Quantifying the co-dependencies of ph, temperature, and speciation on iron(ii) oxidation kinetics in oxygenated aqueous solutions: analytical advances and environmental implications
File(s)
Author(s)
Saleesongsom, Sarawud
Type
Thesis
Abstract
Oxidation of ferrous iron, Fe(II), in oxygenated aqueous systems controls iron solubility and bioavailability in natural waters and anthropogenic environments such as acidic aerosols. This study investigates Fe(II) oxidation kinetics across a wide pH range (−1.2 to 8.0) and temperatures from 5 to 65 °C in H₂SO₄ solutions representative of SO₂-derived acidic aerosols. Negative pH values measured using glass electrodes were corrected for acid error and temperature effects, producing a calibration curve extending to pH −7.39. A pH–temperature correction factor enabled cross-electrode calibration, and model predictions agreed closely with experimental measurements (R² > 0.97) for pH values down to −3.
Fe(II) oxidation kinetics in oxygenated H₂SO₄ solutions were parameterised using a four-parameter logistic model describing the dependence of the apparent rate constant (kapp) on pH and temperature. At pH < 3.5, kapp is largely independent of pH but increases with temperature at (7.44 ± 0.33) × 10⁻³ °C⁻¹. At 25 °C under strongly acidic conditions, log kapp = −7.03, corresponding to a pseudo-first-order rate constant of 9.25 × 10⁻⁸ s⁻¹ and a half-life of ~87 days. The logistic amplitude (A = 0.90 ± 0.28) and growth constant (k = 0.94 ± 0.03) are temperature independent, whereas the inflection point shifts toward lower pH with increasing temperature.
Additional oxidation experiments in HCl and HNO₃ solutions, combined with literature data for HClO₄, were used to derive intrinsic rate constants for seven Fe(II) species. At low pH (<3.5), ligand coordination enhances oxidation rates relative to uncomplexed Fe²⁺ (FeCl⁺ > FeHSO₄⁺ > FeSO₄ > Fe²⁺), whereas hydroxo complexes dominate at higher pH (>5). Density functional theory calculations show a strong inverse linear relationship between vertical ionisation energies and intrinsic rate constants (R² = 0.95). Together, these results provide a reliable prediction of Fe redox kinetics across wide ranges of pH, temperature, and complexation.
Fe(II) oxidation kinetics in oxygenated H₂SO₄ solutions were parameterised using a four-parameter logistic model describing the dependence of the apparent rate constant (kapp) on pH and temperature. At pH < 3.5, kapp is largely independent of pH but increases with temperature at (7.44 ± 0.33) × 10⁻³ °C⁻¹. At 25 °C under strongly acidic conditions, log kapp = −7.03, corresponding to a pseudo-first-order rate constant of 9.25 × 10⁻⁸ s⁻¹ and a half-life of ~87 days. The logistic amplitude (A = 0.90 ± 0.28) and growth constant (k = 0.94 ± 0.03) are temperature independent, whereas the inflection point shifts toward lower pH with increasing temperature.
Additional oxidation experiments in HCl and HNO₃ solutions, combined with literature data for HClO₄, were used to derive intrinsic rate constants for seven Fe(II) species. At low pH (<3.5), ligand coordination enhances oxidation rates relative to uncomplexed Fe²⁺ (FeCl⁺ > FeHSO₄⁺ > FeSO₄ > Fe²⁺), whereas hydroxo complexes dominate at higher pH (>5). Density functional theory calculations show a strong inverse linear relationship between vertical ionisation energies and intrinsic rate constants (R² = 0.95). Together, these results provide a reliable prediction of Fe redox kinetics across wide ranges of pH, temperature, and complexation.
Version
Open Access
Date Issued
2025-11-01
Date Awarded
2026-04-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
Plancherel, Yves
Weiss, Dominik
Sponsor
Thailand
Publisher Department
Department of Earth Science & Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)