Determining the Effect of pH on Iron Oxidation Kinetics in Aquatic Environments: Exploring a Fundamental Chemical Reaction To Grasp the Significant Ecosystem Implications of Iron Bioavailability

Understanding the controls of the oxidation rate of iron (Fe) in oxygenated aquatic systems is fundamental for students of the Earth and Environmental Sciences as it defines the bioavailability of Fe, a trace metal essential for life. The laboratory experiment presented here was successfully developed and used during a third-year undergraduate lab course at Imperial College London for several years. It employs ultravioletvisible (UVvis) spectroscopy calibrated externally with 0 to 50 mu M Fe2+ standards created in a 492 mu M ferrozine and 0.43 M acetate matrix. The students conducted the oxidation experiments in stirred batch reactors at equilibrium with atmospheric oxygen. The solution contained 40.5 mu M initial Fe2+ concentration and a 5.1 mM imidazole buffer. The pH was adjusted to values between 7.22 and 7.77. The students observed a pseudo-first-order reaction with respect to Fe2+ concentration. Plotting the logarithms of the apparent rate constants (k') at different pH values leads to a gradient of 2.2 +/- 0.2 min-(1) pH(-1), indicating a second-order reaction with respect to OH concentration, in agreement with published literature. The oxidation reaction occurred rapidly (tens of seconds to tens of minutes) indicating that in oxygenated aquatic systems, Fe3+ will be the dominant oxidation state, significantly reducing the bioavailability of Fe. The simple laboratory experiment presented here allows the students to learn about kinetic parameters for a fundamental chemical reaction. It allows the students to explore the significant implications this has for aquatic ecosystems.