Environmental Engineering Reference
In-Depth Information
increases the yield of oxidant species in the reaction. Under acidic conditions,
POM can mediate the electron transfer from nanoparticle zerovalent iron or Fe(II)
to oxygen, thereby increasing the production of H
2
O
2
. The latter is subsequently
converted to HO
•
through the Fenton reaction (Lee et al.
2008
).
3.4 HO
•
Production from the Photo-Fenton Reaction
The reactivity of the Fenton's reagent, and in particular of Fe
3
+
+
H
2
O
2
is greatly
enhanced by UV/Visible irradiation (
λ
< 580 nm), which can for instance increase
the extent of mineralization of organic pollutants (Zepp et al.
1992
; Voelker et al.
1997
; Southworth and Voelker
2003
; Nakatani et al.
2007
; Vermilyea and Voelker
2009
). The photo-Fenton reaction is defined as the reaction of photoproduced Fe
2
+
with H
2
O
2
to form the highly reactive HO
•
(Eqs.
3.17
,
3.18
). The main chemical
reactions occurring in the photo-Fenton system are the following:
Fe
3
+
+
h
υ →
Fe
2
+
+
O
2
+H
+
(3.17)
h
υ
(3.18)
Fe
2
+
−→
Fe
3
+
+
HO
•
+
HO
−
+
H
2
O
2
Irradiation of Fe(III) species causes the production of Fe
2
+
and possibly of
HO
•
(e.g. in the photolysis of the complex FeOH
2
+
). In the presence of H
2
O
2
,
further HO
•
is produced by the Fenton process between H
2
O
2
itself and the pho-
togenerated Fe
2
+
. The latter reaction yields again Fe(III), but continuing irradia-
tion may recycle Fe(III) to Fe(II) in the reaction media. Therefore, it is possible
to continuously generate HO
•
under irradiation without any net consumption of
Fe(II), which significantly accelerates the overall reaction rate in the photo-Fenton
system. The HO
•
formation in the photo-Fenton system is limited only by the
availability of radiation and by the content of H
2
O
2
in the reaction medium. The
photo-Fenton reaction is observed in open ocean whereas vertical profiles of Fe(II)
show maxima consistent with the plume of the iron infusion whilst H
2
O
2
profiles
demonstrate a corresponding minima showing the effect of oxidation of Fe(II) by
H
2
O
2
(Croot et al.
2005
). Observations also show that the detectable Fe(II) con-
centrations can exist for up to 8 days after an iron infusion (Croot et al.
2005
).
3.5 HO
•
Production from Photo-Ferrioxalate/H
2
O
2
Reaction
The photo-ferrioxalate/H
2
O
2
reaction is an advanced modification of the photo-
Fenton reaction and an effective technique of generating Fe(II) in the reac-
tion media. The addition of oxalate to the photo-Fenton system significantly
accelerates the HO
•
production under UV/visible irradiation (
λ
< 550 nm)
(Eqs. 3.19-3.25; Table
3
) (Hislop and Bolton
1999
; Jeong and Yoon
2005
;
