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Rare earth elements (REEs)
What are REEs?
Rare earth elements (REEs), also known as rare earths, or rare earth metals are a group of 17 metallic elements that comprise 39yttrium (Y) and 15 Lanthanides: 57lanthanum (La), 58cerium (Ce), 59praseodymium (Pr), 60neodymium (Nd), 61promethium (Pm), 62samarium (Sm), 63europium (Eu), 64gadolinium (Gd), 65terbium (Tb), 66dysprosium (Dy), 67holmium (Ho), 68erbium (Er), 69thulium (Tm), 70ytterbium (Yb) and 71lutetium (Lu) (Figure 1.1). Even though 21Scandium (Sc) has been considered as one of the REEs, Voncken (2016) pointed out that the geochemical behaviour of Sc is more similar to the ferromagnesian transition elements such as Iron (Fe), Vanadium (V), Chromium (Cr), Cobalt (Co) and Nickel (Ni) than the other REEs and that Sc should not be considered as an REE.
The 15 lanthanides share very similar physical properties, and the presence of the lanthanides in crystalline compounds are mostly in 3+ oxidation states with the possibility of 2+ or 4+ states for some of lanthanides (Haire and Eyring 1994). The element Y also belongs to REEs due to the similar chemical behaviour and the natural presence of 3+ state (Voncken 2016).
The REEs were firstly discovered in the late 18th century, when a Finnish chemist Johan Gadolin examined a rock containing about 55.5% of an unknown oxide and named the new oxide as gadolinite and then further changed it to ytterbite. In 1803, the Swedish chemist Jöns Jakob Berzelius and scientist Wilhelm Hisinger have analysed the ‘tungsten (heavy stone) of Bastanäs’ and have isolated elements similar to gadolinite but also distinct. At the same time, the German chemist Martin Klaproth sampled the ‘tungsten from Bastanäs’ and made the same conclusion. He also named this new element as ochroite because of the yellow-brown (ochra in Greek) colour of the element. And this element was later named as cerium. At the end of the 19th century, except lutetium (Lu) and promethium (Pm), all the other REEs had been discovered and named (Voncken 2016). Lu was identified in the early 20th century, and Pm was discovered in nuclear reactions in 1947 (Marinsky et al. 1947).
Figure 1.2 Abundance of the elements given in atom fraction as a function of the atomic number. The rare earth elements are indicated in blue. Image from Haxel et al. (2005), courtesy of the U.S.
The term REE was given to Sc, Y and lanthanides due to the discovery of those elements. Most of the REEs were discovered at the end of 19th century and until then the only REE known deposit was in Ytterby in Sweden and therefore, REEs were believed to be rare. Most of the REEs exploited in the early age were oxides and in the major scientific languages in the 19th-century oxide of an element shared the same word with the earth in French, similarly in German written as “Erde” (Voncken 2016). The technology advances and the knowledge on REEs have increased with time, but even knowing that the REEs are actually not rare in the earth crust, their name has been kept unchanged.
As mentioned before, the REEs are actually not rare concerning their concentrations.
The most abundant REEs are Ce, Nd, La and Y. They are comparable to the level of copper (Cu), nickel (Ni) and zinc (Zn). All of the REEs are more abundant in the earth crust than some elements we are familiar with, such as cadmium (Cd) and mercury (Hg) (Figure 1.2). As has been reported in previous studies, the average abundances of La (30 mgkg-1) and Ce (60 mgkg-1) in the crust are comparable to the common metals such as Zn (70 mgkg-1) and Cu (55 mgkg-1) (Tyler 2004a; Kim et al. 2015).
According to the electron configuration (Table 1.1), the REEs are often classified as light rare earth elements (LREEs) and heavy rare earth elements (HREEs). The LREEs include La, Ce, Pr, Nd, Pm, Sm, Eu and Gd. The HREEs comprise Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y (Figure 1.3). Y is classified as HREE due to its similarities in ionic radius and chemical properties with the other HREEs. In general, the LREEs are more abundant in the earth crust and are easier to be exploited in many deposits. Despite the high demand, HREEs are in general less discovered than LREEs and more difficult to be extracted.
The REEs are very electropositive, therefore, it is really easy for REEs to form ionic compounds. REEs mainly exist as metallic form, easily forming oxides, halides, carbonates, phosphates and silicates, borates or arsenates. This is one of the reasons why REEs were discovered rather late. Most of REE metals are grey or silver metals with high luster but easily tarnish in the air. They have good electrical conductivity but low solubility (Voncken 2016).
During the past decades, REEs have become a hot topic of discussion in different mass media and have been more and more familiar to people. REEs have been used into lots of technological products and are mostly crucial in industries and high technologies. REEs are, for this reason, highly demanded in modern society.
Most of the REEs have good magnetic properties, catalytic abilities and some have fluorescent abilities. Some of the REEs can improve other metal abilities in metal alloys or improve non-metallic substance ability as additives. The major applications of each REEs are summarized in Table 1.2.
Tm does not have wide application due to its high price.
Yb has little application due to the low production. Mainly in optical lenses of electronic devices.
Lu is also a rare metal and is mainly used in advanced computer technologies.
The early application of Sm was used in permanent magnets. It was firstly developed in early 1960s using Sm alloyed with cobalt (SmCo). However, this application was replaced by neodymium iron boron (NdFeB) magnets in the 1980s as Nd magnets are stable up to 300 – 400 °C while Sm-Co magnets are stable even about 700 °C. The Nd magnets are still used now (British Geological Survey report 2011). Even though with less industrial applications, the high magnetic strength at a higher temperature enables SmCo magnets to be widely applied in the precision-guided system in the military utilisations (Dushyantha et al. 2020). Another important application of Sm is industrial catalysts in plastic decompositions and ethanol productions. Sm triflate is considered as one of the most efficient catalyst promoting Friedel−Crafts alkylation with alkenes (Hajra et al. 2007).
Source of REEs
The principal rare earth ore minerals are monazite, bastnaesite and xenotime. The most known monazite is CePO4. Beside Ce, some light REEs such as La, Pr, Nd and Sm also occur in monazite. Monazite also contains Thorium (Th) and/or Uranium (U) but with a considerably low amount. Monazite is generally present as a minor part in granites, granodiorites, associated pegmatites and some metamorphic rocks (Voncken 2016). Bastnaesite is a major rare earth ore containing mainly LREEs and low proportion of HREEs. Bastnaesite is widely spread all over the world, generally related to carbonatite intrusions. Therefore, bastnaesite was usually found with the presence of carbonatite at the same time (Atwood 2012; Voncken 2016). The general formula of xenotime YPO4, different from monazite or bastnaesite, is mainly composed of HREEs. Xenotime contains a large amount of rare earth oxides (up to 67%) (Voncken 2016). The most commonly occurring elements are Dy, Yb, Er and Gd. Similar to monazite, xenotime also contains Th and/or U, depending on the situations of different locations, and Th and U can be a by-product (Voncken 2016).
REE reserves and major mining deposits
REE reserve worldwide
The REE reserve was estimated to more than 100 million tonnes all over the world (Chen 2011). Even though the REEs are widely distributed and rather abundant in the earth crust with the total REE concentration reaching 100-200 mgkg-1 (Tyler 2004a; Liang et al. 2005), they are not evenly distributed. According to United States Geological Survey (USGS) 2020 report (USGS 2020), Asia holds the largest REE reserve among all the continents. China, being the biggest REE reserve country, holds about 37% of the world REE reserve. Vietnam holds 22 million tonnes of REE reserve, accounting for 18% of the world reserve, then followed by India who holds 6% of the total reservoir. A large number of rare earth reserves was found in South America, mainly in Brazil who holds 18% of the world reserve and has a long rare earth production history. Comparatively, North America holds less REE reserve only about 1% in the USA and less than 1% in Canada. In eastern countries, several countries have REE deposits such as Russia (10% world reserve), Greenland (1% world reserve), Estonia, Sweden and so on. REE deposits have also been found in Australia with 3.3 million tonnes accounting for 3% of world reserve (Figure 1.4 & 1.5). Except for Northern Europe, rare earth occurrences were found with no intensive mining activities. In France, there is little information about REE occurrence. Few REE enrichments were found in east France located in low grade metamorphosed black shales of Dinantian (Châteauneuf-du-Faou, Corlay) and Ordovician age (le Grand-Fougeray, Châteaubriand) and in south France (EuRare.eu) (Figure 1.6).
Major REE deposits in the world
REE deposits can be formed through primary or secondary geological processes. Primary processes include magmatic, hydrothermal and/or metamorphic processes, during which apatite, monazite, and synchysite are formed. Secondary REE minerals are Fe-oxyhydroxides, secondary monazite, churchite, and plumbogummite-group which are formed due to the erosion and weathering of primary deposits (Voncken 2016; Dushyantha et al. 2020). In the rare earth exploitation, REE mineralization is classified as REE deposit or REE occurrence depending on its economic interest and possible exploitation (EUrare website).
Mountain Pass deposit is an open-pit mine located in California, United States. It has the longest exploitation history of REEs. Mountain Pass deposit was the major rare earth deposit in the USA containing about 8 to 12% rare earth oxides (USGS 2013). From the mid 1960s to the mid 1980s, Mountain Pass was the dominant source of REEs, and the USA were largely self-sufficient in REEs.
Bayan Obo Deposit, China Since 1985, the REE production in China has increased largely due to the exploitation of the Bayan Obo REE deposit (Figure 1.5). Bayan Obo (白云鄂博 in Chinese, also Baiyun-Obo or Baiyun’ebo) Mine is the largest known rare earth mine worldwide, located in the inner Mongolia region of China, bordering Mongolia and Russia. Bayan Obo deposit is formed by large carbonate and hydrothermal rocks and was discovered in 1927 as an iron deposit but not been exploited as rare earth mine until the REE minerals were discovered in 1936. The ore reserves of Bayan Obo are about 1500 million metric tonnes iron ore with an average grade of 35%, 1 million tonnes of niobium ore with an average grade of 0.13% and more than 48 million tonnes of REE ore with an average grade of 6% (Voncken 2016). The REE minerals are bastnaesite, parasite, and monazite. Due to the multi-mineral reserves, the Bayan Obo deposit is also called as the Bayan Obo iron-niobium-REE deposit. The intensive REE exploitation of the Bayan Obo deposit area started at the end of 1970s and China began to dominate the rare earth global supply (Figure 1.5).
Mount Weld, Australia
Mount Weld deposit is located 35 km south of Laverton in South-West Australia with laterite and carbonatite rocks (Figure 1.5). This deposit was discovered in 1988 (Voncken 2016). The Mount Weld deposit was not exploited until REE-crisis in the late 2000s. The rare earth ores are mainly apatite, monazite, synchysite, and churchite. According to the Critical Mineral Resources of the United States, Mount Weld holds 23.9 million tonnes of REE ore at an average grade of 7.9 %.
Ilímaussaq Alkaline Complex, South Greenland Ilímaussaq complex, also called as Ilimaussaq Alkaline Complex, is located in Southwest Greenland (Figure 1.5). The complex is made of alkaline igneous rocks which are defined in terms of the alkali and silica content (Voncken 2016). Ilímaussaq has a large amount of uranium, rare earth elements and zinc resources. The complex is located in southern Greenland with more favourable weather compared to the northern Greenland and more intensive population which made it easier to exploit. A large scale rare earth project, Kvanefjeld project, was launched to produce rare earth oxide as the main goal and uranium, zinc and fluorspar as by-products (Greenland Minerals and Energy 2019).
REE deposits and occurrences in France
There are few rare earth occurrences found in France. The relatively intensive REE deposits that have been reported are located in the monazite nodules of detrital origin in shales from Central Bretagne (EUrare project). According to previous studies, some REE deposits were located in north-western France in the Bay of Morlaix, Corlay, Châteauneuf-du-Faou, Craon, Châteaubriant, Le Grand Fougeray. The Le Grand Fougeray deposit is the most studied area with a large number of monazite nodules which contains REE oxides (Figure 1.6). Some deposits were reported in southern France, close to the border with Spain, in Arize, Trimouns, Mountagne Noire and Le Vigan. Other known REE occurrences like Massif Central, Armorican Massif, Estérel, Pyrenées are also reported to have mineralogical interest and even resource potential (EUrare project).
Figure 1.8 illustrates the history of REE production since the early 1900s. With a limited utilisation of REEs, rare earth mines have not been exploited much worldwide before the 1960s. From the mid 1960s to the mid 1980s, Mountain Pass was the dominant source of REE (Figure 1.8). Since the 1980s, China provides more than 80% of the worldwide supply of REEs (Kynicky et al. 2012; Chao et al. 2016). For the year 2019, China still dominated the REE production globally, took up 62% of the world production. Followed by USA, Burma, Australia, accounting for 12%, 10% and 10% respectively (Figure 1.7 & 1.8).
Table of contents :
Chapter 1. State of the art and objectives
1.1 Rare earth elements (REEs)
1.1.1 What are REEs?
1.1.2 REE applications
1.1.3 Source of REEs
1.1.4 REE reserves and major mining deposits
1.1.5 REE production
1.2 REEs in the environment
1.2.1 REEs in soil
1.2.2 REE transfer and toxicity to microorganisms
1.2.3 REE transfer and toxicity to aquatic environment
1.2.4 REE transfer and toxicity to plants
1.2.5 REE transfer and toxicity to humans
1.3 Arbuscular mycorrhizae
1.3.2 Effects of AMF on plants
1.3.3 Effect of AMF on plant in metallic polluted soils
2 Materials and methods
2.1 Soil, plant and mycorrhizal fungi
2.1.3 Mycorrhizal fungi
2.2 Experiment design
2.2.1 Seed germination
2.2.2 Pot experiments
2.2.3 Compartment experiments
2.2.4 Growth conditions and watering
2.2.5 Spore germination tests
2.3.1 Bioavailability of REEs in soil
2.3.2 Total REE and major elements analysis in soil and plant
2.3.3 Root mycorrhizal colonization
2.3.4 Data analysis
Chapter 3. Bioavailability and transfer of elevated Sm concentration to alfalfa in spiked soils .. 55 Bioavailability and transfer of elevated Sm concentration to alfalfa in spiked soils
Chapter 4. No significant transfer of the rare earth element samarium from spiked soil to alfalfa by Funneliformis mosseae
Chapter 5. Transfer of La, Ce, Sm and Yb to alfalfa and ryegrass from spiked soil and contribution of mycorrhizal inoculation with Funneliformis mosseae
Chapter 6. Availability and toxicity to AM fungi of REE contaminated soils from ionic mine tailings in China
Chapter 7. General discussion, conclusion and perspectives
7.1 General discussion
7.1.1 What are the bioavailability of REEs in the soil and their effects on plants?
7.1.2 Effect of an AM fungus on plants growing in an REE contaminated soil and on the transfer of REEs to the plant?
7.1.3 Effects of an AM fungus on plants growing in a soil with multiple REE contamination and on the transfer of these REEs to plants?
7.1.4 What are the bioavailability of REEs in historically contaminated mining tail soils and their effects on AMF?