Lanthanide accumulation

Bayer ME and Bayer MH (1991) Lanthanide accumulation in the periplasmic space of Escherichia coli B. J. Bacteriol 173 141-149. 

The overall distribution of lanthanides in bone may be influenced by the reactions between trivalent cations and bone surfaces. Bone surfaces accumulate many poorly utilized or excreted cations present in the circulation. The mechanisms of accumulation in bone may include reactions with bone mineral such as adsorption, ion exchange, and ionic bond formation (Neuman and Neuman, 1958) as well as the formation of complexes with proteins or other organic bone constituents (Taylor, 1972). The uptake of lanthanides and actinides by bone mineral appears to be independent of the ionic radius. Taylor et al. (1971) have shown that the in vitro uptakes on powdered bone ash of 241Am(III) (ionic radius 0.98 A) and of 239Pu(IV) (ionic radius 0.90 A) were 0.97 0.016 and 0.98 0.007, respectively. In vitro experiments by Foreman (1962) suggested that Pu(IV) accumulated on powdered bone or bone ash by adsorption, a relatively nonspecific reaction. On the other hand, reactions with organic bone constituents appear to depend on ionic radius. The complexes of the smaller Pu(IV) ion and any of the organic bone constituents tested thus far were more stable (as determined by gel filtration) than the complexes with Am(III) or Cm(III) (Taylor, 1972). 

The apparent failure of trivalent and tetravalent cations to enter plants could result from the interaction of the cations with the phospholipids of the cell membranes. Evidence for such interactions is provided by the use of lanthanum nitrate as a stain for cell membranes (143) while thorium (IV) has been shown to form stable complexes with phospholipid micelles (144). However, it is possible that some plant species may possess ionophores specific to trivalent cations. Thomas (145) has shown that trees such as mockernut hickory can accumulate lanthanides. The proof of the existence of such ionophores in these trees may facilitate the development of safeguards to ensure that the actinides are not readily transported from soil to plants. 

Non-stoichiometry is a very important property of actinide dioxides. Small departures from stoichiometric compositions, are due to point-defects in anion sublattice (vacancies for AnOa-x and interstitials for An02+x )- A lattice defect is a point perturbation of the periodicity of the perfect solid and, in an ionic picture, it constitutes a point charge with respect to the lattice, since it is a point of accumulation of electrons or electron holes. This point charge must be compensated, in order to preserve electroneutrality of the total lattice. Actinide ions having usually two or more oxidation states within a narrow range of stability, the neutralization of the point charges is achieved through a Redox process, i.e. oxidation or reduction of the cation. This is in fact the main reason for the existence of non-stoichiometry. In this respect, actinide compounds are similar to transition metals oxides and to some lanthanide dioxides. 

Monazite is a mixed lanthanide orthophosphate proposed as a host phase for ACTs and REEs. Natural monazite contains <27 wt% U02 + Th02 and remains crystalline in spite of high accumulated a-doses (Boatner Sales 1988 Weber et al. 1998). However, 1.5 MeV Kr+ irradiation amorphizes monazite at a dose of 2.56 x 1018ions/m2 (Meldrum et al. 1996). Even fully amorphized monazite demonstrated low leachability, and the leach rate remained at the same level as that of unirradiated samples (Sales Boatner 1988 Weber et al. 1998). 

A compound that is able to influence the relaxation times of water protons has to be paramagnetic. In the Periodic System paramagnetic ions are to be found amongst the transition metals and the rare earth metals (lanthanides). However, it was well known, that the free ions of heavy metals are toxic. Lanthanide ions form soluble complexes with ligands such as phospholipids, amino acids and proteins that are present in plasma. The liver and the skeleton are the major sites of accumulation of free metal ions. Uptake in the liver is mediated by the hepa-tocytes [2]. 

The results and experience that have accumulated from studies of the exchange reactions of the aminopolycarboxylates of different lanthanides are very useful in the assessment of the kinetic stabilities of the Gd3+ chelates. In the body fluids where the CAs are administered, the Gd3+ chelates are surrounded by various endogenous metal ions and ligands. Some of these metal ions can react with the Gd3+ chelate by displacing Gd3+ in a metal-metal exchange reaction ... 

Figure 14 Inductively coupled plasma mass spectra of a mixture of lanthanides, Y, and Th (a) without stored waveform inverse Fourier transform ion trap (SWIFT) excitation and (b) with the selective accumulation of Ce+ via SWIFT. (From Ref. 58.)...

Weltje L. 2002. Bioavailability of lanthanides to freshwater organisms. Speciation, accumulation and toxicity [dissertation]. Delft DUP Science. 

II 000 cm-1). Nevertheless, the photophysics of lanthanide porphyrinates is attractive because it could be of great help in medicine. For instance, hematoporphyrin derivatives are known to accumulate in malignant tumours and are used in medical diagnosis and photodynamic therapy of cancer. It is noteworthy that the Yb(III) complex with meso-tetra(3-pyridyl)porphyrin displays a substantial quantum yield (1.4%) when inserted into micelles formed by the non-ionic surfactant Triton X-100, a medium that can be considered as a model for biological tissues. 

The lighter lanthanides preferentially accumulate in the liver while heavier lanthanides prefer bone [157], This trend is particularly evident when lanthanide citrate complexes are administered. When lanthanides are introduced as chelate complexes, the absorption by the body is complete and the excretion rates also increase [158]. The excretion depends upon the stability of the chelate complex. If there is no exchange with the physiological ligands the excretion is rapid and complete. 

Biological indicator species such as fresh water sponges, clams, mussels, insect larvae that sorb water-borne pollutants are difficult to assess over time periods of years. The lanthanides may be used as indicator species utilizing the adventitious roots of stream-side trees [204-206]. The ability of tree roots to sorb both chelated and non-chelated lanthanides from water indicate that aquatic roots accumulate, concentrate and retain lanthanides to such an extent that the roots can be used as indicators of aquatic pollution, and that lanthanides can be studied as analogues of toxic elements and compounds. 

It should be pointed out that not all the ions discussed here are affected by the outer fields. In fact, lanthanide ions may be affected by solvents or coordination fields in chemical reactions. For example, E and E will change because of the coordination effect of water or organic molecules in an extraction. In addition, the amount of change would be different in different media. The tetrad effect would thus be different in different systems. The tetrad effect not only relates to the electronic configurations of lanthanide elements but is also affected by the surrounding conditions. Currently it is still not possible to predict the tetrad effect or to calculate it quantitatively. Tetrad effect theory still needs to be improved and further data need to be accumulated. 

Wedler G (1982) Lehrbuch der PhysikaUschen Chemie. Verlag Chemie, Weinheim/Deerfield Beach/Basel Weltje L (2003) Bioavailability of Lanthanides to freshwater organisms. Spedation, accumulation and toxicity. Ph.D. thesis. Technical University Delft, NL Wharton D (2002) Life at the limits. Organisms in extreme environments. Cambridge University Press, Cambridge White R (1991) Chemistry of atmospheres, 2nd edn. Clarendon, Oxford... 

Although new information has accumulated on this aspect of the chemistry of the earlier actinide elements, there still are many uncertainties that ought to be resolved. The area is intrinsically interesting because these elements have chemistries that combine lanthanide and ordinary transition metal characteristics, the latter being more marked in these low oxidation states. Work in the next few years will probably lead to a much clearer picture. 

Different forms of lanthanide have different organ distribution and excretion rates. Intravenously injected chelated lanthanide is transiently accumulated in the kidney and most of the injected dose is excreted in the urine. However, intravenously injected soluble salt is taken up by the reticuloendothelial cells, with most of the dose accumulating in the liver and spleen. The result of this intravenous exposure is liver necrosis. 

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