USE OF CARBONATION FOR TREATMENT OF COAL MINE DRAINAGE TO INCREASE LIMESTONE DISSOLUTION AND GENERATE ALKALINITY

Anoxic limestone drains (ALDs) are a common component used for the passive treatment of 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 (>12 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. Our research team is testing carbonation efficacy by injecting CO₂ gas into full scale ALDs in Pennsylvania. System changes from CO₂ injection at each site are evaluated via measurement of pH, dissolved inorganic carbon (DIC), CO₂, net acidity, and dissolved metal concentrations. These data allow us to construct a DIC mass balance for each sampling day and calculate CO₂ transfer efficiencies. Preliminary results show that carbonation of full scale ALDs containing 270 to 1,100 metric tons of limestone increases alkalinity concentrations to net alkaline conditions. Carbonation increased effluent alkalinity concentrations by up to 115 mg/L as CaCO3. Additionally, Howe Bridge effluent had an increase of up to 5.6 mmol/L CO₂, 8.8 mmol/L DIC, and a pH decrease of up to 0.4 following carbonation. Clarion Park effluent had a CO₂ increase of up to 3.2 mmol/L and a pH decrease of up to 0.2 following carbonation. DIC mass balances show CO₂ transfer efficiency to be approximately 30%; 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.