy metals ions, and thermosensitivity. The
adsorption kinetics of the nanoparticles synthesized was drastically improved, as
compared to that of their bulk analogue. Nonetheless, the attractiveness of utilizing
P(NIPAAm) nanoparticles for heavy metal removal remains low, due to the fact that
the main functional group, the isopropylacrylamide group, does not display any
favorable heavy metal removal ability.
6.3.2.2 Poly(styrene)-Based Nanoparticles
Similar to P(NIPAAm) nanoparticles, polystyrene nanoparticles do not
display any significant heavy metal removal capacity in its unmodified form. The
polystyrene polymers enjoy high popularity among polymer scientists and
technologists, as the precursor materials are inexpensive and the industrial processes
producing them are well-established. Antonietti et al . (1995) explored the synthesis of
metal-chelating poly(styrene)-based nanoparticles using one-step functionalization
via a mini-emulsion polymerization approach. Attempt to directly polymerize pyridyl
monomers was unsuccessful. With careful formulation, polymeric nanoparticles of 13
to 19 nm was obtained successfully via copolymerization of the pyridyl monomer
with styrene. The bipyridine-based metal-chelating groups are mainly located on the
surface of the nanoparticles, and hence, highly accessible. It was observed that the
removal of heavy metal ions from the contacting aqueous phase could be completed
within minutes. In addition, the nanoparticles remained colloidally stable after heavy
metal ion adsorption, which indicates that the cooperative binding event occurred
between every two neighboring bipyridine groups solely.
These polymeric nanoparticles could be further chemically modified via
standard “wet chemistry” routes, to enhance their adsorption capacity or confer these
nanoparticles with ability for sensing and sequestration of specific target heavy metal
ions present in contaminated water. Recent advances in polymer science and
technology provides a great variety of tools for surface-functionalization of polymeric
nanoparticles or latexes (Wang et al . , 2003, and references therein). Specific ligands
or a second polymer layer could be covalently grafted onto the nanoparticle surface,
resulting in a change in surface functionalities and morphologies. The ligands grafted
are covalently bonded and accessible to the multivalent ions or molecules in the
surrounding environment. Amigoni-Gerbier et al . (1999; 2002) explored the synthesis
and application of cyclam-grafted nanoparticles as recoverable chelating agents. Due
to the small size (13 - 20 nm) of the nanoparticles, the nanoparticle solution was
stable and transparent. The cyclam ligands bonded copper ions specifically with high
selectivity, even in the presence of other cations. The loading of cyclam ligands of
0.73 meq/g is one-fold higher than those obtained by other approaches reported
elsewhere (Amigoni-Gerbier et al . , 2002). Fast chelation kinetics on minute scale was
observed, and 85-90% utilization of the available ligands was easily achieved. The
nanoparticles remained stable after dialysis or Cu(II) complexation. The binding