Environmental Engineering Reference
In-Depth Information
Bielski et al.
1985
; Mulazzani et al.
1986
; Jeong and Yoon
2004
; Buxton et al.
1988
; Pignatello
1992
; Zuo and Hoigné
1992
; Sedlak and Hoigné
1993
; Balmer
and Sulzberger
1999
). In the photo-Fenton reaction, the HO
•
radical is formed
photolytically from Fe(OH)
2
+
(Eq. 3.27). The relevant reaction mainly
takes place at pH 2.5-5 (Eq. 3.26), but its quantum yield is relatively low:
ϕ
Fe(II)
=
0.14
±
0.04 at 313 nm and
ϕ
HO
=
0.195
±
0.03 at 310 nm (Hislop
and Bolton
1999
). When Fe(III) is complexed with a carboxylic anion (e.g. oxa-
late), the quantum yield of Fe(II) production (
ϕ
Fe(II)
) is significantly increased to
ϕ
Fe(II)
=
1.24, at 300 nm, pH
=
~2 and 6 mM ferrioxalate (Murov et al.
1993
).
This result is accounted for by the considerable photosensitive nature of the
ferrioxalate complex [Fe(C
2
O
4
)
3
]
3-
, which combines elevated absorption of visible
radiation with a very high quantum yield of Fe
2
+
photoproduction. Interestingly,
the photolysis of the ferrioxalate complex generates an additional reactive radical
species, the carbon dioxide radical anion (CO
2
•
-
) (Eqs. 3.19-3.21; Table
3
).
•
-
can produce Fe(II) via reaction (Eq. 3.21) and by other
reaction pathways (Eqs. 3.23, 3.24). The kinetic and equilibrium constants for
the photo-ferrioxalate/H
2
O
2
reaction are shown in Table
3
. CO
2
The radical CO
2
•
-
can react
•
-
) (Eq. 3.22), which can further
enhance the quantum yield for the generation of Fe
2
+
(Eqs. 3.23, 3.24, 3.28) and
contributes to the production of H
2
O
2
(Eq.
3.17
). When ferrioxalate is irradiated
in the presence of H
2
O
2
under ideal conditions, a radical HO
with oxygen to form the superoxide anion (O
2
•
is produced by
the Fenton reaction per every Fe(II) generated (Eq 3.25, Table
3
). In the reaction
media (Eq. 3.25), both uncoordinated Fe
2
+
and Fe
II
(C
2
O
4
) can react with H
2
O
2
.
Therefore one gets an overall, apparent second-order rate constant for the reac-
tion between Fe(II) and H
2
O
2
. In the presence of excess oxalate, Fe(III) will be
coordinated with either two or three oxalate ligands. Fe(III) is recycled to Fe(II) in
both the photo-Fenton and the photo-ferrioxalate/H
2
O
2
reaction. In the latter case
the formation of HO
•
depends on the availability of radiation, H
2
O
2
and oxalate,
the latter two components being consumed by the reaction. The enhancement of
HO
•
photoproduction that is observed upon addition of oxalate depends on the
very high photolysis quantum yield of the Fe(III)-oxalate complex(es), which
largely compensates for the facts that the photolysis of Fe(III)-oxalate, unlike that
of FeOH
2
+
, does not yield HO
•
•
, and that oxalate is a HO
scavenger.
3.6 HO
•
Production from Photocatalytic Metal Oxide
(TiO
2
) Suspensions
Titanium dioxide is the most frequently used metal oxide photocatalyst, which
undergoes excitation at near-UV wavelengths. The irradiation by sunlight of aque-
ous suspensions of TiO
2
can induce very significant generation of HO
•
in aqueous
•
solution. Below it is reported a general scheme of HO
photo-production, pro-
posed in early studies to describe the behavior of aqueous suspensions of TiO
2
in
the presence of DOM (Konstantinou and Albanis
2004
; Murov et al.
1993
; Tseng
