Abstract: Phosphate-induced metal stabilization involving the reactive medium Apatite II(TM) [Ca10-xNax(PO4)6-x(CO3)x(OH)2], where x < 1, was used in a subsurface permeable reactive barrier (PRB) to treat acid mine drainage in a shallow alluvial groundwater containing elevated concentrations of Zn, Pb, Cd, Cu, SO4 and NO3. The groundwater is treated in situ before it enters the East Fork of Ninemile Creek, a tributary to the Coeur d'Alene River, Idaho. Microbially mediated SO4 reduction and the subsequent precipitation of sphalerite [ZnS] is the primary mechanism occurring for immobilization of Zn and Cd. Precipitation of pyromorphite [Pb10(PO4)6(OH,Cl)2] is the most likely mechanism for immobilization of Pb. Precipitation is occurring directly on the original Apatite II. The emplaced PRB has been operating successfully since January of 2001, and has reduced the concentrations of Cd and Pb to below detection (2 μg L-1), has reduced Zn to near background in this region (about 100 μg L-1), and has reduced SO4 by between 100 and 200 mg L-1 and NO3 to below detection (50 μg L-1). The PRB, filled with 90 tonnes of Apatite II, has removed about 4550 kg of Zn, 91 kg of Pb and 45 kg of Cd, but 90% of the immobilization is occurring in the first 20% of the barrier, wherein the reactive media now contain up to 25 wt% Zn. Field observations indicate that about 30% of the Apatite II material is spent (consumed).

Abstract: Continued mining development in the world's largest lead producing area has generated and increased concern over effective mine water treatment in Missouri's New Lead Belt. A new type of mine/mill wastewater treatment system was constructed which consisted of a tailings pond followed by a series of artificially constructed meandering biotreatment channels and a polishing lagoon. This system provided additional retention time and distance for the removal of heavy metals by abundant aquatic plants and sedimentation. Seasonal field sampling and analytical testing that evaluated the present system confirmed that it provided good treatment for removal of heavy metals within the company property and produced a final effluent within the state and federal regulatory guidelines. On average, greater than 95% of zinc and manganese in the drainage water were removed by the biotreatment system, while lead and copper were 50 to 60%. A chemical equilibrium model, MINTEQ, was also used to identify various species of lead and zinc in the biotreatment system. The model predicted that the major species of carbonates and hydroxides would be the predominant complexes of lead and zinc for the pH and alkalinity values reported in the biotreatment system. These results were also supported by the literature.