CHAPTER 3 . Photooxidation of Dissolved Organic Matter
85
oxidation kinetics in samples from Lake Greifen, a eutrophic lake in Switzerland, in
the pH range between 6.8 and 8.3. The pH was varied by bubbling with CO2 'and synthetic air. Fe(II) concentrations were followed using an automated flow injection analysis system employing luminol-based chemiluminescence detection of Fe(II) (King et al.
1995). Above pH 7.4, apparent rate constants were consistent with the rate law determined in pure carbonate systems (Stumm and Lee 1961; Singer and Stumm 1969;
Millero et al. 1987; Davison and Seed 1983; Millero 1988; King 1998). Between pH 6.8 and
7.3, however, the apparent rate constant was independent of pH (Fig. 3.6). Emmenegger
and co-workers hypothesize that some naturally occurring (organic, colloidal, or surface) ligand(s), L, may accelerate the oxidation of Fe(II).
The apparent rate constant, k app, is the sum of the pseudo first -order rate constants
of Fe(II) oxidation by O2 and by H20 2:
Emmenegger and co-workers (1998) have also determined the second-order rate
constants koz and k HzOz . The latter was assessed by adding excess H20 2 (100-1 000 JlM)
to water samples from Lake Greifen. With help of the experimentally determined rate
constants kapp and kHzOz (see Fig. 3.6 and 3.7), koz was calculated using Eq. 3.14. While
log k HzOz exhibited a pH dependence as expected for pure carbonate systems (Millero
and Sotolongo 1989) in the whole measured pH range, log koz became independent of
pH below pH 7.3 (Fig. 3-7).
1.0
0.5
0.0
C:
-0.5
·s
-8: - 1.0
",,"
l -1.5
-2.0
-2.5
-3.0
6.5
•
7.0
7.5
pH
•
. .
Davison (1983)
•
This study
King (1 997)
8.0
8.5
Fig. 3.6. Apparent rate constants of Fe(1I) oxidation in unfiltered lake water as a function of pH.
T = 25 DC, Poz = 0.209 atm, alkalinity = 3.88 mM, [HzOz] = 20-50 nM, initial [Fe(lI)] = 30 nM. Also
plotted, assuming steady-state concentrations of [HzOz], are the range for log k app given by Davison
and Seed (1983), and log k app calculated using the model by King (1998) (from Emmenegger et al.
1998)
85
oxidation kinetics in samples from Lake Greifen, a eutrophic lake in Switzerland, in
the pH range between 6.8 and 8.3. The pH was varied by bubbling with CO2 'and synthetic air. Fe(II) concentrations were followed using an automated flow injection analysis system employing luminol-based chemiluminescence detection of Fe(II) (King et al.
1995). Above pH 7.4, apparent rate constants were consistent with the rate law determined in pure carbonate systems (Stumm and Lee 1961; Singer and Stumm 1969;
Millero et al. 1987; Davison and Seed 1983; Millero 1988; King 1998). Between pH 6.8 and
7.3, however, the apparent rate constant was independent of pH (Fig. 3.6). Emmenegger
and co-workers hypothesize that some naturally occurring (organic, colloidal, or surface) ligand(s), L, may accelerate the oxidation of Fe(II).
The apparent rate constant, k app, is the sum of the pseudo first -order rate constants
of Fe(II) oxidation by O2 and by H20 2:
Emmenegger and co-workers (1998) have also determined the second-order rate
constants koz and k HzOz . The latter was assessed by adding excess H20 2 (100-1 000 JlM)
to water samples from Lake Greifen. With help of the experimentally determined rate
constants kapp and kHzOz (see Fig. 3.6 and 3.7), koz was calculated using Eq. 3.14. While
log k HzOz exhibited a pH dependence as expected for pure carbonate systems (Millero
and Sotolongo 1989) in the whole measured pH range, log koz became independent of
pH below pH 7.3 (Fig. 3-7).
1.0
0.5
0.0
C:
-0.5
·s
-8: - 1.0
",,"
l -1.5
-2.0
-2.5
-3.0
6.5
•
7.0
7.5
pH
•
. .
Davison (1983)
•
This study
King (1 997)
8.0
8.5
Fig. 3.6. Apparent rate constants of Fe(1I) oxidation in unfiltered lake water as a function of pH.
T = 25 DC, Poz = 0.209 atm, alkalinity = 3.88 mM, [HzOz] = 20-50 nM, initial [Fe(lI)] = 30 nM. Also
plotted, assuming steady-state concentrations of [HzOz], are the range for log k app given by Davison
and Seed (1983), and log k app calculated using the model by King (1998) (from Emmenegger et al.
1998)