Innovative technologies of phytoremediation for contaminated soils

Abstract

Heavy metals pollution has become a global problem in all industrialized countries. Since the industrial revolution, a continuous release of heavy metals has led to a severe contamination of the soil. There are many techniques available for the remediation of contaminated soils, that to date represent a constantly evolving field, absorbing a lot of resources for research and development. Phytoremediation is a technique that uses plants to clean up metals and other contaminants from the soil or to make them harmless or less dangerous. Cynara cardunculus L. (cardoon), a perennial species from Asteraceae family, native to Mediterranean countries, is a crop studied as a metal accumulator in several researches. In this work, two experiments were performed to evaluate the effects of Cadmium (Cd) and Arsenic (As) on growth of different cardoon subspecies and to determine if this crop can be used for the remediation of polluted soils, combining this application with energy production. Different As and Cd concentrations were tested in Cynara cardunculus L. var. altilis in Experiment 1, with the aim to study the biological response of cardoon to heavy metals stress. In cardoon, the biomass production and Cd and As concentrations were determined in 4 different stages of the biological cycle in different parts of plant. The results showed that the cardoon was a plant that could tolerate the presence of Cd and As, even in high doses. Under Cd treatment, the Cd concentration decreased in the roots while increased in the leaves over time. Under As treatment, the As concentration in cardoon tissues increased with increasing As concentration; in particular the plants showed a several reduction in the production of biomass and the highest dose of As caused the death of the plants. In the combined Cd and As treatments, the plants improved resistance to As and Cd and the presence of Cd increased the ability of cardoon to tolerate As up to 45 days after artificial contamination. In the second study (Experiment 2), three accessions belonging to var. altilis (Gen 1) and var. sylvestris (Gen 2 and Gen 3) were compared and different concentrations of As, Cd and As+Cd were used. The aim of this work was to assess the concentration and bioaccumulation of As and Cd in the soil and in different parts of the plant, to understand the effects of Cd and As comparing different varieties and genotypes of cardoon plants and to study the specific speciation of As and Cd into plants. The results showed that plants were considerable tolerant to Cd and As, suggesting that this species was able to tolerate low doses of these toxic elements. Moreover, as shown in the values of bioaccumulation factor, cardoon plants had the ability to accumulate large quantities of metal contaminants in its tissue. The results regarding the speciation of As and Cd suggested that exposure of plants to toxic metals appeared to induce the synthesis of sulfur-rich ligands such as phytochelatins, a cysteine-rich oligopeptide, that strongly bound metals. The presence of As upregulated the production of these specific proteins/ligands that bound and traslocated Cd into the plant tissue suggesting that the two metals interacted to magnify phytochelatin production, leading to sequestration of both metals and consequently increasing the tolerance to both. In conclusion cardoon was a plant that could tolerate the presence of heavy metals including Cd and As. The combination of As+Cd treatment, however, increased the resistance of plants allowing them to survive. Furthurmore, Cynara Cardunculus var. sylvestris was the best subspecies that could tolerate high levels of As and Cd in its tissues and bioaccumulate greater concentrations of both metals than var. altilis. It would be useful to continue the trials with the selected Genotype 3 in future works, with the aim to test for more years, its remediation efficiency in polluted soils and exploit its biomass for energy purposes.