AERATION OF NET ALKALINE MINE DRAINAGE TO DEGAS CO2, INCREASE PH AND IRON OXIDATION RATES

Flow-through reactor field experiments were conducted at two large net alkaline mine discharges in central Pennsylvania. The goal was to drive off CO₂, increase pH, and accelerate Fe(II) oxidation rates. Both discharges were low Mn, low Al, net alkaline discharges with pH of ≈ 5.6 and Fe(II) concentration of ≈ 16 mg L^-1. Flow rates were ≈ 3000 and 15000 L min^-1. For both discharges, 3-hour aeration experiments in a 14-L reactor resulted in pH increases from 5.7 to greater than 7, temperature increases 12 to 19 ºC, dissolved oxygen increases to saturation with respect to the atmosphere, and Fe(II) concentration decreases to less than 0.05 mg L^-1. The same experiment at one of the sites with a 13-hour run time and no active aeration caused pH to change from ≈ 6.1 to 6.3 and Fe(II) concentration to decrease from 16.3 mg to 13.8 mg L^-1. Results from an Fe(II) oxidation model written in a differential equation solver matched the field experiments very well using pH, temperature, dissolved oxygen, and initial Fe(II) concentration. The maximum oxidation rate was 9 x 10^-8 mol L^-1 s^-1. The oxidation model was also modified to include predict alkalinity, P_CO2, and pH changes based on initial conditions and aeration rate. This modified model also fit the data well, is more predictive than the first model, and should serve as a tool for predicting pond size needed for aerated Fe(II) oxidation at the field scale. Using a published Fe removal rate of 20 g m^-2 day^-1 and Fe loading from field data, we predict that aerated treatment ponds can be as much as 10 time smaller than passive treatment ponds, decreasing Fe(II) concentrations to less than 1 mg L^-1 at summer and winter temperatures for both sites. The use of active aeration for some net alkaline discharges can result in considerably reduced treatment area and may lower treatment costs. The reduced capital cost for earthmoving will need to be compared to energy and maintenance costs for aeration.