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 tre atment of coal mine drainage. Little work has been done to determine the ki netics of oxidation in such field systems, and no models of passive treatme nt systems explicitly consider iron oxidation kinetics. A Stella II model u sing 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 ba sed 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 r easonably accurately for conditions in this study. Measured T. ferrooxidians concentrations were approximately 10(7) times low er 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 mo del 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) co ncentrations, and predicted oxidation rates also agree well with field-meas ured rates. From pH 2.8 to approximately pH 5, Fe(II) oxidation rates are n egatively 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 co rrelated with pH. Above pH 6.4, oxidation appears to be independent of pH. Above pH 5, treatment efficiency is affected most by changing design parame ters in the following order: pH > temperature approximate to influent Fe(II ) > pond volume approximate to O-2. Little to no increase in Fe(II) oxidati on 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 r emoval. (C) 1999 Elsevier Science Ltd. All rights reserved.