Speciation of rare earth elements in soil by sequential extraction then HPLC coupled with visible and ICP–MS detection

Abstract

The distribution of rare earth elements (REE) in a pooled soil sample collected from Zhangzhou, Fujian Province, China, was screened by a five-step sequential extraction procedure coupled with ICP–MS determination after preconcentration of REE and removal of the matrix by extraction with 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (HPMBP). The results showed that the distribution of REE in the different fractions of the pooled soil sample studied followed the order soluble species (46.76%) > species bound to organic matter (22.08%) > species in the residue (16.77%) > species bound to Fe–Mn oxides (2.02%). An effective method for speciation of REE, which utilized weak cation-exchange HPLC separation hyphenated with post-column derivatization and visible or on line ICP–MS detection, was, moreover, developed and successfully applied to the speciation of REE in the soluble extract of the pooled soil sample. The stability of known complexes of lanthanum during the HPLC separation was investigated with fluoride, citrate, and ethylenediamine tetraacetic acid (EDTA) chosen as ligands modeling those in the soil. REE in the soluble extract of the pooled soil sample were subsequently classified into three types of species –≤ + 1 charged complexes (negatively charged, neutral, and +1 charged), + 2 charged complexes, and “free” REE species. This method is expected to be useful for identification of bioavailable (or toxic) species of REE in environmental samples.