Environmental Engineering Reference
In-Depth Information
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 con-
sidering 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
):
K
P
,
[
P
]
−→
(4.36)
HO
•
+
P
rection products
K
S
,
[
S
]
−−−−−→
HO
•
+
S
scavenging products
(4.37)
•
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
scav-
enging 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
):
S]
}
[ HO
•
(4.38)
R
S
={
K
P
[P]
+
K
S
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
):
HO
•
(4.39)
ss
=
r
HO
/
k
s
[S]
The oxidation rate (Ms
-1
) of the probe compound in an irradiated system (con-
version per unit time) is described as (Eq.
4.40
) (Zepp et al.
1992
):
−
d [P]
/
dt
=
k
P
HO
•
ss
[P]
=
k
[P]
(4.40)
If the concentrations of the photoactive Fe(III) species, H
2
O
2
, and the scaven-
gers 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 pseudo-
first 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
):
r
HO
=
k
k
s
[S]
/(
k
P
)
(4.41)