Helicrysum italicum (roth) G. Don, a promising species for the phytostabilization of polluted mine sites: A case study in the Montevecchio mine (Sardinia, Italy)

Mine exploitations worldwide have generated a great amount of tailings, which still contain large quantities of Potentially Harmful Elements (PHEs) able to contaminate soil, water, air, wildlife, and the food chain. Phytoremediation is an option to immobilize and/or extract PHEs from polluted mining areas. This study aims at assessing the phytoremediation properties of Helicrysum italicum (roth) G. Don, and in particular the capacity to absorb, transfer and accumulate some PHEs, such as Cd, Cu, Ni, Pb and Zn, in the plant tissues. A restricted literature review (7 papers) is also proposed in order to outline the H. italicum's behaviour and verify its possible use in phytoremediation strategy of polluted mine soils. A number of 22 contaminated sites from Montevecchio mine area (Sardinia, Italy) were sampled and the results compared with 6 uncontaminated sites. In each site both rhizospheric soil materials and H. italicum plants were sampled. Total composition and bioavailable fractions were analysed on soil samples. Helicrysum italicum roots, stems and leaves were separately analysed to extract PHEs; root/soil and leaf/root ratios were calculated to elucidate plant behaviour. Results show that Cd and Zn are the most bioavailable PHEs in contaminated sites compared to uncontaminated ones (300 and 500 folds, respectively), while Cd, Cu, Pb and Zn exceed the thresholds of the Italian environmental legislation when aqua regia extraction is executed. Helicrysum italicum plants growing on contaminated soils accumulate significantly more Cd, Ni, Pb and Zn than plants growing on uncontaminated soils, while no significant differences are found for Cu. For all considered PHEs the root/soil ratios are >1 in both contaminated/uncontaminated sites meaning that H. italicum can be defined as a tolerant species for remediation of metal-polluted soils. The leaf/root ratios weakly >1 of Zn, Pb and Cu and <1 of Ni and Cd indicate H. italicum not suitable for phytoextraction. Our results are aligned with the available literature indicating H. italicum a tolerant species, especially for Cd, Pb and Zn. The low leaf/root ratios, along with its being a spontaneous and perennial species able to propagate seeds directly on contaminated soils, recommended the use of H. italicum as pioneering strategy for the phytostabilization.