Advancing phytomining: Harnessing plant potential for sustainable rare earth element extraction

Rare earth elements (REEs) are pivotal for advanced technologies, driving a surge in global demand. Import dependency on clean energy minerals raises concerns about supply chain vulnerabilities and geopolitical risks. Conventional REEs productionis resource-intensive and environmentally harmful, necessitating a sustainable supply approach. Phytomining (agromining) utilizes plants for eco-friendly REE extraction, contributing to the circular economy and exploiting untapped metal resources in enriched soils. Critical parameters like soil pH, Casparian strip, and REE valence influence soil and plant uptake bioavailability. Hyperaccumulator species efficiently accumulate REEs, serving as energy resources. Despite a lack of a comprehensive database, phytomining exhibits lower environmental impacts due to minimal chemical usage and CO2 absorption. This review proposes phytomining as a system for REEs extraction, remediating contaminated areas, and rehabilitating abandoned mines. The phytomining of REEs offers a promising avenue for sustainable REEs extraction but requires technological advancements to realize its full potential.

Influence of subsoil and soil volume on the accumulation of nickel by Odontarrhena corsica grown on a serpentine soil

Odontarrhena corsica was grown for three months on Chrome loam topsoil and subsoil from near Reisterstown, MD, to examine the effects of varying soil masses (2.8 and 5.6 kg pot-1) and soil layers (topsoil vs. subsoil) on plant growth and Ni accumulation. The subsoil position effect was simulated by placing a pot of topsoil on top of a pot filled with subsoil. Shoot Ni concentrations were similar for all treatments at 7 g Ni kg-1. Shoot yield was significantly higher in the 5.6 kg treatments compared to the 2.8 kg treatments (>18 g pot-1 vs. ∼12 g pot-1) and also greater in the topsoil treatment compared to the subsoil treatment (24.0 g pot-1 vs. 18.6 g pot-1), resulting in significantly higher phytomining. Soil depth had no statistically significant effect on shoot and root yield. Subsoil fertilization increased yield (25.8 g pot-1 vs. 19.7 g pot-1), enough to suggest that further research is warranted to optimize Ni phytomining. This study confirms the importance of soil volume and root access to the subsoil when evaluating the potential for Ni phytomining by Odontarrhena species. The use of small pots may lead to an underestimation of phytomining potential.

Maximizing trace metal phytoextraction through planting methods: Role of rhizosphere fertility and microbial activities

Brownfields are a widespread problem in the world. The poor quality of these soils and the potential presence of contaminants can pose a significant threat to plant establishment and growth. However, it may be possible to improve their establishment with an appropriate agricultural practice. In this paper, the effects of two common planting strategies, seeding and transplanting, on the establishment and growth of the hyperaccumulator species Noccaea caerulescens and on its phytoextraction capacity were investigated. A field experiment was conducted by direct sowing of N. caerulescens seeds on a plot of contaminated Technosols in Jeandelaincourt, France. At the same time, seeds were sown on potting soil under controlled conditions. One month later, the seedlings were transplanted to the field. One year later, the results showed that transplanting improved the establishment and growth of N. caerulescens. This was due to a decrease in soil pH in the rhizosphere, which subsequently increased nutrient availability. This change in rhizosphere properties also appeared to be the key that improved microbial activities in the rhizosphere soil of transplanted plants. The observed improvement in both rhizosphere nutrient availability and microbial activities, in turn, increased auxin concentrations in the rhizosphere and consequently a more developed root system was observed in the transplanted plants. Furthermore, the Cd and Zn phytoextraction yield of transplanted plants is 2.5 and 5 times higher, respectively, than that of sown plants. In conclusion, N. caerulescens transplantation on contaminated sites seems to be an adequate strategy to improve plant growth and enhance trace metal phytoextraction.

Farming for battery metals

Globally, there is a major shift to electric vehicles to combat climate change and these vehicles are currently powered by lithium-ion batteries that contain nickel cobalt manganese oxide materials. This technological change from internal combustion engines means that demand for battery minerals will need to increase by factors of >20 for the critical metals required for batteries in the next three decades. If this scenario plays out, it will require a dramatic increase in the worldwide capacity to produce nickel, manganese, cobalt, and lithium raw materials of sufficient purity. This demand could partly be met by agromining technology, which is a 'green technology' that extracts valuable products, including high-purity metal salts useful for the battery industry, from selected plants known as 'metal crops'. Farming for nickel, cobalt, and manganese is currently within reach, whereas lithium agromining has not yet been developed but has potential. SYNOPSIS: Agromining offers a sustainable approach to economically produce battery-grade raw materials from unconventional sources, thus, producing 'green technologies' from 'green sources'.

Phytohormone based biostimulant combined with plant growth promoting endophytic fungus enhances Ni phytoextraction of Noccaea goesingensis

To improve the efficiency of Ni phytoextraction, the metal hyperaccumulator N. goesingensis was subject to treatment with a combination of a Ni uptake stimulating microorganism and the commercially available, IAA- based biostimulating seaweed extract - Kelpak. Additionally, we compared the plant growth promoting and Ni uptake capabilities of the two biofertilizers. Treatment with the Kelpak alone had no significant effect on plant growth or Ni accumulation. Inoculation of N. goesingensis with Phomopsis columnaris significantly improved the biomass of the hyperaccumulating plant and Ni yield per plant and improved several plant biometric features such as fresh and dry weight and several others related to leaf and root size. However, the combination of the two treatments yielded the best results; plants treated with the two growth promoting agents yielded 85% more biomass compared to not treated plants and accumulated 48% more Ni per plant. To verify plant inoculation with the fungus we generated a GFP expressing strain of P. columnaris and visualized the fungus in both plant leaves and roots. To trace the development of the fungus in planta and to evaluate the effect of biostimulant treatment on mycelium development fungal translational elongation factor 1α (tef1α) DNA was quantified with qPCR. Upon biofertilizer the abundance P. columnaris in plant leaves increased nearly 5-fold. The utilization of plant growth stimulating microorganisms, endophytic fungi in particular, can significantly improve Ni phytoextraction in hyperaccumulator N. goesingensis.

The X-ray fluorescence screening of multiple elements in herbarium specimens from the Neotropical region reveals new records of metal accumulation in plants

Plants have developed a diversity of strategies to take up and store essential metals in order to colonize various types of soils including mineralized soils. Yet, our knowledge of the capacity of plant species to accumulate metals is still fragmentary across the plant kingdom. In this study, we have used the X-Ray Fluorescence technology to analyze metal concentration in a wide diversity of species of the Neotropical flora that was not extensively investigated so far. In total, we screened more than 11 000 specimens representing about 5000 species from herbaria in Paris and Cuba. Our study provides a large overview of the accumulation of metals such as manganese, zinc and nickel in the Neotropical flora. We report 30 new nickel hyperaccumulating species from Cuba, including the first records in the families Connaraceae, Melastomataceae, Polygonaceae, Santalaceae and Urticaceae. We also identified the first species from this region of the world that can be considered as manganese hyperaccumulators in the genera Lomatia (Proteaceae), Calycogonium (Melastomataceae), Ilex (Aquifoliaceae), Morella (Myricaceae) and Pimenta (Myrtaceae). Finally, we report the first zinc hyperaccumulator, Rinorea multivenosa (Violaceae), from the Amazonas region. The identification of species able to accumulate high amounts of metals will become instrumental to support the development of phytotechnologies in order to limit the impact of soil metal pollution in this region of the world.

Treasure from trash: Mining critical metals from waste and unconventional sources

To meet future technological demands of our growing global community new sources of industry critical metals need to be identified. To meet these demands, extracting minerals from larger, lower grade deposits across most commodities is required, which in turn generates ever increasing amounts of mine wastes. We propose that agromining could be used to enables access to unconventional resources not viable using existing minerals processing techniques. This innovative technique relies on so-called hyperaccumulator plants to bio-concentrate high levels of metals into living biomass which can then be extracted from the harvested bio-ore. Producing critical metals, such as nickel, cobalt and thallium, efficiently and sustainably using agromining appears to be well within reach, but this technology needs industrial champions to develop demonstration sites that are scaled appropiately in areas where it is feasible.

Fertilization regimes affecting nickel phytomining efficiency on a serpentine soil in the temperate climate zone

Phytomining of nickel (Ni) refers to cropping of selected Ni hyperaccumulator plants on Ni-rich serpentine soils. In this study, the effect of different fertilization regimes on the Ni yield of Odontarrhena chalcidica (syn. Alyssum murale) was evaluated within a field experiment on an Austrian serpentine site. Odontarrhena chalcidica was planted in six treatments: control, fertilized by mineral fertilizer, cow manure, pig manure, compost, and planted at higher plant density. A positive fertilization effect was observed: plants treated with NPK and pig manure produced significantly higher biomass (1.9 t ha^-1 for both treatments). Nickel yields showed a clear trend for enhancement upon fertilization (cow manure: 22.7 kg Ni ha^-1, pig manure: 21.3 kg Ni ha^-1, NPK: 20.6 kg Ni ha^-1), but were not significantly different from the control. As a result of Ni accumulation in plants, DTPA-extractable Ni pools were significantly lower after harvesting (average 37.3 mg kg Ni-DTPA^-1) compared to the time of planting (average 45.6 mg kg Ni-DTPA^-1) in organic fertilization treatments and plots of higher plant density. The application of organic fertilizers contributed also to improved soil quality. We conclude that fertilization can increase the phytomining potential of field-grown Ni hyperaccumulator plants in a soil-friendly manner.

Nickel phytomining from industrial wastes: Growing nickel hyperaccumulator plants on galvanic sludges

Nickel (Ni) is used in numerous industrial processes, with large amounts of Ni-rich industrial wastes produced, which are largely sent to landfill. Nickel recovery from waste materials that would otherwise be disposed is of particular interest. Nickel phytomining represents a new technology in which hyperaccumulator plants are cultivated on Ni-rich substrates for commercial metal recovery. The aim of this study was to investigate the possibility of Ni transfer from industrial waste into plant biomass, to support recovery processes from bio-ores. Different industrial galvanic sludges (containing 85-150 g kg^-1 Ni) were converted into artificial substrates (i.e. technosols) and the Ni hyperaccumulator Odontarrhena chalcidica (formerly Alyssum murale) was cultivated on these Ni-rich matrices. A greenhouse pot experiment was conducted for three months including an ultramafic soil control and testing fertilized (NPK) and unfertilized replicates. The results showed that fertilization was effective in improving plant biomass for all the substrates and that O. chalcidica was capable of viably growing on technosols, producing a comparable biomass to O. chalcidica on the control (ultramafic soil). On all technosols, O. chalcidica achieved Ni shoot concentrations of more than >1000 mg Ni kg ^-1 and maximum Ni uptake was obtained from one of the technosols (26.8 g kg ^-1 Ni, unfertilized; 20.2 g kg ^-1 Ni, fertilized). Nickel accumulation from three of the technosols resulted to be comparable with the control ultramafic soil. This study demonstrated the feasibility of transferring Ni from toxic waste into the biomass of Odontarrhena chalcidica and that phytomining from galvanic sludge-derived technosols can provide similar Ni yields as from natural ultramafic soils.

Phytoremediation of Heavy Metal-Contaminated Sites: Eco-environmental Concerns, Field Studies, Sustainability Issues, and Future Prospects

Environmental contamination due to heavy metals (HMs) is of serious ecotoxicological concern worldwide because of their increasing use at industries. Due to non-biodegradable and persistent nature, HMs cause serious soil/water pollution and severe health hazards in living beings upon exposure. HMs can be genotoxic, carcinogenic, mutagenic, and teratogenic in nature even at low concentration. They may also act as endocrine disruptors and induce developmental as well as neurological disorders, and thus, their removal from our natural environment is crucial for the rehabilitation of contaminated sites. To cope with HM pollution, phytoremediation has emerged as a low-cost and eco-sustainable solution to conventional physicochemical cleanup methods that require high capital investment and labor alter soil properties and disturb soil microflora. Phytoremediation is a green technology wherein plants and associated microbes are used to remediate HM-contaminated sites to safeguard the environment and protect public health. Hence, in view of the above, the present paper aims to examine the feasibility of phytoremediation as a sustainable remediation technology for the management of metal-contaminated sites. Therefore, this paper provides an in-depth review on both the conventional and novel phytoremediation approaches; evaluates their efficacy to remove toxic metals from our natural environment; explores current scientific progresses, field experiences, and sustainability issues; and revises world over trends in phytoremediation research for its wider recognition and public acceptance as a sustainable remediation technology for the management of contaminated sites in the twenty-first century.

A nickel phytomining field trial using Odontarrhena chalcidica and Noccaea goesingensis on an Austrian serpentine soil

Phytomining of nickel (Ni) is based on the cropping of Ni hyperaccumulators on Ni-rich serpentine soils. The efficiency of this approach is dependent on shoot nickel concentration and harvestable biomass. In a field experiment conducted on an Austrian serpentine site, the phytomining efficiency of the two plant species Odontarrhena chalcidica (syn. Alyssum murale) and Noccaea goesingensis was evaluated. O. chalcidica was planted in three treatments: control, sulphur application (0.46 g S kg^-1 soil) and intercropping with the legume Lotus corniculatus. For N. goesingensis the treatments control, high-density planting (110 plants m^-2) and intercropping were implemented. Given the experimental set-up, shoot biomass, shoot Ni concentration and thus the total amount of harvested Ni were on average higher for O. chalcidica. The highest Ni yield was achieved with O. chalcidica, reaching 55 kg Ni ha^-1 in the sulphur treatment. N. goesingensis showed the maximum yield in the high-density treatment with 36 kg Ni ha^-1. However, high-density planting of N. goesingensis and sulphur application to O. chalcidica plots did not significantly increase the Ni yield compared to the control. Intercropping with L. corniculatus tended to decrease the shoot biomass of both species. Planting of the hyperaccumulators led to a decrease of DTPA-extractable Ni and to an increase of soil pH, with the exception of sulphur-amended plots. Likewise, rhizosphere soil pH was higher than bulk soil values. Our data suggest that in particular O. chalcidica is suitable for Ni phytomining on the tested site. Measures to further increase the Ni yield and to optimise crop management will be evaluated in follow-up experiments.

Community diversity and potential functions of rhizosphere-associated bacteria of nickel hyperaccumulators found in Albania

Ultramafic (i.e. serpentine) soils are widespread in the Balkans and particularly in Albania. They account for a large part of plant endemism in that region and host several hyperaccumulator species, which are characterized by leaf nickel concentrations frequently above 1%. This rich nickel hyperaccumulating flora could serve as candidate to be used in phytoextraction and agromining. Despite recent interest in metal hyperaccumulating plants and agromining, very few studies have investigated the bacterial diversity and the influence of environmental factors on microbial gene profiles in the rhizosphere of hyperaccumulator plants growing on ultramafic soils. Because rhizospheric bacteria could be crucial to the success of phytoremediation, we studied a total of 48 nickel-hyperaccumulating plants which were sampled from four species that are widespread in Albania: Noccaea ochroleuca, Odontarrhena smolikana, O. rigida and O. chalcidica. All samples were taken from the ultramafic regions of Librazhd and Pogradec in eastern Albania in October 2015. Our study shows that Proteobacteria, Actinobacteria and Acidobacteria dominated the soil bacterial communities. Of these three phyla, only Proteobacteria was relatively abundant. This study underlines the influence of soil Cation Exchange Capacity on the bacterial community's diversity and structure. Based on the predicted metagenomes, the genes belonging to amino acid, lipid and carbohydrate metabolisms were identified as major gene families. Our study sheds some light on our understanding of how bacterial communities are structured within and affect the rhizosphere of hyperaccumulator plants from ultramafic soils in Albania.

Can organic amendments replace chemical fertilizers in nickel agromining cropping systems in Albania?

In Albania, ultramafic outcrops cover 11% of the surface and have the potential to support nickel phytomining. In a large-scale in-situ experiment on an ultramafic Vertisols in Pojskë we are studying the influence of agronomical practices on Ni phytoextraction yield of Odontarrhena chalcidica (syn. Alyssum murale). Three cropping systems were compared in three plots in 2016-2017; POJ-1 Plot (0.3 ha) was established with plants that had germinated spontaneously without any treatments; POJ-2 plot (0.3 ha) was covered by plants that had germinated spontaneously and was treated with mineral fertilizer (N50P50K50 kg ha^-1); and POJ-3 Plot (400 m²) was divided in four sub plots, where O. chalcidica was planted at a density of 4 plants m^-2 on which, we neither applied fertilizer, nor NPK fertilizer (N65P65K65), pig (FPM; N260:P105:K260 + 15 kg ha^-1N, P, K) or chicken manure (FCHM; N260:P390:K260 +15 kg ha^-1 N, P, K. Irrigation and mechanical control of weeds was done on POJ-3. After 8 months, shoot Ni concentration, biomass, and Ni yields were higher in O. chalcidica treated with manure and the cost of biomass production was smaller. Nickel yield was more promising (145 kg ha^-1) than in previous field trials. This study highlights that, using manure, the Ni yield increases Ni phytomining net values, thus agromining can become an economically justifiable agricultural cropping system.

Influence of new agromining cropping systems on soil bacterial diversity and the physico-chemical characteristics of an ultramafic soil

Most of the research dedicated to agromining has focused on cultivating a single hyperaccumulator plant, although plant diversity has been shown to positively modify soil characteristics. Hence, we compared the effect of cropping a nickel-hyperaccumulator Alyssum murale with a legume (Vicia sativa) to A. murale's mono-culture, on the bacterial diversity and physico-chemical characteristics of an ultramafic soil. A pot experiment with 5 replicates was conducted in controlled conditions for 11 months. The treatments studied were: co-cropping and rotation vs. mineral fertilization controls and bare soil. The introduction of legumes induced a clearly positive effect on the soil's microbial biomass carbon and nitrogen. Arylsulfatase and urease activities tended to be enhanced in the co-cropping and rotation treatments and to be lessened in the mineral fertilization treatments. However, β-glucosidase and phosphatase activities were seen to decrease when legumes were used. Our results showed that the rotation treatment induced a higher organic matter content than the fertilized control did. Actinobacteria was the most-represented bacterial phyla and had lower relative abundance in treatments associating legumes. Conversely, the relative abundance of Acidobacteria and Gemmatimonadetes phyla increased but not significantly in treatments with legumes. The relative abundance of Chloroflexi phylum was shown to be significantly higher for the fertilized rotation control. The relative abundance of β-Proteobacteria subphylum increased but not significantly in treatments with legumes. NMDS analysis showed a clear separation between planted treatments and bare soil and between co-cropping and rotation and fertilized controls. Shannon index showed reduction in microbial diversity that was mainly due to chemical inputs in the soil. This study showed that these new cropping systems influenced both the bacterial diversity and the physico-chemical characteristics of an ultramafic soil. In addition, this study provides evidence that mineral fertilization can negatively impact bacterial communities and some of their functions linked to biogeochemical cycles.

Organic amendments for improving biomass production and metal yield of Ni-hyperaccumulating plants

Ni phytomining is a promising technology for Ni recovery from low-grade ores such as ultramafic soils. Metal-hyperaccumulators are good candidates for phytomining due to their extraordinary capacity for Ni accumulation. However, many of these plants produce a low biomass, which makes the use of agronomic techniques for improving their growth necessary. In this study, the Ni hyperaccumulators Alyssum serpyllifolium ssp. lusitanicum, A. serpyllifolium ssp. malacitanum, Alyssum bertolonii and Noccaea goesingense were evaluated for their Ni phytoextraction efficiency from a Ni-rich serpentine soil. Effects of soil inorganic fertilisation (100:100:125kgNPKha(-1)) and soil organic amendment addition (2.5, 5 or 10% compost) on plant growth and Ni accumulation were determined. All soil treatments greatly improved plant growth, but the highest biomass production was generally found after addition of 2.5 or 5% compost (w/w). The most pronounced beneficial effects were observed for N. goesingense. Total Ni phytoextracted from soils was significantly improved using both soil treatments (inorganic and organic), despite the decrease observed in soil Ni availability and shoot Ni concentrations in compost-amended soils. The most promising results were found using intermediate amount of compost, indicating that these types of organic wastes can be incorporated into phytomining systems.

Challenges and opportunities in the phytoremediation of heavy metals contaminated soils: A review

Mining operations, industrial production and domestic and agricultural use of metal and metal containing compound have resulted in the release of toxic metals into the environment. Metal pollution has serious implications for the human health and the environment. Few heavy metals are toxic and lethal in trace concentrations and can be teratogenic, mutagenic, endocrine disruptors while others can cause behavioral and neurological disorders among infants and children. Therefore, remediation of heavy metals contaminated soil could be the only effective option to reduce the negative effects on ecosystem health. Thus, keeping in view the above facts, an attempt has been made in this article to review the current status, challenges and opportunities in the phytoremediation for remediating heavy metals from contaminated soils. The prime focus is given to phytoextraction and phytostabilization as the most promising and alternative methods for soil reclamation.