Nanoparticles for Treatment of Arsenic - Nanotechnologies for Water Environment Applications - page 129

The CN of carbon atoms are one and two for MMA and DMA, respectively. EXAFS
data analysis confirmed the tetrahedral geometry of MMA and DMA where the
hydroxyl groups of As(V) are replaced by the methyl group. The primary structure
change in the adsorbed organic arsenic on TiO 2 is demonstrated by the appearance of a
distant atomic shell in the FT spectra at 3.32-3.37 Å as shown in Figure 5.13. The
coordination number of this As-Ti shell is 1 and 2 for DMA and MMA, respectively.
The results indicate that MMA and DMA formed bidentate and monodentate inner
sphere surface complexes with TiO 2 , respectively. However, the As-Ti distance (3.37 ±
0.04 Å) for the DMA surface complex is shorter than the algebraic sum of As-O (1.69 Å)
and Ti-O (1.90 Å). This could be explained by the formation of hydrogen bonds between
the oxygen atom in DMA and the adjacent (protonated) hydroxyl group on TiO 2 surface,
which would attract the As tetrahedra closer to the surface.

Figure 5.14 illustrates a significant shift of the isoelectric point (IEP) to lower pH
values for MMA and DMA loaded TiO 2 due to the anion adsorption behavior of MMA
and DMA. The adsorption of MMA results in a greater decrease in the potential than
that of DMA, which may be caused by the bidentate vs. monodentate surface
complexation.

20

Ti O 2
DMA
MMA

10

0

2 4 6 8 10

pH

-10

-20

-30

Figure 5.14 Zeta potential of blank, MMA, and DMA adsorbed TiO 2 as a function of
pH in 0.04 M KNO 3 solution.

The granular TiO 2 adsorbent has been applied in groundwater arsenic
remediation at a superfund site exhibiting a total arsenic concentration of 994 μg/L and a
pH of 5.84. The groundwater contains 367.5, 200.3, 121.8, and 304.7 μg/L of As(V),
DMA, MMA, and As(III), respectively. A study has been performed with the purpose of