EXPERIMENTS AND MODELING OF FEII OXIDATION RATE FOR DESIGN OF REMEDIATION OF NET ALKALINE COAL-MINE EFFLUENT, PENNSYLVANIA, U.S.A

Coal-mine drainage (CMD) from the Pine Knot Tunnel and Oak Hill Boreholes is a major source of base flow to the West Branch Schuylkill River near Pottsville in eastern Pennsylvania. Combined, the two CMD sources discharge 1 ton of dissolved metals daily and account for 30% of the annual loading of metals from abandoned mines to the entire Schuylkill River. To evaluate potential remediation strategies, field experiments were conducted in June and July 2013 at the Oak Hill Boreholes, which have high concentrations of Fe^II (12-21 mg/L) and Mn (3.1-4.5 mg/L) but are net alkaline (acidity -238 to -59.4 mg/L CaCO₃; pH 5.8 to 7.0). To test the hypothesis that aeration could be effective for degassing CO₂, increasing pH, and increasing the rate of abiotic Fe^II oxidation, three different aeration rates (Aer1 12.6 ml/s; Aer2 16.8 ml/s; Aer3 25.0 ml/s) were evaluated by bubbling air through porous stone diffusers immersed in 20 liters of CMD in cylindrical coolers. For comparison, a hydrogen-peroxide treatment (5 ml 3% H₂O₂) and a control with no mechanical or chemical aeration were monitored. The temperature, pH, specific conductance, and dissolved oxygen were recorded at 1-min intervals; serial samples collected during the 5-6 hr experiments were titrated for alkalinity and filtered (0.45 μm) for cation and anion analysis.

During the experiments, pH increased from 6.3 to 7.3, 7.8, and 8.2 for Aer1, Aer2, and Aer3, respectively, but decreased from 6.3 to 6.2 for H₂O₂ and was unchanged for the control. Fe^II removal increased with aeration rate; H₂O₂ addition resulted in almost instantaneous removal of Fe^II; the control exhibited no change. Upon oxidation, Fe^III particles settled quickly in the H₂O₂ test and more slowly in aerated tests. Kinetic modeling with PHREEQC was used to evaluate interdependence of dissolved CO₂ (degassing), pH, and Fe^II oxidation rate during the Aer3 tests. After temperature correction, the Fe^II oxidation rate indicated by the PHREEQC model equaled the Singer-Stumm reference value of k_H = 3 x 10^-12 mol/L/min.

Thus, for efficient treatment of net alkaline CMD: (1) mechanical or hydraulic aeration may be incorporated to degas CO₂, thereby increasing pH and rates of Fe attenuation, or (2) H₂O₂ treatment may be an effective alternative. In both cases, oxidation ponds may be followed by wetlands to remove Fe^III particulate.