with contributions from Claudia Cocozza, Werther Guidi Nissim, Massimo Fagnano, Antonio Saracino, Giorgio Vacchiano

We are well acquainted with the benefits that trees provide to humanity: from the production of food and building materials to the absorption of carbon dioxide, from hydrogeological protection to global climate regulation. But there is another special task we can entrust to certain trees, particularly in urbanised areas: remediating polluted environments. The mechanisms that make plants capable of absorbing and immobilising pollutants are still being studied, but many believe that these "phytotechnologies" could become of great importance in the years ahead, as an alternative to conventional remediation techniques.

Remediation through trees

Despite growing sensitivity to environmental issues, numerous productive activities are still responsible for the contamination of land and water. The mining sector and the chemical and petrochemical industry are just some of the many examples of activities that have caused, and continue to cause, serious environmental contamination problems in many parts of the world. Many countries have adopted strict regulations for the monitoring and recovery of contaminated sites.

Traditionally, remediation involves excavating, transporting and subsequently disposing of contaminated substrates in landfill. In some cases, the contaminated matrix can be treated through physical or chemical processes and returned to a state close to its original condition. These techniques are effective, but have two major drawbacks: they are costly (several hundred thousand euros per hectare remediated) and carry risks of contamination from the polluted matrix, which may disperse into the environment both during transport (from the site to the landfill) and during storage. These are also interventions with a very low level of social acceptability: few communities are willing to receive contaminated matrices within their territory, a phenomenon expressed in technical language by the term Nimby ("Not In My BackYard"). All of these factors have hindered the spread of remediation at a global level, with the result that thousands of hectares of contaminated land remain to this day in a state of abandonment.

But are there forms of remediation that are more competitive from an economic standpoint and less socially problematic? Enter phytoremediation.

Trees and plants: what is phytoremediation?

Phytoremediation (or phytosanation) is the technique that uses plants and the micro-organisms associated with them to eliminate, contain or render less toxic the pollutants present in water or soil. The fact that the natural world is capable of metabolising toxic substances may seem paradoxical. Yet vascular plants appeared on Earth around 400 million years ago: since then, through evolution, they have had time to develop the adaptive mechanisms necessary for survival in difficult environmental conditions. Among the various forms of adaptation and resilience, there are plants capable of accumulating and tolerating in their organs certain substances, such as heavy metals, at concentrations that would normally be harmful to other living organisms, including humans. It is therefore possible to base environmental recovery techniques on the capacity of certain groups of plants to absorb and store the toxic substances found in contaminated soils.

This technique has undeniable advantages: lower costs, low environmental impact and high social acceptability, also thanks to the fact that phytoremediation can improve the aesthetic appearance of heavily degraded environments and their overall ecological value, encouraging, for example, an increase in biodiversity. There are, of course, weaknesses too: the technique cannot be applied where soils are so heavily contaminated as to preclude plant life, and the time required for remediation is much longer than that required by conventional methods.

But how does phytoremediation work?

This term in fact encompasses a wide range of techniques, distinguished according to the strategies plants use to carry out the purification work and the nature of the contaminant itself. Thus one speaks of phytoextraction when the plant is able to absorb contaminants from the soil and transfer them to the stem and leaves; by harvesting these, it is possible to gradually reduce (especially after several harvesting cycles repeated over the years) the levels of pollutants. Phytoextraction does not involve the chemical transformation of the contaminant and therefore its physical elimination; it does, however, make it possible to concentrate pollutants originally dispersed in the soil (and the quantities of land to be treated by conventional methods are enormous!) into a modest amount of plant material, which can be treated more easily. The efficiency of phytoextraction depends on certain basic requirements that plants must possess: a high rate of pollutant absorption, rapid growth, high biomass production, a dense and extensive root system and, naturally, tolerance towards the contaminants present in the soil.

Another technique is phytostabilisation. This is a "passive" method — it is not a true remediation technique so much as a containment strategy. One speaks of stabilisation when the plant produces substances that immobilise (i.e. render insoluble) the pollutant in the soil, thereby preventing its dispersal into the surrounding environment. This action can also be physical in nature: plant roots can act as a "net" and limit soil erosion, which is responsible for the dispersal of pollutants.

Phytodegradation, on the other hand, is a technique that lends itself well to contaminants of organic origin, such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and other by-products of petrochemical activity contained in the soil, often persistent and difficult to degrade. In this case, plants capable of directly degrading these compounds into molecules that are less persistent and less dangerous to human health and the environment are employed, thanks to the substances produced by their roots. Maize, for example, is well known for this property. Plants with a dense and compact root system often host a large number of micro-organisms, which further promote the degradation of these complex molecules. In this case, certain grasses and poplars prove highly effective.

Finally, plants can contribute to the aerial degradation of certain pollutant compounds through the process of phytovolatilisation. This means that the plant is able to absorb contaminants, transfer them to the aerial parts and release them into the atmosphere through transpiration. Certain trees with high evapotranspiration rates (poplars and willows) are particularly suited to this technique and, during the transfer process, are even capable of degrading some molecules, preventing the contaminant from being transferred unchanged from the soil to the atmosphere.

Applications in Italy

These techniques are currently being trialled in Italy as well, in one of the best-known areas for soil pollution: the Terra dei fuochi. Here, phytoremediation tree plantations were tested at pilot scale, thanks to the Life Ecoremed project (2012–2017), using Arundo donax on compacted and degraded soils, Phragmites australis on salinised soils, and poplar clones (P. nigra x deltoides) on sites subject to illegal waste dumping.

In 2015, building on the project's success, two full-scale interventions were carried out: the first on a 3.5-hectare industrial site contaminated with lead at a battery recycling plant in Marcianise (known as "Il bosco in fabbrica"), and the second on a six-hectare agricultural site seized from an organised crime figure who had been using it to dispose of tannery sludge (San Giuseppiello).

The main objective of these plantations was containment — that is, preventing particles of contaminated soil from dispersing into the air and water, and from there threatening human health. Accordingly, a high-density poplar planting design (3 x 1 metres) was chosen in order to create rows of trees capable of reducing wind speed and thereby preventing soil particles from being lifted and dispersed. Between each row of trees, Bermuda grass (Cynodon dactylon) was sown to ensure complete ground cover during dry periods and thus enhance the reduction of wind erosion. From the third or fourth year onwards, the grass cover was progressively replaced by leaf litter, which continued to retain soil particles, as evidenced by particulate matter measurements and their chemical analysis carried out in collaboration with Arpa Campania.

At the agricultural site of San Giuseppiello, following detailed environmental characterisation and risk analyses, an area of 3,000 square metres was found to be contaminated with bioavailable cadmium, which could pose a health risk to consumers as it is capable of translocating into the edible parts of leafy vegetables grown on site. Today, poplar trees are continuing their remediation work there, removing the bioavailable fraction of cadmium, which according to analyses carried out in 2000 has been halved compared to initial values. The accumulation of cadmium in the leaves of vegetables (chicory, rocket, lettuce and spinach), used as biological test species for risk analysis, is also decreasing linearly, suggesting that the risk could be completely eliminated within another four to five years. The remainder of the plot (57,000 square metres) shows high concentrations of chromium and zinc, but has not been classified as "contaminated" because analyses have shown that these two minerals do not threaten human health, both on account of their low toxicity and their low bioavailability, which prevents root uptake and accumulation in fruit and vegetable products at concentrations dangerous to consumers' health.

The now decade-long experience of phytoremediation carried out in Campania has also highlighted that the plantations provide a series of "collateral" ecosystem benefits: the biomass produced, which is a by-product of phytoremediation, can be used as an industrial fuel, replacing fossil fuels in lead smelters; the accumulation of organic carbon in the soil contributes to climate change mitigation; the root uptake of pollutants reduces their percolation through the soil and lowers the risk of groundwater contamination. Furthermore, wildlife has begun to frequent these areas, thereby enriching biodiversity.

Phytoremediation thus transforms a degraded landscape into a pleasant environment with high naturalistic and aesthetic value. Today, the remediated sites are used for social, educational and artistic activities proposed by local residents and associations, thereby completing the objective of restoring our territories and making them available to the community.