233
Photoinduced Generation of Hydroxyl Radical in Natural Waters
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. oxalate), 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).
The radical CO 2
•– 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
•– can react
with oxygen to form the superoxide anion (O 2
•– ) (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
• 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 reaction 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 aqueous 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, proposed 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
Photoinduced Generation of Hydroxyl Radical in Natural Waters
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. oxalate), 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).
The radical CO 2
•– 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
•– can react
with oxygen to form the superoxide anion (O 2
•– ) (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
• 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 reaction 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 aqueous 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, proposed 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