Remediation of heavy metal-contaminated iron ore tailings by applying compost and growing perennial ryegrass (Lolium perenne L.)

Improper management and disposal of heavy metal-contaminated iron ore tailings pose a significant risk to the surrounding environments. Adding compost and growing ryegrass could be a cost-effective long-term solution for remediation of heavy metal-contaminated iron ore tailings. A glasshouse study was conducted to investigate the impact of compost amendment (0, 50, 75 and 100% w/w) on growth and accumulation of heavy metals (Cu, Fe, Mn, Pb, Ni and Zn) and As in shoots of perennial ryegrass (Lolium perenne L.) grown in two iron ore tailings (Christmas Creek and Solomon mines, Pilbara, Western Australia). Ryegrass was harvested 45 and 60 days after sowing. Christmas Creek iron ore tailings had total concentrations of Fe (449 g kg-1), Mn (6.9 g kg-1), Zn (109 mg kg-1), Co (16 mg kg-1) and As (7.3 mg kg-1). Solomon mine tailings were characterised by the total concentrations of Fe (457 g kg-1), Ni (21 mg kg-1), Zn (109 mg kg-1) and As (45 mg kg-1). Both tailings had low cation exchange capacity, organic matter content, air porosity and near-neutral pH, but varied in particle size distribution (Christmas Creek–clay loam and Solomon–sandy loam). Ryegrass germination was higher in the tailings extract of Solomon than Christmas Creek tailings. Increasing the compost-to-tailings ratio increased dry shoot biomass at 45 days. The heavy metal/metalloid concentrations in shoots significantly decreased with increasing compost-to-tailings ratio, except for Cu and Zn. The bioconcentration factor (BCF) of heavy metals (defined as metal concentration in shoot/total metal concentration in the substrate) significantly decreased with an increasing proportion of compost in the growth substrate. The BCF was >1 for Zn and Cu, and <1 for other heavy metals. A high concentration of organic matter in the treatments with compost likely contributed to the enhanced mobilisation of Cu and Zn for plant uptake. In contrast, compost stabilised other metals and metalloids in the tailings to decrease their uptake by ryegrass and maintain plant growth despite relatively high Mn, Fe, As and Pb concentrations in iron ore tailings.