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Title: Quantification of Coexisting Inner- and Outer-Sphere Complexation of Sulfate on Hematite Surfaces

Abstract

Sulfate adsorption on hematite surfaces controls sulfate mobility and environmental behavior but whether sulfate forms both inner- and outer-sphere complexes and the type of the inner-sphere complexes remain contentious. With ionic strength tests and S K-edge X-ray absorption near-edge structure spectroscopy, we show that sulfate forms both outer- and inner-sphere complexes on hematite surfaces. Both S K-edge extended X-ray absorption fine structure spectroscopy and the differential pair distribution function analyses determine the S–Fe interatomic distance (~3.24 Å) of the inner-sphere complex, suggesting bidentate-binuclear complexation. A multivariate curve resolution (MCR) analysis of the attenuated total reflection–Fourier-transform infrared spectra of adsorption envelope samples shows that increasing ionic strength does not affect the inner-sphere but decreases the outer-sphere complex adsorption loading, consistent with the ionic strength effect. The extended triple layer model directly and successfully models the MCR-derived inner- and outer-sphere surface loadings at various ionic strengths, indicating weaker sulfate inner-sphere complexation on hematite than on ferrihydrite surfaces. Results also show that sample drying, lower pH, and higher ionic strength all favor sulfate inner-sphere complexation, but the hematite particle size does not affect the relative proportions of the two types of complexes. Sulfate adsorption kinetics show increasing ratio of exchanged OH– to adsorbedmore » sulfate with time, attributed to inner- and outer-sphere complexation dominating at different adsorption stages and to the changes of the relative abundance of surface OH and H 2O groups with time. This work clarifies sulfate adsorption mechanisms on hematite and has implications for understanding sulfate availability, behavior and fate in the environment. Our work suggests that the simple macroscopic ionic strength test correlates well with directly measured outer-sphere complexes.« less

Authors:
 [1];  [2];  [3];  [4]; ORCiD logo [4]; ORCiD logo [5]
  1. Huazhong Agricultural Univ., (China); Univ. of Wyoming, Laramie, WY (United States)
  2. Louisiana State Univ., Baton Rouge, LA (United States)
  3. Univ. of Saskatchewan, Saskatoon, SK (Canada)
  4. Huazhong Agricultural Univ., (China)
  5. Univ. of Wyoming, Laramie, WY (United States)
Publication Date:
Research Org.:
Argonne National Laboratory (ANL), Argonne, IL (US). Advanced Photon Source (APS)
Sponsoring Org.:
USDOE
OSTI Identifier:
1434729
Grant/Contract Number:  
AC02-06CH11357
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
ACS Earth and Space Chemistry
Additional Journal Information:
Journal Volume: 2; Journal Issue: 4; Journal ID: ISSN 2472-3452
Publisher:
American Chemical Society
Country of Publication:
United States
Language:
ENGLISH
Subject:
sulfate; hematite; inner- and outer-sphere complexation; S K-edge X-ray absorption spectroscopy; differential pair distribution function analysis; ATR-FTIR spectroscopy; MCR analysis; surface complexation modeling

Citation Formats

Wang, Xiaoming, Wang, Zimeng, Peak, Derek, Tang, Yadong, Feng, Xionghan, and Zhu, Mengqiang. Quantification of Coexisting Inner- and Outer-Sphere Complexation of Sulfate on Hematite Surfaces. United States: N. p., 2018. Web. doi:10.1021/acsearthspacechem.7b00154.
Wang, Xiaoming, Wang, Zimeng, Peak, Derek, Tang, Yadong, Feng, Xionghan, & Zhu, Mengqiang. Quantification of Coexisting Inner- and Outer-Sphere Complexation of Sulfate on Hematite Surfaces. United States. doi:10.1021/acsearthspacechem.7b00154.
Wang, Xiaoming, Wang, Zimeng, Peak, Derek, Tang, Yadong, Feng, Xionghan, and Zhu, Mengqiang. Fri . "Quantification of Coexisting Inner- and Outer-Sphere Complexation of Sulfate on Hematite Surfaces". United States. doi:10.1021/acsearthspacechem.7b00154. https://www.osti.gov/servlets/purl/1434729.
@article{osti_1434729,
title = {Quantification of Coexisting Inner- and Outer-Sphere Complexation of Sulfate on Hematite Surfaces},
author = {Wang, Xiaoming and Wang, Zimeng and Peak, Derek and Tang, Yadong and Feng, Xionghan and Zhu, Mengqiang},
abstractNote = {Sulfate adsorption on hematite surfaces controls sulfate mobility and environmental behavior but whether sulfate forms both inner- and outer-sphere complexes and the type of the inner-sphere complexes remain contentious. With ionic strength tests and S K-edge X-ray absorption near-edge structure spectroscopy, we show that sulfate forms both outer- and inner-sphere complexes on hematite surfaces. Both S K-edge extended X-ray absorption fine structure spectroscopy and the differential pair distribution function analyses determine the S–Fe interatomic distance (~3.24 Å) of the inner-sphere complex, suggesting bidentate-binuclear complexation. A multivariate curve resolution (MCR) analysis of the attenuated total reflection–Fourier-transform infrared spectra of adsorption envelope samples shows that increasing ionic strength does not affect the inner-sphere but decreases the outer-sphere complex adsorption loading, consistent with the ionic strength effect. The extended triple layer model directly and successfully models the MCR-derived inner- and outer-sphere surface loadings at various ionic strengths, indicating weaker sulfate inner-sphere complexation on hematite than on ferrihydrite surfaces. Results also show that sample drying, lower pH, and higher ionic strength all favor sulfate inner-sphere complexation, but the hematite particle size does not affect the relative proportions of the two types of complexes. Sulfate adsorption kinetics show increasing ratio of exchanged OH– to adsorbed sulfate with time, attributed to inner- and outer-sphere complexation dominating at different adsorption stages and to the changes of the relative abundance of surface OH– and H2O groups with time. This work clarifies sulfate adsorption mechanisms on hematite and has implications for understanding sulfate availability, behavior and fate in the environment. Our work suggests that the simple macroscopic ionic strength test correlates well with directly measured outer-sphere complexes.},
doi = {10.1021/acsearthspacechem.7b00154},
journal = {ACS Earth and Space Chemistry},
issn = {2472-3452},
number = 4,
volume = 2,
place = {United States},
year = {2018},
month = {3}
}

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