Phytostabilisation with native plant species has been advocated as a cost-effective approach to rehabilitate mine tailings containing phytotoxic metal-bearing minerals. For this purpose, five Acacia species (natural colonisers) native to north-west Queensland region of Australia were investigated for metal uptake and root exudation characteristics in response to growth for 3 weeks in three different mine tailings (Cu and Cu-Pb-Zn) differing in their degree of weathering. Root tissues of the plant species grown in the three types of tailings accumulated up to 0.9 mg Cd kg–1, 10 mg Co kg–1, 177 mg Cu kg–1, 38 mg Pb kg–1 and 4800 mg Zn kg–1 (DW basis) – being suitable for phytostabilisation purposes. However, elevated levels of root exudates in rhizosphere tailings enhanced the bioavailability of metals, leading to the accumulation of high levels of some metals in Acacia shoots (e.g. shoot concentrations of 140 mg Zn kg–1 in Acacia acradenia F.Muell. and 230 mg Zn kg–1 in Acacia hilliana Maiden). Positive correlations (P < 0.05) between citric acid levels and metal root uptakes suggested that citric acid production in these plant species may be the main driver for metal mobilisation. The results suggest that the native acacia species have the potential to mobilise metals (albeit in mineral forms) in the tailings, when used for phytostabilisation.

Phytostabilization of sulfidic Pb-Zn tailing landscapes may be one of interim options of tailings management, but which is limited by acute phytotoxicity of heavy metals in the tailings. The present study aimed to investigate the effectiveness of soluble phosphate (i.e., K 2 HPO 4) in immobilizing soluble Pb, Cd and Zn and lowering their acute phytotoxicity. The addition of soluble phosphate improved the growth of native plants Acacia chisholmii and survival rate of A. ligulata, where the latter exhibited 100% survival rate. This was in contrast to effects of conventional organic amendment in the tailings on metal solubility (e.g., elevated metal levels in porewater) and plant survival (e.g., only 42%). Organic amendment with mulch did not lower the levels of water-soluble Cd, Pb and Zn and their concentrations in plant tissues after 56 days of plant growth in the treatment. In contrast, the tailings amended with K 2 HPO 4 significantly decreased metal concentrations in the porewater and plant tissues by about 80-92% and 56-88%, respectively. The metal immobilization by phosphate was due to the formation of insoluble or sparingly soluble metal (Pb, Cd and Zn)

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