Open access publications in the SLU publication database

Abstract

Tailings are the fine-grained fraction of waste produced during mining operations. This work was carried out on tailings from the Aitik copper mine in northern Sweden. Establishment of vegetation on the Aitik mine tailings deposit is planned to take place at closure of the mine, using sewage sludge as fertiliser. However, the tailings contain traces of metal sulphides, e.g. pyrite, FeS2, and chalcopyrite, CuFeS2. When the sulphides are oxidised, they start to weather and release metals and strong acid, affecting the plant establishment. This thesis investigated physical and chemical growing conditions for plants in tailings with and without sewage sludge, and how these conditions changed when the tailings were oxidised. Studies on tailings properties were related to growth and metal uptake by barley (Hordeum vulgare) and red fescue (Festuca rubra) cultivated on the tailings. Fine sand particles (diameter 0.06-0.2 mm) and moderately large pores (diameter 30-300 μm) dominated the tailings. Oxidation processes resulted in a slightly higher percentage of fine pores (diameter ≤5μm) due to the formation of aggregating iron(III)hydroxide and in a pH decrease due to the acid produced as sulphides oxidised. As oxidation proceeded and pH decreased from neutral to 4.5, Al, Cd, Cu, Pb and Zn concentrations in tailings solution increased by 2-3 orders of magnitude. Compared with mineral fertiliser, sewage sludge consistently increased plant biomass production, due to the contribution of nutrients and organic matter, and increased the percentage of large pores (30-300 μm) and fine pores (<5 μm). The effect of the sludge on pH was generally limited. In the field trial, higher levels of Al, As, Cr, Cu, Pb and Zn were found in red fescue grown in sludge-treated tailings compared with tailings treated with mineral fertiliser. In the climate chamber experiment, sludge effects on Cu in tailings and plants depended on the degree of oxidation of the material. In unoxidised tailings, sewage sludge slightly increased Cu levels in solutions and fescue shoots, but in oxidised tailings the opposite occurred. In general, the sewage sludge did not counteract the toxic conditions caused by sulphide oxidation, but had beneficial effects on plant growth irrespective of oxidation degree of the tailings material.