251
Photoinduced Generation of Hydroxyl Radical in Natural Waters
4.7.1 Kinetics of the Photo-Fenton Reaction
The kinetics of the photo-Fenton reaction can be determined as a function of
pH, based on the yield of HO
• formed per Fe(II) oxidized by H 2 O 2 , and considering the photoreactions of aqueous organic substrates (Zepp et al. 1992;
Hoigné et al. 1988). Under illumination with constant irradiance of a diluted
probe compound (P) that reacts with HO
• (Eq. 4.36), the hydroxyl radical would
rapidly reach a steady-state concentration. In the presence of P and of other HO
•
scavengers (S), the decay of HO
• can be expressed as follows (Eqs. 4.36, 4.37)
(Zepp et al. 1992):
where k p is the second-order rate constant (M –1 s –1 ) for the reaction of HO
• with
the probe P, and ∑k s [S] is the pseudo-first order rate constant (s –1 ) for HO
• scavenging by all the components present in the reaction medium, except the probe
compound.
The scavenging rate of HO
• can be expressed as (Eq. 4.38) (Zepp et al. 1992):
If the concentration of P or the reaction rate for the P is sufficiently low (i.e.,
∑k s [S] » k P [P]), it is r S = (∑k s [S])[HO
•
]. Under the steady-state condition the rate
of generation of HO
• is r OH = r S , from which the hydroxyl radical concentration
becomes (Eq. 4..39) (Zepp et al. 1992):
The oxidation rate (Ms –1 ) of the probe compound in an irradiated system (conversion per unit time) is described as (Eq. 4.40) (Zepp et al. 1992):
If the concentrations of the photoactive Fe(III) species, H 2 O 2 , and the scavengers show a negligible variation as compared to [P], both r OH and ∑k s [S] (and
[HO
• ] ss as a consequence) would be about constant. That would give a pseudofirst order reaction with rate constant k. If the second-order rate constant, k P and
the scavenging rate constant, ∑k s [S] are known, then r OH can be determined from
k by the following equation (Eq. 4.41) (Zepp et al. 1992):
(4.36)
HO
• + P
K P , [P]
−→ rection products
(4.37)
HO
• + S
K s , [S]
−−−−−→ scavenging products
(4.38)
r S = {k P [P] +
k s
S ]}[ HO
•
(4.39)
HO
•
ss
= r HO /
k s [S]
(4.40)
−d [P] /dt = k P
HO
•
ss
[P] = k [P]
(4.41)
r HO = k
k s [S]
/(k P )
Photoinduced Generation of Hydroxyl Radical in Natural Waters
4.7.1 Kinetics of the Photo-Fenton Reaction
The kinetics of the photo-Fenton reaction can be determined as a function of
pH, based on the yield of HO
• formed per Fe(II) oxidized by H 2 O 2 , and considering the photoreactions of aqueous organic substrates (Zepp et al. 1992;
Hoigné et al. 1988). Under illumination with constant irradiance of a diluted
probe compound (P) that reacts with HO
• (Eq. 4.36), the hydroxyl radical would
rapidly reach a steady-state concentration. In the presence of P and of other HO
•
scavengers (S), the decay of HO
• can be expressed as follows (Eqs. 4.36, 4.37)
(Zepp et al. 1992):
where k p is the second-order rate constant (M –1 s –1 ) for the reaction of HO
• with
the probe P, and ∑k s [S] is the pseudo-first order rate constant (s –1 ) for HO
• scavenging by all the components present in the reaction medium, except the probe
compound.
The scavenging rate of HO
• can be expressed as (Eq. 4.38) (Zepp et al. 1992):
If the concentration of P or the reaction rate for the P is sufficiently low (i.e.,
∑k s [S] » k P [P]), it is r S = (∑k s [S])[HO
•
]. Under the steady-state condition the rate
of generation of HO
• is r OH = r S , from which the hydroxyl radical concentration
becomes (Eq. 4..39) (Zepp et al. 1992):
The oxidation rate (Ms –1 ) of the probe compound in an irradiated system (conversion per unit time) is described as (Eq. 4.40) (Zepp et al. 1992):
If the concentrations of the photoactive Fe(III) species, H 2 O 2 , and the scavengers show a negligible variation as compared to [P], both r OH and ∑k s [S] (and
[HO
• ] ss as a consequence) would be about constant. That would give a pseudofirst order reaction with rate constant k. If the second-order rate constant, k P and
the scavenging rate constant, ∑k s [S] are known, then r OH can be determined from
k by the following equation (Eq. 4.41) (Zepp et al. 1992):
(4.36)
HO
• + P
K P , [P]
−→ rection products
(4.37)
HO
• + S
K s , [S]
−−−−−→ scavenging products
(4.38)
r S = {k P [P] +
k s
S ]}[ HO
•
(4.39)
HO
•
ss
= r HO /
k s [S]
(4.40)
−d [P] /dt = k P
HO
•
ss
[P] = k [P]
(4.41)
r HO = k
k s [S]
/(k P )