Relative contributions of abiotic and biological factors in Fe(II) oxidation in mine drainage

The oxidation of Fe(II) is apparently the rate-limiting step in passive treatment of coal mine drainage. Little work has been done to determine the kinetics of oxidation in such field systems, and no models of passive treatment systems explicitly consider iron oxidation kinetics. A Stella II model using Fe(II)(init) concentration, pH, temperature, Thiobacillus ferrooxidans and O-2 concentration, flow rate, and pond volume is used to predict Fe(II) oxidation rates and concentrations in seventeen ponds under a wide range of conditions (pH 2.8 to 6.8 with Fe(II) concentrations of less than 240 mg L-1) from 6 passive treatment facilities. The oxidation rate is modeled based on the combination of published abiotic and bic,logical laboratory rate laws. Although many other variables have been observed to influence Fe(II) oxidation rates, the 7 variables above allow field systems to be modeled reasonably accurately for conditions in this study. Measured T. ferrooxidians concentrations were approximately 10(7) times lower than concentrations required in the model to accurately predict field Fe(II) concentrations. This result suggests that either 1) the most probable number enumeration method underestimated the bacterial concentrations, or 2) the biological rate law employed underestimated the influence of bacteria, ol both. Due to this discrepancy, bacterial concentrations used in the model for pH values of less than 5 are treated as fit parameters rather than empirically measured values. Predicted Fe(II) concentrations in ponds agree well with measured Fe(II) concentrations, and predicted oxidation rates also agree well with field-measured rates. From pH 2.8 to approximately pH 5, Fe(II) oxidation rates are negatively correlated with pH and catalyzed by T. ferrooxidans. From pH 5 to 6.4, Fe(II) oxidation appears to be primarily abiotic and is positively correlated with pH. Above pH 6.4, oxidation appears to be independent of pH. Above pH 5, treatment efficiency is affected most by changing design parameters in the following order: pH > temperature approximate to influent Fe(II) > pond volume approximate to O-2. Little to no increase in Fe(II) oxidation rate occurs due to pH increases above pH 6.4. Failure to consider Fe(II) oxidation rates in treatment system design may result in insufficient Fe removal. (C) 1999 Elsevier Science Ltd. All rights reserved.