Metal Speciation and Immobilization Reactions Affecting the True Efficiency of Artificial Wetlands to Treat Acid Mine Drainage

The ability of constructed wetlands to lower total metal concentrations and organically complex metals in acid mine drainage (AMD) was investigated under greenhouse and field conditions. In the greenhouse study, Typha plants grown in six different substrates received simulated acid mine drainage of low metal load for five months. Most effluents, especially those from ground flows, showed significant decreases in acidity and metal concentrations. The pine needle and hay substrates most effectively reduced acidity and total Al levels. Effluents from these substrates contained 80% less total Al than respective influents. Organically complexed Al levels were independent of matrix and varied from 10 to 30% of inflow total Al concentrations. Peat and Sphagnum moss most efficiently reduced Fe concentrations but only 10% of the total Fe was organically complexed. Matrix composition had little or no effect on Mn concentrations. Substrates lowered Cu and Zn levels by 40-90% in most effluents, but pine needle and hay mixtures were the most effective. The metal concentration and acidity of a very high metal load AMD were also reduced substantially during the first six months of treatment with a wetland which was constructed by the U.S. Forest Service in McCreary County, KY and used mushroom compost as a substrate. After 8 months of operation, however, and during periods of high flow rates(\u3e 10 gallons/min) the efficiency of the wetland was drastically reduced, apparently due to reduced residence time, insufficient size and metal overloading. No major differences were observed during high flow rate periods between input-output metal concentration, although input concentrations varied due to dilution effects. The majority of Fe, Mn, and Zn in surface effluents was present in inorganic metal species. Nearly 100% of Cu and about 40% of the Al, however, was organically bound. Substrate solutions extracted by centrifugation showed increased organic/inorganic metal species ratios apparently due to increased residence time. A great portion of the metals retained by the greenhouse and field substrates was in residual forms (oxyhydroxides, sulfides, sulfates, carbonates). The metals Fe, Mn, and Zn showed the highest tendency for residual retention, while Al and especially Cu showed high affinity for organic retention. Exchangeable and sorbed forms were present in very small concentrations and in many cases were almost negligible