Phyto-mining to recover critical raw materials from mining wastes

The European Union (EU) is heavily dependent on external sources of critical raw materials (CRMs), essential for many technological applications. To mitigate this dependency, the EU's Critical Raw Materials Act (CRMA) promotes a strategic framework for adopting circular economy models, and valorising mining waste is a viable solution for reducing environmental impacts while recovering valuable minerals.

To this aim, scraps and wastes from abandoned quarries on serpentinite outcrops in Sila Piccola Massif (southern Italy) were collected and characterized by optical microscopy, SEM-EDS and XRF. Results indicate that serpentine rocks, the main lithotypes in the area, are the potential sources of Mg, Mn, Cr, Co and Ni, some of which are found in high amounts within the serpentinite and included in the critical raw materials (CRM) list, considered crucial for the EU economy.

The ReMade@ARI – RECAMP (Recovery of Critical rAw materials from Mining wastes through Plants; PID27426) project, in which we are involved, aims to investigate the potential of phyto-mining, an ecologically sound technique for extracting these valuable elements from contaminated soils where traditional mining is not competitive. Specifically, our research focuses on using Helianthus annuus (sunflower) to uptake and concentrate metals such as nickel, cobalt, and magnesium from serpentinite mining wastes.

To evaluate the capacity of sunflower plants to accumulate metals from serpentinite waste materials, assess the effect of these plants on the bioavailability of heavy metals in the soil, and compare the efficiency of different detection techniques in identifying metal concentrations in plant tissues, sunflower plants were grown on a control pot substrate and on serpentinite waste material (alone or amended by a mycorrhizae fertilizer). After three months sunflower stems and leaves, and serpentinite material were collected and characterized using Laser Induced Breakdown Spectroscopy (LIBS), Particle Induced X-Rays Emission (PIXE), and Inductively Coupled Plasma (ICP).

Preliminary results indicate that while LIBS can detect major elements such as magnesium and calcium in plant biomass, other metals such as Ni and Co are difficult to detect in soils and plant biomass due to the high iron content, as iron peaks could potentially mask the peaks of these elements when their concentrations are low (below 50ppm). Set up adjustments are required to accurately detect the elements of interest, PIXE could be a more reliable technique for some trace elements.  

Regarding phyto-mining, ICP results indicated that sunflower plants exhibited a higher capacity for element uptake, including CRMs, when cultivated in a pot substrate as opposed to serpentinite soils, despite the higher CRM concentration present in the serpentinite. This is likely to be a consequence of the bioavailability of these elements in serpentinite being very low.

To conclude, the use of phyto-mining as a sustainable method for CRM recovery from serpentinite in the context of circular economy could be considered taking into account other plant species and/or other amendments that could increase CRM bioavailability in wastes. By improving detection methods and understanding of plant-metal interactions, we aim to increase the efficiency of this process and thereby support the EU’s strategic goals for resource independence.