Next Article in Journal
Long-Term Strength Evolution in Ambient-Cured Solid-Activator Geopolymer Compositions
Next Article in Special Issue
Geochemistry of Basalts from Southwest Indian Ridge 64° E: Implications for the Mantle Heterogeneity East of the Melville Transform
Previous Article in Journal
Alkali-Hydrothermal Treatment of K-Rich Igneous Rocks for Their Direct Use as Potassic Fertilizers
Previous Article in Special Issue
Mineralogy and Geochemistry of Deep-Sea Sediments from the Ultraslow-Spreading Southwest Indian Ridge: Implications for Hydrothermal Input and Igneous Host Rock
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Minerals
Volume 11
Issue 2
10.3390/min11020142
minerals-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Review

Ocean-Floor Sediments as a Resource of Rare Earth Elements: An Overview of Recently Studied Sites

by
Jelena Milinovic
1,*,
Francisco J. L. Rodrigues
2,
Fernando J. A. S. Barriga
3 and
Bramley J. Murton
4
1
REQUIMTE-LAQV, Department of Chemistry, NOVA School of Science and Technology, NOVA University Lisbon, 2829-516 Caparica, Portugal
2
Department of Communication, Philosophy and Politics, Beira Interior University, 6201-001 Covilhã, Portugal
3
IDL-Instituto Dom Luiz, Faculdade de Ciências, Universidade de Lisboa, Edifício C1, Campo Grande, 1749-016 Lisboa, Portugal
4
National Oceanography Centre, European Way, Southampton SO14 3ZH, UK
*
Author to whom correspondence should be addressed.
Minerals 2021, 11(2), 142; https://doi.org/10.3390/min11020142
Submission received: 1 January 2021 / Revised: 26 January 2021 / Accepted: 27 January 2021 / Published: 30 January 2021
(This article belongs to the Special Issue Genesis and Exploration for Submarine Sulphide Deposits)
Browse Figures
Versions Notes

Abstract

:
The rare earth elements (REE), comprising 15 elements of the lanthanum series (La-Lu) together with yttrium (Y) and scandium (Sc), have become of particular interest because of their use, for example, in modern communications, renewable energy generation, and the electrification of transport. However, the security of supply of REE is considered to be at risk due to the limited number of sources, with dependence largely on one supplier that produced approximately 63% of all REE in 2019. As a result, there is a growing need to diversify supply. This has resulted in the drive to seek new resources elsewhere, and particularly on the deep-ocean floor. Here, we give a summary of REE distribution in minerals, versatile applications, and an update of their economic value. We present the most typical onshore methods for the determination of REE and examine methods for their offshore exploration in near real time. The motivation for this comes from recent studies over the past decade that showed ΣREE concentrations as high as 22,000 ppm in ocean-floor sediments in the Pacific Ocean. The ocean-floor sediments are evaluated in terms of their potential as resources of REE, while the likely economic cost and environmental impacts of deep-sea mining these are also considered.

1. Introduction

As developed economies across the world transition away from carbon-based fossil fuels, much of the alternative technology is dependent on an increasing number of relatively scarce yet critical elements. Rare earth elements (REE) belong to the group of critical elements, since they are vital to modern technologies. With the increased development of technology and the constant need for more advanced technical devices, the demand for rare earths is continuously growing worldwide. Considering the unique properties of REE, which can increase efficiency and performance in many technological products and techniques, they are safely considered as the key for a green energy future. REE are crucial components in many advanced technologies, such as electronics, medical and military applications, etc. [1,2,3,4]. The annual increase of REE consumption is variable, because it strongly depends on the discovery of new fields of REE application [5].
The leading industrialized countries (e.g., EU/Germany and Japan) established different rare earth strategies [4]. As a member of the EU, Germany has no strategy for sustainable REE supply. In general, the EU is strongly dependent on imports of strategically important raw materials and high-tech metals. However, until 2005, the European Commission (EC) did not fully realize that it had a critical dependency on REE. In 2007, a “coherent political approach with regard to raw materials supplies” started to be discussed, but only in 2008 did the EC present the Raw Materials Initiative (RMI) [4]. On this initiative of 41 analyzed minerals and metals, it was found that 14 should be regarded as critical. From the beginning, the RMI was considered as neo-colonial, and the criticism created the situation that missing resource efficiency was the target [4]. Hence, in 2011, a strategy was adopted, updating raw materials with the main three pillars: fair supply from the world market, promotion of sustainable supply, and improved resource efficiency [4]. Finally, the European Rare Earths Competency Network suggested a strategy for the development of the European rare earths scheme, and it focused on opportunities for primary supply in Europe through resource efficiency and recycling [4]. The EU is trying to establish access to raw materials worldwide, but it is missing resource efficiency as a target. In comparison to the EU approach, the Japanese strategy is more sustainable. Although Japan is the third largest consumer of industrial metals, it is also a resource-poor country [4]. Since 2001, a recycling law was in force, and in 2012, Japan implemented the “Rare Earth Recycling Act”. The electronic waste conversion described as “urban mining” permits the recycling of e-waste at a large scale. Nevertheless, typically only 1% of the REE are recycled from end products, while the rest is deported to waste [6].
In 2017, the EC prepared a report to highlight the importance of critical raw materials (CRM) for the more resource-efficient and more circular economy [7]. Light rare earth elements (LREE) and heavy rare earth elements (HREE) have been listed at the top of the CRM list, showing their supply risk at the maximum level [7]. The parameter “substitution index”, described in the EC report as a measure of difficulty in substituting the material, was scored (for both economic importance and supply risk) with the maximum values between 0.9 and 1.0 for all REE.
China is the most influential country in terms of the global supply of the majority of CRM, i.e., rare earths [7]. Estimated mine production of REE from China accounted for approximately 84% of the global world production between 2011 and 2017, and ≈63% between 2018 and 2019 [8]. Among many deposits in China, only a few are of significant economic importance: LREE from the Himalayan Mianning-Dechang orogenic belt, Bayan Obo in Inner Mongolia, and most importantly, HREE from ion adsorption in clays in the southern part of the country [1,9,10,11,12]. Recently, China has reduced export quotas to combat environmental problems associated with REE mining and to cope with its growing internal demand [9,10]. Despite the fact that until the 1990s, the United States was the world’s largest REE producer—the Mountain Pass mine (in SE California), in the late 1980s, China started mining terrestrial deposits and quickly gained control of global REE production [8]. Meanwhile, in 2002, the mining at the Mountain Pass deposit was stopped because of considerable leaching of radioactive by-products from the mine tailings (mainly Th and U) after the extraction of REE [11]. Thus, since 2009, China has been an effective monopoly producer of rare earths, despite the Mountain Pass mine resuming production between 2012 and 2015, making a total contribution of only about 4% of the world REE supply in that period [8]. This imposes increased global demands for REE and appropriate rare earth oxides (REO). An additional conflict with exportation strategy may appear because the actual bilateral scenario between the two world’s biggest economies, China and the United States, is going toward radical changes. This fact together with the critical role of rare earths in high technology has triggered an urgent need for a better knowledge of the global natural resources of the REE [13]. Therefore, countries should re/start the REE mines to gradually reduce dependence on China’s supply. To achieve this, not only land-based but also resources from the deep sea should be considered to meet demand for the years and decades to come [14]. World demand for REE has tripled in the last decade, and active exploration strategies for new resources can successfully substitute supply shortfall [9].
The resources of REE are concentrated in carbonatite-related deposits, alkaline igneous rocks, pegmatites, monazite and apatite veins, ion adsorption clays, and in some deep-ocean sediments [10]. In Figure 1, the global REO reserves worldwide estimated for 2019 [15] are presented by a statistical diagram. As expected, China remained the dominant country with REE resources, with a total reserve equal to 44 Mt (39%) of corresponding oxides of rare earths. The reserves of Brazil and Vietnam follow, each with an estimated 22 Mt (each half of China, i.e., 20%). Russia is the fourth country with a resource of REO (around 12 Mt, corresponding to 11%), while India was estimated to have reserves of approximately 6.9 Mt (6%) of REO. Thus, as can be seen from the diagram, Europe and the United States need to develop the technical capabilities to assess and extract REE resources if they are to decrease their dependency on imports.
Compared to the global terrestrial (land-based) reserves, REO including Y and Sc can be in significantly higher amounts in the deep oceans, which is particularly known in the Indian and Pacific ocean bottom [2,16]. Notably, it is acknowledged that the two main terrestrial deposits have less than 1% of HREE, whereas in the Pacific cobalt-rich ferromanganese (Fe-Mn) crusts and polymetallic nodules, the amount of HREE can reach a level of as much as 26% [16]. Mining of the main minerals in Fe-Mn nodules and crusts from the bottom of the oceans can recover a very significant amount of REE as by-products [17]. In India, the principal ore mineral for LREE, monazite [(LREE)PO4], is mined mainly in beach placers (70–75%), and the rest are mined in the inland and offshore areas [2]. Other REE minerals in India include ilmenite, sillimanite, garnet, zircon, and rutile, which are all dominantly containing LREE. In general, monazite and xenotime [Y,HREE(PO4)] are two main potential resources for REE (LREE and HREE, respectively) that are present in a wide variety of rock types, but the concentration of extracted REE cannot support economies [18] because REE are typically dispersed; i.e., they are not concentrated in economically exploitable ore deposits. For mineral exploration and mining processes, of special interest is the detection of deposits (such as carbonatites, silico-carbonatites, peralkaline granites, pegmatites) enriched in REE. Although deep-sea mineral deposits, such as polymetallic nodules and cobalt-rich crusts, have been known for almost 50 years, only recent studies have indicated that unconsolidated ocean-floor sediments can also be good candidate resources for REE. So far, only a few investigations have focused on a quantitative assessment of the deep-sea floor for critical metal resources [19,20,21,22,23,24,25,26,27]. The estimations emphasize the importance of marine sediments and other classes of deposit (Fe-Mn crusts and polymetallic nodules) as promising alternative supplied for REE, compared with land-based resources [2,19]. Recently, there have been strong efforts to explore the potential supply of REE by cost-effective, low-carbon extraction and separation methods.
Within the EU, Portugal has a high area of jurisdiction (4127.408 km2) and a very large ratio between the total areas of maritime zone and the continental shelf (M/C = 42.1); thus, exploration strategies should be more focused on the res1ources available at the bottom of the Atlantic Ocean [28]. In addition to Portugal, several other European and non-European countries with significant areas of jurisdiction have similarly large M/C ratios and thus can explore their potential more. As an example, the United Kingdom (M/C = 28), New Zealand (M/C = 24.7), France (M/C = 16.9), and Japan (11.9) have also the possibilities to explore minerals and REE deposits in the deep-ocean floors. Several other countries have smaller ratios (M/C < 5), such as Chile (M/C = 4.9), Indonesia (M/C = 3.2), Australia (M/C = 1.4), and the USA (M/C = 1.2). The rest of the countries have a very small M/C ratio; i.e. M/C < 1. Unfortunately, to date, the sequences of mining of strategically important deposits of REE are poorly understood, and obviously, they are influenced by the mineralogy of the deposits [29]. The planning of the exploitation of REE deposits would benefit from a comprehensive mineralogical analysis [30]. The EU-FP7-funded project ‘Blue Mining-Breakthrough solutions for sustainable deep-sea mining’ produced much scientific and technical knowledge about the ocean-floor sediments, and it established standards for the best practices for the exploitation and economic use of mineral resources at the Trans-Atlantic Geotraverse (TAG) area in the Mid-Atlantic Ridge (MAR) [30,31].
In this review, the distribution of REE in minerals (as minor or major constituents), their increased importance, and their versatile applications in modern life have been summarized. An overview of the most typical onshore methods for the determination of REE are reviewed, as well as methods for offshore mineral and geochemical exploration (based on spectral data and statistical modeling of previously studied mining area), in near real time. The available recent research (i.e., studies performed in the past decade) on rare earth mineral resources in the global ocean floor unconsolidated sediments have been reviewed. Finally, the economic cost, environmental aspects, and future perspectives of ocean-floor mining have been commented on.

2. The Rare Earth Elements (REE)

2.1. The Chemistry of REE

The rare earth elements group includes the 15 lanthanides plus yttrium (Y) and scandium (Sc). REE are typically divided into two main groups, light (LREE), i.e., low-atomic number (Z) elements: lanthanum (La, Z = 57), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm, Z = 62), and heavy (HREE), with high-atomic numbers: europium (Eu, Z = 63), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu, Z = 71) [13]. Nevertheless, the classification is not consistent, and sometimes, the LREE group includes the group of La–Eu and the HREE group comprises the Gd–Lu series [18]. Moreover, the group of elements with mid-atomic numbers (from Sm to Ho) is named middle rare earth elements (MREE) [9,24]. The geochemical behavior of Y is similar to Ho, and hence, Y is included in HREE (average apparent Z = 66, based on its physical properties), forming a group designated as REY [32,33]. Yttrium is widely present in nature [18]. In addition to lanthanides and Y, Sc (Z = 21) is also considered as rare earth; however, it is sometimes excluded in the discussion of REE because it has a smaller cation with geochemical behavior closer to ferromagnesian transition elements, rather than REE [8,34,35].
The lanthanide elements have similar electronic configurations that set them apart from the rest of the elements. Namely, the REE have electrons in the 4f sub-shell, and this has a great influence on the oxidation state of these metals. Mostly, REE show an oxidation state of +3 (donate three electrons), except for the elements with oxidation state +2 (Eu) or the +4 oxidation state (Ce) (Table 1). The decrease in the size of atoms with increased nucleus charge generates small changes between their properties.

2.2. The Abundance and Applications of REE

Although they are described as “rare” earth elements, the abundance of rare earth elements in the earth’s crust is not rare. They exist in nature from trace (negligible) abundances (Pm) to 66.5 ppm (Ce) (see Table 1). Promethium has no natural stable or long-lived radioactive isotopes but possesses around 40 short-lived isotopes 145Pm [8]. On the other hand, Ce usually occurs in substitution to major elements in minerals, and as an example, it is more abundant than copper, Cu (50 ppm), in the earth’s crust. The REE do not exist in free form in nature because of their high reactivity. Rather, they are major or minor constituents of minerals, such as silicates, carbonates, oxides, and phosphates [39]. The main economic sources of REE are bastnaesite [(Ln,Y)(CO3)F], monazite [(Ln,Th)PO4], and loparite [(Ln,Na,Ca)(Ti,Nb)O3]. In total, there are about 250 minerals that contain rare earths [39]. The isolation of REE from minerals is a very complex procedure, including several steps, among which the most important are the extraction concentration, smelting, and refining. Thus, obtaining REE in pure metallic or oxide form from minerals makes them even more valuable.
The cellphones and other high-tech devices are composed of numerous metal components, many of which contain REE [1,4,16]. Namely, more than one-half of the components in cellphones (e.g., its electronics, display, battery, speakers, etc.) are made with a large involvement of REE [40]. According to statistical data, in 2019, about 1.52 billion units of smartphones were sold worldwide, and it is expected that it will increase in 2021 for 37% [41]. All REE (except radioactive Pm) can be found in varieties of smartphones, and some of them give colors to screens or vibrations (Dy and Nd) [42].
In the last three decades, the applications of REE have been extended in technological devices, such as computers, digital versatile discs (DVD), liquid-crystal displays (LCD), rechargeable batteries, autocatalytic converters, super magnets, light-emitting diode (LED) lighting, superconductors, glass additives, fluorescent materials, phosphate binding agents, etc. In the industry (electric vehicles, solar panels, wind turbines, etc.), the importance of REE is also very high. Some of the main applications of REE are listed in Table 1.
In addition to diverse applications in high-tech technologies, REE are also important in medical applications (e.g., medical diagnosis, anti-tumor agents, kidney dialysis medicine, surgical equipment, etc.) and medical instruments, e.g., magnetic resonance imaging (MRI), positron emission tomography (PET) imaging, and X-rays [38,43,44].
Some REE, such as Tb, Ho, Lu, and Y, have been considered as valuable elements in anticancer treatment because of their therapeutic radioisotopes, especially in radio-immunotherapy and photodynamic therapy [45]. Sm-153 and Yb-176 are also used in the treatment of tumors and in medical applications, respectively. Due to its superlative paramagnetic properties in the whole periodic table, gadolinium (Gd) is the most used rare earth in the MRI [29]. Gd3+ improves the contrast in magnetic resonance imaging scans with very low toxicity [46].
Since the REE are very important in all high-tech devices, and they are essential in modern medical applications in very low concentrations, these elements are often called “the vitamins of modern industry” [47].

2.3. Economic Value of REE

As previously stated, the extraction of REE from minerals is a very complex and time-consuming process. The cut-off grade in mining metallurgical operations is influenced by different economic indicators (e.g., market prices of both the primary product and the by-product, mining and processing costs, etc.). Moreover, the economic value of REE is multiplied due to their importance in numerous applications in modern technologies and globally constant growing tendency of the balance between demand and supply in the world market [48]. The separation and purification of individual REE is challenging due to their chemical similarities. In Figure 2, the current market prices of REO are presented according to the mineral price list [49]. The prices are shown per kg of each oxide of rare earth (high purity >99.5%) and as can be seen, they are in the broad range for all REO. As an example, for some REO, the prices higher than 20,500 and 22,000 € (Er2O3 and Gd2O3, respectively), and 38,000 € and 41,600 € (Nd2O3 and Pr2O3), respectively. The remaining REO have lower values, e.g., around 1500 €/kg (CeO2, La2O3, Sm2O3), 2600 €/kg (Y2O3), and from 240 to 540 €/kg for Dy2O3 and Tb4O7, can be noticed respectively. Currently, Eu2O3 and Sc2O3 (25 and 50 €/kg, respectively) are the rare earths with the lowest prices on the market.

3. Ocean-Floor Sediments as a Resource of REE

3.1. Onshore Methods vs. Offshore Exploration of REE in Ocean-Floor Sediments

The accurate analytical techniques for REE quantification are very important for their geochemical exploration, mining, extraction, and monitoring of the environment. With available instrumental techniques, several analytical methods are most commonly used for REE quantification, such as inductively coupled plasma coupled with optical emission spectroscopy (ICP-OES) or mass spectrometry (ICP-MS) detection. The ICP-based techniques are multi-element analytical methods used for the accurate and precise determination of REE at major, minor, and trace concentration levels in complex geological samples, such as sediments, crust samples, hard rocks, ores, etc. [2,50,51]. The ICP-methods are characterized with high sensitivity, covering a wide linear range of concentrations, with minimal interferences and high sample throughput. The techniques can simultaneously measure all rare earths at low concentration levels, after selecting the appropriate spectral lines [52]. ICP-MS is usually the method of choice for REE quantification in environmental and earth matrices because it has low detection limits with excellent sensitivity, of several orders of magnitude higher (10–100,000) in comparison with the ICP-OES method [50,51]. Therefore, the detection limits for the analysis of REE using ICP-MS are typically in ppb-levels (µg·kg−1 of dried material, e.g., sediment or rock sample). After using geological reference materials, it was shown that the limit of detection (LOD) for most of the REE can be even in ppt and sub-ppt ranges with a high precision, <5% relative standard deviation (RSD) [50]. Thus, ICP-MS methods are the best suited for high-precision geochemical studies.
In addition to ICP analyses, a technique such as X-ray fluorescence (XRF) can also be applied for the geochemical analysis as a reliable method for REE at the ppm level. The main advantages of XRF analysis are efficacy and cost [53]. Moreover, one of the main benefits of XRF is the possibility of using light, portable XRF devices for offshore quantification of REE (e.g., Ce, La, Nd, and Pr), which is of great importance for exploration strategies during cruises, i.e., in near real time. Nevertheless, unlike the ICP-analyses, XRF is not REE-specific and has lower sensitivity, with an LOD at least two times less than the ICP-OES method [54]. Therefore, the XRF technique lacks sensitivity and is not suitable for the determination of REE at low concentrations, as is the case in most of the geological samples.
To explore the potential of conducting research at the bottom of the oceans (sediments, cover rocks, etc.), the research teams can apply two strategies: onshore methods and/or offshore exploration, depending on the mission objectives. In general, prior to analysis, after the careful sampling at the ocean bottom by drilling, TV grab, or gravity coring, collected samples can be stored appropriately in a nitrogen atmosphere (to prevent sulfide oxidation). Onshore analysis implies that after the mission is completed, the samples are transported into the conventional, full-fledged laboratories, where they can be pre-treated (e.g., dried) and treated (usually with a mixture of mineral acids in an automated high-temperature microwave digestion system). After digestion, cooled samples are typically transferred to decontaminated flasks and diluted before the running conventional ICP analyses. Thus, most of the available results are based on an onshore analysis in fully fledged laboratory conditions. The onshore identification of minerals is often postponed until long after the cruise is over. The main aim is to get a better knowledge of potential high-tech resources from deep-ocean sources; hence, these highly sensitive results can give insight into current perspectives.
On the other hand, offshore exploration of the collected samples from the bottom of the oceans can be applied synchronized during the mission. The extreme conditions on the cruise require specific technical set-up, which is entirely different from land-based analysis and pre-feasibility study. This type of fieldwork was performed under the Project Blue Mining—Breakthrough solutions for sustainable deep-sea mining, during the expedition of the M127 cruise, with recovered gravity cored sediments from the TAG hydrothermal field along the MAR [31]. Selected samples were analyzed for mineral composition by portable X-ray diffraction (XRD), and all on-board results were validated later, after the mission, by conventional, onshore XRD analysis. It was shown that mineralogy could be efficiently assessed during the cruises at sea; i.e., within 20 h, it was possible to analyze a batch of up to 24 gravity cored sediment samples [55].
Moreover, the rapid, non-destructive, and economically acceptable short-wave infrared reflectance (SWIR) spectral analysis could provide representative statistics for the offshore evaluation of geochemical variables in sediment samples from the deep ocean [55]. Thus, based on the previously established modeling equations, it is possible to substitute time-consuming destructive sample preparation (wet digestion and extraction) and conventional analytical methods (ICP-MS) by the portable infrared mineral analyzer (PIMA) using SWIR spectral analysis to get both qualitative and quantitative results of mineral components and geochemistry (e.g., La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb, and Y) in previously studied ocean-floor areas, with high sensitivity. This new approach highlights the benefit of rapidly obtained spectral data that can be used for near real-time exploration of geochemical deposits in the ocean floor. This mode of work enables the modification of exploration strategies in light of new information as it becomes available in near real time.

3.2. The Concentration of REE in Ocean-Floor Sediments

The dominant resources of REE are available on the ocean floor [2,19,22]. Under aqueous conditions, REE exist mostly as a variety of complexes, with speciation dominated by metal, the predominant ionic form of some REE is trivalent [REE]3+, carbonate [REE(CO3)]+, and di-carbonate [REE(CO3)2]−, with carbonate complexes being the most abundant [8]. An increase in the pH of seawater leads to a preferable adsorption of LREE on suspended particles relative to HREE. The REE absolute concentrations in hydrothermal fluids are highly variable and can reach from a few to several thousand ppm by weight, and this enrichment is evident for LREE, in particular for Eu and Ce, which both exist in geological environments in two oxidation states (Eu2+, Eu3+; Ce3+; Ce4+) [8,24]. The higher accumulation capacity of LREE relative to HREE in hydrothermal fluids can be explained by the higher stability of chloride and fluoride complexes with LREE [32]. In hydrothermal fluids, the REE composition is related to the interaction of oceanic crustal rocks with seawater at high temperatures [32]. Many ore deposits are more enriched with LREE than with HREE, and consequently, HREE became a more valuable commodity [9].
As previously mentioned, besides ocean-floor sediments, polymetallic nodules, cobalt-rich Fe-Mn crusts, and seafloor massive sulfides (SMS) are three other types of REE-rich resources in oceans. Polymetallic nodules occur in ocean-floor sediments at different water depths (of 1640–6500 m) in the oceans [56,57,58,59]. Cobalt-rich Fe-Mn crusts are composed of Fe oxyhydroxide and Mn oxide that precipitate from ocean water onto rock substrates [56]. The concentrations of REE in SMS are influenced by the REE patterns of the sulfide-forming fluids and interactions with subsea-floor rocks [60]. It was established that REE concentrations increase in deep water (from North Atlantic to North Pacific), and abundant REE are enriched in deep-sea sediments, which is mainly caused by the strong sorption capacity of sediments [61,62]. As an example, the concentrations of REE are 10–100 times higher in the deep-sea sediments compared to seawater [61]. Therefore, in this work, results are focused on unconsolidated deep ocean-floor sediments as potential REE-rich resources.
Conventionally obtained results by onshore ICP-MS analysis (performed after the wet digestion of homogenized and dried ocean-floor sediments samples) for the total concentration of REE (min–max range, expressed in ppm) are listed in Table 2. The results over the last decade show that large resources of REE (>1000 ppm) can be found in some ocean-floor locations (Pacific, Indian Ocean, etc.), which indicates that these highly valuable elements could be alternatively exploited at different deep-sea locations, thus diversifying the sources [19,21,22,23,33,53].
In the Mid-Atlantic Ridge, at extensive vent fields, such as Main Lilliput and Roman City (9°32′–9°33′ S, MAR) sediments are composed of Fe-Si-oxyhydroxides, goethite, and ferrihydrite [63]. Selected red sediment samples taken from both hydrothermal fields were characterized with very low contents of REE (14.4–16.6 ppm). REE distribution patterns were characterized with both negative Ce and Eu anomalies and a slight enrichment in LREE compared with HREE [63]. In the Logatchev hydrothermal field (14°45′ N, MAR), sediments contain chlorite, smectite, quartz, lizardite, and anothrite [64]. The low abundance of REE (13.6–130 ppm) exhibit a pattern of weak positive Eu and negative Ce anomalies [64]. The low ΣREE in these fields was attributed to the low-temperature hydrothermal origin of the Fe-Si-oxyhydroxides and a short contact time of precipitating fluid with ambient seawater [63,64,66,67]. Researchers claim they were precipitated from a fluid composed of seawater with hydrothermal fluid, and this was confirmed by venting fluid temperatures (<20 °C) and slightly lower concentrations of Mg in the vent fluids compared to seawater [68].
Another location explored at the MAR, the Saldanha hydrothermal field, showed similarly low levels of REE with the maximum total sum for the concentrations of the REE not exceeding the 40.6 ppm at the gravity cored sediment samples [65]. Depending on the studied depth, they were negative (Ce) or positive (Eu), which showed that the Saldanha hydrothermal field is not simply a low-temperature and diffuse venting field, as focused fluid flow occurs at this site as well [69]. In recently available results of the sediment samples gravity cored at the TAG hydrothermal field (26° N, MAR), a similar concentration of REY was found with the maximum concentration level of 113.9 ppm found in deep-sea sediments [55]. The neutrally buoyant plume overlying the TAG hydrothermal field is characterized by fine-grained Fe-oxyhydroxide particles enriched in Cd, Cu, Ni, Pb, and Zn, which are efficient REE scavengers.
In the Pacific (at 78 sites), several studies showed that sediments contained high concentrations of REE at numerous sites throughout the ES and Central North Pacific, where the total REY-rich deep-sea sediments (“REY-rich mud”) were in the ranges of 1000–2230 ppm and 400–1000 ppm, respectively [19]. The results showed that sediments contained a higher HREE/LREE ratio than the current land reserves of REO, but only after more detailed sampling and analysis of the ocean-floor sediments can this hypothesis be better evaluated [19]. The estimation in the study was very promising to consider ocean-floor sediments as a relatively good resource of REY. Namely, in the selected cores collected from the Pacific, it was calculated that an area of 1 km2 could provide up to 20% of the world annual consumption of these elements [19]. Similar values for the total REE content (200–577 ppm) and total REO (150–1150 ppm) were obtained after analysis of unconsolidated sea-floor sediment samples from the NE Pacific [20,21]. [21] estimated that the studied sediment samples comprised 37.3% of Sm, Eu, Gd, Tb, and Y, 33.5% of La and Ce, and 29.2% of Nd, Pr, and Dy, which allows them to cost twice as much as other studied samples (the polymetallic nodules and Fe-Mn crusts) [66]. Highly REY-rich sediments (2000–5000 ppm) and extremely REY-rich sediments (>5000 ppm) were found in deep-sea sediments collected from the NW Pacific Ocean within the Minamitorishima, Exclusive Economic Zone of Japan [23]. The extremely REY-rich deposits are characterized by abundant phillipsite and biogenic calcium phosphate [27].
In the North Pacific Ocean, coring deep-sea sediments contained over 5000 ppm of the total REE, and after the mineral processing method, samples with as much as 22,000 ppm of REE were recovered [22]. The authors concluded that this new resource of REE should be exploited soon, and thus can represent tremendously a much more promising resource of rare earths than on-land resources.
In addition to sediments from the Pacific, cored sediments from the Indian Ocean also contained high REY content of possible economic significance [33,53]. The next significant prospective resource of rare earths has been found in sediment samples from Deep-Sea Drilling Project (DSDP) and/or Ocean Drilling Program (ODP) sites covering a great area of the Indian Ocean [33,53]. These samples were composed mainly of siliceous ooze with zeolitic clay that contained high abundances of REY. The total concentration of REY was found to be equal to 1113 ppm, which is comparable to the levels found in the Pacific [66]. The authors found that apatite abundance was positively correlated with the sum of REY, and it was assumed that apatite played an important role as a host phase of REY in the sediments [33].
Briefly, ocean-floor sediments have not been sufficiently studied as a (future) REE (REY or REO) resource. Potentially, they can constitute a highly promising REY resource for the near future, but this must be studied in more detail. Therefore, detailed sampling and analysis are needed to properly evaluate the potential benefits at many sites within the bottom of the ocean. The promising REE-bearing sea-floor potential resource, deep-sea sediments deserve further study as possible favorable REE resources in the near future.

3.3. Economic Cost and Environmental Implications of REE Exploitation

The cut-off grade in mining operations is influenced by economic indicators of mining and recovery yield, transportation, and part of the processing of the ocean-floor sediments. Their cost-effective extraction and exploitation would leave a residue enriched in REE. Due to high price of REE, even 50% recovery after extraction will be economical. Economic benefits derived from deep-sea mining activities are to be shared for the benefit of humankind and the balanced growth of international trade [70]. Deep-ocean operations will not impact indigenous or human populations after exploitation: both must be minimal or non-existent. However, deep-sea mining must also preserve the fragile deep marine ecosystem and biodiversity.
For all earth-surface activities, disturbance or even destruction of habitat may happen, so it is important to develop environmentally acceptable practices. However, the overall environmental impact of deep-ocean seabed mineral extraction is little known, and it depends on the deposit type and extraction tools. Environmental risk assessments should be always considered a priori exploitation, and this practice should be an essential part of the decision-making process [16,71,72,73]. The value of an environmental risk assessment is ultimately associated with the evaluation of environmental managers about a project [71]. The deep-sea mining should demonstrate its commitment to prevent serious environmental harm by complying with legal requirements, using good practice and thus minimizing environmental impacts [72,73]. Recently, six components of good practice that could improve deep-sea mining were proposed, and the International Seabed Authority will need to develop additional rules, regulations, and procedures to assure decision concerning the common heritage resources of the areas beyond national jurisdiction (the Area) [74]. The new alternative areas should be prioritized by enhanced scientific research [75].
The damage will occur in the habitat in the path of a mining vehicle, and sediment plume of unknown extent will be created in the bottom-water layer [56]. Nevertheless, it is hard to predict the impact on the surrounding biodiversity and the risk of species because of limited knowledge of the distribution of species [3]. Additional environmental studies are needed before large-scale, commercial exploitation starts; this may include a moratorium on deep-sea mining. Detailed discussions for the funding, planning, and implementation of the strategic goals by International Seabed Authority and the scientific community are crucial to address in the near future, both for rising the deep-sea mining in the Area and to protect the environment.

4. Conclusions

Given that the demands for the REE are growing rapidly, and their economic trade is limited, abundant resource potentials of the rare earths in the global ocean-floor unconsolidated sediments may be an alternative solution for their supply. Reported high concentrations of REE in sea-floor sediments (in Indian and Pacific Oceans ranging up to ≈ 1100 and 22,000 ppm, respectively) guarantee future detailed exploration and studies of their resource potential. To ensure a truly global supply of REE and other critical metals, to enable the advancement of a wide range of REE applications necessary for the transition to a low-carbon future, new sources of these strategic, highly valuable metals will be imperative, and many innovative ideas for their extraction should be considered. For the time being, more than immediate large-scale ocean-floor mining, we need readiness followed by implementing assessment.

Author Contributions

Conceptualization, J.M. and F.J.L.R.; writing—original draft preparation, J.M.; writing—review and editing, F.J.L.R.; supervision, F.J.A.S.B. and B.J.M. All authors have read and agreed to the published version of the manuscript.

Funding

The work was done as part of the project “Blue Mining—Breakthrough Solutions for the Sustainable Exploration and Extraction of Deep-Sea Mineral Resources”, has received funding from the European Union’s Seventh Framework Program (EU-FP7) for research, technological development and demonstration under Grant agreement Nº: 604500. The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020–IDL.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Hurst, C. China’s Rare Earth Elements Industry: What Can the West Learn? Institute for the Analysis of Global Security (IAGS): Washington, DC, USA, 2010; pp. 1–42. [Google Scholar]
  2. Balaram, V.; Banakar, V.K.; Subramanyam, K.S.V.; Roy, P.; Satyanarayanan, M.; Mohan, M.R.; Sawant, S.S. Yttrium and rare earth element contents in seamount cobalt crusts in the Indian Ocean. Curr. Sci. 2012, 103, 1334–1338. [Google Scholar]
  3. Folkersen, M.V.; Fleming, C.M.; Hasan, S. The economic value of the deep sea: A systematic review and meta-analysis. Mar. Policy 2018, 94, 71–80. [Google Scholar] [CrossRef]
  4. Mez, L. Rare earth strategies of Japan and EU/Germany. In The Ecological Modernization Capacity of Japan and Germany. Comparing Nuclear Energy, Renewables, Automobility and Rare Earth Policy. Energy Policy and Climate Protection; Mez, L., Okamura, L., Weidner, H., Eds.; Springer: Berlin/Heidelberg, Germany, 2020; pp. 171–184. [Google Scholar]
  5. Preinfalk, C.; Morteani, G. The Industrial Applications of Rare Earth Elements. In Lanthanides, Tantalum and Niobium; Möller, P., Černý, P., Saupé, F., Eds.; (Special Publication No. 7 of the Society for Geology Applied to Mineral Deposits); Springer: Berlin/Heidelberg, Germany, 1989; pp. 359–370. [Google Scholar]
  6. Jowitt, S.M.; Werner, T.T.; Weng, Z.; Mudd, G.M. Recycling of the rare earth elements. Curr. Opin. Green. Sustain. Chem. 2018, 13, 1–7. [Google Scholar] [CrossRef]
  7. European Commission (EC), COM(2017) 490. Communication from the Commission to the European Parliament, the Council, the European Economic and Social Committee and the Committee of the Regions, on the 2017 List of Critical Raw Materials for the EU; European Commission: Brussels, Belgium, 2017; pp. 1–8. [Google Scholar]
  8. Van Gosen, B.S.; Verplanck, P.L.; Emsbo, P. Rare Earth Element Mineral Deposits in the United States; US Department of the Interior, US Geological Survey: Reston, VA, USA, 2019; pp. 1–14.
  9. Kynicky, J.; Smith, M.P.; Xu, C. Diversity of rare earth deposits: The key example of China. Elements 2012, 8, 361–367. [Google Scholar] [CrossRef]
  10. Simandl, G.J. Geology and market-dependent significance of rare-earth element resources. Miner. Depos. 2014, 49, 889–904. [Google Scholar] [CrossRef]
  11. Massari, S.; Ruberti, M. Rare earth elements as critical raw materials: Focus on international markets and future strategies. Resour. Policy 2013, 38, 36–43. [Google Scholar] [CrossRef]
  12. Wang, X.; Lei, Y.; Ge, J.; Wu, S. Production forecast of China’s rare earths based on the Generalized Weng model and policy recommendations. Resour. Policy 2015, 43, 11–18. [Google Scholar] [CrossRef] [Green Version]
  13. Emsbo, P.; McLaughlin, P.I.; Breit, G.N.; Bray, E.A.; Koenig, A.E. Rare earth elements in sedimentary phosphate deposits: Solution to the global REE crisis? Gondwana Res. 2015, 27, 776–785. [Google Scholar] [CrossRef] [Green Version]
  14. Petersen, S.; Krätschell, A.; Augustin, N.; Jamieson, J.; Hein, J.R.; Hannington, M.D. News from the seabed—Geological characteristics and resource potential of deep-sea mineral resources. Mar. Policy 2016, 70, 175–187. [Google Scholar] [CrossRef]
  15. US Geological Survey. Mineral Commodity Summaries. Rare Earths. 2020. Available online: https://pubs.usgs.gov/periodicals/mcs2020/mcs2020-rare-earths.pdf (accessed on 13 January 2021).
  16. Hein, J.R.; Mizell, K.; Koschinsky, A.; Conrad, T.A. Deep-ocean mineral deposits as a source of critical metals for high- and green-technology applications: Comparison with land-based resources. Ore Geol. Rev. 2013, 51, 1–14. [Google Scholar] [CrossRef]
  17. Hein, J.R.; Conrad, T.A.; Staudigel, H. Seamount mineral deposits, as a source of rare metals for high-technology industries. Oceanography 2010, 23, 184–189. [Google Scholar] [CrossRef] [Green Version]
  18. Bern, C.R.; Shah, A.K.; Benzel, W.M.; Lowers, H.A. The distribution and composition of REE-bearing minerals in placers of the Atlantic and Gulf coastal plains, USA. J. Geochem. Explor. 2016, 162, 50–61. [Google Scholar] [CrossRef]
  19. Kato, Y.; Fujinaga, K.; Nakamura, K.; Takaya, Y.; Kitamura, K.; Ohta, J.; Toda, R.; Nakashima, T.; Iwamori, H. Deep-sea mud in the Pacific Ocean as a potential resource for rare-earth elements. Nat. Geosci. 2011, 4, 534–539. [Google Scholar] [CrossRef]
  20. Zawadzki, D.; Maciag, L.; Abramowski, T.; McCartney, K. Fractionation trends and variability of rare earth elements and selected critical metals in pelagic sediments from Abyssal Basin of NE Pacific (Clarion-Clipperton Fracture Zone). Minerals 2020, 10, 320. [Google Scholar] [CrossRef] [Green Version]
  21. Pak, S.J.; Seo, I.; Lee, K.Y.; Hyeong, K. Rare earth elements and other critical metals in deep seabed mineral deposits: Composition and implications for resource potential. Minerals 2019, 9, 3. [Google Scholar] [CrossRef] [Green Version]
  22. Takaya, Y.; Yasukawa, K.; Kawasaki, T.; Fujinaga, K.; Ohta, J.; Usui, Y.; Nakamura, K.; Kimura, J.I.; Chang, Q.; Hamada, G.; et al. The tremendous potential of deep-sea mud as a source of rare-earth elements. Sci. Rep. 2018, 8, 5763. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  23. Yasukawa, K.; Miyazaki, T.; Vaglarov, B.S.; Chang, Q.; Ueki, K.; Toyama, C.; Kimura, J.I.; Tanaka, E.; Nakamura, K.; Fujinaga, K.; et al. Statistic and isotopic characterization of deep-sea sediments in the Western North Pacific Ocean: Implications for genesis of the sediment extremely enriched in rare-earth elements. Geochem. Geophys. Geosyst. 2019, 20, 3402–3430. [Google Scholar] [CrossRef] [Green Version]
  24. Liao, J.; Sun, X.; Wu, Z.; Sa, R.; Guan, Y.; Lu, Y.; Li, D.; Liu, Y.; Deng, Y.; Pan, Y. Fe-Mn (oxyhydr)oxides as an indicator of REY enrichment in the deep-sea sediments from the central North Pacific. Ore Geol. Rev. 2019, 112, 103044. [Google Scholar] [CrossRef]
  25. Zawadzki, D.; Maciag, L.; Kotlinski, R.A.; Borówka, R.K. Rare earth elements and selected major elements distribution in the siliceous-clayey silts from the NE Pacific abyssal Basin (Clarion-Clipperton Fracture Zone). Harvesting seabed minerals resources in harmony with nature UMI 2014. In Proceedings of the 43rd Underwater Mining Institute Conference, Lisbon, Portugal, 21–28 September 2014. [Google Scholar]
  26. Maciag, L.; Zawadzki, D. Spatial variability and resources estimation of selected critical metals and rare earth elements in surface sediments from the Clarion-Clipperton Fracture Zone, equatorial Pacific Ocean, Interoceanmetal claim area. In Proceedings of the 20th Annual Conference of the International Association for Mathematical Geosciences, IAMG, State College, PA, USA, 10–16 August 2019. [Google Scholar]
  27. Iijima, K.; Yasukawa, K.; Fujinaga, K.; Nakamura, K.; Machida, S.; Takaya, Y.; Ohta, J.; Haraguchi, S.; Nishio, T.; Usui, Y.; et al. Discovery of extremely REY_rich mud in the western North Pacific Ocean. Geochem. J. 2016, 50, 557–573. [Google Scholar] [CrossRef] [Green Version]
  28. Barriga, F.J.A.S. Can recycling and the circular economy render seafloor mining unnecessary? In Economical, Technological and Environmental Aspects: Cooperative Solutions for Future Deep-Sea Mining; UMC, Federation of German Industries (BDI): Berlin, Germany, 2017. [Google Scholar]
  29. Jamieson, H.E.; Walker, S.R.; Parsons, M.B. Mineralogical characterization of mine waste. Appl. Geochem. 2015, 57, 85–105. [Google Scholar] [CrossRef]
  30. Petersen, S. RV METEOR Fahrtbericht/Cruise Report M127 Metal fluxes and Resource Potential at the Slow-spreading TAG Mid-Ocean Ridge Segment (26° N, MAR)—Blue Mining@Sea, Bridgetown (Barbados)—Ponta Delgada (Portugal), 25.05.–28.06. 2016 (extended version). GEOMAR Helmholtz-Zent. Ozeanforschung Kiel 2016. [Google Scholar] [CrossRef]
  31. Milinovic, J.; Dias, Á.A.; Janeiro, A.I.; Pereria, M.F.C.; Martins, S.; Petersen, S.; Barriga, F.J.A.S. XRD identification of ore minerals during cruises: Refinement of extraction procedure with sodium acetate buffer. Minerals 2020, 10, 160. [Google Scholar] [CrossRef] [Green Version]
  32. Dubinin, A.V. Geochemistry of rare earth elements in the ocean. Lithol. Miner. Resour. 2004, 39, 289–307. [Google Scholar] [CrossRef]
  33. Yasukawa, K.; Liu, H.; Fujinaga, K.; Machida, S.; Haraguchi, S.; Ishii, T.; Nakamura, K.; Kato, Y. Geochemistry and mineralogy of REY-rich mud in the eastern Indian Ocean. J. Asian Earth Sci. 2014, 93, 25–36. [Google Scholar] [CrossRef]
  34. International Union of Pure and Applied Chemistry (IUPAC). Nomenclature of Inorganic Chemistry IUPAC Recommendations 2005; Connelly, N.G., Damhus, T., Hartshorn, R.M., Hutton, A.T., Eds.; IUPAC: Cambridge, UK, 2005; p. 51. [Google Scholar]
  35. Weng, Z.H.; Jowitt, S.M.; Mudd, G.M.; Haque, N. Assessing rate earth element mineral deposit types and links to environmental impacts. Appl. Earth Sci. 2013, 122, 83–96. [Google Scholar] [CrossRef]
  36. Lide, D.R. Geophysics, Astronomy, and Acoustics; Abundance of elements in the Earth’s crust and in the sea. In CRC Handbook of Chemistry and Physics, 85th ed.; CRC Press: Boca Raton, FL, USA, 2005. [Google Scholar]
  37. Giese, E.C. Rare earth elements: Therapeutic and diagnostic applications in modern medicine. Clin. Med. Rep. 2018, 2, 1–2. [Google Scholar] [CrossRef]
  38. Barriga, F.J.A.S. Sustainable and responsible mining in the deep oceanic crust. In Colóquio Contaminação Ambiental; Neiva, A., Ed.; Academia das Ciências de Lisboa: Lisbon, Portugal, 2019; pp. 3–28. [Google Scholar]
  39. Dostal, J. Rare earth element deposits of alkaline igneous rocks. Resources 2017, 6, 34. [Google Scholar] [CrossRef]
  40. Jenness, J.E.; Ober, J.A.; Wilkins, A.M.; Gambogi, J. A world of minerals in your mobile device. US Geol. Surv. Gen. Inf. Prod. 2016, 167, 1–2. [Google Scholar]
  41. Statista. Global Smartphone Sales to End Users 2007–2021. Available online: https://www.statista.com/statistics/263437/global-smartphone-sales-to-end-users-since-2007/ (accessed on 28 December 2020).
  42. Rohrig, B. Smartphones: Smart Chemistry. Chem. Matters 2015, 10–12. [Google Scholar]
  43. Riba, J.R.; López-Torres, C.; Romeral, L.; Garcia, A. Rare-earth-free propulsion motors for electric vehicles: A technology review. Renew. Sustain. Energy Rev. 2016, 57, 367–379. [Google Scholar] [CrossRef] [Green Version]
  44. Cui, J.; Kramer, M.; Zhou, L.; Liu, F.; Gabay, A.; Hadjipanayis, G.; Balasubramanian, B.; Sellmyer, D. Current progress and future challenges in rare-earth-free permanent magnets. Acta Mater. 2018, 158, 118–137. [Google Scholar] [CrossRef]
  45. Kostova, I. Lanthanides as anticancer agents. Curr. Med. Chem. Anticancer Agents. 2005, 5, 591–602. [Google Scholar] [CrossRef] [PubMed]
  46. Mitsumori, L.M.; Bhargava, P.; Essig, M.; Maki, J.H. Magnetic resonance imaging using gadolinium-based contrast agents. Top. Magn. Reson. Imaging 2014, 23, 51–69. [Google Scholar] [CrossRef] [PubMed]
  47. Asian Metal. Rare Earths: Uses. Available online: http://metalpedia.asianmetal.com/metal/rare_earth/application.shtml (accessed on 24 April 2020).
  48. Charalampides, G.; Vatalis, K.I.; Apostoplos, B.; Ploutarch-Nikolas, B. Rare earth elements: Industrial applications and economic dependency of Europe. ICOAE 2015, 24, 126–135. [Google Scholar] [CrossRef] [Green Version]
  49. Mineral Prices. Rare Earth Metals. Available online: https://mineralprices.com/rare-earth-metals/ (accessed on 22 April 2020).
  50. Balaram, V.; Rao, T.G. Rapid determination of REE and other trace elements in geological samples by microwave acid digestion and ICP-MS. Atom. Spectrosc. 2003, 24, 206–212. [Google Scholar]
  51. Amaral, C.D.B.; Machado, R.C.; Barros, J.A.V.A.; Virgilio, A.; Schiavo, D.; Nogueira, A.R.A.; Nobrega, J.A. Determination of rare earth elements in geological and agricultural samples by ICP-OES. Spectroscopy 2017, 32, 32–36. [Google Scholar]
  52. Bentlin, F.R.S.; Pozebon, D. Direct determination of lanthanides in environmental samples using ultrasonic nebulization and ICP OES. J. Braz. Chem. Soc. 2010, 21, 627–634. [Google Scholar] [CrossRef] [Green Version]
  53. Yasukawa, K.; Nakamura, K.; Fujinaga, K.; Machida, S.; Ohta, J.; Takaya, Y.; Kato, Y. Rare-earth, major, and trace element geochemistry of deep-sea sediments in the Indian Ocean: Implications for the potential distribution of REY-rich mud in the Indian Ocean. Geochem. J. 2015, 49, 621–635. [Google Scholar] [CrossRef]
  54. Nakayama, K.; Nakamura, T. X-ray fluorescence analysis of rare earth elements in rocks using low dilution glass beads. Anal. Sci. 2005, 21, 815–822. [Google Scholar] [CrossRef] [Green Version]
  55. Milinovic, J.; Barriga, F.A.J.S.; Murton, B.J. Analysis of deep-ocean sediments from the TAG hydrothermal field (MAR, 26° N): Application of short-wave infrared reflectance (SWIR) spectra for offshore geochemical exploration. J. Soils Sediments 2020, 20, 3472–3486. [Google Scholar] [CrossRef]
  56. Hein, J.R.; Koschinsky, A. Deep-ocean ferromanganese crusts and nodules. Treatise Geochem. 2014, 2, 273–291. [Google Scholar]
  57. Konstantinova, N.; Cherkashov, G.; Hein, J.R.; Mirão, J.; Dias, L.; Madureira, P.; Kuznetsov, V.; Maksimov, F. Composition and characteristics of the ferromanganese crusts from the western Arctic Ocean. Ore Geol. Rev. 2017, 87, 88–99. [Google Scholar] [CrossRef] [Green Version]
  58. Marino, E.; González, F.J.; Somoza, L.; Lunar, R.; Ortega, L.; Vázquez, J.T.; Reyes, J.; Bellido, E. Strategic and rare elements in Cretaceous-Cenozoic cobalt rich ferromanganese crusts from seamounts in the Canary Island Seamount Province (Northeastern tropical Atlantic. Ore Geol. Rev. 2017, 87, 41–61. [Google Scholar] [CrossRef]
  59. Baturin, G.N.; Dubinchuk, V.T. Mineralogy and chemistry of ferromanganese crusts from the Atlantic Ocean. Geochem. Int. 2011, 49, 578–593. [Google Scholar] [CrossRef]
  60. Zeng, Z.G.; Ma, Y.; Yin, X.B.; Selby, D.; Kong, F.C.; Chen, S. Factors affecting the rare earth element compositions in massive sulfides from deep-sea hydrothermal systems. Geochem. Geophy. Geosy. 2015, 16, 2679–2693. [Google Scholar] [CrossRef] [Green Version]
  61. Chen, S.Y.; Zhang, H.S.; Zhao, P.D. Rare earth element geochemistry of co-rich crust in the mid-Pacific and South China Sea. Mar. Geol. Quat. Geol. 2006, 26, 45–57. [Google Scholar]
  62. Jaireth, S.; Hoatson, D.M.; Miezitis, Y. Geological setting and resources of the major rare-earth-element deposits in Australia. Ore Geol. Rev. 2014, 62, 72–128. [Google Scholar] [CrossRef]
  63. Dekov, V.M.; Petersen, S.; Garbe-Schönberg, C.D.; Kamenov, G.D.; Perner, M.; Kuzmann, E.; Schmidt, M. Fe-Si-oxyhydroxide deposits at a slow-spreading centre with thickened oceanic crust: The Lilliput hydrothermal field (9°33′ S, Mid-Atlantic Ridge). Chem. Geol. 2010, 278, 186–200. [Google Scholar] [CrossRef]
  64. Dekov, V.; Boycheva, T.; Hålenius, U.; Petersen, S.; Billström, K.; Stummeyer, J.; Kamenov, G.; Shanks, W. Atacamite and paratacamite from the ultramafic-hosted Logatchev seafloor vent field (14°45′N, Mid-Atlantic Ridge). Chem. Geol. 2011, 86, 169–184. [Google Scholar] [CrossRef]
  65. Dias, Á.S.; Mills, R.A.; Costa, I.R.; Costa, R.; Taylor, R.N.; Cooper, M.J.; Barriga, F.J.A.S. Tracing fluid-rock reaction and hydrothermal circulation at the Saldanha hydrothermal fluid. Chem. Geol. 2010, 273, 168–179. [Google Scholar] [CrossRef]
  66. Zhang, Z.; Du, Y.; Gao, L.; Zhang, Y.; Shi, G.; Liu, C.; Zhang, P.; Duan, X. Enrichment of REEs in polymetallic nodules and crusts and its potential for exploitation. J. Rare Earths 2012, 30, 621–626. [Google Scholar] [CrossRef]
  67. Usui, A.; Bau, M.; Yamazaki, T. Manganese microchimneys buried in the Central Pacific pelagic sediments: Evidence of intraplate water circulation? Mar. Geol. 1997, 141, 269–285. [Google Scholar] [CrossRef]
  68. Haase, K.M.; Koschinsky, A.; Petersen, S.; Devey, C.W.; Geman, C.; Lackschewitz, K.S.; Melchert, B.; Seifert, R.; Borowski, C.; Giere, O.; et al. Diking, young volcanism and diffuse hydrothermal activity on the southern Mid-Atlantic Ridge: The Lilliput field at 9°33′ S. Mar. Geol. 2009, 266, 52–64. [Google Scholar] [CrossRef] [Green Version]
  69. Barriga, F.J.A.S.; Fouquet, Y.; Almeida, A.; Biscoito, M.J.; Charlou, J.L.; Costa, R.L.P.; Dias, A.; Marques, A.M.S.F.; Miranda, J.M.A.; Olu, K.; et al. Discovery of the Saldanha hydrothermal field on the Famous Segment of the MAR (36°30′ N). Eos Trans. AGU 1998, 79, F67. [Google Scholar]
  70. Ardron, J.A.; Ruhl, H.A.; Jones, D.O.B. Incorporating transparency into governance of deep-seabed mining in the Area beyond national jurisdiction. Mar. Policy 2018, 89, 58–66. [Google Scholar] [CrossRef]
  71. Clark, M.R.; Durden, J.M.; Christiansen, S. Environmental impact assessments for deep-sea mining: Can we improve their future effectiveness? Mar. Policy 2020, 114. [Google Scholar] [CrossRef]
  72. Jones, D.O.B.; Durden, J.M.; Murphy, K.; Gjerde, K.M.; Gebicka, A.; Colaço, A.; Morato, T.; Cuvelier, D.; Billett, D.S.M. Existing environmental management approaches relevant to deep-sea mining. Mar. Policy 2019, 103, 172–181. [Google Scholar] [CrossRef]
  73. Tunnicliffe, V.; Metaxas, A.; Le, J.; Ramirez-Llodra, E.; Levin, L.A. Strategic environmental goals and objectives: Setting the basis for environmental regulation of deep seabed mining. Mar. Policy 2020, 114, 103347. [Google Scholar] [CrossRef]
  74. Durden, J.M.; Lallier, L.E.; Murphy, K.; Jaeckel, A.; Gjerde, K.; Jones, D.O.B. Environmental impact assessment process for deep-sea mining in ‘the Area’. Mar. Policy 2018, 87, 194–202. [Google Scholar] [CrossRef] [Green Version]
  75. Johnson, D.; Ferreira, M.A.; Kenchington, E. Climate change is likely to severely limit the effectiveness of deep-sea ABMTs in the North Atlantic. Mar. Policy 2018, 87, 111–122. [Google Scholar] [CrossRef]
Minerals 11 00142 g001 550
Figure 1. Rare earth oxides (REO) reserves worldwide; estimated statistical data for 2019 [15].
Figure 1. Rare earth oxides (REO) reserves worldwide; estimated statistical data for 2019 [15].
Minerals 11 00142 g001
Minerals 11 00142 g002 550
Figure 2. The actual market prices of rare earths: light rare earth elements (LREE) (La-Sm) + heavy rare earth elements (HREE) (Eu-Er) + Y + Sc. The prices (€) correspond to 1 kg of REO of high purity (>99.5%), updated in February 2020 [49].
Figure 2. The actual market prices of rare earths: light rare earth elements (LREE) (La-Sm) + heavy rare earth elements (HREE) (Eu-Er) + Y + Sc. The prices (€) correspond to 1 kg of REO of high purity (>99.5%), updated in February 2020 [49].
Minerals 11 00142 g002
Table
Table 1. List of the rare earth elements (REE), their relative abundances in the earth’s crust (ppm), and main applications.
Table 1. List of the rare earth elements (REE), their relative abundances in the earth’s crust (ppm), and main applications.
REEAtomic number (Z)Oxidation StateAbundance *
(ppm)		Applications ‡
Lanthanum (La)57+339Camera lenses; catalysts for petroleum refining; MRI; new-generation electric vehicles; optical glass; rechargeable batteries
Cerium (Ce)58+3, +466.5Catalyst for oil refineries; color screen, LCD; metal alloys; PET; polishing powder¸ radiation shielding; UV filters in glass
Praseodymium (Pr)59+39.2Colorant in glasses; color screen LCD; cryogenic refrigerant; energy-efficient lighting; hard disk drives; lasers; magnets; pigments; portable electronics and small motors; radiotherapy techniques
Neodymium (Nd)60+341.5Hard disk drives; new-generation electric vehicles; permanent magnets; portable electronics and small motors; treatment of skin cancers, violet colors in glass and ceramics
Promethium (Pm)61+3trace †Luminous paint; nuclear batteries
Samarium (Sm)62+37.05Lasers; magnets; new-generation electric vehicles; Sm-153 treatment of tumors
Europium (Eu)63+3, +22Bioimaging; color screen LCD; energy-efficient lighting; fluorescent lighting; lasers; portable electronics and small motors
Gadolinium (Gd)64+36.2Lasers; magnetostrictive alloys; memory chips; MRI; neutron capture; steel additive
Terbium (Tb)65+31.2Cancer therapies; color screen LCD; fuel cells; fluorescent lamps; high-powered electric motors; lasers; magnetostrictive alloys; optical computer memories
Dysprosium (Dy)66+35.2Dy-165 treatment of effusions; high-powered electric motors; lasers; lighter vehicles; magnetostrictive alloys
Holmium (Ho)67+31.3Cancer therapies; lasers; magnets; standards for optical spectrophotometers
Erbium (Er)68+33.5Fiber-optic technology; lasers; medical and dental practice; steel
Thulium (Tm)69+30.52Lasers; metal-halide lamps; Tm-167 in portable XRD-devices
Ytterbium (Yb)70+33.2Chemical reducing agent; infrared lasers; stainless steel; Yb-176 medical application
Lutetium (Lu)71+30.8Cancer therapies; catalyst; high-refractive-index glass; LED
Yttrium (Y)39
(66.5) ‖		+333Cancer therapies; ceramics; color screen LCD; compact fluorescent lamps; high-temperature superconductors; laser; LED; microwave filters
Scandium (Sc)21+322Alloys for aerospace components; camera lighting; catalysts; color screen LCD; energy-efficient lighting; radioactive tracing agent in oil refineries; super alloys; X-ray tubes
* [36]; † [8]; ‡ [16,37,38]; ‖ apparent average atomic Number (Z) based on physical properties of the element.
Table
Table 2. Available data (2010–2020) on the total concentration of the REE in ocean-floor sediment samples worldwide.
Table 2. Available data (2010–2020) on the total concentration of the REE in ocean-floor sediment samples worldwide.
LocalitySediment Sample, Water Depth (m)ΣREE,
Min–Max (ppm)		Wet DigestionReference
Atlantic
Main Lilliput, Roman City
(9°32′–9°33′ S,
13°12′ W)		1486–149514.4–16.6HF-HCl-HNO3
aqua regia		[63]
Logatchev
(14°45′ N,
44°58′ W)		292313.6–130HF-HCl-HNO3
aqua regia		[64]
Saldanha
(36°34′ N, MAR)		2125–23006.3–40.6HCl-aqua regia-HF-HBr[65]
TAG
(26° N, MAR)		3407–36542.8–113.9 *Na2O2-HNO3[55]
Pacific
South East
(5°–20° S,
90°–150° W)
Central North
(3°–20° N,
130° W–170° E)		4000–50001000–2230 *
400–1000 *		HNO3-HF-HClO4
aqua regia		[19]
North East
(5°–20° N,
119°–160° W)		4313–4529200–577HNO3-HF-HClO4
aqua regia		[20]
North East(9°30′–10°30′ N,
131°–133° W)		4000–6000150–1150 †NP ‡[21]
North West
(21°–27° N,
150°–158° E)		5720–5735299–6800 *HNO3-HF-HClO4
aqua regiaHNO3-HCl-HF		[23,27]
North
(21°48′–22°15′ N,
153°30′–154°07′ E) 		4700–61005000–22,000 *HClO4-HF-HNO3
aqua regia		[22]
Indian
ODP sites
(5° N, 90° E)		29240.5–1113 *HNO3-HF-HClO4
aqua regia		[33]
DSDP and ODP sites
(1°–30° S, 57°–112° E)		2832–56090.5–920 *HNO3-HF-HClO4
aqua regia		[53]
Southern
DSDP site
(59° S, 104° E)		452272–114 *HNO3-HF-HClO4
aqua regia		[33]
* ΣREY; † ΣREO; ‡ Not provided.
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Milinovic, J.; Rodrigues, F.J.L.; Barriga, F.J.A.S.; Murton, B.J. Ocean-Floor Sediments as a Resource of Rare Earth Elements: An Overview of Recently Studied Sites. Minerals 2021, 11, 142. https://doi.org/10.3390/min11020142

AMA Style

Milinovic J, Rodrigues FJL, Barriga FJAS, Murton BJ. Ocean-Floor Sediments as a Resource of Rare Earth Elements: An Overview of Recently Studied Sites. Minerals. 2021; 11(2):142. https://doi.org/10.3390/min11020142

Chicago/Turabian Style

Milinovic, Jelena, Francisco J. L. Rodrigues, Fernando J. A. S. Barriga, and Bramley J. Murton. 2021. "Ocean-Floor Sediments as a Resource of Rare Earth Elements: An Overview of Recently Studied Sites" Minerals 11, no. 2: 142. https://doi.org/10.3390/min11020142

APA Style

Milinovic, J., Rodrigues, F. J. L., Barriga, F. J. A. S., & Murton, B. J. (2021). Ocean-Floor Sediments as a Resource of Rare Earth Elements: An Overview of Recently Studied Sites. Minerals, 11(2), 142. https://doi.org/10.3390/min11020142

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop