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 in 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 most cost effective CMD treatment technologies, alkalinity generation is sometimes insufficient to produce net alkaline conditions. One way to increase calcite dissolution and alkalinity generation is the addition of CO₂ to ALDs. In this study, we tested carbonation efficacy by injecting CO₂ gas into two full-scale ALDs (265 and 1,100 metric tons of limestone aggregate) 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 increases alkalinity concentrations to achieve net alkaline conditions. Alkalinity increases covaried with CO₂ flow rate and added up to 100 mg/L CaCO₃ of alkalinity. At the Howe Bridge site, effluent had 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. At the Clarion Park site, effluent had 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. For both sites, CO₂ transfer efficiencies (the percentage of gaseous CO₂ injected that was transferred to the aqueous phase) ranged from -36% to 99% and were dependent on CO₂ flow rates. While 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.