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

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