Rare Earth Elements: A Review of Production, Processing ...

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Rare Earth Elements Review Appendix A – Selected Chemical Properties Appendix A Selected Chemical Properties of REEs It was once thought that rare earth elements (REEs) naturally occur as oxides, referred to as “earths,” but it is now understood that these elements do not naturally occur in the form of an oxide in natural deposits. Commonly, modern geochemists refer to REEs as the “lanthanide group elements,” or “lanthanides,” since lanthanide is positioned first in a series of elements that all share similar chemical properties. The elements scandium and yttrium have almost identical physical and chemical properties to that of the lanthanide group elements; therefore, these are also included in the lanthanide group. All the rare earths except promethium (Pm) are available in nature. Promethium occurs by thermal neutron fission of uranium (U 235 ) or by bombarding neodymium with neutrons produced from uranium fission in a nuclear reactor. The natural geochemistry and physico-chemical properties of REES that control background concentrations of these metals in the environment are not well understood, nor are the factors influencing bioavailability. Studies in the literature appear limited at the present time. Measurement of rare earth metals (REMs) in environmental media is also problematic due to the physico-chemical properties that make them unique. Predicting the environmental human and ecological health risks associated with anthropogenic activities that have the potential to release REMs to the environment is difficult and not straightforward. Some of the physico-chemical characteristics of lanthanides are presented below that likely affect the occurrence and bioavailability of these metals. Although not included here, an extensive tabulation of the properties for the lanthanide group metals is provide by Gupta and Krishnamurthy (2004) and others. In general, metals typically exist in the environment in several different forms, depending on ambient chemical conditions (e.g., pH, organic carbon, presence of other chemicals). The chemical form of a metal, or its speciation, can play a large role in its fate and bioavailability and, thus, in its potential to produce toxic effects (Langmuir et al., 2004). Because of the high variability in both biotic and abiotic factors (e.g., pH) affecting the speciation and bioavailability of metals within and between environmental media, risks posed by metals often can vary by orders of magnitude depending on the speciation. Environmental conditions can change significantly, even over small spatial scales (e.g., a few centimeters in a sediment transitioning from an aerobic zone downward to an anaerobic zone), and such significant shifts in conditions can lead to significant changes in metals speciation and risk. Evidence found pertaining to the toxicity and health effects of lanthanides is provided in Section 6 of the main document to this appendix. The rare earths are so chemically similar to one another that separation of individual elements from the host minerals is difficult (see Section 4). Their similarity is also demonstrated by their occurrence together in the same geological deposits; however, that does not imply that these metals respond equally to changes in natural systems (Weber, 2008). It is believed that the lanthanide group elements also have very similar electronic configurations; however, due to the complexity of the electron spectra and difficulty of analysis the electron configuration is not known with complete certainty (Gupta and Krishnamurthy, 2004). There are observed distributions for lanthanide group elements in the natural environment indicating that they will, under the right conditions, separate from the each other. Preferences for the occurrence of certain lanthanides have been observed in different mineral types. In aqueous solution, separation also occurs due to variable stability constants, indicating the strength of the chemical bond of lanthanideligand complexes (Weber, 2008). Observations like these have suggested a subdivision of the lanthanides A-1
Rare Earth Elements Review Appendix A – Selected Chemical Properties into various groupings as presented by Gupta and Krishnamurthy (2004) from other sources and that are presented below. Previously the lanthanides were categorized into three groups according to Kramers in 1961: � Light (or cerium) lanthanide group – lanthanum (La) through samarium (Sm); � Middle (or terbium) lanthanide group – europium (Eu) through dysprosium (Dy); � Heavy (or yttrium) lanthanide group – holmium (Ho) through lutetium (Lu) and including yttrium. More recently, two other attempts have been made to subdivide the lanthanides into groups: (after Jackson and Christiansen, 1993): � Light (or cerium) lanthanide group – lanthanum (La) through gadolinium (Gd); and � Heavy (or yttrium) lanthanide subgroup – terbium (Tb) through lutetium (Lu), including yttrium. (after Sabot and Maestro, 1995) � Light lanthanide group – lanthanum (La) through neodymium (Nd); � Middle lanthanide group – samarium (Sm) through dysprosium (Dy); � Heavy lanthanide group – holmium (Ho) through lutetium (Lu) and including yttrium. Other groupings continue to be developed based on newer studies and observations and according to purpose. The chemical properties of metals are determined by the valence electrons or the number of bonds that can be formed by an atom. Rare earths differ from other metals since their valence electrons are not located in the outermost shell of the atom, rather the valence electrons of the lanthanide group are positioned in the (4f) subshell that is shielded by two larger closed (or full) subshells (5s2 and 5p6). Regardless of the attempted groupings, this atomic configuration supporting stable outermost electron shells results in the very similar chemical properties of the lanthanide group in general and the difficulty in their separation during processing. Contrary to the general trend in the periodic table, it has been observed in the lanthanide group that the atomic radii of the elements and their ions decrease slightly as the atomic numbers increase (starting with cerium). Lanthanum has the largest radius and lutetium the smallest, owing to the incremental increase of 4f orbital electrons by one across the group. In very general terms, a current theory considers that the atomic nucleus of the lanthanide elements is poorly shielded, and therefore with the increase in atomic number, the nuclear charge experienced by the 4f orbital electrons increases as well. The effect of this shielding pattern results in a contraction of the 4f shell as that subshell’s electrons are pulled closer to the nucleus. This reduction in ionic radii across the lanthanides with increasing ionic charge is referred to as the “lanthanide contraction.” The difference in ionic radius of adjacent rare earths is very small. For example, the ionic radius of Ce +3 is 1.06 Å and that of Lu +3 is 0.85 Å. The lanthanide contraction controls many of the features observed for REE chemistry. The chemistry of lanthanides is predominantly ionic and is determined primarily by the size of the trivalent ion. All the REMs typically occur in the trivalent (M 3+ ) state in terrestrial environments. However, a few of the REMs occur in other ionic forms such as samarium (Sm 2+ ), thulium (Tm 2+ ), and ytterbium (Yb 2+ ), but only the alternate forms of cerium (cerous, Ce 4+ ), and Europium (Eu 2+ ), are commonly found in natural systems (Railsback, 2008). Since the outer orbitals of lanthanides have higher energy, they tend not to form covalent bonds (Weber, 2008). Some covalency is exhibited and increases slightly with increasing A-2
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Rare Earth Elements Review Table of
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Foreword The U.S. Environmental Pro
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Abstract Rare earth elements (REEs)
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Table of Contents List of Acronyms
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List of Tables 2-1. Abundance of El
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Rare Earth Elements Review List of
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Rare Earth Elements Review Section
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