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Journal Article

Direct Visualization of Arsenic Binding on Green Rust Sulfate

Authors
/persons/resource/jpperez

Perez,  J.P.H.
3.5 Interface Geochemistry, 3.0 Geochemistry, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum;

/persons/resource/freeman

Freeman,  Helen
3.5 Interface Geochemistry, 3.0 Geochemistry, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum;

Brown,  Andy P.
External Organizations;

van Genuchten,  Case M.
External Organizations;

Dideriksen,  Knud
External Organizations;

S’ari,  Mark
External Organizations;

Tobler,  Dominique J.
External Organizations;

/persons/resource/benning

Benning,  Liane G.
3.5 Interface Geochemistry, 3.0 Geochemistry, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum;

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Citation

Perez, J., Freeman, H., Brown, A. P., van Genuchten, C. M., Dideriksen, K., S’ari, M., Tobler, D. J., Benning, L. G. (2020): Direct Visualization of Arsenic Binding on Green Rust Sulfate. - Environmental Science and Technology, 54, 6, 3297-3305.
https://doi.org/10.1021/acs.est.9b07092


Cite as: https://gfzpublic.gfz-potsdam.de/pubman/item/item_5001694
Abstract
“Green rust” (GR), a redox-active Fe(II)–Fe(III) layered double hydroxide, is a potential environmentally relevant mineral substrate for arsenic (As) sequestration in reduced, subsurface environments. GR phases have high As uptake capacities at circum-neutral pH conditions, but the exact interaction mechanism between the GR phases and As species is still poorly understood. Here, we documented the bonding and interaction mechanisms between GR sulfate and As species [As(III) and As(V)] under anoxic and circum-neutral pH conditions through scanning transmission electron microscopy (STEM) coupled with energy-dispersive X-ray (EDX) spectroscopy and combined it with synchrotron-based X-ray total scattering, pair distribution function (PDF) analysis, and As K-edge X-ray absorption spectroscopy (XAS). Our highly spatially resolved STEM–EDX data revealed that the preferred adsorption sites of both As(III) and As(V) are at GR crystal edges. Combining this data with differential PDF and XAS allowed us to conclude that As adsorption occurs primarily as bidentate binuclear (2C) inner-sphere surface complexes. In the As(III)-reacted GR sulfate, no secondary Fe–As phases were observed. However, authigenic parasymplesite (ferrous arsenate nanophase), exhibiting a threadlike morphology, formed in the As(V)-reacted GR sulfate and acts as an additional immobilization pathway for As(V) (∼87% of immobilized As). We demonstrate that only by combining high-resolution STEM imaging and EDX mapping with the bulk (differential) PDF and extended X-ray absorption fine structure (EXAFS) data can one truly determine the de facto As binding nature on GR surfaces. More importantly, these new insights into As–GR interaction mechanisms highlight the impact of GR phases on As sequestration in anoxic subsurface environments.