Page 398 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials as Adsorbents 381

surrounding the arsenic. In agreement with the literature and the
study of reference compounds, the presence of 3.2 oxygen atoms at
1.76 Å, highlights that arsenic remains under the trivalent form during
adsorption at the maghemite nanoparticles surface and XAS experi-
ments. The second peak on the FT corresponds to the presence of iron
atoms in the second coordination sphere of As. Previous EXAFS stud-
V III
ies have described the adsorption of As and As onto iron oxide using
multiple As-Fe shells: As-Fe 3.55–3.60 Å for monodentate mononu-
clear complex, As-Fe 3.25–3.40 Å for bidentate binuclear complex,
and As-Fe 2.8–2.9 Å for bidentate mononuclear complex (Arai et al.,
2001; Ladeira et al., 2001; Manceau, 1995; Manning et al., 1998;
Manning and Goldberg, 1996; Pierce and Moore, 1982; Sherman and
Randall, 2003; Thoral et al., 2005; Waychunas et al., 1993; Waychunas
et al., 1995). In the case of arsenate, the distance As-Fe at 2.8–2.9 Å
is controversial and several authors suggest that it corresponds to
multiple scattering of the photoelectron involving O-O pairs with the
V
As O tetrahedron (Arai et al., 2001; Manceau, 1995; Manning et al.,
4
1998; Waychunas et al., 1995; Waychunas et al., 1993). In our partic-
ular case, a contribution of 1.7 iron atoms at 3.33 Å from As is neces-
sary to satisfactorily reproduce the experimental spectra. This As-Fe
III III
distance is very close to the average As -Fe inter-atomic distances
III
of 3.25–3.40 Å attributed to As -O-Fe linkages through double corner
sharing.

Magnetite nanoparticles for arsenic removal. The effectiveness of using
nanoscale magnetite to treat arsenic-contaminated drinking water was
investigated. The adsorption and desorption behavior of arsenite and
arsenate to magnetite nanoparticles brings some very interesting
responses on the size effect on the surface properties. Three types of mag-
2 2
netite nanoparticles were studied: 300 nm (3.7 m /g) and 20 nm (60 m /g)
2
commercially made nano-magnetites, and 11.72 nm (98.8 m /g) monodis-
persed magnetite synthesized in the laboratory.
III
The adsorption of As to 20 nm and 300 nm magnetite nanoparti-
V
cles is dependent on pH and SSA (Table 10.1). The adsorption of As to
20 nm and 300 nm nanoscale magnetite decreases with increasing pH,
because arsenate ionicity is pH dependent and the surface of the mag-
netite is positively charged at pH values below 6.8 (the point of zero
III
charge for magnetite). The adsorption isotherm (Figure 10.4) of As and
V
As onto 11.72 nm magnetite reveals dramatic changes (Al-Abadleh and
Grassian, 2003; Axe and Trivedi, 2002; Dixit and Hering, 2003; Zhang
et al., 1999). Compared with other literature results (Dixit and Hering,
2003; Morel and Hering, 1993), the maximum adsorption densities for
2
11.72 nm magnetite are very high (~18 and ~23 µmol/m or ~10 and ~15
2
As atoms/nm ).