CHAPTER 3 . Photochemical Processes in the Euphotic Zone of Sea Water
99
been observed by several research groups, not only in acidic lakes and rivers (McKnight
et al.1988), but also in sea waters. The concentration of Fe(II) is much higher in acidic
surface waters than in sea water because of the much slower oxidation of Fe(II) at low
pH-values as compared to pH-values of sea water. Oxidation kinetics of Fe(II) by O2
and H20 2 depends in fact strongly on the Fe(II) speciation.
Organically and inorganically complexed Fe(III) can be reduced under the influence of sunlight. Although the average steady-state concentration of Fe(II) may be very
low in the euphotic zone of sea water, considerably higher Fe(II) concentrations can
be expected upon illumination with high intensity solar radiation.
The cycling of iron between its reduced and oxidized forms and between particulate and dissolved species in natural waters has a number of environmental consequences beyond iron's role as a nutrient and adsorbent. Iron photo-redox cycling has
been shown to catalyse the oxidation of dissolved organic matter (Miles and Brezonik
1981). This process could represent a significant sink of biologically refractory materials, such as humic substances, and is closely coupled to the cycling of reactive transient species such as H20 2, ·OH, and H02/·O;:,(Faust and Zepp 1993).
Within the acidic pH range, the oxidation of dissolved Fe(II) by oxygen is expected
to be very slow compared to other processes. However, the irradiation of humic substances in the presence of oxygen results in the formation ofH02/·0;: (Blough and Zepp
1995; Hoigne et al. 1989). The mechanism of this reaction is not known but may occur
via reduction of oxygen by aqueous electrons or triplet state, both formed when humic substances are photo-excited. The end product of H02/·O;: dismutation is hydrogen peroxide. This reaction is catalysed by the presence of dissolved iron by the reactions:
H02/·O;: + Fe(II) -7 Fe(III) + H20 2
(3.15)
H02/·O;: + Fe(IIl) -7 Fe(II) + O2
(3.16)
The hydrogen peroxide formed from H02/·O;: is another potentially significant oxidant of Fe(II). It is clear that both iron and light have an accelerating effect on the
oxidation of humic substances by oxygen. Voelker et al. (1997) have observed two photooxidation processes of fulvic acid. The first, which does not seem to require the presence of metals, results in the reduction of oxygen to H02/·O;:.
The second process, photo induced ligands to metal charge transfer reaction of
Fe(III)-fulvate complex, occurs both in solution and on the surface of the iron oxide
and results in the reduction of Fe(III). Dissolved Fe(III) is continuously supplied via
re-oxidation of Fe(II) by both H0 2 /·O;: and H20 2 • The OH radicals produced by the
reaction of Fe(II) with H20z further oxidize fulvic acid.
3.4.3
Interactions Between Iron and Chloride
Recently chlorine was identified and measured in coastal areas at concentrations up
to 150 parts per trillion. This is a much larger concentration than can be attributed to
known reactions of sea salt particles, suggesting that there must be an unrecognized
source producing elz. Photocatalytic reactions involving metal oxides have been stud-
99
been observed by several research groups, not only in acidic lakes and rivers (McKnight
et al.1988), but also in sea waters. The concentration of Fe(II) is much higher in acidic
surface waters than in sea water because of the much slower oxidation of Fe(II) at low
pH-values as compared to pH-values of sea water. Oxidation kinetics of Fe(II) by O2
and H20 2 depends in fact strongly on the Fe(II) speciation.
Organically and inorganically complexed Fe(III) can be reduced under the influence of sunlight. Although the average steady-state concentration of Fe(II) may be very
low in the euphotic zone of sea water, considerably higher Fe(II) concentrations can
be expected upon illumination with high intensity solar radiation.
The cycling of iron between its reduced and oxidized forms and between particulate and dissolved species in natural waters has a number of environmental consequences beyond iron's role as a nutrient and adsorbent. Iron photo-redox cycling has
been shown to catalyse the oxidation of dissolved organic matter (Miles and Brezonik
1981). This process could represent a significant sink of biologically refractory materials, such as humic substances, and is closely coupled to the cycling of reactive transient species such as H20 2, ·OH, and H02/·O;:,(Faust and Zepp 1993).
Within the acidic pH range, the oxidation of dissolved Fe(II) by oxygen is expected
to be very slow compared to other processes. However, the irradiation of humic substances in the presence of oxygen results in the formation ofH02/·0;: (Blough and Zepp
1995; Hoigne et al. 1989). The mechanism of this reaction is not known but may occur
via reduction of oxygen by aqueous electrons or triplet state, both formed when humic substances are photo-excited. The end product of H02/·O;: dismutation is hydrogen peroxide. This reaction is catalysed by the presence of dissolved iron by the reactions:
H02/·O;: + Fe(II) -7 Fe(III) + H20 2
(3.15)
H02/·O;: + Fe(IIl) -7 Fe(II) + O2
(3.16)
The hydrogen peroxide formed from H02/·O;: is another potentially significant oxidant of Fe(II). It is clear that both iron and light have an accelerating effect on the
oxidation of humic substances by oxygen. Voelker et al. (1997) have observed two photooxidation processes of fulvic acid. The first, which does not seem to require the presence of metals, results in the reduction of oxygen to H02/·O;:.
The second process, photo induced ligands to metal charge transfer reaction of
Fe(III)-fulvate complex, occurs both in solution and on the surface of the iron oxide
and results in the reduction of Fe(III). Dissolved Fe(III) is continuously supplied via
re-oxidation of Fe(II) by both H0 2 /·O;: and H20 2 • The OH radicals produced by the
reaction of Fe(II) with H20z further oxidize fulvic acid.
3.4.3
Interactions Between Iron and Chloride
Recently chlorine was identified and measured in coastal areas at concentrations up
to 150 parts per trillion. This is a much larger concentration than can be attributed to
known reactions of sea salt particles, suggesting that there must be an unrecognized
source producing elz. Photocatalytic reactions involving metal oxides have been stud-