Аннотация
The oxidation of Fe(II) by molecular oxygen at nanomolar levels has
been studied using a UV-Vis spectrophotometric system equipped with
a long liquid waveguide capillary flow cell. The effect of pH (6.5-8.2),
NaHCO3 (0.1-9 mM), temperature (3-35 degrees C), and salinity (0-36)
on the oxidation of Fe(II) are presented. The first-order oxidation
rates at nanomolar Fe(II) are higher than the values at micromolar
levels at a pH below 7.5 and lower than the values at a higher pH.
A kinetic model has been developed to consider the mechanism of the
Fe(II) oxidation and the speciation of Fe(II) in seawater, the interactions
between the major ions, and the oxidation rates of the different
Fe(II) species. The concentration of Fe(II) is largely controlled
by oxidation with O-2 and O-2(.-) but is also affected by hydrogen
peroxide that may be both initially present and formed from the oxidation
of Fe(II) by superoxide. The model has been applied to describe the
effect of pH, concentration of NaHCO3, temperature, and salinity
on the kinetics of Fe(II) oxidation. At a pH over 7.2, Fe(OH)2 is
the most important contributing species to the apparent oxidation
rate. At high levels Of CO32- and pH, the Fe(CO3)(2)(2-) species
become important. At pH values below 7, the oxidation rate is controlled
by Fe2+. Using the model, log k(i) values for the most kinetically
active species (Fe2+, Fe(OH)(+), Fe(OH)(2), Fe(CO3), and Fe(CO3)(2)(2-))
are given that are valid over a wide range of temperature, salinity,
and pH in natural waters. Model results show that when H2O2 concentrations
approach the Fe(II) concentrations used in this study, the oxidation
of Fe(II) with H2O2 also needs to be considered.
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