Assessing the bioavailability of dissolved rare earths and other trace elements: Digestion experiments with aquatic plant species Lemna minor (“duckweed” reference standard BCR-670)

Abstract

Reference material BCR-670 (Lemna minor, “duckweed”) is one of the very few plant reference materials for which certified data for all rare earths and yttrium (REY) are available. However, biological samples like plants or fungi often include detrital inorganic material because substrate particles (e.g., soil or atmospheric dust) may be closely associated with the organic component. Hence, the digestion approach used may strongly affect element yields. We here present concentration data for major, minor and trace elements with a focus on REY (with and without off-line preconcentration and matrix separation) following high-pressure high-temperature HNO3-HCl-HF decomposition and low-pressure low-temperature HNO3 extraction, respectively.Rare earths and yttrium concentrations after HNO3-HCl-HF decomposition are in good agreement with certified data but corroborate that preconcentration and matrix separation prior to quadrupole ICP-MS analyses result in improved data quality compared to data obtained without preconcentration and matrix separation. The results of both the decomposition and the extraction protocol show excellent reproducibilities, respectively, but the REY concentrations of the HNO3 extractions are lower than the certified bulk data and are accompanied by considerably lower concentrations of Al, Zr and Th. This suggests that the HNO3 extraction does not dissolve trace elements from detrital (alumino)silicate particles associated with the plant material.Our results expand the existing data set for BCR-670 not only for REY but also for a range of other elements (e.g., Li, Co, Rb, Sr, Ba, Mg, Mn, Na, P) including elements for which no or hardly any information had hitherto been available. Therefore, our dataset helps to improve the overall characterisation of BCR-670.The new data obtained from HNO3 extraction may be used to assess analytical quality in studies focusing on the chemical composition of the biological component of plants and fungi, which is a prerequisite to quantify the bioavailability and bioaccumulation of trace elements such as the REY in plants and fungi.