CHAPTER 3 • Photooxidation of Dissolved Organic Matter
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An additional, presumably iron-independent pathway is direct photo oxidation resulting in reduction of O2 to OZ', (see Table 3.1). Of the redox processes, where iron is
directly involved, photo oxidation of adsorbed SRFA under reduction of surface Fe(III)
is the most important process in systems containing initially lepidocrocite, both at pH 3
and 5 (Fig. 3.4). With regard to the oxidation of SRFA by dissolved Fe(III), photochemical and thermal oxidation play almost equally significant roles at both pH values.
The relative rates of the individual processes shown in Fig. 3.4 were assessed by
measuring net formation of dissolved Fe(II), total dissolved Fe, and H20 2 and by simulating the data with kinetic modelling (Fig. 3.5) (for more information on the kinetic
modelling, see Voelker and Sulzberger 1996, and Voelker et al. 1997).
3.4.1 Photooxidation of SRFA by Fe(lII)
Voelker and collaborators (1997) hypothesize that photo oxidation of SRFA with Fe(III)
as oxidant occurs through photolysis of Fe(III)-SRFA surface and solution complexes
involving carboxyl-groups of SRFA (Leenheer et al.1995a, b), in a similar way as photolysis of Fe(III)-oxalate or other simple Fe(III)-carboxylate complexes (in the following shown for photolysis of the Fe(III)-fulvate surface complex, where the symbol i
stands for the surface lattice of lepidocrocite):
1. Ligand-to-metal charge-transfer transition creating a charge-transfer state, which
is in part thermally deactivated,
iFeIIIOOCR ~ R + iFeIIO·OCR *
and in part by formation of primary photoproducts which are surface Fe(II) and the
fulvate radical, RCOO·:
iFeIIO·OCR*~ iFe(II) + RCOO·
2. Detachment of surface Fe(II) from the crystal lattice of lepidocrocite and reconstitution of the surface site:
(3-7)
3. Decarboxylation of the fulvate radical to form CO2 and the radical R·:
RCOO·~ R· + CO2
If photooxidation of humic substances by Fe(III) occurs indeed in a similar way as
photooxidation of simple carboxylic acids, then one would expect formation of CO 2
as the sole carbon gas. Carbon monoxide production (Valentine and Zepp 1993; Tarr
et al. 1995) and COS production (Andreae and Ferek 1992; Zepp and Andreae 1994)
have also been observed upon DOM photo oxidation, presumably without the involvement of iron. Carbon monoxide formation is thought to occur from reactions involving ketones, aldehydes, or other organic carbonyls that make up natural organic matter (Averett et al.1990; Zafiriou 1994 pers. comm.; Zepp 1994). These functional groups
81
An additional, presumably iron-independent pathway is direct photo oxidation resulting in reduction of O2 to OZ', (see Table 3.1). Of the redox processes, where iron is
directly involved, photo oxidation of adsorbed SRFA under reduction of surface Fe(III)
is the most important process in systems containing initially lepidocrocite, both at pH 3
and 5 (Fig. 3.4). With regard to the oxidation of SRFA by dissolved Fe(III), photochemical and thermal oxidation play almost equally significant roles at both pH values.
The relative rates of the individual processes shown in Fig. 3.4 were assessed by
measuring net formation of dissolved Fe(II), total dissolved Fe, and H20 2 and by simulating the data with kinetic modelling (Fig. 3.5) (for more information on the kinetic
modelling, see Voelker and Sulzberger 1996, and Voelker et al. 1997).
3.4.1 Photooxidation of SRFA by Fe(lII)
Voelker and collaborators (1997) hypothesize that photo oxidation of SRFA with Fe(III)
as oxidant occurs through photolysis of Fe(III)-SRFA surface and solution complexes
involving carboxyl-groups of SRFA (Leenheer et al.1995a, b), in a similar way as photolysis of Fe(III)-oxalate or other simple Fe(III)-carboxylate complexes (in the following shown for photolysis of the Fe(III)-fulvate surface complex, where the symbol i
stands for the surface lattice of lepidocrocite):
1. Ligand-to-metal charge-transfer transition creating a charge-transfer state, which
is in part thermally deactivated,
iFeIIIOOCR ~ R + iFeIIO·OCR *
and in part by formation of primary photoproducts which are surface Fe(II) and the
fulvate radical, RCOO·:
iFeIIO·OCR*~ iFe(II) + RCOO·
2. Detachment of surface Fe(II) from the crystal lattice of lepidocrocite and reconstitution of the surface site:
(3-7)
3. Decarboxylation of the fulvate radical to form CO2 and the radical R·:
RCOO·~ R· + CO2
If photooxidation of humic substances by Fe(III) occurs indeed in a similar way as
photooxidation of simple carboxylic acids, then one would expect formation of CO 2
as the sole carbon gas. Carbon monoxide production (Valentine and Zepp 1993; Tarr
et al. 1995) and COS production (Andreae and Ferek 1992; Zepp and Andreae 1994)
have also been observed upon DOM photo oxidation, presumably without the involvement of iron. Carbon monoxide formation is thought to occur from reactions involving ketones, aldehydes, or other organic carbonyls that make up natural organic matter (Averett et al.1990; Zafiriou 1994 pers. comm.; Zepp 1994). These functional groups