A method for predicting bioavailability of rare earth elements in soils to maize

A single-extraction procedure using low-molecular-weight organic acids (LMWOAs) as extractant and the first and second steps of a three-step extraction procedure recommended by the European Community Bureau of Reference (BCR; now European Community Standards, Measurement and Testing Programme, Brussels, Belgium) were performed to extract the light rare earth elements (LREEs) La, Ce, Pr, and Nd from wet rhizosphere soil. The extracted soil solutions were successively filtered through membranes with a pore size of less than 0.45 mum and a molecular weight cutoff of less than 1 kDa, which were termed colloidal and truly dissolved fractions, respectively. Apoplastically and symplastically bound LREEs in maize roots were experimentally distinguished by ultrasound-assisted desorption with 1 mM CaCl2 solution at 0degreesC in ice-cooled water bath. When the LMWOAs extraction method was used, a good correlation was obtained between LREEs in soil colloidal and truly dissolved fractions and LREEs bound to apoplasm and symplasm of maize root. Both apoplastically and symplastically bound LREEs are the result of bioavailability. However, a poor correlation was obtained between LREEs in fractions water soluble, exchangeable and carbonate bound (B1) and Fe-Mn oxide bound (B2) of the BCR method and LREEs in apoplasm and symplasm and in intact roots. Hence, the LMWOAs extraction method is recommended for measuring the bioavailability of LREEs in soils.