Monitoring, field experiments, and geochemical modeling of Fe(II) oxidation kinetics in a stream dominated by net-alkaline coal-mine drainage, Pennsylvania, USA

• Aeration promoted CO2 outgassing thereby increasing pH and Fe(II) oxidation rate.
• Kinetic model simulates variations in pH, Fe(II), alkalinity, and dissolved gases.
• CO2 outgassing and O2 ingassing rates are estimated as 1st-order asymptotic functions.
• Fe(II) oxidation rate is accurately estimated by abiotic homogeneous oxidation rate law.

Watershed-scale monitoring, field aeration experiments, and geochemical equilibrium and kinetic modeling were conducted to evaluate interdependent changes in pH, dissolved CO2, O2, and Fe(II) concentrations that typically take place downstream of net-alkaline, circumneutral coal-mine drainage (CMD) outfalls and during aerobic treatment of such CMD. The kinetic modeling approach, using PHREEQC, accurately simulates observed variations in pH, Fe(II) oxidation, alkalinity consumption, and associated dissolved gas concentrations during transport downstream of the CMD outfalls (natural attenuation) and during 6-h batch aeration tests on the CMD using bubble diffusers (enhanced attenuation). The batch aeration experiments demonstrated that aeration promoted CO2 outgassing, thereby increasing pH and the rate of Fe(II) oxidation. The rate of Fe(II) oxidation was accurately estimated by the abiotic homogeneous oxidation rate law −d[Fe(II)]/dt = k1·[O2]·[H+]−2·[Fe(II)] that indicates an increase in pH by 1 unit at pH 5–8 and at constant dissolved O2 (DO) concentration results in a 100-fold increase in the rate of Fe(II) oxidation. Adjusting for sample temperature, a narrow range of values for the apparent homogeneous Fe(II) oxidation rate constant (k1′) of 0.5–1.7 times the reference value of k1 = 3 × 10−12 mol/L/min (for pH 5–8 and 20 °C), reported by Stumm and Morgan (1996), was indicated by the calibrated models for the 5-km stream reach below the CMD outfalls and the aerated CMD. The rates of CO2 outgassing and O2 ingassing in the model were estimated with first-order asymptotic functions, whereby the driving force is the gradient of the dissolved gas concentration relative to equilibrium with the ambient atmosphere. Although the progressive increase in DO concentration to saturation could be accurately modeled as a kinetic function for the conditions evaluated, the simulation of DO as an instantaneous equilibrium process did not affect the model results for Fe(II) or pH. In contrast, the model results for pH and Fe(II) were sensitive to the CO2 mass transfer rate constant (kL,CO2a). The value of kL,CO2a estimated for the stream (0.010 min−1) was within the range for the batch aeration experiments (0–0.033 min−1). These results indicate that the abiotic homogeneous Fe(II) oxidation rate law, with adjustments for variations in temperature and CO2 outgassing rate, may be applied to predict changes in aqueous iron and pH for net-alkaline, ferruginous waters within a stream (natural conditions) or a CMD treatment system (engineered conditions).

Comparison of measured (symbols) and PHREEQC simulated time-series (curves) for pH, Fe(II), Pco2, Po2, and alkalinity during batch aeration experiments on CMD from the Oak Hill Boreholes. In the legend, aeration condition (Aer0, not aerated, control; H2O2, not aerated, hydrogen peroxide added; and, aerated, in order of increasing aeration rate, Aer1: kL,O2a = 0.014 min−1, kL,CO2a = 0.006 min−1; Aer2: kL,O2a = 0.042 min−1, kL,CO2a = 0.013 min−1; Aer3: kL,O2a = 0.071 min−1, kL,CO2a = 0.033 min−1) and multiplication factors for the apparent homogeneous oxidation rate constant, k1′ (0.5–1.0×), and heterogeneous oxidation rate constant, k2′ (0×), after temperature correction, are given for each simulation.Figure optionsDownload as PowerPoint slide