Use of Carbonation for Treatment of Coal Mine Drainage to Increase Limestone Dissolution and Generate Alkalinity

Abstract Use of Carbonation for Treatment of Coal Mine Drainage to Increase Limestone Dissolution and Generate Alkalinity Dietrich Kuhlmann IV Anoxic limestone drains (ALDs) are a common component used within passive treatment systems for coal mine drainage (CMD). Contact between limestone aggregate and CMD results in limestone dissolution, producing carbonate alkalinity. Though ALDs are one of the least expensive CMD treatment technologies, alkalinity generation is limited by limestone solubility. This necessitates a long retention time (\u3e12 hours) and sometimes results in net acidic effluent which requires additional alkalinity generation steps downstream. One way to increase calcite dissolution and alkalinity generation is the addition of CO2 to ALDs. This work tested carbonation efficacy by injecting CO2 gas into two full-scale ALDs in Pennsylvania over approximately six weeks at each site. System changes from CO2 injection at each site were evaluated via measurement of pH, dissolved inorganic carbon (DIC), CO2, alkalinity, net acidity, and dissolved metal concentrations. These data allowed for construction of a DIC mass balance for each sampling day and calculation of CO2 transfer efficiencies. Results show that carbonation of full scale ALDs containing 265 to 1,100 metric tons of limestone can increase alkalinity concentrations to achieve net alkaline conditions. CO2 injection increased effluent alkalinity concentrations by up to ca. 100 mg/L as CaCO3. Additionally, Howe Bridge effluent had an average CO2 increase from 3.33 mmol/L to 11.6 mmol/L, an average DIC increase from 7.27 mmol/L to 18.3 mmol/ L, and an average pH decrease from 6.27 to 6.09 following carbonation. Clarion Park effluent had an average CO2 increase from 4.99 mmol/L to 8.35 mmol/L, an average DIC increase from 9.57 mmol/L to 12.3 mmol/L, and an average pH decrease from 6.12 to 5.97 following carbonation. DIC mass balances show CO2 transfer efficiency to be approximately 50% for Howe Bridge and 25% for Clarion Park; further work is required to increase CO2 transfer efficiency. The development of an optimized carbonated limestone system could offer an inexpensive treatment technology that generates high concentrations of alkalinity to treat severely polluted CMD