Letter pubs.acs.org/journal/estlcu

Structure of Sulfate Adsorption Complexes on Ferrihydrite Mengqiang Zhu,†,‡ Paul Northrup,§ Chenyang Shi,∥ Simon J. L. Billinge,∥,⊥ Donald L. Sparks,@ and Glenn A. Waychunas*,† †

Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States Department of Ecosystem Science and Management, University of Wyoming, Laramie, Wyoming 82071, United States § Department of Geosciences, Stony Brook University, Stony Brook, New York 11794, United States ∥ Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States ⊥ Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, United States @ Department of Plant and Soil Sciences and Delaware Environmental Institute, University of Delaware, Newark, Delaware 19716, United States ‡

S Supporting Information *

ABSTRACT: Sulfate adsorption on mineral surfaces is an important environmental chemical process. However, the structure of sulfate adsorption complexes has remained uncertain. In this study, we have determined the S−Fe distance of sulfate inner-sphere adsorption complexes on iron (Fe) oxyhydroxide (ferrihydrite) surfaces under air-dried conditions using sulfur K-edge extended X-ray absorption fine structure spectroscopy and differential X-ray pair distribution function analysis. Both approaches indicate that the S− Fe interatomic distance of the sulfate adsorption complexes is 3.22−3.25 Å, suggesting that sulfate forms bidentate−binuclear adsorption complexes on ferrihydrite surfaces. Outer-sphere complexes are also observed using infrared spectroscopic analysis. This finding clarifies the long-standing debate over the structure of sulfate adsorption complexes and has important implications for understanding the chemistry of sulfate at environmental interfaces.

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INTRODUCTION Sulfate plays an important environmental role, particularly in sulfate-enriched acidic soils and acid mine drainage-impacted areas where sulfate templates the formation of schwertmannite, a highly reactive material for the sorption of toxic elements,1−4 or enhances or inhibits adsorption of metals, radionuclides, anions, and organic molecules on mineral surfaces.5−12 Sulfate adsorption on mineral surfaces also affects the atmospheric chemistry of aerosols as sulfate coatings on mineral dust aerosols significantly weaken their ice nucleation abilities.13 Mechanistic understanding of these chemical processes requires knowledge of how sulfate interacts with surfaces and, specifically, the binding geometry of sulfate surface complexes. This information is also critical to quantifications of sulfate adsorption on mineral surfaces using surface complexation modeling (SCM).14 Infrared (IR) spectroscopy has been the major technique for characterization of sulfate surface complexes on ferric iron (Fe) oxides, one group of the most important environmental minerals, on the basis of the number of observed S−O IRactive vibrational bands.2,12,15−28 It has been suggested that each type of sulfate complex has its own unique symmetry and, accordingly, a characteristic number of sulfate vibration bands. On the basis of this conjecture, IR analyses have revealed that © 2013 American Chemical Society

sulfate forms both inner-sphere and outer-sphere complexes on iron oxides and that their relative proportions depend on pH, ionic strength, surface coverage, and hydration.19,20 However, a consensus has not been reached about the nature of the sulfate inner-sphere complexes, i.e., monodentate−mononuclear (MM) versus bidentate−binuclear (BB) complexes, because of the uncertainties in interpreting the IR spectral features.18,22,24−27 The difficulty comes from the fact that sulfate can undergo concomitant protonation and H-bonding reactions that also alter sulfate symmetry and hence affect the number of observed vibrational bands. S K-edge X-ray absorption near edge structure (XANES) spectroscopy has potential for characterizing sulfate surface complexes as the formation of inner-sphere complexes on Fe oxides gives rise to a pre-edge feature because of orbital hybridization.2,28−30 Nevertheless, pre-edge analysis has not been able to further verify the structure of the inner-sphere complexes, although the positions and amplitudes of the pre-edge peaks for ferric iron Received: Revised: Accepted: Published: 97

August 27, 2013 September 29, 2013 September 30, 2013 September 30, 2013 dx.doi.org/10.1021/ez400052r | Environ. Sci. Technol. Lett. 2014, 1, 97−101

Environmental Science & Technology Letters

Letter

Figure 1. Sulfur K-edge EXAFS spectra and their fits for the three sulfate adsorption samples and the solid and solution references: k2-weighted spectra (a) and the Fourier transform magnitude (b) and imaginary components (c).

Table 1. Structural Parameters of Sulfate Adsorbed on Ferrihydrite and Reference Samples Determined by EXAFS and d-PDF Analysesa EXAFS Spectroscopy S−O CN

σ (Å )

d (Å)

CN

4 4d 4 4.1 (0.9) 4.1 (0.7) 4.1 (0.7)

0.0003 (0.001)