AERATION TO DEGAS CO2, INCREASE PH AND IRON OXIDATION RATES, AND DECREASE TREATMENT POND SIZE IN TREATMENT OF NET ALKALINE MINE DRAINAGE1

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 document increased Fe(II) oxidation rates compared to passive treatment methods. Both discharges were low Mn, low Al, net alkaline discharges with pH of ≈ 5.7 and Fe(II) concentration of ≈ 16 mg/L. Flow rates were ≈ 3000 and 15000 L/min. Three- hour aeration experiments with flow rates scaled to a 14-L reactor resulted in pH increases from 5.7 to greater than 7, temperature increases from 12 to 22 ºC, dissolved oxygen increases to saturation with respect to the atmosphere, and Fe(II) concentration decreases to less than 0.05 mg/L. The same experiment at one of the sites with a 13-hour run time and no active aeration had a pH change from 6.1 to 6.3 and decrease in Fe(II) concentration from 16.3 to 13.8 mg L-1. Results from an Fe(II) oxidation model, using field-measured pH, temperature, dissolved oxygen, and initial Fe(II) concentration and written in a differential equation solver, were the same as the field experiments within analytical uncertainty. The maximum oxidation rate was 1.3 x 10-4 mol L-1 sec-1. The model was also modified to predict alkalinity, PCO2, and pH changes based on initial conditions and aeration rate. This modified model also matched the data within analytical uncertainty, 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 without the need for field pilot studies. Using a published Fe removal rate of 20 g m-2 day-1 and Fe loading from field data, 3.6 x 103 and a 3.0 x 104 m2 passive oxidation treatment ponds would be required for Site 21 and Packer 5 discharges, respectively. Fe(II) oxidation modeling of actively aerated systems predicted that a 1 m deep pond with 10 times less area would be adequate to lower Fe(II) concentrations to less than 1 mg L-1 at summer and winter temperatures for both sites. The use of active aeration for net alkaline discharges with high CO2 concentrations can result in considerably reduced treatment area for oxidation and may lower treatment costs, but settling of iron hydroxides was not considered in this study. The reduced capital cost for earthmoving will need to be compared to energy and maintenance costs for aeration.