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Probing the colloid-water interface with rare earth elements


IPGP - Îlot Cuvier


Séminaires thème Système Terre

Salle 310

Rémi Marsac

Géosciences Rennes

A large variety of contaminants (e.g. metals and metalloids, radionuclides, pesticides, hydrocarbons, drugs) are currently found in surface waters, groundwaters, sediments and soils. These contaminants are of special concern given their extensive use and their impact on human health, aquatic life and environment. Their mobility and environmental transfer are largely controlled by their interaction with colloids in response to their extremely reactive surface towards contaminants. Colloids are ubiquitous environmental (nano)particles, which can be transported by waters over large distances. Regarding their small size, structural diversity and chemical heterogeneity, their characterization and the quantification of processes occurring at their surface are extremely challenging. This clearly complicates the prediction of the dynamics of contaminants in the environment. Because of their similar and coherent chemical properties, naturally occurring rare earth elements (REE) have fascinated geochemists for decades, and are now considered as emerging contaminants due to their increasing use. While all REE are most stable under +III oxidation state in low temperature surface environments, cerium is sensitive to redox conditions (Ce(III)/Ce(IV)), which drastically affects its geochemical behavior and produces so called Ce anomaly on REE patterns used as tracer of paleo-redox conditions. Furthermore, due to subtle differences in their chemical behavior, REE fractionation between water and natural colloids/particles occurs. Hence, by contrast with other contaminants whose speciation can only be determined by advanced analytical/spectroscopic techniques, systematic investigations of all REE (as REE patterns) directly show REE speciation. After a brief illustration of some of the various processes controlling (in)organic ions binding to colloids and (nano)particles, taken from the recent and ongoing work of the invited speaker (e.g. 1,2,3), this conference will show how REE can be used as probes of surface mechanisms, such as binding site heterogeneity, cation competition or redox reaction, when combining experiments and geochemical speciation modeling. Given their analogous geochemical behavior, when present under the same oxidation state, comparisons with actinides will often be made.4,5 Implementing these concepts into geochemical speciation codes might improve their use as tools to predict the environmental fate of a large range of contaminants. References (1) Jungcharoen et al. (2022) Environ. Sci.: Nano, 9 (7), 2363–2371. (2) Luo et al. (2022) Environ. Sci. Technol., 56 (4), 2378–2385. (3) Tesfa et al. (2022) Environ. Sci. Technol., 56 (14), 10494-10503. (4) Marsac et al. (2015) Chem. Geol., 400, 1–10. (5) Marsac et al. (2017) Dalton Trans., 46 (39), 13553–13561.