Phytoremediation is a cost-effective and sustainable technology used to clean up pollutants from soils and waters through the use of plant species. Indeed, plants are naturally capable of absorbing metals and degrading organic molecules. However, in several cases, the presence of contaminants causes plant suffering and limited growth. In such situations, thanks to the production of specific root exudates, plants can engage the most suitable bacteria able to support their growth according to the particular environmental stress. These plant growth-promoting rhizobacteria (PGPR) may facilitate plant growth and development with several beneficial effects, even more evident when plants are grown in critical environmental conditions, such as the presence of toxic contaminants. For instance, PGPR may alleviate metal phytotoxicity by altering metal bioavailability in soil and increasing metal translocation within the plant. Since many of the PGPR are also hydrocarbon oxidizers, they are also able to support and enhance plant biodegradation activity. Besides, PGPR in agriculture can be an excellent support to counter the devastating effects of abiotic stress, such as excessive salinity and drought, replacing expensive inorganic fertilizers that hurt the environment. A better and in-depth understanding of the function and interactions of plants and associated microorganisms directly in the matrix of interest, especially in the presence of persistent contamination, could provide new opportunities for phytoremediation.
Phytoextraction is a low-cost technology with negligible environmental impacts. A major issue at the field scale is the heterogeneity of contaminant concentration since the entire site needs to be treated evenly even though zones may need different incisiveness in the treatment. The concentration ratio (C shoot /C soil ) is generally used to evaluate plant species performance and it includes for simplicity an assumption of linearity in the uptake behavior, although deviation from linearity has been observed in several studies. This work describes a phytoextraction feasibility test, conducted at a greenhouse scale for the remediation of an arsenic-contaminated site. Since a feasibility test should also provide an uptake model that accounts for plant growth in heterogeneous areas, the investigation focused on defining the uptake behavior of the various selected species growing in a site with homogeneous soil properties, but with considerable differences in arsenic concentration. Among the many models selectable to describe the soil-to-plant transfer, the Freundlich-like approach was tested. While remaining easy to handle, the non-linear model selected proves to be adequate to predict the arsenic uptake despite the complex contamination considered, thus allowing a more realistic prediction of the potential of a field-scale phytoremediation procedure.
Lead (Pb) is one of the most common metal pollutants in soil, and phytoextraction is a sustainable and cost-effective way to remove it. The purpose of this work was to develop a phytoextraction strategy able to efficiently remove Pb from the soil of a decommissioned fuel depot located in Italy by the combined use of EDTA and endophytic bacteria isolated from indigenous plants. A total of 12 endophytic strains from three native species (Lotus cornicolatus, Sonchus tenerrimus, Bromus sterilis) were isolated and selected to prepare a microbial consortium used to inoculate microcosms of Brassica juncea and Helianthus annuus. As for B. juncea, experimental data showed that treatment with microbial inoculum alone was the most effective in improving Pb phytoextraction in shoots (up to 25 times more than the control). In H. annuus, on the other hand, the most effective treatment was the combined treatment (EDTA and inoculum) with up to three times more Pb uptake values. These results, also validated by the metagenomic analysis, confirm that plant-microbe interaction is a crucial key point in phytoremediation.
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