501
Processes in the Oceans
The values of k 1 , k 2 , and k 3 as a function of salinity (S) and temperature (T K) have been
fitted to the equations (pH = 8.2):
ln k 1 = 26.90 + 0.0322 S – 8123.21/T
(10.94)
ln k 2 = 14.91 + 0.0524 S – 1764.68/T
(10.95)
ln k 3 = 28.92 + 0.0369 S – 8032.68/T
(10.96)
These equations should be valid for estuarine and seawaters over a wide range of salinity and temperature. This kinetic model can be used to predict the product distribution
for the oxidation of sulfide in natural waters with low concentrations of trace metals. The
agreement between the model and the observed distribution of reaction products does not
provide conclusive proof that the reaction pathways of the overall model actually describe
the series of elemental reactions that occur. The detailed mechanisms might involve many
elemental reaction steps.
The rates of oxidation of hydrogen sulfide, the effect of metals, and the intermediates
formed have been examined in a number of natural anoxic basins, such as the Black Sea,
the Cariaco Trench, the Chesapeake Bay, and the Framvaren Fjord. The field measurements of the rates of oxidation of H 2 S were found (Figure 10.86) to be in good agreement
with those estimated from laboratory studies at the same concentration of Fe 2+ . The levels
of Fe 2+ are high enough in most anoxic environments to increase the rates of oxidation of
H 2 S. A kinetic model has been used to analyze the distribution of products (SO 3
2– , S 2 O 3
2– ,
SO 4
2– ) formed during the oxidation in the Framvaren Fjord and the Cariaco Trench. The
rate constants for the production of SO 3
2– (k 1 ), SO 4
2– (k 2 ), and S 2 O 3 (k 3 ) estimated for these
waters were in reasonable agreement with the predicted values at the same level of Fe 2+ .
The values of k 2 estimated for the Framvaren Fjord and Cariaco Trench were slightly
higher than the predicted values. This could have been due to errors in our estimation of
the concentration and form of iron in these waters. Direct measurements of iron and manganese in anoxic waters should be made in all future kinetic studies to avoid this problem.
The estimates based on only the concentration of iron may also lead to some errors. The
concentrations of Mn 2+ below the oxic–anoxic interface reached levels of 0.5 and 15 μM in
the Cariaco Trench and Framvaren Fjord, respectively. This Mn 2+ came from reduction of
MnO 2 sinking from above the oxic–anoxic interface. Reoxidation of Mn 2+ to MnO 2 by bacteria occurs when Mn 2+ diffuses up to the oxic layer. This cycling of manganese between
Mn 2+ and MnO 2 is an important feature of the oxic–anoxic interface and probably affects
the distribution of products in the field. The products (S 2 O 3
2– , SO 3
2– , and SO 4
2– ) formed during the oxidation in the Framvaren also have been examined and used to determine k 1 , k 2 ,
and k 3 (Table 10.12). The measured results are compared in Figure 10.86, with the estimated
values at the same level of Fe. The agreement is quite good.
Table 10.12
Comparison of the Rate Constants for the Oxidation
of Hydrogen Sulfide in Different Waters
Rate
Constant
Gulf Stream
Cariaco Trench
Surface
Mixed
Deep
k 1
1.7
3.1
18.4
36.3
k 2
48,000
48,000
72,000
240,000
k 3
30
15
180
360
Processes in the Oceans
The values of k 1 , k 2 , and k 3 as a function of salinity (S) and temperature (T K) have been
fitted to the equations (pH = 8.2):
ln k 1 = 26.90 + 0.0322 S – 8123.21/T
(10.94)
ln k 2 = 14.91 + 0.0524 S – 1764.68/T
(10.95)
ln k 3 = 28.92 + 0.0369 S – 8032.68/T
(10.96)
These equations should be valid for estuarine and seawaters over a wide range of salinity and temperature. This kinetic model can be used to predict the product distribution
for the oxidation of sulfide in natural waters with low concentrations of trace metals. The
agreement between the model and the observed distribution of reaction products does not
provide conclusive proof that the reaction pathways of the overall model actually describe
the series of elemental reactions that occur. The detailed mechanisms might involve many
elemental reaction steps.
The rates of oxidation of hydrogen sulfide, the effect of metals, and the intermediates
formed have been examined in a number of natural anoxic basins, such as the Black Sea,
the Cariaco Trench, the Chesapeake Bay, and the Framvaren Fjord. The field measurements of the rates of oxidation of H 2 S were found (Figure 10.86) to be in good agreement
with those estimated from laboratory studies at the same concentration of Fe 2+ . The levels
of Fe 2+ are high enough in most anoxic environments to increase the rates of oxidation of
H 2 S. A kinetic model has been used to analyze the distribution of products (SO 3
2– , S 2 O 3
2– ,
SO 4
2– ) formed during the oxidation in the Framvaren Fjord and the Cariaco Trench. The
rate constants for the production of SO 3
2– (k 1 ), SO 4
2– (k 2 ), and S 2 O 3 (k 3 ) estimated for these
waters were in reasonable agreement with the predicted values at the same level of Fe 2+ .
The values of k 2 estimated for the Framvaren Fjord and Cariaco Trench were slightly
higher than the predicted values. This could have been due to errors in our estimation of
the concentration and form of iron in these waters. Direct measurements of iron and manganese in anoxic waters should be made in all future kinetic studies to avoid this problem.
The estimates based on only the concentration of iron may also lead to some errors. The
concentrations of Mn 2+ below the oxic–anoxic interface reached levels of 0.5 and 15 μM in
the Cariaco Trench and Framvaren Fjord, respectively. This Mn 2+ came from reduction of
MnO 2 sinking from above the oxic–anoxic interface. Reoxidation of Mn 2+ to MnO 2 by bacteria occurs when Mn 2+ diffuses up to the oxic layer. This cycling of manganese between
Mn 2+ and MnO 2 is an important feature of the oxic–anoxic interface and probably affects
the distribution of products in the field. The products (S 2 O 3
2– , SO 3
2– , and SO 4
2– ) formed during the oxidation in the Framvaren also have been examined and used to determine k 1 , k 2 ,
and k 3 (Table 10.12). The measured results are compared in Figure 10.86, with the estimated
values at the same level of Fe. The agreement is quite good.
Table 10.12
Comparison of the Rate Constants for the Oxidation
of Hydrogen Sulfide in Different Waters
Rate
Constant
Gulf Stream
Cariaco Trench
Surface
Mixed
Deep
k 1
1.7
3.1
18.4
36.3
k 2
48,000
48,000
72,000
240,000
k 3
30
15
180
360