Nanoparticles for Treatment of Arsenic - Nanotechnologies for Water Environment Applications - page 123

with the first dissociation constant of H 3 AsO 3 (pK 1 = 9.3). Such adsorption behavior has
often been observed for anions of weak acids and specifically for As(III) adsorbed on
iron (hydr)oxides (Meng et al., 2000).

100

80

60

As(III)

40

20

As(V)

0

2 4 6 8 10 12 14

pH

Figure 5.6 The experimental adsorption edge for As(V) (closed triangle) and As(III)
(open diamond) on 0.2 g/L TiO 2 in 0.04 M NaCl. Total As(V) and As(III) concentration
= 1.0 mg/L.

To further understand the adsorption mechanisms of As(V) and As(III) at the
solid-water interface of nanocrystalline TiO 2 , the electrophoretic mobility (EM)
measurements, Fourier transform infrared (FTIR) spectroscopy, and extended X-ray
absorption fine structure (EXAFS) spectroscopy were employed.

Figure 5.7 shows the zeta potentials of TiO 2 in the presence and absence of 50 or
100 μg/L As(V) or As(III). The adsorption of either 50 μg/L As(V) or As(III) decreases
the pH pzc from 5.8 for pristine TiO 2 to approximately 5.2. In the presence of the higher
As(V) or As(III) concentration, the PZC of TiO 2 is shifted to an even lower pH value.
With increasing As concentrations, the pH pzc of TiO 2 shifted from 5.8 to 5.2 and 4.8 in
the presence of 50 and 100 μg/L As, respectively. The pH pzc of metal oxides is
determined by protonation and deprotonation of surface hydroxyl groups. The formation
of outer-sphere surface complexes cannot shift the pH pzc . The shift of pH pzc to a lower
pH range is evidence of the formation of anionic negatively charged surface complexes.
Therefore, the results imply that both As(V) and As(III) formed negatively charged
inner-sphere complexes on TiO 2 . The formation of negatively charged As(V) surface
complexes was expected because anionic H 2 AsO 4 - and HAsO 4 2- are the predominant
As(V) species in the pH range between 3 and 11. However, the formation of negatively
charged As(III) surface species was not expected because the predominant aqueous
As(III) species is neutral H 3 AsO 3 at pH < 9. It could be possible that the adsorption
reaction enhanced the deprotonation of surface As(III).