Page 394 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials as Adsorbents 377

surface hydroxyl groups (≡MOH) are the functional groups of metal
oxides. They have a double pair of electrons with a dissociable hydro-
gen atom that enables the metal oxide to react with ions in solution and
to form surface complexes. The following are acid-base equilibria reac-
tions of metal oxides in water (K and K : equilibrium constants that rep-
2
1
resent the concentration of surface species [moles/kg] of metal oxide)
(Cornell and Schwertmann, 1996; Sawyer et al., 2003).

≡MOH : ≡MO H where K 1 [≡MO ][H ]/[≡MOH]

≡MOH H : ≡MOH 2 where K 2 [≡MOH 2 ]/[≡MOH][H ]
Surface sensitive techniques can be used to provide direct information
about the mode of attachment of adsorbates to the surface of nanopar-
ticles at the atomic scale. In general, electron-based spectroscopy, vibra-
tional spectroscopy, and synchrotron-based X-ray techniques are among
the common structural methods in use (see Chapter 4). Spectroscopic
studies reveal that sorption on a crystalline surface may result from the
molecular mechanisms shown in Figure 10.1.
It is difficult to precisely study sorption reactions without spectro-
scopic evidence. Spectroscopic data provide a better understanding of the
adsorption mechanisms at the atomic scale and enable more meaning-
ful interpretation of the adsorption isotherm experiments. In addition
to the increasing importance and the remarkable improvement of the
experimental surface methods, theoretical calculations are also becom-
ing an indispensable tool in chemical research, particularly in the case
of nano-systems. The computational and calorimetric techniques yield
rich insight into the structure and the adsorption processes at the sur-
face of the nanoparticles.

Nanomaterial-Based Adsorbents
for Water and Wastewater Treatment

As an example of the use of nanomaterials as adsorbents in water treat-
ment, we will consider the problem of arsenic removal from water. Arsenic
in drinking water is a priority pollutant of considerable interest world-
wide. In the Bangladesh crisis, approximately 30 to 36 million people are
exposed to carcinogenic levels of arsenic (Bates et al., 1992; Hossain,
2006). Arsenic mobility in aqueous media and its removal by various
water treatment technologies have been the subject of considerable study.
Much of this work has addressed the adsorption of arsenic on iron-
containing solids. The sorption of inorganic or organic contaminants at the
surface of iron (hydr)oxide minerals may affect the mobility, reactivity,
bioavailability, and toxicity of these particles in natural waters (Al-Abadleh
and Grassian, 2003; Johnson and Hallberg, 2005; Stipp et al., 2002).