Phytoremediation of metal-contaminated soils by industrial crops
Linum usitatissimum (flax), Brassica napus var. oleifera (oilseed rape), Miscanthus x giganteus (miscanthus) and Urtica dioica (nettle) were investigated to assess their potential as phytoremediation crops. Germination experiments using flax and oilseed rape established that seedling germination was not inhibited by exposure to metals in solution except at the highest concentrations considered. Germination was, however, not a reliable indicator of plant metal tolerance as metal toxicity to emerged seedlings was evident in contaminated soil treatments exhibiting good germination rates. Four plant species were grown in soils containing six metals at both highly and marginally spiked levels, to reproduce genuine contaminated soils whilst allowing the study of each metal in isolation. A sewage sludge treated soil with a high metal and organic matter content was also included in the study. Miscanthus was the species most tolerant of the highly contaminated soils. The highest tissue concentrations recorded in plants exposed to the highly contaminated soils were (969 mg Zn/kg) in stems of miscanthus and (919 mg Cd/kg) in stems of nettle, but plant growth in these soils was generally poor. The plant species survived well in the sewage sludge soil, although metal uptake from this matrix was low. Oilseed rape and nettle accumulated the highest tissue metal concentrations in the study of marginally contaminated soils. Indeed the highest tissue concentration recorded for plants grown in all of the soils was found in nettle grown in the marginally contaminated Zn soils (1937 mg/g). Miscanthus, was able to remove a greater weight of metal from the soil owing to its higher biomass, despite having a lower tissue metal concentration than the other species. Flax, miscanthus, nettle and oilseed rape have been shown to have potential to act as part of a phytoremediation programme, however, more work with these crops is required before film advice can be given on commercial application of the crops in contaminated land remediation.