Nanoscale Porous Materials for Water Treatment: Advances and Challenges - Nanotechnologies for Water Environment Applications - page 329

During the last decade, TiO 2 nanoparticles have emerged as promising
photocatalysts for water purification (Adesina, 2004). This is mainly due to their
versatile reactivity, being able to serve both as oxidative and reductive catalysts for
organic and inorganic pollutants, and its ability to completely mineralize toxic and
nonbiodegradable organics to non-toxic end products such as CO 2 , H 2 O, and inorganic
constituent (Yamashita et al., 2007). Recently, several reviews on the utilization of
photocatalysts for water treatment have been reported (Kabra et al., 2004; Savage and
Diallo, 2005; Narr et al., 2007). In general, TiO 2 nanoparticles have been used
extensively to (a) degrade organic compounds (e.g., chlorinated alkanes and benzenes,
dioxins, furans, PCBs, etc.) and (b) reduce toxic metal ions [e.g., Cr(VI), Ag(I) and
Pt(II)] in aqueous solutions under UV light (Savage and Diallo, 2005). Chitose et al.
(2003) reported the removal of total organic carbon from water contaminated with
organic wastes using TiO 2 nanoparticles in the presence of ultraviolet light. One of the
most cited studies in the field is the report published by Ashasi et al. (2001). They
synthesized N-doped TiO 2 nanoparticles that were capable of photodegrading methylene
blue under visible light. Visible light-activated TiO 2 nanoparticles based on TiO 2
modified by ruthenium-complex sensitizers have synthesized. Bae and Choi (2003)
prepared Pt/TiO 2 /RuIIL 3 nanoparticles which drastically enhanced the rate of reductive
dehalogenation of trichloroacetate and carbon tetrachloride in aqueous solutions under
visible light (Bae and Choi, 2003). Similarly, Kominami et al. (2003) prepared
Titanium(IV) oxide (TiO2) nanoparticles with various physical properties by
hydrothermal crystallization in organic media (HyCOM) and post-calcination. The
products were used for photocatalytic decomposition of malachite green (MG) in an
aqueous suspension under aerated conditions. The amount of MG adsorbed on TiO 2
([MG](ad)) increased as the surface area of HyCOM TiO 2 increased. Recently,
Venkatachalam et al. (2007) prepared nano TiO 2 having a surface area of 69107 m 2 /g
and a size of 8.317nm and evaluated its photocatalytic oxidation by taking bisphenol-A
as a model compound. Under the optimum conditions, nano TiO 2 showed higher
photocatalytic activity for the degradation of bisphenol-A than commercial TiO 2
(Degussa P-25).
Recently, Alvaro et al. (2006) synthesized a variety of mesoporous TiO 2
nanoparticles in combination with tetraethyl orthosilicate using neutral pluronic or
cationic cetyltrimethylammonium as templates. The pore diameter was ranged between
3.8 and 10.9 nm, and the BET surface area varied from 99 to 584 m 2 /g. The
photocatalytic activity of these samples for the degradation of phenol in aqueous
solution has been compared with that of standard P-25 TiO 2 . Even though the activity of
these mesostructured materials is lower than those found for P-25 TiO 2 , the turnover
frequency of the photocatalytic activity (moles of phenol degraded per Ti atom present
at initial reaction time) is much higher for the mesoporous titania, particularly with low
titanium contents for those materials (mpTiO 2 -5 and TiO 2 SBA15-5). Similarly, Pena et
al. (2006) reported arsenate [As(V)] and arsenite [As(III)] interactions at the solid-water