The overall amount of metals and their variety used for technical applications has been subject to a steep increase during the past decades and is forecast to further develop. Growing metal demand for a wide range of high technology applications, including so-called ‘green’ technologies (e.g. PGM, REEs, Te) drives the economic value of these metals and the related mining efforts. For some of these metals, anthropogenic metal fluxes have outcompeted natural biogeochemical cycles, including plate tectonics (Sen & Peucker-Ehrenbrink, 2012). Dispersion and loss, inherent to their cycle between production and use (e.g. PGM from automobile catalytic converters), limit their overall recycling rates and/or end-of-life recovery is uncertain as collection and recycling still need development (e.g. REE, Yang et al. 2017). Their presence in all environmental compartments, including remote areas, makes these metals emerging contaminants and warrants systematic surveillance. However, the geochemical backgrounds, often at ultra-trace levels, and anthropogenic contributions of the critical elements are still widely under-documented, as many analytical challenges persist. Studying exposure and effects in complex environmental matrices, including natural waters or biota, at environmentally relevant contamination levels, is a prerequisite to the assessment of exposure risks.

References

Sen, I.S., Peucker-Ehrenbrink, B., 2012. Anthropogenic disturbance of element cycles at the Earth’s surface. Environ. Sci. Technol. 46, 8601–8609. https://doi.org/10.1021/es301261x

Yang, Y., Walton, A., Sheridan, R. et al. REE Recovery from End-of-Life NdFeB Permanent Magnet Scrap: A Critical Review. J. Sustain. Metall. 3, 122–149 (2017). https://doi.org/10.1007/s40831-016-0090-4

Siderophores are important biogenic chelators produced by plants, microbes and fungi, which promote the (bio-)availability of iron and other highly-charged cations in the natural environment. The hydroxamate siderophore desferrioxamine B (DFO-B) enhances the mobilization of certain trace elements that hydrolyze easily and hence are traditionally assumed as being ‘immobile’ during water-rock interaction. Leaching of different rock material with DFO-B under ambient conditions, for example, facilitates the formation of a very pronounced positive Ce anomaly in bulk-normalized patterns and fractionates the Th-U element pair, which we tentatively attributed to an oxidation of Ce(III) to Ce(IV) and U(IV) to U(VI). We here reports results of an investigation into the effects of solution pH, fO₂ and weathering state of different rocks on the mobilization of redox-sensitive trace elements and their isotopes during water-rock interaction in presence of DFO-B. The impact of natural organic ligands on redox-sensitive elements may be largely underestimated. Siderophores are omnipresent today and may also have been present in the geological past. Our preliminary results indicate that the impact of solution pH on fractionation of redox-sensitive trace elements is rather small, but that fractionation is strongly controlled by oxygen fugacity and by the weathering state of the studied rock. Siderophores have the potential to significantly catalyze the oxidation of these elements even under strongly hypoxic conditions.

Rare Earths and Yttrium (REY) have become pivotal constituents of many high-technology products and processes. Their widespread use has led to a growing release of (“anthropogenic”) REY into the environment and hence rising concerns about their (bio)geochemical and (eco)toxicological behaviour. Yet, information on REY transfer, fractionation and bioaccumulation and -magnification in the food web is still scant.

Here, we present REY data for naturally grown duckweeds and ambient waters. Duckweeds are small, rapidly-growing macrophytes inhabiting many lentic water bodies worldwide. Duckweed is increasingly used as protein-rich animal feed and food supplement for vegans. The REY concentrations of duckweeds are in the µg/kg range (dry matter) and exceed those of ambient waters by several orders of magnitude, revealing strong bioaccumulation. Their shale-normalised (_SN) REY patterns are rather flat and show little variation regardless of sampling site and season. By contrast, the REY_SN patterns of all 0.2 µm-filtered water samples are characterised by an increase from light REY (LREY) to heavy REY (HREY) and some show large anthropogenic positive Gd_SN anomalies. Such anomalies have become common in Germany and can be attributed to the application of Gd-based contrast agents (Gd-CAs) in magnetic resonance imaging. The absence of anomalous Gd enrichment in all duckweed samples suggests that Gd-CAs are not incorporated by these macrophytes but corroborates their conservative behaviour in the environment. Moreover, partition coefficients between duckweeds and ambient waters show that the duckweeds preferentially incorporate LREY over HREY, possibly due to stronger complexation of HREY with dissolved ligands.

Rare Earths and Yttrium (REY) are widely used in many domains, resulting in anthropogenic input into the environment. However, still little is known about their uptake and bioavailability towards aquatic organisms.

We studied REY bioavailability by quantifying their concentrations in the aragonitic shells of Corbicula fluminea, which are precipitated from the extrapallial fluid (EPF) of the mussel. Both shells and ambient water samples were collected from the Elbe and Weser rivers which are known to carry anthropogenic gadolinium (Gd) from Gd-based contrast agents (Gd-CAs) applied in magnetic resonance imaging. The shells were grouped according to their size, meticulously cleaned, acid-digested and pre-concentrated before ICP-MS measurement. Analytical quality was monitored by using REY-poor reference material JLs-1.

Total REY concentrations in the shells decrease with increasing shell size, indicating that REY uptake occurred most rapidly during the juvenile age of mussels. Shale-normalized REY patterns show a continuous increase from light REY (LREY) to heavy REY (HREY) and a slightly inverse V-shape for shells from the Elbe and Weser rivers, respectively. Compared with the 0.2 µm-filtered waters from the same locations, the shells show between 2 to 4 magnitudes higher total REY concentrations. Despite significant anthropogenic Gd enrichment in the river waters, no Gd anomaly is observed in the shells suggesting long environmental half-life and poor bioavailability of the Gd-CAs. Partition coefficients between shells and water reveal a preferential uptake of LREY over HREY in mussel shells. These observations complement and corroborate the results of previous research on shells from the Rhine River.

Uranium (U) isotopes are suggested to monitor the success of (bio)remediation relying on the reduction of soluble and mobile U(VI) to less soluble U(IV)¹. However, the subsurface stability of U(IV), typically present as solid-phase non-crystalline U, may be affected by complexation or oxidation. Understanding these processes and their impact on U isotope fractionation is important to correctly interpret field U isotope signatures.

We investigated U mobilization by complexation and oxidation and measured the associated U isotope fractionation in laboratory batch experiments. Non-crystalline U(IV) was produced as the starting material by reducing a U(VI) isotope standard with Shewanella oneidensis MR-1². Subsequently, U(IV) was mobilized: 1) anoxically, with ligands (EDTA, citrate, or bicarbonate), 2) by oxidation with Fe(III), or 3) with molecular oxygen at low pH in the presence of the bacterium Acidithiobacillus ferrooxidans.

All ligands mobilized U(IV) and enriched ²³⁸U in the complexed fraction (δ²³⁸U: 0.2 to 0.6 ‰). Oxidative U mobilization both, with Fe(III) or with At. ferrooxidans biomass, resulted in insignificant U isotope fractionation. Either isotope fractionation during all involved reaction steps was very small or cancelled eachother out. The latter may be indicated by the observation of high aqueous δ²³⁸U values (~0.8 ‰) in corresponding abiotic control experiments (without biomass), which may be the result of adsorption effects after oxidative U mobilization.

(1) Bopp et. al. Environ. Sci. Technol. 2010, 44 (15), 5927–5933.

(2) Stylo et al. Environ. Sci. Technol. 2013, 47 (21), 12351–12358.