Clean polluted soil with plants: now it can be done. Eni is developing methods for the remediation of soils contaminated by heavy metals and organic compounds using combinations of plants and particular microorganisms.
by
Luca Longo
07 August 2020
6 min read
by
Luca Longo
07 August 2020
6 min read
Highly toxic heavy metals, organic and chlorinated compounds... These are just some of the substances that can remain in the soil as residues from industrial activity and become responsible for environmental pollution and the poisoning of plants, animals and, potentially, of man.
To reclaim a site using traditional methods, it is necessary to dig up the contaminated soil and dump it somewhere else, or to take it to a plant where a series of chemical, physical, thermal or biological treatments will extract the pollutants or degrade them into less dangerous compounds.
However, by using these methods, the technologies that are used to clean the soil actually add to the existing pollution. The dust that is displaced by removing the contaminated soil and transporting it, by truck or other means, to the reclamation plant contributes to dispersing polluting substances into the environment, together with the fine dust.
However, there is an alternative on the horizon: At Eni, we are developing low environmental impact reclamation processes that can reduce - if not eliminate - the pollutants present in the soil to levels that are no longer dangerous to human health. if not altogether eliminate them.
The research is carried out at the Environmental Technologies unit of the Research Centre for Renewable Energy and the Environment in Novara, in collaboration with the Research Institute on Terrestrial Ecosystems (IRET)at CNR in Pisa, on behalf of Eni Rewind, the six-legged dog company dedicated to environmental remediation.
From the in situ reclamation technologies available today, we have identified a very promising one. It is called phytoremediation and takes advantage of the natural purifying ability of plants to extract heavy metals from the soil and eliminate organic compounds. Plants capture the energy of the sun and do their work on the spot, without the need to move the soil.
This natural process improves the chemical-physical characteristics of the soil until true environmental and landscape regeneration has been achieved.
There are two main mechanisms: in the first, called phytoextraction, plants extract heavy metals from the soil and store them in their roots and leaves. In the second, called hytorizodegradation, the synergy between plants and so called rhizospheremicroorganisms, present around and inside their roots, is exploited to biodegrade organic contaminants transforming them into other, simpler, and less toxic molecules which are metabolised (i.e. eliminated) by the plants themselves. Enhanced phytoremediation occurs when their action is supported by particular bacteria that promote growth (Plant Growth Promoting Rhyzobacteria)
Thanks to laboratory, greenhouses and field tests carried out by biologists and microbiologists from the Research Centre for Renewable Energy and the Environment, we are identifying the optimal conditions for using enhanced phytoremediation in areas that are contaminated by heavy metals and hydrocarbons.
Assisted phytoremediation testing in Novara laboratories
We have identified the best plant species for the different types of contaminants and defined the microorganism/plant combinations with the highest yield. Once the effectiveness of the technology has been demonstrated, the next step is to determine protocols for using them in the field, together with the public bodies responsible for environmental and health protection.
Phytoextraction represents a valid alternative to physical and thermal treatments thanks to the great biodiversity in the plant kingdom and the numerous species capable of taking up and storing heavy metals, even in contaminated soil. These are the main ones that we are working on:
All the selected species have already been proven to be able to extract and store significant quantities of different metals in their roots and leaves, with efficiencies ranging from 35% to 40%. It is possible to hypothesise that in the field, we can completely eliminate the bioavailable fraction of dangerous metals after 3-5 successive seasonal cycles.
Rhizospheric microorganisms play a fundamental role. The extraction process was amplified thanks to metal-tolerant bacterial strains, i.e. ones that are able to survive in the presence of those particular metals. Where are they found? Precisely in some of the same contaminated soils, where they have adapted to live! Once detected, we identified them, cultivated them in the laboratory and - finally - inoculated them in the soil.
Why is their role so important? When added to the soil sown with the different plants, these microorganisms have made it possible to significantly improve the performance of the plants, both in terms of growth and phytoextraction yield. This has increased by 40-50%, enabling the extraction of up to 60% of the bioavailable polluting metals in a single season and therefore meaning that remediation targets can be met much sooner. But there’s more: the same beneficial microorganisms are also potentially hydrocarbon-oxidants, which means that they enable the biodegradation of organic pollutants. The result is an efficient, sustainable and cost-effective environmental recovery compared to conventional chemical-physical techniques.
When the metals that pollute the soil are particularly precious, such as aluminium, lead or zinc, their life does not end in the plants that have captured them. On the contrary, they can be recovered from the ashes of the plants using a process called phytomining and reused.
Not just environmental benefits. The enhancement of biomass produced for energy purposes can also be associated with the soil reclamation process. If burned in a controlled way, this can produce thermal energy, to be transformed into biogas or biofuel or be reused for the production of recycled materials.
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