Integrated multidisciplinary study of Helichrysum microphyllum Cambess. subsp. tyrrhenicum Bacch., Brullo & Giusso for phytoremediation in abandoned mining areas

This PhD thesis is a multidisciplinary study about the use of Helichrysum microphyllum subsp. tyrrhenicum for phytoremediation in abandoned mining areas. The study area was the abandoned mine dump of Campo Pisano (SW Sardinia). The main pollutants are Zn, Pb and Cd. The soil-plant system and the seed germination properties of this species were studied to understand its behaviour towards these pollutants. The specific objectives of this work were the evaluation of: • the bioaccumulation of Zn, Pb and Cd in spontaneous specimens and on those subjected to laboratory growing test; • the growth of plant specimens under control conditions which reflect in situ ones; • the germination capability of seeds and the survival of seedling under Zn and Pb stress; • the presence of biominerals in the roots-soil interface and the mechanisms ruling bioavailability of metals. A preliminary study on spontaneous specimens growing on mine waste and not were carried out. Samples of soil and plant were analysed to obtain Zn, Pb and Cd concentration. The results showed that this species tolerates high concentration of these metals and it is able toinfluence the minerals in the rhizosphere. The phytoremediation laboratory experiment were carried out for six months under controlled climatic conditions using several specimens growing in a nursery. These were planted in three kind of substrate: • an unpolluted reference soil; • the Campo Pisano mine waste; • Campo Pisano mine waste amended with 10% of compost. The study confirms its metal tolerance and highlighted that compost helps in the survival of the species (improving soil properties and decreasing the accumulation of metals in plant tissues) Seed germination tests under different Zn and Pb concentrations were carried out to elucidate the germination capability of this species under heavy metals stress. The results showed that the seeds coming from the mine dump are better adapted to the metal stress. The best germination percentage and survival data were observed for Zn test. Nonetheless, the germination was not completely inhibited on seeds of the unpolluted site. The investigation about the mineralogy and its evolution from rhizosphere to plant compartments were carried out to shed light on possible adaptation mechanism. Some samples of soils, rhizosphere and plant compartments were collected in four sampling site. X-ray diffraction, scanning electron microscopy and synchrotron-based techniques were used. The results showed the presence of Al - Si biominerals on the roots surface forming a rim. This formation can be explained as a defence mechanism towards the highly Zn polluted substrates. Combining these results, Helichrysum microphyllum subsp. tyrrhenicum appears a good candidate for future phytostabilization actions of abandoned mine areas.