Page 403 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

386 Environmental Applications of Nanomaterials

Inorganic nanoparticles with an
organic shell
The combination of inorganic nanoparticles with an organic shell has
attracted considerable attention because of the potential applications in
many fields, such as separation processes, optoelectronics, catalysts and
sensors, biotechnology, medical diagnostics, and therapy. Several nano-
systems using iron oxide nanoparticles as the core and an organic layer
on the surface have been studied to improve the water and liquid waste
treatment. In such systems, the binding strength between the coating
layer and the surface of nanoparticles and the affinity of the organic sur-
face for specific pollutants play preponderant roles (Yamaura et al., 2002).

Organic-coated magnetite nanoparticles. Attention has been given to the
functionalization of the surface of magnetite nanoparticles to develop new
nano-adsorbents for the removal of ions from solution. For instance, Liao
et al. (2003) coated magnetite nanoparticles (12 nm) with polyacrylic acid
(PAA). The PAA coating was chosen for its strong affinity for cationic
solutes with large molecular weights. The PAA coating layer attaches to
the surface of magnetite nanoparticles through covalent linkage (Liao
et al., 2003). This nano-adsorbent exhibits a high adsorption capacity and
fast adsorption and desorption rates (Huang et al., in press, Liao et al., 2003;
Liu et al., 1999). However, the PAA surface layer is not effective for the
adsorption of metallic ions. Therefore, magnetite nanoparticles (13.5 nm)
were surface-functionalized with chitosan (Chang et al., 2006; Chang and
Chen, 2005). Chitosan is a natural polysaccharide with many useful char-
acteristics, such as hydrophilicity, antibacterial properties, and affinity for
heavy metal ions. Vibrational spectroscopic experiments have demon-
strated the covalent attachment of the carboxymethyl chitosan at the sur-
face of magnetite nanoparticles through the carboxylate functions. The
resulting chitosan-coated nanoparticles were efficient for the fast removal
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of Co (0.47 mmol/g) and Cu ions (0.34 mmol/g) from aqueous solution
(Chang et al., 2006; Chang and Chen, 2005). As for the PAA-coated
nanoparticles, the kinetics of adsorption and desorption reactions were
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fast—one minute to reach equilibrium for Cu ions. The fast removal of
ion from solution can be explained by the small internal diffusion within
the structure of the nanoparticles. The chitosan coating may prevent a
direct contact between the contaminants and the surface of the nanopar-
ticle. Moreover, magnetic analysis indicates that once chitosan-coated,
magnetite nanoparticles remain superparamagnetic, which could allow
them to be separated from water under a magnetic field and reused.

DMSA-coated maghemite nanoparticles. To improve the efficiency of
maghemite nanoparticles in the removal of arsenic from polluted waters,
these nanoparticles were surface-coated with DMSA [meso-2,3-
dimercaptosuccinic acid, (SH) (CH) (COOH) ]. DMSA is an effective
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