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Kinetics of iron (II) oxidation determined in the field

Applied Geochemistry
Publication Date
DOI: 10.1016/j.apgeochem.2011.05.018
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Abstract This paper presents the results of extensive field trials measuring rates of Fe(II) oxidation at a number of Fe-bearing mine drainage discharges in the UK. Batch experiments were carried out with samples taken at regular intervals and Fe(II) concentration determined spectrophotometrically using 2′2-bipyridyl as the complexing agent. Initial concentrations for Fe(II) were 5.65–76.5 mg/L. Temperature, pH and dissolved O 2 (DO) were logged every 10 s, with pH at the start of the experiments in the range 5.64–6.95 and alkalinity ranging from 73 to 741 mg/L CaCO 3 equivalent. A numerical model based on a fourth order Runge–Kutta method was developed to calculate values for k 1, the rate constant for homogeneous oxidation, from the experimental data. The measured values of pH, temperature, [Fe(II)] and DO were input into the model with resulting values for k 1 found to be in the range 2.7 × 10 14–2.7 × 10 16 M −2 atm −1 min −1. These values for k 1 are 1–3 orders of magnitude higher than previously reported for laboratory studies at a similar pH. Comparison of the observed Fe(II) oxidation rates to data published by other authors show a good correlation with heterogenous oxidation rates and may indicate the importance of autocatalysis in these systems. These higher than expected rates of Fe oxidation could have a significant impact on the design of treatment schemes for the remediation of mine drainage and other Fe-bearing ground waters in the future.

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