HO 2
•
þ H 2 O 2 ! H 2 O þ O 2 þ HO
•
ð1:15Þ
Termination : HO
•
þ HO
•
! H 2 O 2
ð1:16Þ
HO
•
þ HO 2
•
! H 2 O þ O 2
ð1:17Þ
HO 2
•
þ HO 2
•
! H 2 O 2 þ O 2
ð1:18Þ
H 2 O 2 molecules can act as a scavenger to consume HO
• and subsequently produces oxygen and water molecules according to Eq. (1.19), so the demand concentration of H 2 O 2 must be high to generate a sufficiently high concentration of HO
• for
decomposition and mineralization of organic target pollutants.
2HO
•
þ 3H 2 O 2 ! 4H 2 O þ 2O 2
ð1:19Þ
UV/H 2 O 2 /O 3
From ozonation discussed above, although UV absorptivity of O 3 is much higher
than H 2 O 2 , the self-decay rate of O 3 is approximately 1000 times higher than that of
H 2 O 2 . This limitation can be overcome by the addition of H 2 O 2 into UV/O 3 process
for enhancement of the decomposition of O 3 , which is called “UV/H 2 O 2 /O 3 ”
process.
Even though homogeneous photocatalysis has many advantages, e.g., high oxidation properties, it is, however, not popular in various photocatalytic applications.
This is because it is difficult to separate the photocatalysts from the solution; the
photocatalysts have a low potential to reuse; purification of products is necessary;
and almost homogeneous photocatalysts absorb narrowly light within the solar
spectrum (Karimian et al. 2015; Zhu and Wang 2017). In addition, the photocatalytic
activity and stability of homogeneous photocatalysts are limited due to the instability
inherent to the molecular nature of their structures (Limburg et al. 2016; Ye et al.
2016).
Heterogeneous Photocatalysis
Many years ago, heterogeneous photocatalysis was found by Fujishima and Honda
due to an electrochemical photocatalysis of water at a semiconductor electrode. In
heterogeneous photocatalysis, the catalyst is totally separate from the reactants. It
occurs by emerging materials with supremacy properties (Klavarioti et al. 2009). On
the basis of band gap energy – the differential energy between the valence band (the
1 Photocatalytic Remediation of Organic Pollutants in Water
7
•
þ H 2 O 2 ! H 2 O þ O 2 þ HO
•
ð1:15Þ
Termination : HO
•
þ HO
•
! H 2 O 2
ð1:16Þ
HO
•
þ HO 2
•
! H 2 O þ O 2
ð1:17Þ
HO 2
•
þ HO 2
•
! H 2 O 2 þ O 2
ð1:18Þ
H 2 O 2 molecules can act as a scavenger to consume HO
• and subsequently produces oxygen and water molecules according to Eq. (1.19), so the demand concentration of H 2 O 2 must be high to generate a sufficiently high concentration of HO
• for
decomposition and mineralization of organic target pollutants.
2HO
•
þ 3H 2 O 2 ! 4H 2 O þ 2O 2
ð1:19Þ
UV/H 2 O 2 /O 3
From ozonation discussed above, although UV absorptivity of O 3 is much higher
than H 2 O 2 , the self-decay rate of O 3 is approximately 1000 times higher than that of
H 2 O 2 . This limitation can be overcome by the addition of H 2 O 2 into UV/O 3 process
for enhancement of the decomposition of O 3 , which is called “UV/H 2 O 2 /O 3 ”
process.
Even though homogeneous photocatalysis has many advantages, e.g., high oxidation properties, it is, however, not popular in various photocatalytic applications.
This is because it is difficult to separate the photocatalysts from the solution; the
photocatalysts have a low potential to reuse; purification of products is necessary;
and almost homogeneous photocatalysts absorb narrowly light within the solar
spectrum (Karimian et al. 2015; Zhu and Wang 2017). In addition, the photocatalytic
activity and stability of homogeneous photocatalysts are limited due to the instability
inherent to the molecular nature of their structures (Limburg et al. 2016; Ye et al.
2016).
Heterogeneous Photocatalysis
Many years ago, heterogeneous photocatalysis was found by Fujishima and Honda
due to an electrochemical photocatalysis of water at a semiconductor electrode. In
heterogeneous photocatalysis, the catalyst is totally separate from the reactants. It
occurs by emerging materials with supremacy properties (Klavarioti et al. 2009). On
the basis of band gap energy – the differential energy between the valence band (the
1 Photocatalytic Remediation of Organic Pollutants in Water
7