Environmental Engineering Reference
In-Depth Information
and Yoon
2005
; Balmer and Sulzberger
1999
). First, an increase in pH may
enhance the conversion of dominant ferric complexes such as [Fe
III
(C
2
O
4
)]
+
and
[Fe
III
(C
2
O
4
)
3
]
3
−
(Eq. 3.19). It subsequently enhances the overall reaction rates
through the chain reactions of CO
2
•
-
that form H
2
O
2
and, through formation of
Fe
2
+
(Eqs. 3.21, 3.23, 3.24), produce HO
•
as a consequence (Eqs. 3.22, 3.43,
3.45, 4.23, 4.25). Second, the formation of the Fe
II
(C
2
O
4
) complex increases
with pH and subsequently enhances the Fenton reaction, because Fe
II
(C
2
O
4
) can
react with H
2
O
2
at a faster rate (Eq. 3.25) than Fe
2
+
(Eq. 4.18). Thus, without
addition of H
2
O
2
the rate-determining step for the production of HO
•
is the for-
•
-
or HO
2
•
mation of H
2
O
2
. The latter is formed upon reaction of Fe(II) with O
2
,
•
•
-
is formed from
or from O
2
-
+
H
+
and chain propagation within HO
2
. O
2
•
-
or from the reaction of O
2
with photoinduced electron (e
-
), emitted upon
photo-ionization of organic compounds. Therefore, an increase in pH will favor
the occurrence of Fe(II), CO
2
CO
2
•
-
, which leads to higher amounts of H
2
O
2
and of Fe
II
(C
2
O
4
). Then, the reaction between H
2
O
2
and Fe
II
(C
2
O
4
) favors the
formation of HO
•
-
and O
2
•
•
. It is obvious that higher HO
photoproduction causes faster
degradation of the dissolved organic substrates.
With addition of H
2
O
2
to the photo-ferrioxalate system, the formation of HO
•
depends on the concentration of H
2
O
2
(Hislop and Bolton
1999
; Jeong and Yoon
2005
). With H
2
O
2
above 10 mM the reaction rate may decrease, but the addition
of H
2
O
2
from 0.1 to 1 mM may enhance the degradation of organic substances.
Therefore, a high concentration of H
2
O
2
is a major factor for decreasing the over-
all formation rate of HO
•
. First of all, excess H
2
O
2
can contribute significantly to
•
-
. The
latter species are then able to oxidize Fe(II), which might be kept low so that the
formation rate of HO
•
•
the HO
scavenging, consuming hydroxyl radicals and producing HO
2
/O
2
•
•
in the Fenton process is decreased. Lower HO
formation
•
and higher HO
consumption by H
2
O
2
can inhibit the degradation of dissolved
organic compounds; the inhibition would be higher at higher pH, where the oxida-
tion of Fe(II) by HO
2
•
-
is favored.
On the other hand, relatively low levels of H
2
O
2
(0.1 to 1 mM) can enhance
degradation, because the addition of hydrogen peroxide would by-pass the slow
step of H
2
O
2
formation in the photo-ferrioxalate system without H
2
O
2
. Moreover,
low H
2
O
2
levels would not be able to scavenge HO
•
/O
2
•
significantly, nor to cause
Fe(II) oxidation.
5 Significance of HO
•
in Natural Waters
•
The HO
radical is the most reactive transient among the reactive oxygen species
(ROS) that can be present in natural waters. It is an effective, nonselective and
strong oxidant in natural waters for the following reasons:
•
(i) Photoinduced transformation of DOM by HO
into bioavailable compounds.
An example is the degradation of persistent organic substances, which are
