Metal mobilization under alkaline conditions in ash-covered tailings

• Published in: Journal of environmental management 2014-06, Vol.139, p.38-49
• Main Authors: Lu, Jinmei, Alakangas, Lena, Wanhainen, Christina
• Format: Article
• Language: English
• Subjects: Alkalinity; Animal, plant and microbial ecology; Applied ecology; Applied Geology; Ash cover; Biological and medical sciences; Coal Ash; Conservation, protection and management of environment and wildlife; Dissolution; Fly ash; Fundamental and applied biological sciences. Psychology; General aspects; Geochemistry; Hydrogen-Ion Concentration; Industrial Waste - analysis; Malmgeologi; Metal mobilization; Metals - analysis; Metals - chemistry; Mineralogy; Minerals - analysis; Minerals - chemistry; Mining; Ore Geology; Oxidation-Reduction; Sediment transport; Sewage; Sludge; Sulfide oxidation rate; Sulfide tailings; Sulfides - analysis; Sulfides - chemistry; Tillämpad geologi
• ISSN: 0301-4797; 1095-8630; 1095-8630
• DOI: 10.1016/j.jenvman.2013.12.036

The aim of this study was to determine element mobilization and accumulation in mill tailings under alkaline conditions. The tailings were covered with 50 cm of fly ash, and above a sludge layer. The tailings were geochemically and mineralogically investigated. Sulfides, such as pyrrhotite, sphalerite and galena along with gangue minerals such as dolomite, calcite, micas, chlorite, epidote, Mn-pyroxene and rhodonite were identified in the unoxidized tailings. The dissolution of the fly ash layer resulted in a high pH (close to 12) in the underlying tailings. This, together with the presence of organic matter, increased the weathering of the tailings and mobilization of elements in the uppermost 47 cm of the tailings. All primary minerals were depleted, except quartz and feldspar which were covered by blurry secondary carbonates. Sulfide-associated elements such as Cd, Fe, Pb, S and Zn and silicate-associated elements such as Fe, Mg and Mn were released from the depletion zone and accumulated deeper down in the tailings where the pH decreased to circum-neutral. Sequential extraction suggests that Cd, Cu, Fe, Pb, S and Zn were retained deeper down in the tailings and were mainly associated with the sulfide phase. Calcium, Cr, K and Ni released from the ash layer were accumulated in the uppermost depletion zone of the tailings. •The weathering rate of sulfides increased at extreme alkaline condition (pH > 12).•Weathering of silicates and carbonates occurred at pH > 11.•Metals such as Cd, Fe, Mg, Mn, Pb and Zn were mobilized at pH > 11.•Landfill of highly alkaline waste will increase the weathering of native minerals.