21HIGHLIGHTS 2020

Direct Visualization of Arsenic Binding on Green Rust Sulfate, J.P. Perez (a,b), H.M. Freeman (a,c), A.P. Brown (c), C.M. van Genuchten (d), K. Dideriksen (d), M. S ari (c), D.J. Tobler (e) and L.G. Benning (a,b,f), Environ. Sci. Technol. 54, 3297-3305 (2020);

https://doi.org/10.1021/acs.est.9b07092. (a) GFZ German Research Center for Geosciences, Potsdam (Germany) (b) Department of Earth Sciences, Freie Universität Berlin (Germany) (c) School of Chemical and Process Engineering, University of Leeds (UK)

(d) Geological Survey of Denmark and Greenland (GEUS), Copenhagen (Denmark) (e) Nano-Science Centre, Department of Chemistry, University of Copenhagen (Denmark) (f) School of Earth and Environment, University of Leeds (UK)

[1] M. Usman et al., Chem. Rev. 118, 3251-3304 (2018). [2] P.L. Smedley et al., Appl. Geochem. 17, 517-568 (2002). [3] J.P. Perez et al., Sci. Total Environ. 648, 1161-1170 (2019). [4] J.P. Perez et al., J. Hazard. Mater. 401, 123327 (2021). [5] C.M. van Genuchten et al., Environ. Sci. Process Impacts 21, 1459-1476 (2019). [6] J. Jönsson et al., Chem. Geol. 255, 173-181 (2008). [7] Y. Wang et al., Environ. Sci. Technol. 44, 109-115 (2010).

THE MISSING VOLATILES ON EARTH COULD BE TRAPPED IN ITS SOLID DEEP INTERIOR

Noble gases are important geochemical markers that allow the reconstruction of atmospheric formation processes but their behaviour at deep-Earth interior conditions are poorly constrained. High-pressure and -temperature experiments determined their distribution behaviour between solid and liquid phases, revealing that a large fraction of noble gases could be trapped in Earth s lower mantle.

PRINCIPAL PUBLICATION AND AUTHORS

REFERENCES

Shell-by-shell fitting of the Fourier-transformed extended X-ray absorption fine structure (EXAFS) spectra of the As(III)-reacted GRSO4 sample (Figure 8, red) revealed that the As(III) bound at the GRSO4 particles identified from STEM- EDX maps were predominantly adsorbed as bidentate binuclear (2C) inner-sphere complexes. The data do not suggest the formation of multi- nuclear As(III) oligomers at the GRSO4 particle edges under circum-neutral pH. Nonetheless, it is possible that their formation can be promoted at other conditions (i.e., pH, ionic strength).

Fig. 8: Arsenic K-edge XAS of As-reacted GR samples recorded at ~77 K. a) XANES spectra of the reacted solids and parasymplesite, As(III) and As(V) adsorbed onto ferrihydrite. b) k3-weighted c(k) EXAFS spectra. c) Fourier-transformed EXAFS spectra. The shell-by-shell fitting output (solid line) is superimposed on the experimental data (open circles).

Meanwhile, EXAFS analysis of As(V)-reacted GRSO4 (Figure 8, blue) indicated that most As(V) were immobilised as the newly formed parasymplesite (~87%), formed through the partial dissolution of GRSO4. The remaining As(V) were adsorbed at the GRSO4 particle edges as 2C inner-sphere surface complexes. The formation of parasymplesite provides an additional immobilisation pathway and can minimise potential As re-release in anoxic contaminated environments since it is a highly stable and poorly soluble phase.

Geochemical models predict that during Earth s formation, a large amount of volatile elements (water, carbon, nitrogen and noble gases) may

have been trapped in the deep Earth interior without degassing into the atmosphere [1]. The estimated abundance for water in the deep