Adsorption and Reduction of Arsenate during the Fe ²⁺ -Induced Transformation of Ferrihydrite

Journal Article · Thu Apr 25 00:00:00 EDT 2019 · ACS Earth and Space Chemistry

DOI:https://doi.org/10.1021/acsearthspacechem.9b00031· OSTI ID:1543341

Perez, Jeffrey Paulo H.  ^[1]; Tobler, Dominique J.  ^[2]; Thomas, Andrew N. ^[3]; Freeman, Helen M. ^[4]; Dideriksen, Knud ^[5]; Radnik, Jörg  ^[6]; Benning, Liane G. ^[7]

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1. GFZ German Research Center for Geosciences, Telegrafenberg, 14473 Potsdam, Germany, Department of Earth Sciences, Free University of Berlin, 12249 Berlin, Germany
2. Nano-Science Center, Department of Chemistry, University of Copenhagen, 2100 Copenhagen, Denmark
3. Institute of Applied Geosciences, Karlsruhe Institute of Technology, 76137 Karlsruhe, Germany
4. GFZ German Research Center for Geosciences, Telegrafenberg, 14473 Potsdam, Germany, School of Chemical and Process Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom
5. Nano-Science Center, Department of Chemistry, University of Copenhagen, 2100 Copenhagen, Denmark, Geological Survey of Denmark and Greenland (GEUS), 1350 Copenhagen K, Denmark
6. Federal Institute for Materials Research and Testing (BAM), 12205 Berlin, Germany
7. GFZ German Research Center for Geosciences, Telegrafenberg, 14473 Potsdam, Germany, Department of Earth Sciences, Free University of Berlin, 12249 Berlin, Germany, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, United Kingdom

Iron (oxyhydr)oxides play an important role in controlling the mobility and toxicity of arsenic (As) in contaminated soils and groundwaters. Dynamic changes in subsurface geochemical conditions can impact As sequestration and remobilization since the fate of As is highly dependent on the dominant iron mineral phases present and, specifically, the pathways through which these form or transform. To assess the fate of arsenate [As(V)] in subsurface settings, we have investigated the Fe²⁺-induced transformation of As(V)-bearing ferrihydrite (As(V)-FH) to more crystalline phases under environmentally relevant anoxic subsurface conditions. Specifically, we examined the influence of varying Fe²⁺_(aq)/Fe(III)_solid ratios (0.5, 1, 2) on the behavior and speciation of mineral-bound As species during the transformation of As(V)-FH to crystalline iron-bearing phases at circumneutral pH conditions. At all Fe²⁺_(aq)/Fe(III)_solid ratios, goethite (GT), green rust sulfate (GR_SO4), and lepidocrocite (LP) formed within the first 2 h of reaction. At low ratios (0.5 to 1), initially formed GR_SO4 and/or LP dissolved as the reaction progressed, and only GT and some unreacted FH remained after 24 h. At Fe²⁺_(aq)/Fe(III)_solid ratio of 2, GR_SO4 remained stable throughout the 24 h of reaction, alongside GT and unreacted As(V)-FH. Despite the fact that majority of the starting As(V)-FH transformed to other phases, the initially adsorbed As was not released into solution during the transformation reactions, and ~99.9% of it remained mineral-bound. Nevertheless, the initial As(V) became partially reduced to As(III), most likely because of the surface-associated Fe²⁺-GT redox couple. The extent of As(V) reduction increased from ~34% to ~40%, as the Fe²⁺_(aq)/Fe(III)_solid ratio increased from 0.5 to 2. Overall, our results provide important insights into transformation pathways of iron (oxyhydr)oxide minerals in As contaminated, anoxic soils and sediments and demonstrate the impact that such transformations can have on As mobility and also importantly oxidation state and, hence, toxicity in these environments.

• Research Organization: Argonne National Laboratory (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES); European Commission (EC)
• Grant/Contract Number: AC02-06CH11357; 675219
• OSTI ID: 1543341
• Alternate ID(s): OSTI ID: 1545881
• Journal Information: ACS Earth and Space Chemistry, Journal Name: ACS Earth and Space Chemistry Vol. 3 Journal Issue: 6; ISSN 2472-3452
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
arsenic
ferrihydrite
goethite
green rust
mineral transformation
XAS
XPS