80
B. Sulzberger
i. What are the various pathways of CDOM photooxidation in iron-rich, sunlit aquatic
systems?
ii. Which are the major reactive oxygen species formed concommitantiy with CDOM
photo oxidation in the presence of iron?
iii. Does the iron-related oxidation of humic substances result in the formation of bioavailable carbon compounds, as does direct photooxidation of humic substances, or
does it lead to further polymerization of the biologically refractory humic substances?
In the experimental study described in the next section, these questions are addressed.
3.4
Experimental Study on the Roles of Iron in eDOM Oxidation in
Irradiated Aqueous Systems (Voelker et al. 1997)
In this experimental study, lepidocrocite (y. FeOOH) was chosen as a model solid Fe(III)
phase for crystalline Fe(III)-(hydr)oxides present in surface waters, and Suwannee
River Fulvic Acid (SRFA) as a model compound for humic substances. In illuminated,
aerated systems containing lepidocrocite and SRFA, four pathways or fulvic acid oxidation are linked to the redox cycling of iron (Fig. 3.4). These pathways are:
i. Photo oxidation of SRFA that is adsorbed on the surface of lepidocrocite
ii. Photooxidation of SRFA via photolysis of dissolved Fe(III)-fulvate complexes
iii. Thermal oxidation of SRFA by dissolved Fe(III)
iv. Oxidation of SRFA by HO· that is formed in the Fenton reaction
5.8
2.1
•
OH· +oFe(ll)
hv
3.3
2.2
HP2
SRFA
Fe(lI!)
HOf /Oi '
1.2
1.0
Fig. 3.4. Pathways of oxidation of SRFA in illuminated, aerated aqueous systems, containing 40 f1M Fe
in the form of initially y-FeOOH and 10 mg rl SRFA. The numbers in italics indicate the amount of iron
(in f1M) oxidized or reduced by each of the depicted processes after 300 min of illumination at pH 3
(upper numbers) and at pH 5 (lower numbers). The rate of photoreduction of surface Fe(III) was assumed to be equal to the rate of photoreductive dissolution in deaerated suspensions. The amount of
surface Fe(II) that reoxidized instead of detaching was calculated as the difference in the amount of
total dissolved iron (normalized to total Fe(III)-hydroxide concentration) formed in the deaerated and
aerated suspensions (modified from Voelker et al. 1997)
B. Sulzberger
i. What are the various pathways of CDOM photooxidation in iron-rich, sunlit aquatic
systems?
ii. Which are the major reactive oxygen species formed concommitantiy with CDOM
photo oxidation in the presence of iron?
iii. Does the iron-related oxidation of humic substances result in the formation of bioavailable carbon compounds, as does direct photooxidation of humic substances, or
does it lead to further polymerization of the biologically refractory humic substances?
In the experimental study described in the next section, these questions are addressed.
3.4
Experimental Study on the Roles of Iron in eDOM Oxidation in
Irradiated Aqueous Systems (Voelker et al. 1997)
In this experimental study, lepidocrocite (y. FeOOH) was chosen as a model solid Fe(III)
phase for crystalline Fe(III)-(hydr)oxides present in surface waters, and Suwannee
River Fulvic Acid (SRFA) as a model compound for humic substances. In illuminated,
aerated systems containing lepidocrocite and SRFA, four pathways or fulvic acid oxidation are linked to the redox cycling of iron (Fig. 3.4). These pathways are:
i. Photo oxidation of SRFA that is adsorbed on the surface of lepidocrocite
ii. Photooxidation of SRFA via photolysis of dissolved Fe(III)-fulvate complexes
iii. Thermal oxidation of SRFA by dissolved Fe(III)
iv. Oxidation of SRFA by HO· that is formed in the Fenton reaction
5.8
2.1
•
OH· +oFe(ll)
hv
3.3
2.2
HP2
SRFA
Fe(lI!)
HOf /Oi '
1.2
1.0
Fig. 3.4. Pathways of oxidation of SRFA in illuminated, aerated aqueous systems, containing 40 f1M Fe
in the form of initially y-FeOOH and 10 mg rl SRFA. The numbers in italics indicate the amount of iron
(in f1M) oxidized or reduced by each of the depicted processes after 300 min of illumination at pH 3
(upper numbers) and at pH 5 (lower numbers). The rate of photoreduction of surface Fe(III) was assumed to be equal to the rate of photoreductive dissolution in deaerated suspensions. The amount of
surface Fe(II) that reoxidized instead of detaching was calculated as the difference in the amount of
total dissolved iron (normalized to total Fe(III)-hydroxide concentration) formed in the deaerated and
aerated suspensions (modified from Voelker et al. 1997)