Using trees to extract heavy metals from polluted soils. What a great idea! The scientific community has been successfully working on this idea for more than twenty years. Among the woody species studied, the willow and its many species and sub-species offer promising prospects. However, the health of these trees that can successfully concentrate heavy metals in their biomass while still producing wood in abundance might actually be under threat. This knowledge is important for the managers of polluted sites… 

Today, we have a much more detailed knowledge of those plants that are capable of accumulating-sometimes at very high concentrations-heavy metals such as zinc, cadmium, copper, lead and nickel. This is a veritable godsend because most of these metals (described by scientists as MTEs or « Metal Trace Elements ») are toxic to the ecosystem and, ultimately, to humans. Thus the idea was born: why not use these plants, and therefore shrubs and trees equipped with the same properties for the remediation of soils polluted by industrial or domestic activity? Even better: by using woody species we could kill two birds with one stone. That is to say, we could remove the pollution from soils and produce biomass, wood that could be used as fuel, for example.

In 1983, the term “phytoremediation” appeared in scientific literature for the first time. It describes all the techniques involved in the use of plants in the fight against the risks associated with pollutants. These techniques are based on tolerance mechanisms in the sense that, despite the stresses caused by high levels of metals, some plant species clearly succeed in surviving and reproducing without the slightest hitch.  “For a quarter of a century, research into phytoremediation has mainly been concentrated on phytoextraction. This involves the tolerance mechanisms by which the plant succeeds in extracting pollutants from the soil and concentrating them in the harvestable parts, the twigs and leaves.  Yet other plants, on the other hand, can conserve the metals in the roots and the rhizosphere which is the part of the soil that is influenced by the roots and their associated microorganisms. These plants are to be prioritized for phytostabilisation”, comments Aricia Evlard, of the Laboratory of Environmental Toxicology of Gembloux Agro-Bio Tech. A distinction needs to be made between these two concepts. 

The willow tree to the rescue of our watercourses

In order to properly understand the doctoral thesis defended last May by Aricia Evlard, it would be necessary to give a brief history of the context in which she got the idea for her work. During 2009, the young researcher was immersed in the ECOLIRIMED project. Spread over the period between 2008 and 2012, this European initiative, placed under the banner of Interreg IV and coordinated by the Walloon Centre for Agronomic Research (CRA-W), aims to identify the best local woody species to consolidate and remediate riverbanks in the North of France, Belgium and Luxembourg. 

After consulting the volume of information that has been collected over the last few years on the willow (a species reputed for its quick growth and its faculties of adaptation to diverse ecosystems), Aricia Evlard asks the following questions. Could it be that we are on the wrong track? Is it possible that we are persisting in a vain search for the “miracle tree”, a sort of idealized “great photovoltaic pump” which would be capable both of depolluting the environment by accumulating metals and producing great quantities of biomass over a period of many years?  “If we were to consider phytoremediation alone as a means to clean up polluted soil, the species of trees offered to the managers of polluted sites would require a period of 30 to 600 years to finish the job. Yet management officials have to answer short-term objectives: depollution without delay! Likewise, studies carried out on cleaning by woody plants do not always include the “health” of the tree in question. As trees can live a very long time, those that are used to accumulate metals could prove to be intolerant to pollution and, after a few years could weaken and die. What would happen if we scientists advised managers to use these metal-accumulating but intolerant trees in the long term? It is therefore crucial in the general context of phytoremediation, to include a monitoring program to assess the tolerance of these woody species to metals”.