Phytoremediation of soils contaminated by heavy metals

Phytoremediation of soils contaminated by heavy metals: nature as an ally in environmental remediation

Soils contaminated by heavy metals represent one of the most serious environmental emergencies in Europe.
However, nature offers us powerful and sustainable “biological” solutions: natural substances, plants, fungi, and various microorganisms can be used to “clean up” contaminated soils.
This approach is known as phytoremediation.
Phytoremediation itself exploits the ability of certain plants (metallophytes and hyperaccumulators) and the microorganisms associated with their roots to absorb, biotransform, or immobilize heavy metals and contaminants.
This low-cost, low-environmental-impact strategy is particularly suitable for large degraded areas.

Main Strategies –
• Phytoremediation: enzymatic degradation of organic compounds by plants (e.g., Populus sp., Myriophyllium spicatum).
• Phytostabilization: metal binding in the soil through enzymes and/or root exudates (e.g., Haumaniastrum, Alyssum, Gladiolus).
• Phytoextraction: absorption and accumulation in aerial tissues (e.g., Pteris vittata for As, Thlaspi caerulescens for Zn/Cd, Alyssum bertolonii for Ni).
• Phytovolatilization: absorption and gaseous release of contaminants such as Hg, Se, and As (Stanleya pinnata, transgenic Arabidopsis thaliana).
• Rhizofiltration: removal of heavy metals through the roots (Phragmites australis, Brassica juncea, Salix sp.).
• Rhizodegradation: degradation of contaminants by bacteria in the rhizosphere (Pseudomonas spp.) or the endosphere.
Numerous plant species have been identified that can tolerate and accumulate metals such as As, Pb, Zn, Cr, Cu, Ni, and even U.

For example –
• Digitalis purpurea, Cistus ladanifer, and Pinus pinaster are effective at accumulating As, Sb, Pb, and W.
• Holcus lanatus and Agrostis castellana are bioindicators of Cu, Pb, Fe, and Zn.
• Juncus effusus and Pteridium aquilinum are accumulators of Zn and Pb in humid areas.
• Helichrysum stoechas and Hypochaeris radicata are even tolerant to uranium.
• Alyssum serpyllifolium, Linaria spartea, and Lavandula stoechas hyperaccumulate Ni and Cr.

Benefits of Phytoremediation –
• Natural adaptation to poor and contaminated soils
• Low water and nutrient requirements
• High biomass for extraction or stabilization
• Potential use as bioindicators for agroecological analyses and bioprospecting.
Fungi (especially mycorrhizal fungi) and bacteria present in the rhizosphere (e.g., Pseudomonas, Rhizobium) can also improve the effectiveness of phytoremediation by promoting the absorption and detoxification of heavy metals.
Bio-based strategies, such as environmental phytoremediation of heavy metals, therefore represent a concrete and easily applicable opportunity for ecological soil regeneration.
With the right combination of plants, microorganisms, and agronomic techniques, we can transform contaminated sites into sustainable productive environments.

Francesco Di Lorenzo