Chapter 11
Photo-Fenton Reaction
Fenton reaction was named after Henry J. Fenton, who first reported the activation of
H 2 O 2 by Fe
2+ for the oxidization of tartaric acid [1]. The reaction system has now
expanded to heterogeneous system based on metal or metal oxide and versatile
peroxides including H 2 O 2 , persulfate, monopersulfate, etc. It has become the most
useful method for the degradation of organic pollutants in aqueous solution.
The classical mechanism of Fenton process is a simple redox reaction, where Fe
2+
is oxidized to Fe
3+ and H 2 O 2 is reduced to hydroxide ion (OH
À ), simultaneously
forming the hydroxyl radical (HO
• ) with strong oxidation potential (E
0
¼ 2.73 V vs
NHE, Eq. 11.1). In the conventional Fenton reaction, the ferric ion produced in
Eq. 11.1 can be reduced back to ferrous ion by H 2 O 2 (Eq. 11.2). The Fe
3+ reduction
(0.02 M
À1 s
À1 , Eq. 11.2) is much slower than the Fe
2+ oxidation (40–80 M
À1 s
À1 ,
Eq. 11.1), which causes the slowing down of the Fenton reaction and requires the
great amount of Fe
2+ in the initial step to achieve the complete mineralization of
organics [2]. Hydroxyl radical can be scavenged by H 2 O 2 (Eq. 11.3) and Fe
2+
(Eq. 11.4).
Fe
2þ
þ H 2 O 2 ! Fe
3þ
þ HO
•
þ OH
À
ð11:1Þ
Fe
3þ
þ H 2 O 2 þ H 2 O ! Fe
2þ
þ H 3 O
þ
þ HO 2
• À
ð11:2Þ
HO
•
þ H 2 O 2 ! HO 2
• À
þ H 2 O
ð11:3Þ
HO
•
þ Fe
2þ
! Fe
3þ
þ OH
À
ð11:4Þ
For the degradation of organic molecules, the optimum pH for the Fenton reaction
is typically in the range of 2–3 and the optimum mass ratio of iron to H 2 O 2 is ca. 1.5.
Due to the slow regeneration of Fe
2+ , a great amount of iron ion needs to be used,
thus resulting in the large-scale sludge generation from iron and poor reusability of
the catalyst. It was later realized that Fenton reactions were markedly accelerated by
light (photo-Fenton reactions) concerning the reaction speed and the mineralization
degree of organics. This chapter discusses the complex mechanism of photo-Fenton
© Springer Nature Singapore Pte Ltd. 2018
J. Zhang et al., Photocatalysis, Lecture Notes in Chemistry 100,
https://doi.org/10.1007/978-981-13-2113-9_11
259
Photo-Fenton Reaction
Fenton reaction was named after Henry J. Fenton, who first reported the activation of
H 2 O 2 by Fe
2+ for the oxidization of tartaric acid [1]. The reaction system has now
expanded to heterogeneous system based on metal or metal oxide and versatile
peroxides including H 2 O 2 , persulfate, monopersulfate, etc. It has become the most
useful method for the degradation of organic pollutants in aqueous solution.
The classical mechanism of Fenton process is a simple redox reaction, where Fe
2+
is oxidized to Fe
3+ and H 2 O 2 is reduced to hydroxide ion (OH
À ), simultaneously
forming the hydroxyl radical (HO
• ) with strong oxidation potential (E
0
¼ 2.73 V vs
NHE, Eq. 11.1). In the conventional Fenton reaction, the ferric ion produced in
Eq. 11.1 can be reduced back to ferrous ion by H 2 O 2 (Eq. 11.2). The Fe
3+ reduction
(0.02 M
À1 s
À1 , Eq. 11.2) is much slower than the Fe
2+ oxidation (40–80 M
À1 s
À1 ,
Eq. 11.1), which causes the slowing down of the Fenton reaction and requires the
great amount of Fe
2+ in the initial step to achieve the complete mineralization of
organics [2]. Hydroxyl radical can be scavenged by H 2 O 2 (Eq. 11.3) and Fe
2+
(Eq. 11.4).
Fe
2þ
þ H 2 O 2 ! Fe
3þ
þ HO
•
þ OH
À
ð11:1Þ
Fe
3þ
þ H 2 O 2 þ H 2 O ! Fe
2þ
þ H 3 O
þ
þ HO 2
• À
ð11:2Þ
HO
•
þ H 2 O 2 ! HO 2
• À
þ H 2 O
ð11:3Þ
HO
•
þ Fe
2þ
! Fe
3þ
þ OH
À
ð11:4Þ
For the degradation of organic molecules, the optimum pH for the Fenton reaction
is typically in the range of 2–3 and the optimum mass ratio of iron to H 2 O 2 is ca. 1.5.
Due to the slow regeneration of Fe
2+ , a great amount of iron ion needs to be used,
thus resulting in the large-scale sludge generation from iron and poor reusability of
the catalyst. It was later realized that Fenton reactions were markedly accelerated by
light (photo-Fenton reactions) concerning the reaction speed and the mineralization
degree of organics. This chapter discusses the complex mechanism of photo-Fenton
© Springer Nature Singapore Pte Ltd. 2018
J. Zhang et al., Photocatalysis, Lecture Notes in Chemistry 100,
https://doi.org/10.1007/978-981-13-2113-9_11
259