Phytostabilisation of severely contaminated mine tailings using halophytes and field addition of organic and inorganic amendments.

Phytostabilisation strategies have proven to be an efficient remediation option for mine tailings, but the adequate plant species and amendments have to be carefully selected. A remediation experiment was carried out at the semi-field level in tailings (pH 3.2, ≈1100, 4700 and 5000 mg kg-1 of As, Pb and Zn, respectively) from the mining district of La Unión-Cartagena (SE Spain). A red mud derivative (Fe/Al oxides), its combination with compost, and hydrated lime (Ca hydroxide) were applied in field plots of 0.25 m2. After four months of field stabilisation, tailings were transferred unaltered to a plant growth facility, and Atriplex halimus and Zygophyllum fabago (halophytes) were sown. Three months later, trace element (TE) solubility, plant accumulation and chemical speciation in the tailings pore water were studied. In unamended tailings, soluble TEs concentrations were very high (e.g., 40 mg Zn l-1), the dominant species being free ions and SO42-- complexes (>70%). The addition of amendments increased tailings pH (6.7-7), reduced TEs solubility and extractability (>80-99%) and changed the dominant species of soluble Al, Cu, Pb and Zn to hydroxides and/or organo-metallic complexes, but increased slightly the extractable As and soluble Tl concentrations. Plants were able to grow only in amended tailings, and both species presented low levels of Al, As, Cd and Zn. Therefore, the use of combined red mud derivative and compost and halophytes was shown to be a good phytostabilisation strategy, although the dose applied must be carefully chosen in order to avoid possible solubilisation of As and Tl.