Nanoparticles as Sorbents for Removal of Heavy Metal Ions from Aqueous Solutions - Nanotechnologies for Water Environment Applications - page 153

extensively in various areas such as biomedicine and water/wastewater treatment (Li
and Bai, 2006). As the magnetite nanoparticles have little surface sites that could
serve as the anchor point for chitosan polymers, Chang and Chen (2005) modified the
chitosan polymer and grafted carboxylate groups; the carboxylated chitosan was then
covalently bonded to magnetite nanoparticles. Qi and Xu (2004) took another
approach and obtained chitosan nanoparticles through a simple ionic-gelation
nucleation method, wherein tripolyphosphate (TPP) acted as an ionic crosslinker.
Both research groups reported high adsorption capacity for heavy metals. For
example, Qi and Xu (2004) reported that the chitosan nanoparticles synthesized has
sequestration capacity towards Pb(II) as high as 398 mg/g.

However, chitosan nanoparticles obtained via ionic-gelation has a critical
drawback: they gradually disintegrate in aqueous media over days or aggregate in
alkaline solution (pH = 9.0) (Lpez-Len et al . , 2005). This is mainly due to the weak
electrostatic interactions between chitosan chains and TPP molecules, as shown by
the complete disintegration of spherical chitosan nanoparticles into dissolved polymer
chains in solution of high ionic strength (125 mM of KNO 3 ). Like any other
adsorbents, a systematic physicochemical examination should accompany the
remedial assessment of nanoparticles in order to fully understand their potentials and
pitfalls.

6.3.2.4 Polymeric Nanoparticles for in-situ Heavy Metal Removal

Currently, the environmental applications of polymeric nanoparticles are still
in the research phase despite the fact that the preparation methods are well-
established and industrial facilities capable of producing them are in place. In-situ

remediation of heavy metal contaminated groundwater or soil by polymer
nanoparticles is rarely explored. However, Tungittiplakorn et al . (2004, 2005)
reported one of the first few bench-scale applications of polymeric nanoparticles for
remediation of hydrophobic organic pollutants. They demonstrated that the
polyurethane-based nanoparticles could be engineered to desorb and transport the
organic pollutants with the hydrophobic core of the nanoparticles, whilst the particle
mobility in porous medium such as soil is enhanced due to the hydrophilic shell.
These experimental findings confirmed that polymeric nanoparticles are highly
versatile and customizable. These results would definitely provide thrusts for further
development in environmental applications of polymeric nanoparticles, particularly in
removal of heavy metal ions.

6.4 Conclusions

Significant progresses in environmental applications of nanoparticles have
been obtained, partly due to the emergence of novel nanoparticles with specifically
tailored physicochemical properties. Both iron-based nanoparticles and polymeric
nanoparticles have been demonstrated to be technically capable of sequestering heavy
metal ions from aqueous solutions. Extensive studies have been, and are continuously