Paper No. 3
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 FeII
(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 FeII
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% H2
) 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. FeII removal increased with aeration rate; H2O2 addition resulted in almost instantaneous removal of FeII; the control exhibited no change. Upon oxidation, FeIII 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 FeII oxidation rate during the Aer3 tests. After temperature correction, the FeII oxidation rate indicated by the PHREEQC model equaled the Singer-Stumm reference value of kH = 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 FeIII particulate.