Iron and Arsenic Speciation During As(III) Oxidation by Manganese Oxides in the Presence of Fe(II): Molecular-Level Characterization Using XAFS, Mössbauer, and TEM Analysis
[3];
[5];
- Univ. of Delaware, Newark, DE (United States)
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
- Johns Hopkins Univ., Baltimore, MD (United States)
- Brookhaven National Lab. (BNL), Upton, NY (United States)
- Univ. of Delaware, Newark, DE (United States); Nanjing Univ. (China)
The redox state and speciation of the metalloid arsenic (As) determine its toxicity and mobility. Knowledge of biogeochemical processes influencing the As redox state is therefore important to understand and predict its environmental behavior. Many previous studies examined As(III) oxidation by various Mn-oxides, but little is known concerning environmental influences (e.g., coexisting ions) on the process. As such, in this study we investigated the mechanisms of As(III) oxidation by a poorly crystalline hexagonal birnessite (δ-MnO2) in the presence of dissolved Fe(II) using X-ray absorption spectroscopy, Mössbauer spectroscopy, and transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS). The As K-edge X-ray absorption near edge spectroscopy (XANES) analysis revealed that, at low Fe(II) concentration (100 μM), As(V) was the predominant As species on the solid phase, whereas at higher Fe(II) concentrations (200–1000 μM), both As(III) and As(V) were sorbed on the solid phase. As K-edge extended X-ray absorption fine structure spectroscopy (EXAFS) analysis showed an increasing As–Mn/Fe distance over time, indicating As prefers to bind with the newly formed Fe(III)-(hydr)oxides. Both As(III) and (V) adsorbed on Fe(III)-(hydr)oxides as a bidentate binuclear corner-sharing complex. Both Mössbauer and TEM-EDS investigations demonstrated that oxidized Fe(III) products formed during Fe(II) oxidation by δ-MnO2 were predominantly ferrihydrite-, goethite-, and ferric arsenate-like compounds. However, Fe EXAFS analysis also suggested the formation of a small amount of lepidocrocite. The Mn K-edge XANES data indicated that As(III) oxidation occurs as a two electron transfer with δ-MnO2 and that the observed Mn(III) is due to conproportionation of surface-sorbed Mn(II) with Mn(IV) in the δ-MnO2 structure. This study reveals that the mechanisms of As(III) oxidation by δ-MnO2 in the presence of Fe(II) are very complex, involving many simultaneous reactions, and the formation of Fe(III)-(hydr)oxides plays a very important role in reducing As mobility.
- Research Organization:
- Pacific Northwest National Laboratory (PNNL), Richland, WA (United States). Environmental Molecular Sciences Laboratory (EMSL); Brookhaven National Laboratory (BNL), Upton, NY (United States). National Synchrotron Light Source
- Sponsoring Organization:
- National Science Foundation (NSF); 1000 Youth Talent Program for Outstanding Young Scientists; USDOE Office of Science (SC), Basic Energy Sciences (BES); USDOE Office of Science (SC), Biological and Environmental Research (BER)
- Grant/Contract Number:
- AC05-76RL01830; EPS0814251; AC02-98CH10886
- OSTI ID:
- 1427920
- Report Number(s):
- PNNL-SA-123255; 44685; KP1704020
- Journal Information:
- ACS Earth and Space Chemistry, Vol. 2, Issue 3; ISSN 2472-3452
- Publisher:
- American Chemical SocietyCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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