253
Photoinduced Generation of Hydroxyl Radical in Natural Waters
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 formation of H 2 O 2 . The latter is formed upon reaction of Fe(II) with O 2
•– or HO 2
•
,
or from O 2
– + H + and chain propagation within HO 2
• . O 2
•– is formed from
CO 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
•– and O 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
• . 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 overall formation rate of HO
•
. First of all, excess H 2 O 2 can contribute significantly to
the HO
• scavenging, consuming hydroxyl radicals and producing HO 2
•
/O 2
•– . The
latter species are then able to oxidize Fe(II), which might be kept low so that the
formation rate of HO
• 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 oxidation of Fe(II) by HO 2
• /O 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
• 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
Photoinduced Generation of Hydroxyl Radical in Natural Waters
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 formation of H 2 O 2 . The latter is formed upon reaction of Fe(II) with O 2
•– or HO 2
•
,
or from O 2
– + H + and chain propagation within HO 2
• . O 2
•– is formed from
CO 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
•– and O 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
• . 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 overall formation rate of HO
•
. First of all, excess H 2 O 2 can contribute significantly to
the HO
• scavenging, consuming hydroxyl radicals and producing HO 2
•
/O 2
•– . The
latter species are then able to oxidize Fe(II), which might be kept low so that the
formation rate of HO
• 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 oxidation of Fe(II) by HO 2
• /O 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
• 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