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

Highlights

• REY in BCR-670 (duckweed) before and after matrix separation and preconcentration.

• Off-line preconcentration produces smoother REY_SN pattern and improved data quality.

• Highly reproducible datasets for total decomposition and simple HNO₃ extraction.

• BCR-670 data for elements without certified values, such as Mg, Mn, Rb, Sr and Ba.

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.

Introduction

Rare earth elements and yttrium (REY) are essential constituents of many high-technology products and processes, including catalysts, lasers, super magnets and pharmaceuticals (Campbell, 2014; Gonzalez et al., 2014, and references therein). Further examples of REY utilisation comprise agricultural fertilisers and food additives in animal husbandry (He et al., 2010; Liang et al., 2005). Concomitant with their increasing application is the growing emergence of anthropogenic REY in the environment (e.g., Kulaksız and Bau, 2013; Merschel et al., 2015, and references therein), which implies a greater availability of REY for potential uptake by plants and eventually their introduction into the food chain (e.g., Merschel and Bau, 2015; Schmidt et al., 2019). Calcium and especially light REY (LREY: La–Sm) share similar ionic radii (Shannon, 1976). Hence, REY³⁺ may compete with and replace Ca²⁺ in biological systems (Thomas et al., 2014), which highlights the need for a reasonably good understanding of the general environmental behaviour of the REY, their bioavailability and bioaccumulation potential, and their possible (eco)toxicological effects. However, the topic has been subject to comparably little research in the past and, thus, our knowledge of the biogeochemistry of REY and their impact on plants is still limited (Gonzalez et al., 2014; Hu et al., 2004).

Trace element concentrations in plants are generally rather low (Tyler, 2004). Prior to the development of analytical methods with sufficient sensitivity, it was, therefore, often not possible to measure REY in plant matrices (Tyler, 2004; Markert and De Li, 1991). Nowadays, quadrupole inductively coupled plasma mass spectrometry (ICP-MS) is the favoured method for the determination of REY in plant materials, resulting from the high sensitivity and selectivity of ICP-MS, the wide linear concentration range that is covered, and the possibility of simultaneous analysis of many elements and different isotopes (Bulska et al., 2012; Dressler et al., 2007). However, despite numerous technical advances, the reliable determination of REY in biological samples remains challenging. Rare earth elements and yttrium are characterised by very similar chemical and physical properties, which results in their very coherent behaviour in most natural systems. Their determination in geological, environmental and biological sample material is affected by isobaric and polyatomic interferences during ICP-MS analysis that necessitate careful corrections (Dressler et al., 2007; Dulski, 1994; Krachler et al., 2002; Schmidt et al., 2019). Moreover, a matrix separation and preconcentration procedure is frequently required to precede spectrometric analysis due to very low REY levels in samples and potential interferences from matrix elements (e.g., Bau et al., 2010; Merschel and Bau, 2015; Zocher et al., 2018; Schmidt et al., 2019; Barrat et al., 2020).

Progress in research on REY in plants is also hampered by the lack of well-defined plant-based certified reference materials (CRMs) for REY, which can mainly be attributed to the fact that potential users, such as legislators and commercial laboratories, until recently showed little interest in such CRMs (Kramer et al., 2002).

Existing reference materials of plant origin are often not certified for REY or certified for only a few REY (Bulska et al., 2012), which is in marked contrast to many geological reference standards. An exception is the CRM BCR-670 (aquatic plant powder; Lemna minor) from the Institute for Reference Materials and Measurements (IRMM; Geel, Belgium). For BCR-670 certified values are available for the whole suite of REY (and Sc, Th, and U) (Kramer et al., 2001). The plant material for BCR-670 was collected in the canals of the Beemster polder in The Netherlands (Kramer et al., 2002), i.e., BCR-670 consists of naturally grown plant material. Due to the scarcity of such CRMs, it is of particular importance that the few CRMs for which complete REY data are certified, such as BCR-670, are well-characterised and provide reliable data. Despite the thorough homogenisation and certification campaigns CRMs undergo before certified values are accepted, it is hence essential and in the interest of all potential users that the (certified) values of such CRMs are (re)produced within as many different studies as possible. However, the number of published REY data sets of BCR-670 is still small. For example, the online database GeoReM (Jochum et al., 2005; accessed on Feb. 1st, 2021) contains only a small number of individual REY data for BCR-670, ranging from a minimum of only five literature values for Tm to a maximum of thirteen literature values for Nd. Moreover, only a few studies provide BCR-670 data for REY obtained after application of matrix separation and preconcentration techniques (e.g., Bulska et al., 2012; Danko et al., 2008; Dybczyński et al., 2007).

Apart from the certified values, indicative values of only eight other elements are provided for BCR-670 (Kramer et al., 2001), and do not include information on, for example, essential major plant elements, such as Mg and P. The current database of certified values also lacks elements that could be indicative of detrital silicates, such as Zr and Hf, which may be intimately associated with the biological sample material and affect the concentrations determined (e.g., Schier et al., 2021). Accordingly, determinations of element concentrations other than REY are crucial for a better overall characterisation of BCR-670.

Here, we present major, minor and trace element concentrations of BCR-670 for two different digestion methods with a focus on REY concentrations obtained after matrix separation and preconcentration. While one of the two digestion methods achieves full decomposition of the sample powder, the other is not able to decompose (alumino)silicate and oxide phases, but rather extracts what is associated with the biological material in addition to bioavailable elements that are only loosely bound to particles. Inorganic geological material is often present in biological samples like plants or mushrooms, because substrate particles from soil or atmospheric dust may be attached to or incorporated into the organic component. Depending on the aim of an investigation, however, it may either be important to know about the composition of the total sample (e.g., when studying animal fodder that is digested by livestock) or only of the biological component (e.g., when studying bioavailability and/or bioaccumulation). For both types of (biogeo-)chemical study, analytical quality control is essential and reference materials and analytical protocols are needed. Our aim is to contribute to the improvement of BCR-670 as a reliable reference material not only for REY but for a larger set of elements, and to provide a means for approximating in a reproducible way the amount of REY “truly” incorporated into plant tissue, i.e., the biological material.