Selection of Reactive Mixtures for Use in Permeable Reactive Walls for Treatment of Mine Drainage

Porous, permeable, geochemically reactive walls, installed in situ in the path of migrating groundwater, present a promising passive treatment alternative for remediating metal-contaminated groundwater derived from oxidized mine wastes. Contaminants are removed from the groundwater as a result of bacterial sulfate reduction and the subsequent precipitation of sparingly soluble sulfide solids. The permeability and reactivity of eight organic-carbon reactive mixtures were assessed. The permeability ranged between <10 - 4 and 10 - 2 cm/s. Batch tests conducted using simulated mine drainage indicated that, within 40−70 days, SO 4 concentrations decreased from initial concentrations of 1200−4800 mg/L to final concentrations of <10 mg/L in four mixtures. Iron concentrations decreased from initial concentrations of between 105 and 1400 mg/L to concentra tions between 0.1 and 50 mg/L within 0.1−65 days. Concentrations of 480 mg/L Ni and 135 mg/L Cd decreased to below 0.05 mg/L within 10 days. pH and alkalinity values increased from initial pH values of <6 to values of 6.5−7 and alkalinity values (as CaCO 3 ) of <15 mg/L to >1000 mg/L. Geochemical model calculations suggest precipitation of Fe and Zn sulfide phases and the Mn carbonate phase rhodochrosite. The reactivity of the mixtures varied with those containing several organic sources being most reactive. Results obtained from these studies indicate that levels of reactivity and permeability suitable for remediating tailings-impacted groundwater can be attained.