A Comparative Study of Methods of the Dynamic Fractionation of Rare Earth Elements in Soils

Rare earth elements (REEs) are currently used in fertilizers, but their behavior in the soil-plant system remains poorly understood. The assessment of the binding of REEs to various organomineral phases of soils remains an important task. Using soddy-podzolic soil and typical chernozem as examples, we performed a comparative study of two procedures of the fractionation of REEs in soils in the dynamic extraction mode. According to a five-stage procedure using a 0.05 M Ca(NO3)(2) solution, 0.43 M CH3COOH, a 0.1 M NH2OH.HCl solution (pH 3.6), a 0.1 M K4P2O7 solution (pH 11.0), and a 0.1 M (NH4)(2)C2O4 solution (pH 3.2), five extractable fractions were isolated, which can be conventionally named, exchangeable, specifically sorbed, associated with manganese oxides, associated with organic matter, and associated with amorphous and weakly crystallized oxides of iron and aluminum, respectively. A four-stage procedure, using a 0.05 M Ca(NO3)(2) solution, a 0.1 M citric acid solution, a 0.05 M NH2OH.HCl solution (pH 2.0), and 1.4 M HNO3, enabled the isolation of exchange ions and fractions bound to carbonates, reducible, and acid-soluble, respectively. The concentration of elements in the initial samples and extracts was determined by atomic emission spectrometry and inductively coupled plasma mass spectrometry. In a five-stage procedure, the main extractable REE species (up to 40% of the total content) is provided by organometallic complexes extracted with a 0.1 M solution of potassium pyrophosphate. Using a four-stage procedure, the main fraction of REEs (up to 30%) is extracted with 1.4 M nitric acid. Using La, Ce, and Nd as examples, two procedures were compared. The concentration of REEs extracted by nitric acid (four-stage procedure) corresponds to the concentration of REEs extracted by pyrophosphate (five-stage procedure). In addition, the use of a 0.05 M NH2OH.HCl solution at pH 2.0 (four-stage procedure) and a 0.1 M (NH4)(2)C2O4 solution at pH 3.2 (five-stage procedure) leads to similar results. Thus, both procedures can be used for the dynamic fractionation of REE species in soils. However, the five-stage procedure is preferable, because the reagents used are sufficiently selective with respect to the dissolved organomineral phases of soils, which more clearly reflects the pattern of the distribution of REE species.