Nanoparticles for Treatment of Arsenic - Nanotechnologies for Water Environment Applications - page 130

predicting the TiO 2 adsorption capacity for these four arsenic species simultaneously.
The adsorption isotherm has been established as shown in Figure 5.15.

100

80

60

MMA As(III)

40

20

As(V) DMA

0

0 100 200 300 400

As concentration (μg/L)

Figure 5.15 Adsorption isotherms for As(V), As(III), MMA and DMA in a groundwater
sample.

Experimental results indicate that TiO 2 is not effective for DMA removal in this
case. Equilibrium adsorption can be reached at approximately 55, 70 and 90 mg/kg for
MMA, As(III), and As(V), respectively (Figure 5.15). The groundwater analysis
throughout the period of investigation indicates that the average concentrations for iron,
manganese, phosphate, and silicate are 1.2, 0.037, 0.074, and 2.6 mg/L, respectively.
The adverse effect of the competitive ions for the available surface adsorption sites has
been well documented (Meng et al. 2000; Meng et al. 2002).
5.3.6 TiO 2 Photocatalysis for Arsenic Removal

Photocatalytical oxidation of As(III), MMA and DMA to As(V) using
nanocrystalline TiO 2 has been well studied (Bissen et al., 2001; Lee and Choi, 2002;
Dutta et al., 2005; Pena et al., 2005; Ferguson et al., 2005, Ferguson and Hering, 2006;
Xu et al., 2007). TiO 2 is a semiconductor, and adsorption of light with wavelength
shorter than 387.5 nm leads to a charge separation due to an electron promotion to the
conduction band and a generation of a hole (h + ) in the valence band. These charge
carriers may migrate to the particle surface where they are trapped. Usually, electron
transfer to dissolved oxygen, which acts as a primary electron acceptor, is the rate-
determining step in photocatalysis. The photogenerated electrons react with adsorbed