Several trace metals have gained societal and economic importance due to their limited and uncertain supply as well as their crucial role in high-tech applications, including various enabling technologies. These Technology-Critical Elements (TCE) are now recognized in the critical raw materials lists that are regularly published by entities such as the European Union and the United States Geological Survey. Notable examples are the rare earth elements, the platinum group elements, antimony, gallium, germanium and scandium.

In natural waters and soil solutions, many of these metals are found, due to their strong particle-reactivity, only in extremely minute concentrations, ranging from ng kg^-1 to pg kg^-1. However, their booming application in diverse technologies leads to a strongly increasing input from anthropogenic sources into the environment. At the same time, we face considerable knowledge gaps in their mobility, reactivity and bioavailability. The various chemical forms in which these metals are employed further complicate sound predictions on the mobility and bioavailability of these emerging contaminants in the environment. In this contribution, I will summarize the state of the art and challenges in constraining natural background concentrations as well as anthropogenic contaminations and will showcase research on their mobility, reactivity and bioavailability, with a special emphasis on the potential relevance of natural biomolecules (metallophores) for TCE mobilization and a potential rehabilitation. Such ligands are naturally produced by a range of microorganisms, plants and fungi to cope with the scarcity of nutrient metals, but may also actively promote the mobilization of TCE in the environment.

The ever-increasing application of rare earth elements and yttrium (REY) in diverse sectors has led to their emergence as environmental contaminants. In this study of the Oder and Vistula rivers in Poland, total concentrations of REY (ƩREY) in the dissolved phase (0.2 µm-filtered water samples) decrease from the river’s upper reaches (98.1 and 139 ng/kg) to mid-sections (52.4 and 48.4 ng/kg) but rise again near the estuaries (65.1 and 61.4 ng/kg). The upper reaches exhibit high levels of REY due to the impacts of high population density, strong industrial activity, and the input of acid mine drainage. The rise at their lower reaches indicates input from additional sources, possibly from phosphogypsum tailings. The Gd anomalies (Gd_SN/Gd_SN^*: 4.92 - 44.6) found at the studied sites reveal various Polish regions as hotspots of anthropogenic Gd microcontamination (where >95% of Gd is of anthropogenic origin), resulting in a significant flux into the Baltic Sea. Ultrafiltrates show HREY enrichment over LREY in the truly dissolved REY pool (<1 kDa), enhancing the trend seen in the dissolved phase as LREY preferentially associate with nanoparticles and colloids (NPCs: 0.2 µm - 1 kDa). The anthropogenic Gd is related to MRI contrast agents released with the effluents of wastewater treatment plants, displays negligible particle-reactivity, and resides almost exclusively in the truly dissolved REY fraction. Our results underscore the urgency of monitoring and understanding the anthropogenic impacts causing elevated REY levels and positive Gd anomalies in the Oder and Vistula rivers to protect the environment.

The reactivity and fate of Technology Critical Elements (TCEs) in rivers are still widely unknown. We present yearly monitoring results for Rare Earth Elements (REEs) and less known TCEs such as tellurium (Te) and Thallium (Tl), for three contrasting German watersheds: the Rhine, the Neckar and the Danube rivers. Monthly samples of water (0.45 vs 0.02 µm) and suspended particulate matter were analysed directly and after digestions via ICP-MS (iCAP series, Thermo®). Results show contrasting behavior between watersheds. For instance, only the Neckar River shows increased downstream transport of REEYs in the dissolved phase, reflecting the impact of dams along its course. In contrast, only the Danube River shows a mixed signal between geogenic and anthropogenic Gd, the latter disappearing completely during flood conditions. In any case, flood conditions enhance an overall transport of REEs in the truly dissolved phase for all rivers, increasing the amount released into the system, decreasing the log Kd values and causing a characteristic Tm negative anomaly. The opposite occurs for Tl and anthropogenic Gd, suggesting for Tl an anthropogenically dominated regime, especially at the Neckar and Danube rivers. All this information provides insights for developing scenarios for potential risk assessment of current and future anthropogenic releases of TCEs in aquatic environments.

Acknowledgements: This work was funded as part of the Excellence Strategy of the German Federal and State Governments. The authors also acknowledge the extensive contribution of LUBW (Germany) and AUE (Switzerland) for the collection of water and suspended sediment samples.

Rare Earth Elements and Yttrium (REY) are considered Technology-Critical Elements and contaminants of emerging concern due to their wide application but limited supply. The growing use of REY poses an environmental threat due to increasing entry into natural surface waters and the food web. Therefore, it is crucial to better understand speciation and complexation of REY under a wide variety of natural surface water conditions. Saline-alkaline lakes are present on all continents, cover up to 20 % of the area of all lakes worldwide¹ and up to 75 % in the East African Rift². Here we report REY from (saline) alkaline lakes and hotsprings from the Kenyan part of the East African Rift. The samples are characterized by alkaline pH values ranging between 7.24 and 9.91, which increases with increasing electrical conductivity as well as high DOC in lake waters (up to ~98 mg/L). Shale-normalized REY patterns show typical features of alkaline lake waters such as strong HREY enrichment over LREY. Preliminary inorganic speciation modelling suggests that carbonate is the dominant inorganic ligand. However, the alkaline rift valley lakes do not show the positive Ce anomaly that was previously reported for other alkaline lakes. The high DOC content of the lake waters (i.e. low molecular weight organic matter) might suppress typical Ce redox behavior. In marked contrast, alkaline hotsprings show similarly strong HREY over LREY enrichments, but positive Ce anomalies.

¹Wurtsbaugh et al. (2017) Nat. Geosci. 10 (11), 816.

²Butturini et al. (2020) Water Research, 173

The potential mobility and availability of trace elements in sediments/soils is commonly assessed via selective extractions. In this work, three contrasting sediments, i.e., Rhine riverbank sediment, alkaline volcanic heavy mineral sand, and mine tailings from a Mississippi-valley type ore deposit, are extracted with an oxalate-based solution and compared in terms of trace element mobility from both amorphous and crystalline Fe-Mn-carrier phases. Major and trace elements are characterized in the supernatants via ICP-OES and (HG-)ICP-MS, whereas the residual material is characterized via XRD. The obtained Fe/Si and Mn/As ratios highlight distinct sediment behavior over time. Overall, our results show a significant kinetic component, increasing concentrations for some elements like Fe, Mn, Ti, Si, Al, Mg, Sr, Na, Ge, Sc, and HREE (for the black sand), and decreasing for Ca, K, Y, REE after reaching supersaturation, e.g., formation of a characteristic whitish precipitate. Even though the protocol was successful at destroying most of the Fe-Mn(-Ti) minerals, the crystallization and sedimentation of oxalate salts may be due to the absence of stabilizing agents in solution. A similar situation occurs in the case of urolithiasis, where supersaturation of calcium oxalate in the kidneys, if not inhibited, promotes precipitation. Thus, the findings of this study not only highlight the environmental mobility of technologically critical elements in aquatic systems, but also their potential retention/accumulation in kidney stones.

Acknowledgements:

This work was funded as part of the Excellence Strategy of the German Federal and State Governments.

Rare earth elements (REEs), a group of elements with similar physical properties and coherent geochemical behaviour, are widely used as proxies for many biogeochemical processes. Interpretation of REE data is primarily based on distribution patterns in normalised graphs. Therefore, missing REEs can alter the appearance of the REE patterns and, consequently, the interpretation. Data for certain REEs may be missing for various reasons, e.g., they could not be measured (neutron activation analysis, isotope dilution techniques), the measurement was near to or below the limits of quantification or certain REEs were used as spikes. To address this, we introduce a novel method that leverages REEs' characteristically smooth distribution patterns to reconstruct missing REE data. Our approach provides accurate and precise REE data (<10% deviation) well within common analytical uncertainties of modern analytical techniques (e.g., ICP-MS). The accuracy and precision were determined using a method verification dataset of >13,000 mafic and ultramafic rock samples. The re-modelled REE data can be used to interpret the overall pattern and to quantitatively determine anomalies, one of the most important tools in REE research. Furthermore, our method offers new opportunities for REE data handling and processing, enabling researchers to assess the usability and reliability of REE data, whether self-produced or data from journal publications and data repositories. We implemented our method into our software tool GeoArmadillo, which facilitates geochemical data processing, evaluation and assessment.