Solar driven advanced oxidation processes for water decontamination and disinfection - Solar Energy Sciences and Engineering Applications - page 408

to the range of experimental conditions. Development of a standardized testing proto-
col must be considered another interesting research challenge to the field deployment
of solar driven photocatalytic processes.

13.5.3 Visible light active photocatalyst materials
As a result of its large band gap (3.2 eV), TiO 2 semiconductors exhibits photocatalytic
activity only within UV radiation wavelengths ( 400 nm). This specific characteristic

limits the photosensitivity to the UV part of the solar spectrum and is an important tech-
nological limitation. Since sunlight consists of only about 5% UV radiation, efficiency
enhancements are needed to enhance the viability of SDHPC processes. A recent emerg-
ing field of research is the development of photocatalytic materials excitable by visible
solar radiation, which account for 45% of the solar spectrum. Several modifications to
TiO 2 have produced visible radiation active materials with improved photosensitivity
and quantum yield. Several ways to get this so-called daylight photocatalysis have been
reported recently including dye sensitization, coupling TiO 2 with other semiconduc-
tors possessing favorable band gaps and potentials, surface deposition of metal clusters,
and doping the crystal lattice with metals (Fe, Co, Ag) and/or nonmetal foreign atoms
(N, C, F, S).
According to the literature, one of the more promising approaches to achieve vis-
ible light active TiO 2 is by doping with nonmetal elements including N and S. After
initial reports of visible-light photo-active nitrogen-doped TiO 2 , many groups have
demonstrated that anion-doped TiO 2 has extended optical absorbance into the visible
region (Asahi et al., 2001). However, the number of publications concerning the pho-
tocatalytic activity of these materials for SDPC processes is still limited. In many cases,
the UV activity of undoped TiO 2 has been reported much greater than the visible light
activity of the doped material (Reginfo-Herrera and Pulgarin, 2010). Therefore, pho-
tocatalysts developed for SDHOC applications should be tested under simulated solar
irradiation or, preferably, under real sun conditions. However, in agreement with these
authors, N-doped TiO 2 materials did not exhibit enhanced photocatalytic degradation
of phenol or the photocatalytic inactivation of E. coli under simulated solar light, as
compared to Degussa P25. They suggest that although N, or N-S co-doped TiO 2 may
show visible light response, the localized states responsible for visible light absorption
are not important in the photocatalytic activity. Other studies, however, report that
although solar visible radiation displays a lower activity than solar UV radiation it is
possible to observe interesting photocatalytic activity for N-doped TiO 2 under these
conditions and the overall effect of using complete (UV+visible) radiation is higher than
that observed for regular TiO 2 using only UV solar radiation (Castillo et al., 2011).
More research is required to determine if visible light active materials can deliver an
increased efficiency of photocatalytic processes under solar radiation.

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13.6 CONCLUSIONS

Solar driven AOPs were demonstrated as cost-effective emerging methodologies for
water decontamination and disinfection with very interesting advantages compared
with conventional water and wastewater treatment processes such as higher efficiency,