Environmental Engineering Reference
In-Depth Information
Many studies have reported the possible mechanisms of bacterial disintegration. Photo-
catalysis produces hydroxyl radicals, which are responsible for the disintegration of bacteria.
Cho et al. (2004) reported that the relation between hydroxyl radicals and
E. coli
inactivation
is linear. Apart from hydroxyl radicals, superoxide radicals also oxidize the microorgan-
isms. The catalyst particles should be in contact with the
E. coli
because the hydroxyl radicals
produced have a very short lifetime in the range of nanoseconds. The disintegration starts
from the outer membrane and then the cytoplasmic membrane is degraded (Sunada et al.,
2004). These ions are leaked from the cell, which causes changes in the nucleotide of
E. coli
(Chung et al., 2009). The cell organelles get exposed, and the radicals oxidize and completely
mineralize the cell. Thus, the changes occurring to the cell are irreversible.
N-doped TiO
2
can be used for treating water using solar radiation since it gets photocat-
alytically activated under visible light. Antimicrobial activity of N-doped TiO
2
was stud-
ied using
E. coli
since it is a common indicative organism. The degradation studies were
carried out in batch and continuous reactors. Comparison of the activity of Degussa P-25
and N-doped TiO
2
was done. Degussa P-25 showed higher photocatalytic eficiency under
UV light compared with visible light. N-doped TiO
2
gave good results under visible light.
Seventy percent of the bacteria were degraded in just 10 min. The bacterial concentration
was completely removed within 1 h. The reaction followed pseudo-irst-order kinetics. The
activity of N-doped TiO
2
under UV and visible light is given in Figure 25.13.
Studies were also conducted in the immobilized form of catalyst (Arya, 2012). The cata-
lyst was coated onto a Pyrex glass cylinder of area 169 cm
2
and was kept inside the batch
reactor. The coated catalyst concentration was 0.4 mg/cm
2
. There was only a marginal
variation between the activity of the suspended and immobilized forms of catalyst.
Photocatalytic experiments were carried out in a thin ilm continuous photoreactor
under sunlight with N-doped TiO
2
. The schematic diagram of continuous reactor is given
in Figure 25.14.
The low rate of the raw water was optimized to 40 mL/min. It was found that a mini-
mum of 30 min was needed for the eficient removal of bacteria, with an initial concentra-
tion of 1000 CFU/mL. The outlet concentration of bacteria remained the same throughout
the operation of the reactor. The effect of turbidity, bicarbonate ions, and organic matter
1
UV
N TiO
2
Visible alone
N-doped under UV
0.8
0.6
0.4
0.2
0
0
40
60
100
140
20
80
120
Time (min)
FIGURE 25.13
Effect of N-doped TiO
2
and light source on bactericidal inactivation.
