Nanoparticles for Treatment of Arsenic - Nanotechnologies for Water Environment Applications - page 125

Table 5.1 EXAFS fitting results for As(V) and As(III) adsorption on TiO 2 .

As-O Shell As-Ti Shell

2 (Å 2 ) 2 (Å 2 ) R(Å) CN R(Å) CN

As(V) 1.69 ± 0.01 3.8 ± 0.7 0.0014 ± 0.0015 3.30 ± 0.02 2.5 ± 1.0 0.0065 ± 0.0070

As(III)

1.77 ± 0.01 3.0 ± 0.3 0.0029 ± 0.0074 3.35 ± 0.03 1.7 ± 0.8 0.0105 ± 0.0039

R(Å): interatomic distance; CN: coordination number; 2 : Debye-Waller parameter.

The first peak in the FT spectra (Figure 5.8-B) is the result of backscattering
from the nearest neighbor As-O shell. The average As-O distance is 1.69 Å and 1.77 Å
for As(V) and As(III) samples, respectively. The average coordination number (CN) of
oxygen is determined to be 3.8 and 3.0 for As(V) and As(III), respectively. EXAFS data
analysis has confirmed that the geometry of As(V) and As(III) is not changed by the
formation of TiO 2 surface complexes. The As-O interatomic distance and CN obtained
for the As(III) sample suggest that oxidation to As(V) has not occurred. The second peak
in the FT is attributed to As-Ti bonding with interatomic distances of 3.30 Å and 3.35 Å
for As(V) and As(III) samples, respectively. Fitting the As-Ti peak was completed in
both k space and R space using a single As-Ti shell, resulting in a CN of 2.5 and 1.7 for
the As(V) and As(III) samples, respectively. The EXAFS results show that As(V) and
As(III) form bidentate binuclear inner-sphere complexes on the surfaces of TiO 2 .

Density functional theory (DFT) is a quantum mechanical modeling approach
based on expressing the total energy of a system as a function of the electron density.
DFT has proven to be useful for studying arsenic adsorption processes on iron oxide
surfaces (Sherman and Randall, 2003; Zhang et al., 2005). Figure 5.9 illustrates the
molecular structure for As(V) adsorption on TiO 2 optimized using DFT calculation at
B3LYP/6-311+G(d, p) level. For simplicity, the four water molecules used in the cluster
are not shown in the figure. The DFT calculated As-O distance is 1.71 Å, which is an
average value of two uncomplexed bonds and two complexed bonds at 1.66 Å and 1.76
Å, respectively. The DFT estimated As-Ti distance is 3.35 Å. The DFT results are in
good agreement with EXAFS.

The FTIR spectra for As(V) adsorption on TiO 2 exhibited two bands at 808 and
830/cm corresponding to v(As-OTi) and v(As-O), respectively (Figure 5.10). This
observation is coincident with EXAFS and DFT results. The 830/cm corresponds to the
uncomplexed As-O bond which has an atomic distance at 1.66 Å. The peak at a lower
wavenumber of 808/cm, due to the stretching of the complexed As-OTi bond, has an As-
O distance at 1.76 Å.

The granular TiO 2 adsorbent, produced by agglomerating nanocrystalline
anatase, has been synthesized (Meng et al., 2005). The particle size of the granulated