Comparative Evaluation of Adsorption of Rare-Earth Elements in Some Soil Types

Resumo

Competitive adsorption of scandium, yttrium, and lanthanides was studied for soddy-podzolic and gray forest soils and for leached chernozem. The chemical behavior of scandium significantly differs from that of other rare-earth elements (REEs) by the quick reaching of equilibrium in the soil–solution system and by the fact that its adsorption is not described by the Langmuir equation unlike yttrium and lanthanides. Individual chemical properties of REEs are the most pronounced in the experiments on competitive adsorption. In the yttrium–lutetium sequence, there is a tendency for a gradual decrease in the maximal adsorption capacity (from 0.025 to 0.010 mmol/kg in soddy-podzolic soil and from 0.100 to 0.050 mmol/kg in leached chernozem) and more pronounced increase in the Langmuir adsorption constants (from 5000 to 25 000 L/mmol in soddy-podzolic soil and from 5000 to 40 000 L/mmol in leached chernozem). This reflects the competition between different elements for soil sorption sites, which is directly related to the change in ionic radii of the elements (so-called “lanthanide contraction”). Unlike other soils, leached chernozem absorbs a large amount of REEs and provides their stronger fixation.