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Phytoextraction of potentially toxic elements
Phytoextraction of potentially toxic elements (PTEs) is eco-friendly and cost-effective for remediating agricultural contaminated soils, but plants can only take up bioavailable forms of PTEs, thus meaning that bioavailability is the key for the feasibility of this technique. The soil-plant interactions can change the bioavailable forms of PTE in soil, which are in dynamic equilibrium, leading to a continuous re-equilibration process between these forms.
The phytoremediation approach represents an economically more realistic and cost-effective option than excavation, soil washing and in situ or off site soil disposal or even soil sealing, especially for agricultural areas producing food for the increasing world population.
Soil contamination by potentially toxic elements (PTEs) is of great concern due to their harmfulness for the biota at certain concentrations and to their persistence in the environment . When contaminant concentrations in a site overcome the screening values stated by the national legislations, the site has to be subjected to a site-specific risk analysis for calculating risk thresholds. When contaminant concentrations overcome also risk thresholds, the site must be subjected to remediation projects . In the case of agricultural soils, it is crucial to also analyze the risks that a contaminant can enter the food chain, for this purpose chemical  and biological essays  have been proposed.
Various soil remediation techniques exist, but many of them are complex and expensive. In recent years, much research has been done on soil phytoremediation because it is an in situ eco-friendly and cost-effective method with respect to the others . The main phytoremediation mechanisms are the phytostabilization, which reduces contaminant mobility toward other environmental compartments, and the phytoextraction, which exploits the ability of some plants to take up potentially toxic elements from the contaminated soil . The best suitable plants for phytoextraction are the hyperaccumulators, i.e., plants capable of growing in soils with very high concentrations of PTEs they accumulate in extremely high levels in their tissues . As an alternative, fast-growing high biomass crops that accumulate moderate levels of PTEs in their shoots can allow a greater phytoextraction efficacy since the high biomass yield can more than compensate for the lower PTE concentration in plant tissues . In any case, the contaminant to be absorbed by the plants must be in a bioavailable form in the soil . PTEs in the soil from anthropogenic sources tend to be more mobile and bioavailable than the geogenic ones . This may promote their transfer to other environmental compartments and to the food chain. Therefore, a realistic remediation objective through phytoextraction can be the progressive reduction of the contaminant to safety levels of its bioavailable portion rather than its total removal . However, the bioavailability of a PTE is closely linked to the nature of the element, to the chemical forms in which it occurs in soil, and to the chemical and physical characteristics of the soil itself. This determines the repartition of the element between the various soil geochemical fractions and the soil solution (in which the element is in a readily bioavailable form) . This repartition is controlled by dynamic equilibria between different forms elements and soil fractions, that must be considered in planning strategies of phytoremediation. After short-term phytoextraction (or only few cycles of phytoextraction), the readily bioavailable fraction of the respective element may be replenished through repartition between soil fractions and soil solution, and the kinetics of replenishment can change over time .
2. Considerations of our research experience
Consecutive croppings of hyperaccumulator plants on polluted soils, achieved a significant reduction of the bioavailable metal concentrations in soil along with changes of the soil pH and equilibrium between PTEs pools. The different bioavailability of PTEs could be reflected in their uptake and translocation by the hyperaccumulator plants, which extract PTEs from the soil and can move them from roots to shoots and leaves. Across the time the PTEs extraction changes, because there are modifications of soil properties due to the previous plant growth.
This observation highlights that bioavailability in soil-plant systems is a dynamic concept changing with soil, plant, and time. So, the legislations in future remediation projects should start to consider to reduce the concentration of metal bioavailable fraction rather than the total.
In this perspective, the monitoring of specific soil-plant interaction can be an important step in soil remediation programs, providing valuable indications of the phytoextraction techniques efficiency.
This entry is adapted from 10.3390/agronomy10060880
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