Chemistry Reference
In-Depth Information
by contributing to the production of
•
OH radicals. Experimental and model
calculations have shown the production of ferryl (Fe
IV
O
2+
) as an active inter-
mediate in the Fenton reaction [319-321]. Formation of ferryl species has also
been suggested in the photoassisted Fenton reaction [322, 323].
The following sections present basic chemistry and reactivity of the high-
valent iron species.
6.3.1 Iron(IV) and Iron(V)
6.3.1.1 Ferryl(IV) Ion.
The ferryl ion, FeO
2+
, has been generated by the
oxidation of Fe
2+
by ozone in aqueous acidic solution (pH 0-3) (Eq. 6.86):
Fe
2
+
+ →
O
FeO
2
+
+
O
k
=
8 2 10
.
×
5
/M/s
.
(6.86)
3
2
96
The spectrum of FeO
2+
includes a small broad peak around 320 nm
(ε
320nm
≈ 500/M/cm) with a continuum that grows in the uV region (Fig. 6.24a)
[321, 324]. The +4 oxidation state for the ferryl ion was confirmed by a Möss-
bauer spectroscopic technique [266, 325]. Experimental data and DFT calcula-
tions were consistent with the high spin (
S
= 2) of the ferryl species [325].
The absorption at 320 nm for the FeO
2+
ion was used to study its decay as
a function of pH in the acidic pH region [178]. An increase in the observed
first-order rate constant,
k
(/s), with an increase in pH was observed. The decay
rate increased approximately 50 times with an increase in pH. The results were
interpreted based on the protolytic equilibrium between the two different
hydrolytic forms of the ferryl ion with p
K
a
≈ 2.0.
The decay of FeO
2+
is expressed by reaction (6.87):
4
FeO
2
+
+
4
H
+
→
4
Fe
3
+
+
O
+
2
H O
.
(6.87)
2
2
The half-life of the ferryl ion is on the order of minutes in the pH range of
0-1 but decreases as the pH is increased. The following sequence of reactions
explains the decay of the FeO
2+
ion [326]:
2
+
3
+
•
−
−
2
FeO
+
H O Fe
→
+
OH OH
+
k
=
1 3 10
.
×
/s
(6.88)
2
98
2
+
•
−
3
+
7
FeO
+
OH OH
+
→
Fe
+
H O
k
=
1 3 10
.
×
/M/s
(6.89)
2
2
99
FeO
2
+
+
H O
→
Fe
3
+
+
HO OH
+
−
k
=
1 0 10
.
×
4
/M/s
(6.90)
2
2
2
100
2
+
3
+
−
6
FeO
+
HO
→
Fe
+
O OH
+
k
=
2 0 10
.
×
/M/s
.
(6.91)
2
2
101
All of the reactions participate at low pH, but only reactions (6.88) and
(6.89) dominate at micromolar concentrations of the ferryl ion. The rate-
determining step of the decay is expressed by reaction (6.88). The activation
energy of the decay of the ferryl ion at pH 0 was determined as 34.0 ± 0.3.0
kJ/mol [326].
Search WWH ::
Custom Search