The development of new adsorbent materials for the removal of toxic contaminants from drinking water is crucial to achieving the United Nations Sustainable Development Goal 6 (clean water and sanitation). The characterisation of these materials includes fitting models of adsorption kinetics to experimental data, most commonly the pseudo-second order (PSO) model. The PSO model, however, provides no sensitivity to changes in experimental conditions such as adsorbate and adsorbent concentrations (C 0 and C s ) and consequently is not able to predict changes in performance as a function of operating conditions. Furthermore, the experimental conditionality of the PSO rate constant, k 2 , can lead to erroneous conclusions when comparing literature results. In this study, we analyse 108 kinetic experiments from 47 literature sources to develop a relatively simple modification of the PSO rate equation, yielding:Unlike the original PSO model, this revised rate equation (rPSO) demonstrates the first-order and zero-order dependencies upon C 0 and C s that we observe empirically. Our new model reduces the residual sum of squares by 66% when using a single rate constant to model multiple adsorption experiments with varying initial conditions. Furthermore, we highlight how the rPSO rate constant k' is more appropriate for literature comparison, highlighting faster kinetics in the adsorption of arsenic onto alumina versus iron oxides. This revised rate equation should find applications in engineering studies, especially since unlike the PSO rate constant k 2 , the rPSO rate constant k' does not show a counter-intuitive inverse relationship with the increasing reaction rate when C 0 is increased.

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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.