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

Abstract

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 ultraviolet−visible (UV−vis) spectroscopy calibrated externally with 0 to 50 μM Fe 2+ standards created in a 492 μM 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 μM initial Fe 2+ 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 Fe 2+ 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, Fe 3+ 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.

A Revised Pseudo-Second-Order Kinetic Model for Adsorption, Sensitive to Changes in Adsorbate and Adsorbent Concentrations

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A Revised Pseudo-Second Order Kinetic Model for Adsorption, Sensitive to Changes in Sorbate and Sorbent Concentrations

“…To investigate the dependency of adsorption kinetics on C0, Cs and particle radius, the method of initial rates was used 19 . Experimental kinetic data from the literature was tabulated and initial reaction rates were determined.…”

“…The order of reaction with respect to C0, Cs or particle radius was determined by calculating the gradient of log(initial rate) versus log(independent variable) 19 , where each data point represents a single kinetic experiment within a given literature study where the influence of either C0 or Cs on adsorption kinetics was investigated.…”

Kinetics and Pathways of the Aqueous Photolysis of Pharmaceutical Pollutants: A Versatile Laboratory or Remote Learning Investigation

“…3 While recognizing the value of context-based learning, Sevian and Talanquer cautioned that in some cases, a focus on the contexts that stimulate student engagement may take precedence over the learning of core chemistry ideas and practices. 4 Several recent reports of laboratory experiments have demonstrated that investigating the reactivity of environmentally significant chemicals by natural and engineered processes such as hydrolysis, 5 oxidation, 6 dissolution, 7 and catalyzed reduction 8 provides a stimulating context while developing an understanding of fundamental concepts and skill in analytical techniques. Pharmaceuticals in particular have garnered much attention as aquatic pollutants over the last two decades.…”