The role of nutrients speciation on the oxidation rate of Fe(II) in seawater

Abstract

The effects of nitrate, phosphate and silicate in the oxidation of Fe(II) by molecular oxygen have been evaluated using a UV-Vis spectrophotometric system equipped with a long liquid waveguide capillary flow cell. The Fe(II) oxidation process was studied in UV-seawater enriched with nutrients (SWEN) and oligotrophic UV-seawater (SW) as a function of pH (7.2-8.2) and temperature (5-35°C). The results showed that the oxidation rates was faster in high nutrient concentrations, reducing the lifetime of Fe(II) in nutrient rich waters in the different experimental conditions. The highest values in the apparent oxida-tion constants, were at pH = 8.2 (log kapp-SWEN = 3.85 + 0.01 M-1 min-1 and log kapp-SW = 3.71 + 0.01 M−1 min−1 at 35 ºC), while the minimum oxidation rates were achieved in the lower values of pH ranges of study (log kapp-SWEN = 1.26 + 0.01 M−1 min−1 and log kapp-SW = 1.32 + 0.01 M−1 min−1 at 15 ºC). The apparent rate constant was fitted under the experimental conditions of pH (free scale) and temperature (ºK), in SW and SWEN samples. The different in the Fe(II) oxidation rate between SWEN and SW (∆log kapp = ∆log kapp-SWEN - ∆log kapp-SW) was constant along the pH range studied. Nevertheless ∆log kapp varied throughout the temperature range exanimated. A kinetic model was applied to the experimental results, in order to account for changes in the speciation and to compute the fractional contribution of each Fe(II) species to the overall rate constant, as a function of pH and temperature. In the Fe(II) oxidation in seawater enriched with nutrients was considerate both the silicate as phosphate effect. The rate constants for the most kinetically active species (Fe2+, FeOH+, Fe(OH)2, FeCO3, Fe(CO3)22−, FeH3SiO4 , FePO−) in seawater and seawater enriched with nutrients have been determinate as a function of temperature. Neither Fe-silicate reactive specie (FeH3SiO+4) nor Fe-phosphate reactive species (FePO4 , FeHPO4 and FeH2PO+4) played a key role in the Fe(II) speciation. At high temperatures, model results shown that when the concentration of silicate is 1.4 x 104 M, FeH3SiO+4 was the specie that dominated the Fe(II) oxidation process between pH 7.4 and 8.0. The thermodynamic effect over the FeH3SiO+4 ion pair constant has been fitted to an equation that provides the theoretical coefficients for seawater (S‰= 37.09). Therefore the effect of nutrients, especially silicate and phosphate, must be considered in any further study in seawater media cultures and eutrophic coastal waters.