Kinetics of iron (II) oxidation determined in the field

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 O2 (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 CaCO3 equivalent. A numerical model based on a fourth order Runge–Kutta method was developed to calculate values for k1, 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 k1 found to be in the range 2.7 × 1014–2.7 × 1016 M−2 atm−1 min−1. These values for k1 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.

► Iron (II) oxidation rates were studied in the field.
► Values for the homogeneous oxidation rate constant k1 were calculated.
► Values for k1 were 1–3 orders of magnitude higher than those observed in laboratory studies.
► Apparent elevation in k1 values may be a result of autocatalytic heterogeneous iron oxidation.