Page 388 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials as Adsorbents 373

nano-cube quartz with an edge of 10 nm should dissolve in 1.07 seconds
under identical conditions; however this is not observed, in general
(Bertone et al., 2003; Lasaga, 1998). The remarkable stability of nanopar-
ticles in solution might be explained by the lack of defects on the surface,
the strong surface passivation, or the altered surface composition due to
adsorption of surfactants or oxidation, as seen with magnetite (Shipley
et al., 2006). While many of the unusual physical properties of the
nanocrystalline materials are now well known (Banfield and Navrotsky,
2003), the influence of size on adsorption, chemical reactivity, and on the
nanoparticles/solution interface needs to be further understood.

Size effect on adsorption capacity
As a particle shrinks to the nanometer range, an increasing fraction of
atoms are exposed to the surface, giving rise to excess energy.
Consequently, nanoparticles are thermodynamically metastable com-
pared to macrocrystalline materials. They tend to approach the mini-
mum free energy state (equilibrium state) through several ways: phase
transformation, crystal growth, surface structural changes, aggrega-
tion, and surface adsorption (Banfield and Navrotsky, 2003; Rusanov,
2005). Therefore, nanoparticles with a higher total energy should be
more prone to adsorb molecules onto their surfaces in order to decrease
the total free energy. Hence, adsorption should be favored on nanopar-
ticles (Banfield and Navrotsky, 2003). This is the case for the adsorp-
tion of arsenic at the surface of iron oxide nanoparticles. For instance,
magnetide with a diameter of 11 nm adsorbs 3 times more arsenic per
2
nm than does magnetide of 20 nm or 300 nm.
However, some experimental results have shown that the sorption capac-
ity of inorganic nanoparticles is not always size-dependent for a given
surface area. This is the case for the adsorption of organic acids (valeric,
acetic, adipic, and oxalic acids) at the surface of titanium nanoparticles
(Zhang et al., 1999). This study explicitly demonstrated that when cor-
rected for surface area there is no size-dependence, while for a given mass
of TiO 2 nanoparticles that are 6 nm in diameter are more prone to adsorb
organic molecules than nanoparticles with 16 nm in diameter (Table 10.1).
In this case a greater interest of using nano-sized particles as adsorbents
is their higher specific surface area (SSA). For instance, the SSAof an oxide
nanoparticle of 10 nm in diameter is ≈100 times larger than the SSAof an
oxide particle of 1 m. It is well known that the surface hydroxyl groups
are the chemically reactive entities at the surface of the solid in an aque-
ous environment. A higher SSA increases the number of available func-
tional groups on the nanoparticle surface. Consequently, for a given
mass, the maximum adsorption capacity of ions in solution is higher for
nanoparticles than for micron-sized particles.