Effect of ocean warming and acidification on the Fe(II) oxidation rate in oligotrophic and eutrophic natural waters

Abstract

The oxidation rates (kapp) of nanomolar levels of Fe(II) were studied in seawater enriched with nutrients (SWEN) in air saturated conditions. The nutrient effect (nitrate, phosphate and silicate), on the oxidation of Fe(II), was evaluated as a function of pH (7.2–8.2), temperature (5–35 °C) and salinity (10–37.09). The oxidation of Fe(II) was faster in the presence of nutrient with the change in the Fe(II) oxidation rates (Δlogkapp) more intensive at higher temperatures over the entire pH range studied. A kinetic model that considers the interactions of Fe(II) with the major ions in seawater, including phosphate and silicate, was applied to the experimental results in order to describe the effect of ocean warming and acidification in the speciation of Fe(II) and to compute the fractional contribution of each Fe(II)-specie to the overall oxidation rate. The inorganic speciation of Fe(II) was controlled largely by pH, either in SW or in SWEN. A greater presence of Fe-nutrient reactive species (FeH3SiO4 + and FePO4 −) in SWEN at higher temperatures explained the changes in the oxidation process. The individual oxidation rates by oxygen, for the Fe(II) most kinetically active species (Fe2+, FeOH+, Fe(OH)2, FeCO3(OH)−, FeCO3, Fe(CO3)2 2−, FeH3SiO3 +, FePO4 −), were fitted as a function of the temperature.