Oxidation of Fe(II) in natural waters at high nutrient concentrations

Abstract

The Fe(II) oxidation kinetic was studied in seawater enriched with nutrients as a function of pH (7.2−8.2), temperature (5−35 °C), and salinity (10−36.72) and compared with the same parameters in seawater media. The effect of nitrate (0−1.77 × 10−3 M), phosphate (0−5.80 × 10−5 M) and silicate (0−2.84 × 10−4 M) was studied at pH 8.0 and 25 °C. The experimental results demonstrated that Fe(II) oxidation was faster in high nutrient concentrations affecting the lifetime of Fe(II) in nutrient rich waters. Silicate displayed the most significant effects on the Fe(II) oxidation rate with values similar to those determined in seawater enriched with all the nutrients.

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 total rate constant as a function of pH. FeH3SiO4+ played a key role in the Fe(II) speciation, dominating the process at pH over 8.4. At pH 8.0, FeH3SiO4+ represented 18% of the total Fe(II) species. Model results show that when the concentration of silicate is 3 × 10−5 M as in high nutrient low chlorophyll areas, FeH3SiO4+ contributed at pH 8.0 by 4% increasing the rate to 11% at 1.4 × 10−4 M. The effect of nutrients, especially silicate, must be considered in any further study in seawater media cultures and eutrophic oceanic areas.