| The alkaline mine drainage may be caused in mine tailings contained polymetallic sulfide minerals for depletion of carbonate minerals through consumption by acid-neutralization reactions. However, some special metals can also transport into water with the alkaline mine drainage in elevated levels and result in serious impact on environment. The paper, based on a case study on Molybdenum mine area in western Liaoning, analyses the forming causation of the alkaline mine drainage, establishes the release and transport model of heavy metals, investigates the migration and transformation of heavy metals in water and ecological risk caused by heavy metals. The results can provide scientific basis for seepage quality assessment, dynamics simulation of water quality, heavy metal pollution control and environmental management.The mineralogical and geochemical results showed Mo tailings in western Liaoning were low sulfide and low carbonates tailings. The sulfide minerals in surface tailings pond were oxidized but the alkaline mine drainage was formed because of carbonate neutralization (Paste pH 7.3-8.24). The hydrolytic equilibrium of HCO3- and CO32- controlled the acid-base environment. The peak concentrations of Fe, Cu, Zn and SO42- were presented at the bottom of oxidation zone, which were 1.4,2.4,3.2 and 3.4 times higher than that of in surface tailings pond, respectively. The Mo concentration increased gradually with depth and the Mo concentration at 2 m depth was 2 times higher than that of in surface tailings pond (1090-1146μg/g). Geochemical species of heavy metals indicated that precipitation, co-precipitation and complexing adsorption mechanism controlled Fe, Cu and Zn released from sulfide minerals in migration process, but not for Mo. The proportion of those four metals of bioavailable fraction at 2 m depth in tailings pond followed the order:Mo>>Fe～Cu～Zn. The alkaline conditions played a crucial role in retention of Fe, Cu and Zn, but promoted effectively the leaching and migration of Mo.The results of batch experiments of leaching kinetics with tailings demonstrated that elution quantity of heavy metals was controlled by neutralization of minerals and heavy metal Mo was major metal in leachate. Oxidation time, pH of simulated solution, temperature, particle size and O2 concentration had effect on the release of Mo from tailings, which further prove the release transport of Mo in tailings. The acid producing potential revealed that acidic condition in tailings would not been formed. Fe, Cu and Zn remaining in the bottom of oxidation zone would not leach from tailings ponds due to the alkaline conditions, but heavy metal Mo would leach from tailings ponds into environment downstream of Mo mining area.The geochemical results of samples from water body in mining area and downstream of mining area indicated that the concentrations of Fe, Cu, Zn and Mo in water were higher than in S0. Mo concentrations in aqueous phase were much higher than the health standard for domestic drinking water by 1.86-16.66 times. All the concentrations of heavy metals in sediments of mining area and downstream of mining area were higher than that of heavy metals at SO by 7.30,10.75,11.42 and 494.33 times, respectively. The high concentration values of heavy metals in sediments had a close relationship with Mo mining activity. The partition coefficients of heavy metals of Fe, Cu and Zn were higher than that of Mo. Sediments did not become the bearing medium for Mo but for Fe, Cu and Zn due to adsorption mechanism. The reduction condition in reservoir sediments played an important role for retention Mo, but the bioavailable Mo fraction presented 18-58% in the Nver River.The conclusions suggested that the pH condition controlled the migration and transformation of heavy metals in different phase. Heavy metal Mo migrated easily in mine tailings and in the Never River for the alkaline condition. The ecological risk from Mo was higher than medium grade in sediments.
|
PDF Full Text Request