By
Türker PASİNLİ
A Dissertation Submitted to the
Graduate School in Partial Fulfillment of the
Requirements for the Degree of
MASTER OF SCIENCE
RARE EARTH ELEMENTS (REEs)
1.1. Introduction to Rare Earth Elements (REEs)
The term rare earth was suggested by Johann Gadolin in 1794. "Rare" because when the first of the REEs was discovered they were thought to be present in the earth's crust only in small amounts, and "earths" because as oxides they have an earthy appearance. Because of chemical similarities among the REEs, their complete isolation and classification took more than a century from their discovery (Evans 1997).
The REEs lie in the last rows of Mendeleev’s periodic table, comprising both lanthanide and actinide series (Figure 1.1). The REE term is mostly employed in chemistry as a synonym of the lanthanide series.
The REEs have similar physicochemical properties, which change periodically with the atomic number. They range from La to Lu (atomic numbers between 57 and 71: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Th), ytterbium (Yb), and lutetium (Lu). Yttrium (atomic number 39) a Group IIIB transition metal, although not a lanthanide is generally included with the REEs as it occurs with them in natural minerals and has similar chemical properties. For the same reason, scandium (atomic number 21) is also included with the REEs. The REEs are usually divided into three groups: light REEs, from La to Pm, the medium REEs, from Sm to Ho, and the heavy REEs, from Er to Lu.
Figure 1.1. Periodic table with REEs and scandium, yttrium and thorium.
Although lanthanides are termed rare-earth elements, they are not rare in nature. Their levels in the earth’s crust are often equal to or higher than some physiologically significant elements, such as iodine, cobalt, silver, gold, platinum and selenium (Brzyska 1996). Cerium (68 mg/kg) and lanthanum (32 mg/kg) are the most common. Lutetium and thulium are the rarest (about 0.5 mg/kg) while the concentrations of the remainder range from 1 to 9 mg/kg. Promethium is an artificial radioactive element with no stable isotopes.
The REEs have similar chemical and physical properties and behave relatively coherently as a group. Chemically, REEs are strong reducing agents and their compounds are generally ionic. REEs are never found as free metals in the earth's crust.
All their naturally occurring minerals consist of mixtures of various REEs and nonmetals. Bastnaesite [(Ce,La)(CO3)F], monazite [(Ce,La,Nd,Th)(PO4)] [(REE)PO4] and xenotime [YPO4] are the three most significant minerals of REEs. The REEs are definitely electropositive metals with the oxidation number of +3. Only cerium, terbium and praseodymium with an oxidation number of +4 and samarium, europium and ytterbium with the oxidation number of +2 form stable compounds. Europium and cerium are the most reactive elements of the REEs (Evans 1997).
1.2. Uses of Rare Earth Elements
Lanthanide compounds frequently have magnetic, catalytic and optic properties and therefore are widely used in industry. Their industrial uses are outlined in Table 1.1.
Table 1.1. Industrial uses of REEs (Pedreira et al. 2002)
In the last twenty years new technologies have been introduced in metallurgical, optical and electronic industries, which increased the role of artificial lanthanide compounds with special physicochemical properties.
At present, metallurgy utilizes about 37% of lanthanides and their compounds which is used to remove oxygen and to enrich steel. Thirty percent of lanthanides are used for catalytic converters, 29% in ceramic industry, and 1% in other industries.
Pure lanthanide compounds are used in electronics and optoelectronics to produce luminophores (oxides of lanthanum, gadolinium, europium and terbium), lasers (e.g., halogens of neodymium, holmium and erbium), optical fibers, components of magnetic memories (e.g., gadolinium-gallium garnet, GGG), permanent magnets (alloys of samarium and neodymium) and high-temperature superconductors.
Also they are used as magnetic resonance imaging (MRI) contrast reagents in medicine, and also lanthanum chloride (LaCl3) is added to chemical fertilizers in China (Liang et al. 1991, Gorbunov et al.1992, Sloof et al. 1993, Brzyska 1996, Evans 1997, Shuai et al. 2000, Liang et al. 2001).
1.3. Biological Effects of Rare Earth Elements
Rare earth elements are released into the environment as a result of their industrial uses (Gorbunov et al. 1992). Continuous exposure to low concentrations of REEs could cause adverse health effects because of their bioaccumulation along the food chain.
Although there is so far no reported incidence of intoxication due to the intake of REEs through the food chain, several deleterious effects due to occupational and environmental exposure to REEs have been reported (Sabbioni et al.1982, Sax 1984).
According to these reports, rare earth elements have both positive and negative effects on human health. For example rare earth elements show benefit in the liver where gadolinium selectively inhibits secretion by Kupffer cells and decrease cytochrome P450 activity in hepatocytes, thereby protecting liver cells against toxic products of xenobiotic biotransformation. Praseodymium ion (Pr3+) produces the same protective effect in liver tissue cultures.
On the other hand, cytophysiological effects of lanthanides appear to result from the similarity of their cationic radii to the size of Ca2+ ions, their high degree of ionic bonding and their donor atom affinities. Trivalent lanthanide ions, especially La3+ and Gd3+, block different calcium channels in human and animal cells. Lanthanides can affect numerous enzymes. Dy3+ and La3+ block Ca2+-ATPase and Mg2+-ATPase, while Eu3+ and Tb3+ inhibit calcineurin. In neurons, lanthanide ions regulate the transport and release of synaptic transmitters and block some membrane receptors, e.g. GABA and glutamate receptors (Palasz et al.2000).
It is likely that lanthanides significantly and uniquely affect biochemical pathways, thus altering physiological processes in the tissues of humans and animals.
1.4. Determination of Rare Earth Elements
As the demand for high purity rare earth compounds is increasing and for environmental protection, the development of new precise and accurate analytical methods for the determination of REEs is required at trace levels.
For many years, the most common analytical techniques for measuring REEs have been neutron activation analysis (NAA) (Orvini et al. 2000, Figueiredo et al. 2002, Minowa et al. 2003) and isotope dilution mass spectrometry (IDMS) (Hoyle et al. 1983, Greaves et al. 1989, and Noemia et al. 1990). These methods are, however, time consuming and require very sophisticated equipment, unviable to most laboratories. The inductively coupled plasma optical emission spectrometry (ICP-OES) is one of the most effective multi-element techniques for the quantitative determination of many trace elements with widely varying matrices, and is often used in the determination of the REEs. Instrumental detection limits are stated to be on the order of 50.0 µg/L (Djingova et al. 2002). The more recent inductively coupled plasma mass spectroscopy (ICP-MS) technique is a powerful method for the direct determination of REEs (detection limits, in the order of 2.0 µg/L (Pedreira et al. 2002), but the equipment is still too expensive for many laboratories.
1.4.1. Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)
Inductively coupled plasma optical emission spectrometry (ICP-OES) is an important analytical tool in a wide range of scientific disciplines. It is used for simultaneous determination of over 70 elements in virtually any sample in less than 2 minutes. Concentrations from parts per billion to weight % can be determined without preconcentration or dilution. The ICP-OES method enables multi-element determination with high levels of precision and accuracy for most elements (<1% RSD). Also it offers low detection limits, rapid analytical procedure, a large dynamic range (four or more orders of magnitude), and complete removal of the analyte from its original matrix in order to minimize interferences with lower matrix background (Settle 1997).
ICP-OES is based on the fact that atoms are promoted to higher electronic energy levels when heated to high temperatures. In fact, the plasma temperature is sufficient to ionize most atoms. For about three-quarters of the elements amenable to the technique, the most sensitive line arises from an ion rather than an atom. As the excited species leave the high-temperature region, the absorbed energy is released as ultraviolet and visible photons when the excited atoms decay to lower energy levels or the ground electronic state. Useful emission lines generally occur in the region between 160 and 900 nm. Atomic and ionic emission lines are very narrow, typically less than 5 pm, and their wavelengths follow well-understood selection rules (Settle 1997).
The ICP is estimated to produce a typical temperature of 6500 0K and this high temperature is sufficient to break virtually all chemical bonds in a sample. Consequently, the emitting atoms and ions are virtually independent of one another. As a result, the technique exhibits high sensitivity, a linear range of four or more orders of magnitude, and much reduced chemical interference relative to AAS or arc/spark emission techniques (Settle 1997).
In an ICP-OES determination, the samples are most commonly introduced into the plasma as aerosols. A wide variety of devices are available for sample introduction. Pneumatic nebulizers are the least expensive and most commonly used in commercial devices. The aerosol produced by the nebulizer is generally passed through a spray chamber to remove large droplets and produce a more homogeneous aerosol. While passing through the plasma, the aerosol is vaporized, atomized, perhaps ionized, and then electronically excited. After leaving the plasma, the sample emits photons, which are sampled through a narrow entrance slit and dispersed with grating monochromotor. The resolved radiations are measured with a photomultiplier tube or array detectors (such as a charge coupled device, CCD or charge injection device, CID), which converts the optical signal into an electrical signal. An electronic interface converts the signal into an appropriate form for measurement and storage by a dedicated computer (Settle 1997).
ICP-OES has been widely used in the determination of trace REEs. For example; Liang et al. (2001) used ICP-OES for the determination of La, Y, Yb, Eu, Dy after preconcentration step with nanometer-sized titanium dioxide micro-column. The technique was used by Shuai et al. (2000) for the determination of rare earth impurities in high-purity lanthanum oxide, by Iwasaki (1986) and Rucandio (1992) for the determination of lanthanides and yttrium in rare earth ores; and by Crock et al. (1982) for determination of rare earth elements in geological materials.
1.4.2. Preconcentration and Separation of Rare Earth Elements
Inductively coupled plasma optical emission spectrometry, as mentioned, offers one of the most suitable techniques for REEs determination. However, the low level of REEs in samples is not compatible with the detection limits exhibited by this technique. Also major constituents, such as organic compounds and inorganic salts, cause matrix effects. In order to achieve accurate and reliable results, efficient preconcentration of REEs and their separation from matrix is required.
One of the most widely used techniques for the separation and preconcentration of trace REEs has been co-precipitation. For example, Roychowdhury et al. (1989) precipitated REEs and Y as oxalates using calcium as a carrier. In another study Greaves et al. (1989) employed a precipitation step with hydrated iron (III) oxide followed by a purification procedure using a single cation-exchange column after which the sorbed species were eluted with hydrochloric and nitric acids. Liquid-liquid extraction is another efficient separation technique used in the studies related with REEs. Wang et al. (2004) applied the technique in the separation of Y from heavy REEs using a novel organic carboxylic acid, s-Nonylphenoxy acetic acid, as extractant in the presence of several other complexing agents such as EDTA, DTPA, or HEDTA. Ion exchange procedures have also been applied successfully for the separation of REEs from geological materials [Navarro et al. 2002]. In this study, the researchers separated REEs with a cation exchange resin; then they eluted the sorbed species with a nitric acid-oxalic acid mixture. Zhu et al. (1998) utilized an ion-exchange microcolumn prepared with Diol silica consisting of a acrylic acid/acrylamide copolymer. Elution of REEs from the microcolumn was realized with 0.25 M HNO3. Möller et. al (1992) used Chelex 100 chelating resin for preconcentration of REEs in sea water by ion exchange chromatography. High performance liquid chromatography (HPLC) was also applied in the determination of REEs. In one such study, Qin et al. (2000) used a 2-ethylhexylhydrogen 2-ethylhexylphosphonate resin as the stationary phase and dilute nitric acid as the mobile phase for the separation of REE impurities in high purity cerium oxide.
1.4.2.1. Solid-Phase Extraction (SPE)
In addition to the preconcentration / separation techniques mentioned above, the solid-phase extraction (SPE) technique has become increasingly popular in recent years (Grebneva et al. 1996, Vicente et al. 1998, Dev et al. 1999, Liang et al. 2001, Hirata et al. 2002). It is an extraction method that uses a solid phase and a liquid phase to isolate one, or one type, of analyte from a solution. It is usually used to clean up a sample before using a chromatographic or other analytical method to quantitate the amount of analyte(s) in the sample. The solid phase extraction technique has several advantages; (i) it is simple to implement, (ii) high preconcentration factors can be obtained by SPE , (iii) it enables rapid phase separation, and (iv) it can be combined with different techniques. The general procedure is to load a solution onto the SPE phase, wash away undesired components, and then wash off the desired analytes with another solvent into a collection tube.
The solid phase extraction method is widely used in chromatographic preconcentration studies that can be performed in two distinct forms, the batch and the column methods. In our project we used the batch method. In the batch mode, a quantity of the chromatographic stationary phase (or sorbent) is added to the sample and the mixture is then shaken for some time. If the conditions are suitable, the analytes of some interest become bound to the sorbent and are then separated from the sample solution by filtration.
Column preconcentration can be performed either off-line or on-line. In the former case, the sample is passed through a suitable column after which the enriched analyte is desorbed from the column and the resultant solution is analyzed by an appropriate procedure. In the on-line method, the sorbent column is coupled directly to the analytical instrument so that the sample enrichment, desorption, and analysis steps can be carried out at the same run automatically.
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