Phosphate amendment of metalliferous tailings, Cannington Ag–Pb–Zn mine, Australia: implications for the capping of tailings storage facilities

This study appraised the use of phosphate fertilisers in immobilising metals in mine tailings to prevent their uptake into Curly Mitchell grass (Astrebla lappacea), when grown on capped, phosphate-amended tailings. Leaching experiments showed that Pb mobility was reduced by both bone meal and superphosphate amendment. Bone meal amendment also reduced Cd mobility. By contrast, Cd, Mn and Zn mobility increased in superphosphate-amended tailings due to increased acid production and, Cu was mobilised in bone meal-amended tailings possibly through the formation of soluble metal-complexing organic compounds. Arsenic and Sb were mobilised in both treatments due to phosphate ligand exchange. Greenhouse trials used Curly Mitchell grass grown on 1-m-high columns stacked with waste materials and different amendments overlain by clean topsoil. Curly Mitchell grass showed substantial uptake of Cd, Mn, Pb and Zn from unamended tailings and waste rock, where these were penetrated by the plant's root system. Addition of phosphate fertilisers to the surface of tailings did not result in reduced metal uptake by Curly Mitchell grass. In tailings capped with limestone, the limestone layer formed an effective physical barrier preventing root penetration into the tailings and led to substantially reduced metal uptake in grass. The study demonstrates that thorough mixing of waste materials and fertilisers as well as irrigation may be required for successful phosphate immobilisation of metals in base metal tailings. Alternatively, the placement of a thin layer of crushed limestone on top of the tailings pile prior to capping may lead to the formation of a chemical and physical barrier and prevent the transfer of environmentally significant elements into the above-ground biomass of Mitchell grasses