Sulfide oxidation and the natural attenuation of arsenic and trace metals in the waste rocks of the abandoned Seobo tungsten mine, Korea

Abstract

Mineralogical examinations were performed to characterize the formation of secondary minerals and natural removal process of dissolved As and trace metals (Pb, Zn and Cu) from sulfide oxidation. Laboratory-based leaching tests were also conducted to determine whether the concentrations of As and trace metals in the leachates from waste-rock materials and contaminated soil could be affected by the presence acids such as acid rainwater or acid mine drainage. Waste-rock materials and contaminated soil were compared by 4-day leaching tests using HNO 3 solutions of increasing acidity (0.00001–0.1 mole/L). Mineralogical studies of the waste rocks confirmed the presence of Fe-(oxy)hydroxides (e.g. goethite), jarosite, elemental S, Fe-sulfates, amorphous Fe–As phases, anglesite and covellite as secondary minerals. These secondary minerals act as mineralogical scavengers of dissolved trace metals, SO 4 2 - and acidity released by sulfide oxidation. Arsenic was attenuated by the adsorption on Fe-(oxy)hydroxides and/or the formation of an amorphous Fe–As phase, with a Fe/As ratio = 1 (maybe scorodite: FeAsO 4 · 2H 2O). Electron probe microanalyses data showed that the Fe-(oxy)hydroxides had high concentrations of Pb (up to 21 wt%), with appreciable amounts of As (up to 7.7 wt%), Zn (up to 4.6 wt%) and Cu (up to 2.5 wt%) indicating that dissolved metals were co-precipitated and adsorbed onto Fe-(oxy)hydroxides, Fe(Mn)-hydroxides and Fe-sulfates. The results of the leaching experiments within the pH-range 3.5–5.0 indicated that acidic rainstorms may leach minor amounts of Pb (ca. 1.7–4.0% of total), Zn (ca. 0.8–2.2% of total), Cu (ca. 0.0–0.2% of total) and As (ca. 0.02–0.1% of total) from waste rocks, including the dissolution of soluble secondary minerals previously formed during prolonged dry periods, while dissolution of these elements was negligible from the contaminated soil. In the pH-range 1.0–3.0, the leaching of Pb (ca. 2.4–31% of total) and As (ca. 0.1–5.8% of total) from the waste rocks was significant, which could influence the concentration of these metals in mine runoff. Strongly acidic solutions may also appreciably dissolve Zn (0.0–48% of total) and Cu (0.0–34% of total) in contaminated soil. Leach tests showed that the formation of less soluble secondary minerals had high retention of As, Pb, Zn and Cu, unless their solubilities were increased after the addition of strongly acidic solutions (pH of below 2.0). The precipitation of secondary minerals and the adsorption of trace metals are efficient mechanisms for decreasing the mobilities of As and other trace metals in the surface environment. Geochemical modeling indicated that the leach solutions were supersaturated with respect to goethite, hematite, and magnetite at pH greater than 2.9. Ferrihydrite, Fe(OH) 3, was not in equilibrium with the solution during the entire experiment, suggesting that amorphous Fe-hydroxide can only form in small amounts. X-ray analyses of Seobo mine-waste samples indicated the presence of jarosite and goethite, which retain As, Pb, Zn and Cu during precipitation and co-precipitation.