246
K. M. G. Mostofa et al.
HO
•
. Therefore, the quantum yield of HO
• photoinduced generation from H 2 O 2
is double compared to that of H 2 O 2 photolysis (Hunt and Taube 1952; Baxendale
and Wilson 1956; Volman and Chen 1959).
4.6 Fenton Reaction: Effect of pH, Temperature and Salinity
The Fenton reaction depends on the presence of Fe 2+ (or Fe 3+ ) and H 2 O 2 in natural waters. The oxidation of Fe(II) with H 2 O 2 in seawater depends on pH (2–8.5),
temperature (5–45 °C) and salinity (0–35 g L –1 ) (Wells and Salam 1968; Moffett
and Zika 1987a, b; Gallard et al. 1998; Bossmann et al. 1998; Duesterberg et al.
2008; Millero and Sotolongo 1989; de Laat and Gallard 1999; Duesterberg
and Waite 2006; Duesterberg et al. 2005; Farias et al. 2007; Jung et al. 2009).
The rate constants of the chain Fenton reactions and the relevant dependence on
pH are presented in Table 7 (Kwan and Voelker 2002). The reaction rate constant
between Fe 2+ and H 2 O 2 significantly increases with pH in the range from 3 to 5
(Eq. 4.18; Table 7), and a similar effect is observed with some of the chain reactions (Eqs. 4.19–4.26; Table 7).
A recent study that has been carried out in the pH range 2.5–4.0, both in the
presence and absence of a target organic substance (formic acid), also highlights the importance of the Fenton system in the catalytic redox cycling of iron
(Duesterberg et al. 2008). Supply of oxygen can enhance the efficiency of the
Fenton oxidation, which is understandably attenuated by competition with the
organic intermediates in the reaction media (Sychev and Isak 1995; Duesterberg
et al. 2005).
It is shown that the addition of phosphotungstate (PW 12 O 40
3− ), a polyoxometalate, extends the working pH range of the Fenton system (Fe 3+ /H 2 O 2 ) up
Table 7 The reaction rate constants for generation of HO• in Fenton reaction at various pH
ranges in the absence of light
Reaction type
pH 3
pH 4
pH 5
Equations
Reaction rate constant (k) (M −1 s −1 )
Fe (II) + H 2 O 2 → Fe (III) + HO
• + OH −
63
1.2 × 10 2 5.7 × 10 2
4.18
Fe (III) + H 2 O 2 → Fe (II) + HO 2
• /O 2
− + H + 2 × 10 −3 2.5 × 10 −3 2.6 × 10 −3 4.19
H 2 O 2 + HO
• → HO 2
• /O 2
− + H 2 O
3.7 × 10 7 3.3 × 10 7 3.3 × 10 7
4.20
Fe (III) + HO 2
•
/O 2
− → Fe (II) + O 2 + H +
7.8 × 10 5 6.8 × 10 6 3.1 × 10 7
4.21
Fe (II) + HO
• → Fe (III) + OH −
3.2 × 10 8 3.2 × 10 8 3.2 × 10 8
4.22
Fe (II) + HO 2
• /O 2
− → Fe (III) + HO 2
•
1.3 × 10 6 2.4 × 10 6 6.6 × 10 6
4.23
HO• + HO
• → H 2 O 2
5.2 × 10 9 5.2 × 10 9 5.2 × 10 9
4.24
HO 2
• /O 2
− + HO 2
•
/O 2
− → H 2 O 2
2.3 × 10 6 1.2 × 10 7 2.3 × 10 7
4.25
HO
• + HO 2
• /O 2
− → H 2 O + O 2
7.1 × 10 9 7.5 × 10 9 8.9 × 10 9
4.26
Data source (Kwan and Voelker 2002)
K. M. G. Mostofa et al.
HO
•
. Therefore, the quantum yield of HO
• photoinduced generation from H 2 O 2
is double compared to that of H 2 O 2 photolysis (Hunt and Taube 1952; Baxendale
and Wilson 1956; Volman and Chen 1959).
4.6 Fenton Reaction: Effect of pH, Temperature and Salinity
The Fenton reaction depends on the presence of Fe 2+ (or Fe 3+ ) and H 2 O 2 in natural waters. The oxidation of Fe(II) with H 2 O 2 in seawater depends on pH (2–8.5),
temperature (5–45 °C) and salinity (0–35 g L –1 ) (Wells and Salam 1968; Moffett
and Zika 1987a, b; Gallard et al. 1998; Bossmann et al. 1998; Duesterberg et al.
2008; Millero and Sotolongo 1989; de Laat and Gallard 1999; Duesterberg
and Waite 2006; Duesterberg et al. 2005; Farias et al. 2007; Jung et al. 2009).
The rate constants of the chain Fenton reactions and the relevant dependence on
pH are presented in Table 7 (Kwan and Voelker 2002). The reaction rate constant
between Fe 2+ and H 2 O 2 significantly increases with pH in the range from 3 to 5
(Eq. 4.18; Table 7), and a similar effect is observed with some of the chain reactions (Eqs. 4.19–4.26; Table 7).
A recent study that has been carried out in the pH range 2.5–4.0, both in the
presence and absence of a target organic substance (formic acid), also highlights the importance of the Fenton system in the catalytic redox cycling of iron
(Duesterberg et al. 2008). Supply of oxygen can enhance the efficiency of the
Fenton oxidation, which is understandably attenuated by competition with the
organic intermediates in the reaction media (Sychev and Isak 1995; Duesterberg
et al. 2005).
It is shown that the addition of phosphotungstate (PW 12 O 40
3− ), a polyoxometalate, extends the working pH range of the Fenton system (Fe 3+ /H 2 O 2 ) up
Table 7 The reaction rate constants for generation of HO• in Fenton reaction at various pH
ranges in the absence of light
Reaction type
pH 3
pH 4
pH 5
Equations
Reaction rate constant (k) (M −1 s −1 )
Fe (II) + H 2 O 2 → Fe (III) + HO
• + OH −
63
1.2 × 10 2 5.7 × 10 2
4.18
Fe (III) + H 2 O 2 → Fe (II) + HO 2
• /O 2
− + H + 2 × 10 −3 2.5 × 10 −3 2.6 × 10 −3 4.19
H 2 O 2 + HO
• → HO 2
• /O 2
− + H 2 O
3.7 × 10 7 3.3 × 10 7 3.3 × 10 7
4.20
Fe (III) + HO 2
•
/O 2
− → Fe (II) + O 2 + H +
7.8 × 10 5 6.8 × 10 6 3.1 × 10 7
4.21
Fe (II) + HO
• → Fe (III) + OH −
3.2 × 10 8 3.2 × 10 8 3.2 × 10 8
4.22
Fe (II) + HO 2
• /O 2
− → Fe (III) + HO 2
•
1.3 × 10 6 2.4 × 10 6 6.6 × 10 6
4.23
HO• + HO
• → H 2 O 2
5.2 × 10 9 5.2 × 10 9 5.2 × 10 9
4.24
HO 2
• /O 2
− + HO 2
•
/O 2
− → H 2 O 2
2.3 × 10 6 1.2 × 10 7 2.3 × 10 7
4.25
HO
• + HO 2
• /O 2
− → H 2 O + O 2
7.1 × 10 9 7.5 × 10 9 8.9 × 10 9
4.26
Data source (Kwan and Voelker 2002)