Solid-phase control on the mobility of potentially toxic elements in an abandoned lead/zinc mine tailings impoundment

Detailed mineralogical and geochemical investigations were conducted within abandoned Pb and Zn flotation tailings at El Fraile impoundments in Taxco, Guerrero, central-southern M6xico. These tailings are divided into an active oxidation zone near the surface, an underlying transition zone and an unoxidized zone. Although these tailings have undergone 30 a of sulfide oxidation, the active oxidation zone has only penetrated to a depth of 0.2 m in the settling pond, and to 0.61.2 in in the dam. The oxidation of sulfide minerals and the insufficiency of pH-buffering minerals have produced low-pH conditions (pH = 1.9-4.4) and high concentrations of dissolved SO42-, As and heavy metals: SO42- (1534-10086 mg L-1), Fe (1.5-2568 mg L-1), Zn (36.7-2435 mg L-1), Cd (0.4-30.6 mg L-1), Pb (< 0.01-0.6 mg L-1), Cu (0.5-38.2 mg L-1) and As (0.01-164 mg L-1). These concentrations of dissolved constituents are attenuated by a series of precipitation and sorption reactions. Precipitation of secondary phases, gypsum, goethite, hematite and K-jarosite has led to the formation of cemented layers within the active oxidation zone in the tailings dam. These cemented layers act as a trap for released, potentially toxic elements from the overlying oxidized tailings. Adsorption and coprecipitation on Fe-precipitates play an important role in the mobilization and attenuation of Zn, Cd, Cu and As within the El Fraile tailings. Additionally to the well-known ability of Fe-precipitates to strongly trap As and heavy metals, this study shows that precipitation of beudantite (PbFe3AsO4SO4(OH)(6)) appears to be one of the solid-phase controls on the natural attenuation of As and Pb and other heavy metals in these tailings. (c) 2006 Elsevier Ltd. All rights reserved.