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

Rare Earth Elements Review Appendix A – Selected Chemical Properties
atomic number; however, bonding is predominantly ionic, even with extra electrons in the d-orbitals of
some lanthanides.
An important chemical property is that the lanthanides will form strong complexes with a number of
different ligands. Water is a strong ligand for trivalent (M 3+ ) lanthanides (Weber, 2008). Trivalent (M 3+ )
lanthanides strong affinity for water allows them to form a hydration shell around the Ln 3+ ion. The
number of molecules of water that a lanthanide ion can bond with in an aqueous solution varies between 8
and 9, depending on the element and species (Weber, 2008). Weber (2008) points out that the basic
physico-chemical properties of lanthanides in natural aqueous systems (e.g., interstitial soil water in the
unsaturated zone) depend significantly on the presence of other coordinating compounds that can displace
the water molecules from around the lanthanide metal ion.
In aqueous environments, the water ligand would only be displaced if ligands with small ionic radius
were present with a high oxidation state, high electronegativity, and with the highest occupied molecular
orbitals having low energy; examples of these chemical species are the hydroxyl anion (OH – ), fluorine
(F – ), chlorine (Cl – ), ammonia (NH 3 ), acetic acid (CH 3 COO – ), carbonic acid (CO3 2– ), nitrate (NO 3- ), sulfate
(SO4 2- ), phosphate (PO4 3- ), oxide (O 2- ), alcohols (R –OH), amines (R-NH2), and others containing highly
electronegative donor atoms such as O and F. These chemical species would tend to form mainly an ionic
bond with the lanthanides molecules within their unoccupied lower high-energy orbitals. Lanthanides
preferentially bond with oxygen atoms in aqueous solutions, but one-to-one bonding between lanthanides
and other compatible ions has been shown to be weak compared to bonding with oxygen or flourine
atoms and easily hydrolyzed by water (Gupta and Krishnamurthy, 2004; Weber, 2008). However,
polydentate ligands, which have multiple pairs of electrons that can bond to the lanthanide metal ion, will
form stronger bonds that may be more stable in aquatic environments. In general, the lanthanide ion
preference for donor atoms is O>N>S (Weber, 2008).
At ambient temperatures, all REMs are not affected by air the same way. The ionization energies (or
strength of the nuclear charge) of lanthanides are comparatively low, meaning it takes less energy to
remove valence electrons. These elements are therefore highly electropositive and, as previously stated,
form compounds that are essentially ionic in nature (Gupta and Krishnamurthy, 2004). Like most metals,
the lanthanides have a gray luster or bright silvery appearance. Five of the elements (lanthanum, cerium,
praseodymium, neodymium, and europium) are very reactive. When exposed to the air, they react with
oxygen to form an oxide coating that tarnishes the surface almost immediately. A block of europium will
be converted to an oxide in a few days or weeks. Under the same conditions, surface corrosion of a
massive block of lanthanum or neodymium will occur in a few days, and a thick crust of oxide will
develop in a few months.
Analytical Methods
There are a variety of methods employed to determine REE concentrations in environmental samples,
such as (1) atomic absorption spectroscopy; (2) neutron activation; (3) isotope dilution thermal ionization
mass spectrometry (ID-TIMS); and (4) inductively coupled plasma-mass spectrometry (ICPMS).
Concentrations of REEs in unpolluted freshwater systems typically range in from parts-per-billion (ppb)
to parts-per-trillion (ppt) (Lawrence et al., 2006). Therefore, a pre-concentration step such as ion
exchange chromatography, Fe(OH)3 co-precipitation, or liquid-liquid extraction may be required prior to
analysis. The specific method selected for analysis will provide guidance on sample preparation prior to
analysis (e.g., an EPA 3000 series method).
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