Speciation of Rare Earth Elements in a Heavy Metal Contaminated Soil Using Geochemical and Isotopic Methods

Marc Steinmann Centre de Géochimie de la Surface (CNRS), 1, rue Blessig, F-67084 Strasbourg Cedex, France


Peter Stille Centre de Géochimie de la Surface (CNRS), 1, rue Blessig, F-67084 Strasbourg Cedex, France


Rare earth elements (REE) appear as natural trace metals in the whole geosphere and have become important geochemical tracers during the last few decades. But some REE isotopes also occur as radionuclides in the decay series of nuclear fission and can therefore be found in nuclear waste or be released by nuclear accidents. Consequently, propagation of radio pollution in the environment is to a considerable extent controlled by the migration behavior of the REE. The behavior of trivalent REE furthermore allows to draw analogies for trivalent actinides.

In order to elucidate REE behavior in soils we study a soil profile which has been contaminated by air particles from a neighbouring brass foundry during more than 80 years. In addition to concentration data we also use Sr, Nd, and Pb isotopes. The samples were sequentially extracted in three steps with acetic acid (HAc) 1N, HCl 1N and HNO3 1N.


The depth distribution of the REE is controlled by two different parameters which are soil pH for the HAc extractable REE and Fe-Mn oxides for the REE of the HCl and HNO3 extracts. A part of the HNO3 extractable REE was besides bound to the organic matter of the topsoil. The REE concentrations of the HAc extracts grow with depth increasing soil pH which indicates that they are derived from the surface and hence of anthropogenic origin. This is confirmed by 143Nd/144Nd isotope ratios which show a mixing between a natural silicate pole at the top and an anthropogenic pole at the bottom of the profile. We assume that the anthropogenic REE were transported in dissolved form as carbonate complexes and then precipitated during downwards migration with increasing soil pH.

The REE concentrations of the HCl and HNO3 extracts are like Mn, Fe and Al positively correlated with 87Sr/86Sr isotope ratios. In the case of the HCl and HNO3 extracts these isotopes give a direct measure for the amount of silicate attacked during leaching. The 87Sr/86Sr ratios rise with growing amount of attacked silicate. Consequently, the correlations demonstrate that the Fe-Mn oxide bound REE of the HCl and HNO3 extracts are in contrast to the HAc extractable REE initially derived from soil minerals and hence, in spite of the pronounced heavy metal contamination, of natural origin.


We conclude that our approach allows to distinguish between natural and anthropogenic REE. The natural REE are immobile and fixed in Fe-Mn oxides. The anthropogenic REE were in contrast to this mobile even under the present alkaline conditions and have migrated through the soil. We suggest that considerable amounts of in particular the heavy REE were washed out into the underlying alluvial gravels. REE are known for their affinity for organics. In spite of this, the anthropogenic REE have in contrast to other anthropogenic heavy metals only partly been retained by the organic matter of the topsoil. We speculate that especially the high concentrations of Cu in the topsoil, which undergoes very stable organic complexes, has led in a competitive process to the displacement of the REE away from the topsoil. This REE mobility is of particular importance for the propagation of accidentally released radioactive REE.