Environmental Engineering Reference
In-Depth Information
The study reported that the batch process SODIS and solar photocatalytic methods are
effective against bacterial spores and fungi. However, protozoan cysts were not degraded
by both the methods. The solar photocatalytic method was 50% more eficient than the
SODIS method.
Another comparative study of SODIS and solar photocatalytic disinfection was con-
ducted by Dunlop et al. (2011). Water samples were taken in low-density polyethylene bags
of 2-L capacity. Degussa P-25 was used to improve the eficiency of SODIS. Artiicial sun-
light was provided by xenon lamps. A 10 7 C F U/m L E. coli solution was taken for the stud-
ies. The effect of turbidity reduced the eficiency but complete inactivation was obtained
under a longer irradiation time.
Gelover et al. (2006) studied the bacterial inactivation using 1000 CFU/mL total coli-
forms and 100 CFU/mL fecal coliforms. The bottles were placed in solar collectors. TiO 2
prepared by a sol-gel process was coated on Pyrex glass cylinders. It was found that addi-
tion of catalyst enhances the reaction. Bacterial regrowth was observed after the SODIS
treatment.
25.11.1 Mechanism of Bacterial Degradation Using TiO 2
When TiO 2 is irradiated using photons of suficient energy, electrons from the valence
band shift to the conduction band. Thus, electron-hole pairs are generated in the semi-
conductor. The electron, when it reaches the surface, reduces the electron acceptor
present in the solution. Similarly the photo-generated holes oxidize the electron donor
present in the water. The holes react with water molecules and produce hydroxyl radi-
cals. These radicals act as the primary oxidants in the photocatalytic system (Herrmann,
1999; Wong and Chu, 2003). Degradation depends on the hydroxyl radicals present in the
system. The relation between hydroxyl radicals and E. coli inactivation is linear (Cho et
al., 2004). If oxygen is present in the system, it is converted to superoxide radicals, thus
preventing electron-hole recombination. The superoxide radical is also reactive and can
oxidize cellular components. Contact between the bacteria and the catalyst particles is
very important. Gogniat et al. (2006) reported that adsorption of bacterial cells to the
TiO 2 particles and subsequent loss of membrane integrity are the key steps in bacterial
disintegration.
Matsunaga et al. (1985) reported that inactivation of E. coli takes place due to the changes
in coenzyme A, which inhibits the respiration of the cell. Studies have shown that dam-
age takes place from outward to inward. Sunada et al. (2003) reported that photokilling of
E. coli takes place in two steps. First, the outer membrane is damaged and then cytoplasmic
membrane damage occurs. After that, complete disintegration takes place. Initially, the cell
wall becomes permeable. Damage of cell wall layers leads to the leakage of ions from the
cell. Once the cell membrane is degraded, TiO 2 particles penetrate into the interior and fur-
ther damage occurs. The membrane damage occurring during photodegradation is shown
in Figure 25.11. It is reported that the nucleotide of E. coli changes because of the leakage
of ions (Chung et al., 2009). After the membrane damage, leakage of ions like K + occurs.
Then larger molecules such as proteins come out of the cell. Thus, irreversible damage to
the cell occurs. The internal components of the cell are completely degraded and complete
mineralization of the cell occurs (Foster et al., 2011). The mechanism of bacterial disintegra-
tion is shown in Figure 25.12.
Search WWH ::




Custom Search