Page 397 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

380 Environmental Applications of Nanomaterials

As-O
8 6 Fe As

k 2 .chi(k) (arbitrary scale) Experimental Radial distribution function 4 2 As-O-Fe
Fe

Fitted
0
3 4 5 6 7 8 9 10 11 0 0.5 1 1.5 2 2.5 3 3.5 4
−1 −1
k (Å ) R-∆R (Å )
Figure 10.3 EXAFS As K-edge spectra (left) and its corresponding Fourier transform
(right) of arsenic adsorbed onto maghemite nanoparticles. Experiments performed on the
FAME beamline at the ESRF synchrotron (Grenoble, France).

be of the order of 1.0
10 5 mol m 2 (or 1.8 mmol g 1 of maghemite or
2
~6 As atoms/nm ). This value can be compared (Table 10.4) with those
obtained with β-FeOOH or HFO taking into account that the specific sur-
2
face area of HFO is certainly lower than 600 m /g. The value of ~6 As
2
atoms/nm corresponds to a complete fulfillment of the surface of the
maghemite nanoparticles.
EXAFS spectroscopy at the As K-edge (see Chapter 4, Rose et al.) was
used to determine the local atomic environment of arsenic adsorbed on
maghemite nanoparticles. The EXAFS oscillations, their corresponding
Fourier transform (FT), and the parameters used to achieve the best
theoretical fit are presented in Figure 10.3 and Table 10.2 and corre-
spond to one monolayer of arsenic around maghemite nanoparticles.
The dominant lower frequency on the EXAFS spectra and the first
peak in the corresponding FT corresponds to oxygen atoms immediately

TABLE 10.2 Structural Parameters for As Contributions Obtained from Fitting
EXAFS As K-edge Spectra of Arsenic Adsorbed onto Maghemite Nanoparticles
Samples Atomic shells N ! 20% R(Å) ! 0.02 Å σ (Å) ! 0.01 Å

As adsorbed onto As-O 3.2 1.76 0.061
maghemite As-Fe* 1.7 3.33 0.102
nanoparticles
N represents the number of backscatters at distance R; σ, the Debye-Waller term, is
the disorder parameter for the absorber-backscatterer pair.
* Corresponds to iron atoms in the second coordination sphere of As.