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Quantification of Coexisting Inner- and Outersphere Complexation of Sulfate on Hematite Surfaces Xiaoming Wang, Zimeng Wang, Derek Peak, Yadong Tang, Xionghan Feng, and Mengqiang Zhu ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.7b00154 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 4, 2018

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ACS Earth and Space Chemistry

1

Quantification of Coexisting Inner- and Outer-sphere Complexation of Sulfate

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on Hematite Surfaces

3

Xiaoming Wang,†,‡ Zimeng Wang,§ Derek Peak,∥ Yadong Tang,‡

4

Xionghan Feng,‡ Mengqiang Zhu*,†

5 6 7

†

Department of Ecosystem Science and Management, University of Wyoming, Laramie, WY

82071 ‡

Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtze River),

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Ministry of Agriculture, College of Resources and Environment, Huazhong Agricultural University,

9

Wuhan 430070 China

10

§

Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge,

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LA 70803

12

∥

13

Canada

Department of Soil Science, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 2V3

14 15 16 17 18

*Corresponding author: Mengqiang Zhu

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Tel: +1 307-766-5523

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Email: [email protected]

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ACS Paragon Plus Environment

ACS Earth and Space Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

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Sulfate adsorption on hematite surfaces controls sulfate mobility and environmental behavior,

24

but whether sulfate forms both inner- and outer-sphere complexes and the type of the inner-sphere

25

complexes remain contentious. With ionic strength tests and S K-edge XANES spectroscopy, we

26

show that sulfate forms both outer- and inner-sphere complexes on hematite surfaces. Both S

27

K-edge EXAFS spectroscopy and the differential pair distribution function (d-PDF) analyses

28

determine the S-Fe interatomic distance (~ 3.24 Å) of the inner-sphere complex, suggesting

29

bidentate-binuclear complexation. A multivariate curve resolution (MCR) analysis of the

30

ATR-FTIR spectra of adsorption envelope samples shows that increasing ionic strength does not

31

affect the inner-sphere but decreases the outer-sphere complex adsorption loading, consistent with

32

the ionic strength effect. The Extended Triple Layer Model (ETLM) directly and successfully

33

models the MCR-derived inner- and outer-sphere surface loadings at various ionic strengths,

34

indicating weaker sulfate inner-sphere complexation on hematite than on ferrihydrite surfaces.

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Results also show that sample drying, lower pH, and higher ionic strength all favor sulfate

36

inner-sphere complexation, but the hematite particle size does not affect the relative proportions of

37

the two types of complexes. Sulfate adsorption kinetics show increasing ratio of exchanged OH- to

38

adsorbed sulfate with time, attributed to inner- and outer-sphere complexation dominating at

39

different adsorption stages and to the changes of the relative abundance of surface OH- and H2O

40

groups with time. This work clarifies sulfate adsorption mechanisms on hematite and has

41

implications for understanding sulfate availability, behavior and fate in the environment. Our work

42

suggests that the simple macroscopic ionic strength test correlates well with directly-measured

43

outer-sphere complexes. 2


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TOC Figure

44

45 46

KEYWORDS: sulfate; hematite; inner- and outer-sphere complexation; S K-edge X-ray absorption

47

spectroscopy; differential pair distribution function analysis; ATR-FTIR spectroscopy; MCR

48

analysis; surface complexation modeling

3



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INTRODUCTION

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Sulfate is a common and abundant oxyanion and nutrient in the environment1. Hematite

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(α-Fe2O3) is the most thermodynamically stable Fe oxide polymorph, and is common and abundant

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in soils and sediments, especially in tropical and subtropical regions2. Sulfate adsorption on

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hematite affects sulfate availability in tropical and subtropical soils, and can promote or hinder

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adsorption of other anions and cations on hematite surfaces3-9. Hematite is also a common mineral

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in aerosols, and the association of sulfate with hematite affects physicochemical properties of

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aerosols10,11. In addition, hematite can catalyze degradation of organic pollutants12,13, and the

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catalytic activity may be influenced by sulfate adsorption on hematite surfaces. Thus, it is important

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to clarify, quantify, and ultimately predict the adsorption mechanisms of sulfate on hematite.

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The complexation mechanisms of sulfate on mineral surfaces reported in the existing literatures

60

are summarized in Table 1, including inner- or outer-sphere complexation and the type of

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inner-sphere complexes. Sulfate forms a mixture of inner- and outer-sphere complexes on surfaces

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of a variety of minerals, including goethite, ferrihydrite, magnetite, and gibbsite/aged alumina

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(Table 1)5,14-21, which is dictated by the relative magnitude of the sulfate hydration and binding

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energies5. Surprisingly, hematite behaves differently from those minerals in adsorbing sulfate, and

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studies have shown that sulfate adsorbs only as inner-sphere complexes on hematite surfaces at pH

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3 - 622,23, possibly with minor outer-sphere complexes at higher pH22. Similar to sulfate, selenate is

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reported to form only inner-sphere complexes on hematite but both outer- and inner-sphere

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complexes on goethite and ferrihydrite24. In addition, sulfate and selenate inner- and/or outer-sphere

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complexation in most previous studies (Table 1) was described only qualitatively.

70 4


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Table1. The adsorption mechanisms of sulfate on various minerals. As selenate has similar

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chemical properties with sulfate, the complexation mechanisms of selenate are also included for

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comparison with those of sulfate. Anions

Minerals

Techniques FTIR and DLM

Hematite

Raman DFT and FTIR

SO42-

Goethite

Ferrihydrite Schwertmannite Magnetite Iron oxides ɣ-Al2O3 Hematite

SeO42-

Goethite

Ferrihydrite

FTIR FTIR FTIR ETLM and CD-MUSIC XAFS ETLM FTIR XAFS FTIR FTIR Raman and FTIR XAFS and FTIR XAFS XAFS XAFS FTIR FTIR XAFS XAFS

Hydration Wet Dry Wet Wet Dry Dry Wet Wet Wet

Complexes Monodentate Monodentate bisulfate or bidentate sulfate Bidentate Bidentate or monodentate Bidentate and/or monodentate bisulfate Bidentate-binuclear Bidentate and monodentate Inner- and outer-sphere Monodentate; Outer-sphere

Ref. 22,25

23

26

27 28,29 14,16,30 15,17,19, 31

Wet and dry Wet Wet Wet and dry Wet Dry Wet

Bidentate; Outer-sphere Monodentate; Outer-sphere Inner- and outer-sphere Bidentate; Outer-sphere Monodentate; Outer-sphere Bidentate Monodentate; Outer-sphere

Wet

Monodentate

Wet Wet and dry Wet Wet Wet Wet and dry Wet

Outer-sphere Bidentate Inner- and outer-sphere Inner- and outer-sphere Bidentate Bidentate Inner- and outer-sphere

5,9 19 21 3 20 27 16

24

32 33 24 16 34 33 24

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Additionally, the structure of sulfate inner-sphere complexes on hematite is not well

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characterized (Table 1). Infrared spectroscopy suggests that both sulfate and selenate predominantly

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form monodentate complexes with or without minor bidentate complexes on hematite under wet

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conditions22,24,28, and sample drying converts monodentate sulfate complexes to either monodentate 5



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bisulfate or bidentate complexes22,25,26. In contrast, Raman spectroscopy suggests formation of only

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bidentate complexes under wet conditions23. As more direct approaches for determining structures

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of surface complexes, S K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy

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and differential X-ray atomic pair distribution function (d-PDF) analyses indicated that sulfate

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forms bidentate-binuclear surface complexes on ferrihydrite surfaces and in the schwertmannite

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structure under both wet and dry conditions, and drying converts outer-sphere to inner-sphere

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complexes3,5,9. These studies further showed that the inner- and outer-sphere complexes have

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distinct S K-edge X-ray absorption near-edge structure (XANES) spectra, which can be used to

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quantify the respective contributions of the two types of complexes to overall sulfate adsorption3,5.

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In the present study, we determined sulfate adsorption kinetics and envelopes on hematite

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surfaces under various conditions, including pH, ionic strength, initial sulfate concentration, sample

89

hydration status (wet or air dried), and hematite particle sizes. For adsorption kinetics, we

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determined simultaneous OH- release during sulfate adsorption. Both the ionic strength test and S

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K-edge XANES spectroscopy are used to determine whether sulfate forms inner-sphere complexes,

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outer-sphere complexes, or both. The structure of sulfate inner-sphere surface complexes was

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determined using S K-edge EXAFS spectroscopy and the d-PDF analysis. Respective adsorption

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loadings of inner- and outer-sphere complexes for the adsorption envelope samples at different

95

ionic strengths were quantified using a multivariate curve resolution (MCR) analysis of the

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ATR-FTIR spectra, and further modeled using the extended triple-layer surface complexation

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model (SCM) with implementation of the determined structure of the sulfate inner-sphere complex.

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EXPERIMENTAL SECTION

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Synthesis and Characterization of Hematite 6


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Two hematite samples with a uniform rhombohedral morphology but different particle sizes (~

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8 nm and ~ 35 nm) were synthesized as described in Mulvaney et al.35 (~ 8 nm) and Madden and

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Hochella36 (~ 35 nm) (Figure S1). X-ray diffraction confirmed the synthesized products to be pure

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hematite (Figure S1a). The corresponding specific surface areas (SSAs) were 75 and 39 m2·g-1 for 8

104

nm and 35 nm particles, respectively. Their points of zero charge (PZC) values both were ~ 8.1. At

105

pH above the PZC, the Zeta potentials were similar for the two types of hematite while the 8-nm

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hematite exhibited a higher Zeta potential at pH below the PZC than the 35-nm hematite (Figure

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S1d). The detailed methods and procedures for synthesis and characterization of hematite were

108

given in the supporting information (Text SI-1).

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Sulfate Adsorption Kinetics

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Sulfate adsorption kinetics on hematite at room temperature (RT, 21 ± 0.5 oC) were determined

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with 2 or 4 mM initial sulfate concentration in 0.05 or 0.5 M NaNO3 solution at pH 3, 4, or 5 under

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constant stirring. One hundred milliliter of 3.6 g·L−1 hematite suspension was dispersed overnight

113

and then equilibrated at each pH for ~ 2 h using a pH auto-titrator under stirring (Figure S2a). Then,

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20 mL sulfate solution with the same pH as that of the suspension was quickly added to the

115

suspension. Upon sulfate adsorption, OH− was released into solution with time, which was

116

monitored using the pH-STAT method (Metrohm 907, Figure S2b) by adding the calibrated 0.0276

117

M HNO3. The amount of OH− released, or alternatively regarded as H+ co-adsorption37, was equal

118

to the amount of HNO3 added. At pre-determined time intervals, 0.8 mL suspension was sampled

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using a 3-mL syringe and immediately filtered through a 0.22-µm membrane filter. The sulfate

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concentration in the filtrate was measured using inductively coupled plasma atomic emission

121

spectroscopy (ICP-AES). The experiments at pH 4 and 5 lasted for ~ 21 h while those at pH 3 for 1 7



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h due to the fast adsorption. All the remaining suspension of each experiment was filtered and the

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solid collected on the membrane was divided into two parts, with one kept wet and the other air

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dried, for S K-edge XAS and ATR-FTIR measurements.

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Sulfur K-edge X-ray Absorption and ATR-FTIR Spectroscopy, and Differential Pair

126

Distribution Function Analysis

127

Both wet and dried solids of selected kinetic samples were subject to S K-edge X-ray

128

adsorption and ATR-FTIR spectroscopy. At least five XANES scans were collected for each sample.

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In terms of S K-edge EXAFS spectroscopy, the sulfate adsorption loadings of most samples were

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too low to obtain usable EXAFS spectra, so only the air-dried sample prepared with 8-nm hematite

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at pH 3 with 4 mM S in 0.05 M NaNO3 was measured, which had the highest sulfate loading. Even

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for this sample, thirty scans (~ 10 h) were required to obtain usable spectra. This sample was also

133

measured with the high-energy X-ray total scattering for the d-PDF analysis. The scattering data

134

were collected at beamline 11-ID-B at the Advanced Photon Source, Argonne National Laboratory.

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The detailed data collection and processing was as described in our previous studies9,38,39. All XAS

136

spectra were collected in fluorescence mode at beamline 4−3 at the Stanford Synchrotron Radiation

137

Laboratory (SSRL). The details about data collection and processing were the same as described in

138

our previous studies3,5.

139

For collection of ATR-FTIR spectra, the wet samples obtained from vacuum filtration or the

140

corresponding air-dried samples were pressed tightly by a pressure head directly on the diamond

141

crystal surface. Thirty-two scans (∼ 1.5 min) were collected, with a resolution of 4 cm−1, against air

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and the diamond background for each sample.

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Sulfate Adsorption Envelopes and Multivariate Curve Resolution (MCR) Analysis 8


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Sulfate adsorption envelopes on 8-nm hematite were obtained across a pH range of 3 - 8 with

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0.5 mM S in 0.02, 0.1, or 0.5 M NaNO3 solution at room temperature. A stock suspension of

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hematite (6 g·L-1) was prepared by dispersing dried powder in the NaNO3 solution by overnight

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stirring. Ten mL of the suspension was then mixed with 10 mL of 1 mM S solution, prepared with

148

the same NaNO3 concentration, in a 50 mL polyethylene centrifuge tube with pH quickly adjusted

149

to the target values. The tubes were shaken on an orbital rotator for ~ 21 h, during which, the

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suspension pH was maintained at the target values by adding NaOH or HNO3. After that, each

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suspension was passed through 0.22-µm membrane filter mounted on a vacuum apparatus. The

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sulfate concentration in the filtrate was measured with ICP-AES.

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All the wet and air-dried samples generated from the adsorption envelope experiments were

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measured for ATR-FTIR spectra as described above. The spectra of the wet samples were analyzed

155

with the MCR analysis to determine the major spectral components using the Unscrambler X 10.4

156

program (CAMO Software). MCR is an iterative least squares method that can extract individual

157

components of a set of mixture spectra without any guidance or knowledge about the spectral

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features of the components21,38,40. The details about applying the MCR analysis to IR spectra have

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been described in our previous study38. The two MCR-derived components from this dataset were

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assigned to sulfate inner- and outer-sphere complexes, respectively, based upon similarity to

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literature values. Sample residuals from MCR (2 - 4 ×10-7) were consistent with those obtained via

162

principle component analysis (PCA) results for a 2-component model in Unscrambler X, suggesting

163

that the MCR analysis was reliable for this system. Assuming that the intrinsic absorptivity of both

164

components was similar, the relative contribution of each component to the spectra was equivalent

165

to the molar fractional contribution of the sulfate complex to the total sulfate adsorption. Thus, 9



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speciation of each adsorption envelope into inner-sphere and outer-sphere complexes was then

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performed based upon the results of the MCR analysis. Although the MCR analysis can quantity the

168

variation of different components, the methodology itself does not allow for estimating uncertainties

169

of the quantification21,38,40.

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Surface Complexation Modeling

171

The MCR-derived respective adsorption envelopes of the sulfate inner- and outer-sphere

172

complexes at three ionic strengths (0.02, 0.1 or 0.5 M NaNO3) were quantitatively described with

173

the Extended Triple Layer Model (ETLM) as applied in previous sulfate-Fe oxide adsorption

174

systems19,41. In this mode, the involvement of water molecules in a surface reaction (i.e., water

175

dipole leaving or approaching a charged surface) was considered for correcting the energetics.

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Implemented by MINEQL + 5.0,42 this model incorporated the surface acid-base reactions of

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hematite, outer-sphere adsorption of the electrolyte ions (Na+ and NO3-), sulfate adsorption (both

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inner- and outer-sphere) on hematite, and aqueous speciation of sulfate. The surface acid-base

179

equilibrium constants, the site density, the specific surface area, two capacitance values, and the

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electrolyte ion outer-sphere adsorption equilibrium constants were directly taken from Sahai and

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Sverjensky,43 which were from a systematic evaluation of surface titration datasets from

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peer-reviewed literatures.

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Our present model quantitatively incorporated the new finding of the EXAFS-determined

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sulfate bidentate-binuclear structure whereas the selected outer-sphere surface complex was

185

referenced to Fukushi and Sverjensky19. The optimal values of the equilibrium constants for the two

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surface reactions were identified by minimizing the residual sum of squared errors between the

187

model prediction and the respective inner-sphere and outer-sphere complex adsorption loadings (in % 10


Page 11 of 31

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of the total sulfate) based on the MCR analysis of the ATR-FTIR spectra. MINFIT44 was used to

189

optimize the two fitting parameters.

190

RESULTS AND DISCUSSION

191

Kinetics of Sulfate Adsorption and Accompanied OH- Release

192

Kinetics of sulfate adsorption and the accompanied OH- release under various conditions are,

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respectively, shown in Figure 1 and S3, and the kinetic parameters of sulfate adsorption are

194

summarized in Table 2. The sulfate adsorption kinetics are well described with the power function

195

(Figure 1). The obtained adsorption rate constant (k1) at unit time increases with decreasing pH,

196

ionic strength and particle size, and with increasing sulfate concentration (Table 2). 120

(b)

140

Sulfate adsorbed (µmol/g)


(a)

120

100 8 nm Hm, I = 0.05 M, 2 mM S pH 3 pH 4 pH 5 Fit

80

100

0

100

200

60 8 nm Hm, pH 5, 2 mM S 40

I = 0.05 M I = 0.5 M Fit

300

400

500

0

1240 1260

100

200

Time (min)

197

300

400

500

1240 1260

Time (min) 120

200

(d) Sulfate adsorbed (µmol/g)

(c)

160

120 8 nm Hm, I = 0.05 M 80

pH 3, 2 mM S pH 3, 4 mM S pH 5, 2 mM S pH 5, 4 mM S Fit

40 0 0

198

80

20 0

60 0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

200

300

400

500

100 pH 5, I = 0.05 M, 2 mM S

80

8 nm Hm 35 nm Hm Fit

60 40 20 0

1240 1260

0

Time (min)

100

200

300

400

500

1240 1260

Time (min)

199

Figure 1. Kinetics of sulfate adsorption on hematite surfaces under various conditions: pH (3, 4,

200

and 5) (a), ionic strength (0.05 and 0.5 M NaNO3) (b), initial sulfate concentration (2 and 4 mM S) 11



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(c), and particle sizes (8 and 35 nm) (d). The associated OH- release with sulfate adsorption and the

202

suspension pH are shown in Figure S3. Kinetics of sulfate adsorption within the entire period are

203

fitted with the power function (red line), ln(qt) = ln(a) + bln(t), or qt = a*tb, where qt is the sulfate

204

adsorption amount at a given time (min), a and b are constants with b < 1. a*b (k1) is also a constant,

205

being the specific adsorption rate at unit time, i.e., when t = 1.45 The fitted results are summarized

206

in Table 2. The adsorption kinetics are also fitted using the first-order kinetic equation as

207

comparison (Figure S4).

208

Table 2. The power function fitting results of sulfate adsorption kinetics on hematite surfaces under

209

various conditions, and the amount of sulfate adsorption and accompanied OH- release, and the

210

ratios of OH- released over sulfate adsorbed, at different adsorption stage. Conditions

8-nm

Parameters

I = 0.05 M, pH 3

2 mM S I = 0.05 M, I = 0.05 M, pH 4 pH 5

I = 0.5 M, pH 5

4 mM S I = 0.05 M, I = 0.05 pH 3 M, pH 5

a b Power a function R12 k1 (a*b) b Q (µmol·g-1) b Q1 (µmol·g-1) b Q2 (µmol·g-1) b Q3 (µmol·g-1) c S1 (µmol·g-1) c S (µmol·g-1) c S* (µmol·g-1) S1/Q1 S*/ Q

117 (1) 0.047 (3) 0.97 5.5 140 115 (82%) 19 (14%) 6 (4%) 48 (55%) 88 102 0.42 0.73

102 (1) 0.041 (2) 0.97 4.2 133 95 (71%) 18 (14%) 20 (15%) 39 (41%) 96 0.41 0.73

39 (1) 0.067 (6) 0.93 2.6 60 32 (53%) 16 (27%) 12 (20%) 17 (35%) 48 0.53 0.8

161 (2) 0.041 (4) 0.94 6.6 188 155 (82%) 23 (12%) 10 (5%) 36 (51%) 71 90 0.23 0.48

81 (2) 0.049 (5) 0.88 4.0 112 68 (61%) 30 (27%) 14 (12%) 29 (32%) 92 0.43 0.82

133 (2) 0.032 (4) 0.84 4.3 162 116 (72%) 33 (20%) 13 (8%) 28 (29%) 98 0.24 0.60

35-nm 2 mM S I = 0.05 M, pH 5 23 (1) 0.049 (5) 0.85 1.1 31 18 (58%) 9 (29%) 4 (13%) 13 (54%) 24 0.72 0.77

a

The power function: ln(Q) = ln(a) + bln(t), or Q = a*tb, where Q is the sulfate adsorption amount at a given time (min), a and b are constants with b < 1. ab (k1) is also a constant, being the specific adsorption rate at unit time, i.e., when t = 1.45 b

Q, Q1, Q2, Q3 are the amounts of sulfate adsorbed during the entire adsorption process, 0 - 0.5 mins, 0.5 - 15 mins, and beyond 15 mins, respectively; c S1, S, and S* are, respectively, the OH- released during 0 – 0.5 mins, and the entire processes before (S) and after 12


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correction (S* at pH 3). The correction was made to account for release of H+ due to sorption of SO42- at pH 3 where HSO4- accounts for ~ 10% (pKa = 1.99, c(HSO4-)/c(SO42-) ≈ 0.1). The H+ reacts with sulfate-exchanged OH-, resulting in that the measured amount of the released OH- is lower than the actual amount of exchanged or released OH-.

211

The ratio of released OH- and adsorbed SO42- (OH-release/SO42-ads or S*/Q) strongly depends on

212

experimental conditions. With increasing pH from 3 to 5, S*/Q slightly increases from 0.73 to 0.82

213

(Table 2), probably due to more hydroxyls being present at higher pH for ligand exchange46. As the

214

sulfate concentration increases from 2 mM to 4 mM (Table 2), the corresponding S*/Q decreases

215

from 0.73 to 0.48 at pH 3 and from 0.82 to 0.6 at pH 5 (Table 2), suggesting that a higher sulfate

216

concentration has a lower efficiency in exchanging OH-. S*/Q does not change much with ionic

217

strength and hematite particle sizes.

218

The OH- release kinetics and the changes of suspension pH (Figure S3) show that once the

219

sulfate is added to the suspension, the pH sharply increases to a high value so that the HNO3

220

solution is quickly injected to decrease the pH to the target value. Due to the positive charge and

221

abundant available sites on hematite surfaces at pH 3 – 5 (Figure S1d), most sulfate adsorption (Q1,

222

53 - 82% of total sulfate adsorption) and OH- release (S1, 32 – 55% of total OH- release) occur

223

during the early stage (0 – 0.5 min). Later on (0.5 - 15 min), the adsorption gradually slows down,

224

and the corresponding amounts of sulfate adsorbed and OH- released significantly decrease,

225

because the overall surface charge becomes less positive and the amount of sites available for

226

sulfate adsorption decreases47. Finally, the sulfate adsorption kinetics under all conditions show a

227

long and slow phase after 15 min of reaction. This phase can be ascribed to the further

228

accumulation of negative charge on the surface and/or the inter-particle diffusion. Moreover, with

229

increasing reaction time, OH-release/SO42-ads significantly increases (Figure S4), which can be related

230

to the formation of inner- and outer-sphere complexes as discussed below. 13



231

Evidence of Outer-sphere Complexation

232

Sulfate adsorption envelopes are shown in Figure 2a. Sulfate adsorption decreases

233

monotonically with increasing pH, consistent with the typical sulfate adsorption behavior on Fe

234

oxides3,19,48. With increasing ionic strength, sulfate adsorption decreases drastically, indicating that

235

at least a portion of sulfate exists as outer-sphere complexes according to the conventional

236

interpretation of the ionic strength effect49. In addition, 8-nm hematite exhibits a higher sulfate

237

adsorption than 35-nm hematite, even after the SSA normalization (Figure S5b). Although the

238

amount of sulfate adsorbed significantly increases with increasing initial sulfate concentration

239

(Figure S5a), the adsorption percentage slightly decreases (Figure S5c). 0.02 M NaNO3

120

(b)

Inner-sphere Outer-sphere

0.1 M NaNO3

100

0.5 M NaNO3

80

Abs (a.u.)

Sulfate Adsorbed (µmol/g)

(a)

60 40 20 0 3

4

5

6

7

8

240

1200

1150 1100 1050 -1 Wavenumber (cm )

1000

(d) 100 Sulfate adsorption loading (µmol/g)

(c) 1.0 0.8 Outer-sphere 0.02 M 0.1 M 0.5 M

0.6 0.4

Inner-sphere 0.02 M 0.1 M 0.5 M

0.2 0.0 3

241

1250

pH

Fraction of inner- and outer-sphere

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 31

4

5

6

7

950

Outer-sphere 0.02 M 0.1 M 0.5 M

80 60 40 20 0 -20

Inner-sphere 0.02 M 0.1 M 0.5 M

3

4

5

6

7

pH

pH

242

Figure 2. Sulfate adsorption envelopes of 8-nm hematite with initial sulfate concentration of 0.5

243

mM at different ionic strengths (a). The multivariate curve resolution (MCR) resolved ATR-FTIR 14


Page 15 of 31

244

spectra (b) and the derived molar fractions (c) and the surface loadings (d) of the inner- and

245

outer-sphere complexes for the wet samples at different pHs and ionic strengths. The original

246

ATR-FTIR spectra used for the MCR analysis are shown in Figure S7. (a)

(b)

pH 3, I = 0.5 M, 2 mM S Dry Wet

8 nm Hm, pH 3, I = 0.05 M, 4 mM S dry wet 8 nm Hm, pH 7, I = 0.05 M, 2 mM S dry wet

Dry

Wet

pH 3, I = 0.05 M, 4 mM S Dry Wet

8 nm Hm, pH 3, I = 0.5 M, 2 mM S dry wet Na2SO4 solution

2475

247

Normalized µ(E)

pH 7, I = 0.05 M, 2 mM S

Normalized µ(E) (a.u.)

2480

2485

2490

2495

2500

2505

2510

2476

2477

2478

Energy (eV)

2479

2480

2481

E (eV)

(c) Normalized µ(E) (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pH 5, I = 0.05 M, 2 mM S, dry 8 nm Hm 35 nm Hm pH 3, I = 0.05 M, 2 mM S, dry 8 nm Hm 35 nm Hm Na2SO4 solution

2475

2480

2485

2490

2495

2500

2505

2510

Energy (eV)

248 249

Figure 3. Sulfur K-edge XANES spectra in the energy range of 2475 - 2510 eV for sulfate

250

adsorption on hematite surfaces: wet versus dry samples (a), and the corresponding pre-edge

251

regions of the difference spectra with respect to the sulfate solution at pH 5.5 (b); and 8-nm versus

252

35-nm hematite (c). The insets in panels “a” and “c” show the enlarged pre-edge (left) and white

253

line peak (right) regions, respectively.

254

In addition to the ionic strength effects, S K-edge X-ay absorption spectroscopy is another

255

useful approach to identify sulfate outer-sphere complexes because sulfate inner- and outer-sphere

256

surface complexes on Fe oxides differ in the pre-edge and post-edge feature and the white-line peak 15



Page 16 of 31

257

position. The outer-sphere complexes do not have the pre-edges, and their white-line position is

258

located at a lower energy compared to the inner-sphere complexes,3,5 as shown in Figure S6. The S

259

K-edge XANES spectra of sulfate adsorbed on hematite surfaces are given in Figure 3. The

260

pre-edge peak is present for all adsorption samples, indicating the presence of sulfate inner-sphere

261

adsorption complexes50. The dried sulfate adsorption samples contain less outer-sphere complexes

262

than the wet ones as the former has a higher pre-edge peak intensity and white-line energy (Figure

263

3a). The difference pre-edges are obtained by subtracting the pH 5.5 sulfate solution pre-edge from

264

those of the adsorption samples under wet and dry conditions (Figure 3b). The difference pre-edge

265

of all the adsorption samples has two peaks, consistent with sulfate binding to Fe3-5. The little

266

variation in the peak position and shape with pH and hydration degree (dry or wet) indicates the

267

formation of the same type of inner-sphere complexes on hematite surfaces regardless of the sample

268

preparation conditions. No significant differences in XANES spectra are observed for sulfate

269

adsorption on 8- and 35-nm hematite (Figure 3c), suggesting that particle sizes do not affect the

270

relative proportions of the two types of complexes.

271

The FTIR spectra of sulfate solution shows a tetrahedral (Td) symmetry with two vibration

272

bands (ν1 and ν3)22. When sulfate is adsorbed on mineral surfaces, the symmetry is decreased due to

273

ν3 band splitting. The extent of the splitting has been related to the relative fractions of the

274

inner-sphere complexes, the type of the inner-sphere complexes (bidentate vs. monodentate),

275

formation of bisulfate, and hydrogen bonding of sulfate with surrounding water molecules or

276

surface functional groups14,22. The IR spectra of sulfate adsorption on hematite surfaces under all

277

conditions in the present study show band splitting, i.e., at ~ 1183 cm-1, 1127 cm-1, 1048 cm-1, and

278

971 cm-1, probably indicating of the existence of inner-sphere complexes22 (Figure 4). The degree 16


Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


279

of the peak splitting in our study is similar to those reported for sulfate adsorption on hematite

280

surfaces previously22,25. In addition, with the physical status of the samples gradually changing

281

from wet to dry, the degree of ν3 band splitting increases while the spectral areas and peak

282

intensities both significantly decrease (Figure 4), suggesting transformation of sulfate outer-sphere

283

to inner-sphere complexes by drying14,22. Moreover, the peak at ~ 1183 cm-1 becomes more

284

remarkable with drying, which was also observed in Hug (1997),22 probably suggesting the

285

formation of adsorbed bisulfate and/or a transition of coordination structure from monodentate to

286

bidentate, but the latter possibility can be excluded by the differential XANES analysis indicating

287

no significant change in the coordination structure by drying (Figure 3b).

288

Sulfate concentration, pH, and ionic strength also affect the relative proportions of the inner-

289

and outer-sphere complexes. When the ATR-FTIR spectra are normalized to the same intensity,

290

with decreasing pH or increasing ionic strength, the degree of spectra splitting significantly

291

increases for both wet and dry samples (Figure 4b, 4f, and S7d), suggesting an increase of

292

inner-sphere complexation. Sulfate concentration may increase the inner-sphere complexation as

293

the spectral splitting slightly increases at the higher sulfate concentration (Figure 4d). Additionally,

294

with drying, decreasing pH, or increasing ionic strength, the peaks shift to higher wavenumber

295

(Figure 4 and Figure S7d), which may be caused by a slight increase of the protonation degree, i.e.,

296

formation of more bisulfate species on the surface; however, the peak shift is not obvious as the

297

initial sulfate concentration changes.

298

The co-existence of both inner- and outer-sphere complexes on hematite surfaces could affect

299

sulfate adsorption kinetics. Outer-sphere complexation occurs via electrostatic interactions and/or

300

hydrogen bonding and is deemed to be faster than inner-sphere complexation via ligand exchange. 17



301

Thus, the increase of the OHrelease/SO42-ads ratio with reaction time (Figure S4) could be due to the

302

dominance of outer-sphere complexation at the early stage, not releasing OH-, and the dominance

303

of inner-sphere complexation at the late stage, releasing OH- via ligand exchange. Although a

304

portion of the adsorbed outer-sphere complexes may further transform to inner-sphere complexes

305

((≡FeOH2+)2--SO42− → ≡Fe2O2SO2 + 2H2O), the transformation may not cause release of OH- as

306

sulfate outer-sphere complexation occurs with the aquo group, i.e., (≡FeOH2+)2--SO42−. The low

307

OHrelease/SO42-ads ratio of the early stage suggests that the rapid sulfate adsorption on hematite

308

surfaces occurs mainly via outer-sphere complexation ((≡FeOH2+)2--SO42−) and/or ligand exchange

309

with the aquo group (SO42− + 2≡FeOH2+ → ≡Fe2O2SO2 + 2H2O). The high ratio of the late

310

adsorption stage can be ascribed to ligand exchange mainly with ≡ Fe-OH1/2-.47 (a)

(b)

Dry pH 3 pH 5 pH 7 1183

1048

1150

1100

1050

1000

1127

Dry

1200

1150

1100

1050

1000

(d)

1127

Dry

Normalized

2 mM S 4 mM S

1048

pH 3, I = 0.05 M 1048

1183

Abs (a.u.)

971

Wet

1250

1200

950

-1

Wavenumber (cm )

2 mM S 4 mM S

312

1250

950

pH 3, I = 0.05 M

2 mM S 4 mM S 1183

1048

pH 3 pH 5 pH 7

-1

(c)

2 mM S, I = 0.05 M

Wet

Wavenumber (cm )

311

1127

971

Abs (a.u.)

Abs (a.u.)

pH 3 pH 5 pH 7

1200

Normalized pH 3 pH 5 pH 7 1183

Wet

1250

Dry

2 mM S, I = 0.05 M

1127

971

Abs (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

971

Wet 2 mM S 4 mM S

1150

1100

1050

1000

950

1250

1200

-1

Wavenumber (cm )

1150

1100

1050

1000

950

-1

Wavenumber (cm )

18


Page 19 of 31

(e)

(f)

1127

Dry I = 0.05 M I = 0.5 M

I = 0.05 M I = 0.5 M

1048

Abs (a.u.)

Wet

Wet I = 0.05 M

I = 0.5 M

I = 0.5 M

1200

1150

1100

1050

1048 971

I = 0.05 M

1250

pH 3, 2 mM S

1183 971

1000

950

1250

1200

-1

Wavenumber (cm )

313

Normalized

1127

Dry

pH 3, 2 mM S

1183

Abs (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1150

1100

1050

1000

950

-1

Wavenumber (cm )

314

Figure 4. ATR-FTIR spectra of sulfate adsorption on hematite surfaces at various pH (a, b), initial

315

sulfate concentration (c, d), and ionic strength (e, f). Both dry and wet samples are compared under

316

each condition.

317

Structure of Inner-Sphere Complexes

318

Figure 5a and 5b show the S K-edge EXAFS spectra and the fits for sulfate adsorbed on

319

hematite at pH 3 with 4 mM sulfate, on ferrihydrite at pH 3, and in the structure of schwertmannite

320

and jarosite. The spectra of the ferrihydrite and schwertmannite systems are reported previously3,5

321

and are provided here for comparison. The obtained structural parameters are listed in Table 3.

322

Compared to the spectra of sulfate solution, other samples have additional oscillations in k space,

323

e.g., at ~ 8.5 Å-1 and ~ 10 Å-1, and the peak at ~ 10.65 Å-1 slightly shifts to a higher k value. These

324

oscillation result from backscattering of the Fe atoms, suggesting the formation of sulfate

325

inner-sphere complexes. These spectral features are much stronger for jarosite than for others

326

because each sulfate ion is coordinated to more Fe atoms (i.e., three) in jarosite4. As a result, the

327

Fourier transform shows a peak at ~ 2.9 Å (R + ∆R) where the Fe atomic shell is located. The

328

EXAFS fitting results for sulfate adsorption on hematite show that the S-O bond length is ~ 1.49 Å,

329

and each SO42- is surrounded by ~ 1.4 Fe atoms located at ~ 3.23 Å from the central atom S. Such

330

coordination environment is similar to those of sulfate adsorbed on ferrihydrite and of structural 19



Data

(a)

Fit 8.5

10.65 10

Schwertmannite

K χ(k) (a.u.)

Ferrihydrite

Hematite

3

Jarosite Sulfate solution

4

sulfate

in

schwertmannite (c)

S-O

(b)

S-Fe

(Table

S-O 1.49 Å

6

8

10

12

14

16

-1

3).

K (Å )

Hm_pH 3_4 mM S S-Fe 3.25 Å

Schwertmannite

-4

Ferrihydrite Hematite

O-O 2.41 Å

dG (r)

331

|χ(R)| (Å ) (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

Jarosite Sulfate solution

0

1

2

3

4

5

6

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

R+ ∆R (Å)

332

r (Å)

333

Figure 5. Sulfur K-edge EXAFS spectra and their fits for a dried sulfate adsorption sample on

334

hematite prepared with 4 mM S in 0.05 NaNO3 solution at pH 3 and references (a, b), and the dG(r)

335

(differential PDF) obtained by subtracting the PDF of the pristine hematite from the PDF of the

336

sulfate adsorption sample on hematite by minimizing the intensity of the Fe-Fe peak at 3.37 Å (c).

337

The reference includes the spectra of sulfate solution, jarosite, schwertmannite, and sulfate

338

adsorption on ferrihydrite.

339

Table 3. The structural parameters of sulfate adsorption on three different minerals determined

340

from S K-edge EXAFS fitting. The numbers in parentheses are error bars. S-O Samples

pH

S-Fe

d (Å)

CN

σ (Å)

d (Å)

CN

σ2b

∆E (eV)

a

2

R

Solution

5.5

1.493 (6)

4

0.0003 (3)

--

--

--

14 (3)

0.030

Jarosite

--

1.485 (7)

4

0.0004 (3)

3.24 (3)

2.7 (9)

0.006

11 (3)

0.027

Hematite

3

1.488 (5)

4

0.0009 (2)

3.23 (3)

1.4 (6)

0.006

13 (2)

0.013 20


Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ferrihydritec

3

1.487 (5)

4

0.0008 (2)

3.24 (3)

1.2 (6)

0.006

12 (2)

0.012

Schwertmannitec 3.2 1.483 (4) 4 0.0015 (2) 3.22 (2) 1.2 (4) 0.006 11 (2) 0.008 The coordination number (CN) are fixed at 4. b The obtained Debye-Waller factor (σ2) from the fitting are around 0.006 for the dried samples and below 0.001 for the wet samples if floated during the fitting. To compare the CNs for different samples, we fixed σ2 at 0.006 for all samples. c The sulfate adsorption on ferrihydrite was prepared at pH 3 with 1 mM sulfate in 0.1 M NaNO3 for 24 h,5 and the schwertmannite was equilibrated at pH 3.2 in 0.05 M NaNO3 for 24 h.3 a

341

The PDFs of the pristine hematite and sulfate-loaded hematite are shown in Figure S8. The

342

difference of the two PDFs is significantly smaller than for the sulfate-ferrihydrite system because

343

of the much lower sulfate adsorption capacity of hematite (188 µmol·g-1) than ferrihydrite (750 -

344

1600 µmol·g-1)9. However, a usable d-PDF can be extracted, showing three major peaks at ~ 1.49,

345

2.41, and 3.25 Å (Figure 5c). These peaks can be assigned to the S-O, O-O, and S-Fe pairs,

346

respectively. The S-O and S-Fe distances are consistent with the EXAFS fitting results (Table 3)

347

and also similar to those of sulfate adsorption on ferrihydrite9.

348

The S-Fe interatomic distance at 3.23 - 3.25 Å agrees with sulfate bidentate-binuclear

349

complexation3,5,9. The smaller CN (1.4) of the Fe shell than the theoretical value of 2 for a

350

bidentate-binuclear complex can be ascribed to the co-existing outer-sphere complexes9 and fitting

351

uncertainties in CNs. Previous studies3,5 indicated that drying process does not change the structure

352

of sulfate inner-sphere complex on ferrihydrite surfaces or in schwertmannite, which is also

353

observed for sulfate adsorption on hematite surfaces (Figure 3b)26. The formation of the sulfate

354

bidentate-binuclear complexes under wet conditions is also proposed with a recent study based on

355

Raman spectroscopy23. Consistent with bidentate-binuclear complexation, the ATR-FTIR spectra of

356

the wet sample can be fitted better with four Gaussian peaks than with three peaks (Figure S9)22,26,51;

357

and the obtained peak positions (Figure S9) also agree well with bidentate-binuclear complexation

358

rather than bidentate mononuclear complexation51. 21



359

Quantification and Predictive Modeling of Surface Complexation Equilibrium

360

The MCR analysis of the ATR-FTIR spectra (Figure S7a-c) derived from the adsorption

361

envelope experiments indicates two main components that can be respectively assigned to inner-

362

and outer-sphere complexes (Figure 2b). The sulfate outer-sphere complex dominates sulfate

363

adsorption, and its fraction increases while the fraction of the inner-sphere complex decreases with

364

increasing pH and decreasing ionic strength (Figure 2c). The result at pH 7 and 0.5 M NaNO3 is off

365

the trend, likely caused by the very weak IR spectra (Figure S7c), leading to a large uncertainty in

366

the MCR analysis. The respective adsorption loadings (Figure 2d) of the two complexes were

367

determined by combining the fractions in Figure 2c with the adsorption envelopes in Figure 2a.

368

With increasing pH at each ionic strength, both inner- and outer-sphere adsorption loadings

369

gradually decrease (Figure 2d). At a given pH, the adsorption loading of the outer-sphere complex

370

decreases drastically with increasing ionic strength, whereas the loading remains essentially

371

unchanged for the inner-sphere complex. The changes of the inner- and outer-sphere complexes

372

with ionic strength strongly support the conventional interpretation of the ionic strength effects for

373

the two types of complexation. Similar observation was made for sulfate adsorption on

374

ferrihydrite5. 120 100

Sulfate adsorbed (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

80

376

100 80

0.02 M NaNO3

0. 1 M NaNO3

60

60

40

40

40

20

20

20

0

0

0

3

375

120

120 Experimental: Predicted: Inner-sphere Inner-sphere Outer-sphere Outer-sphere 100 Total Total 80

4

5

pH

6

7

3

4

5

6

pH

60

7

0. 5 M NaNO3

3

4

5

6

7

pH

Figure 6. Comparison of sulfate adsorption envelopes and surface-complexation modeling results 22


Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


377

for sulfate adsorption on hematite in 0.02 M, 0.1 M, or 0.5 M NaNO3 with initial sulfate

378

concentration of 0.5 mM. The two surface-complexation equilibrium constants are optimized by

379

directly fitting their respective loadings determined by the MCR analysis of the ATR-FTIR spectra.

380

The surface complexation model can simulate the general trends of the inner-sphere and

381

outer-sphere complexation as a function of pH and ionic strength, hence the overall sulfate

382

adsorption (i.e., the adsorption envelops) (Figure 6 and Table 4). As ionic strength increases, the

383

model reproduces the suppression of the overall sulfate adsorption, which is attributed to the

384

outer-sphere complexation as opposed to the inner-sphere complexation. Those trends simulated by

385

the model are consistent with the results from both the macroscopic adsorption experiments and the

386

spectroscopic analyses.

387 388

Table 4. Equilibrium reactions for the surface complexation model for sulfate adsorption on hematite.

ψ0 ψβ LogK0a LogKθb Equilibrium Reactions Hematite (de)protonation and electrolyte adsorption +1 0 5.7 6.9 ≡FeOH + H+ ⇌ ≡FeOH2+ −1 0 −11.3 −10.1 ≡FeOH ⇌ ≡FeO− + H+ −1 +1 −9.7 −8.5 ≡FeOH + Na+ ⇌ ≡FeO−--Na+ + H+ + − + − +1 −1 7.7 8.9 ≡FeOH + H + NO3 ⇌ ≡FeOH2 --NO3 Sulfate innerand outer-sphere surface complexation 0 0 12.1c 17.0 2≡FeOH + 2H+ + SO42− ⇌ (≡FeO)2SO2 + 2H2O +2 −2 20.9d 25.8 2≡FeOH + 2H+ + SO42− ⇌ (≡FeOH2+)2--SO42− Aqueous Reactions N.A. N.A. 1.98 N.A. SO42− + H+ ⇌ HSO4− N.A. N.A. 0.88 N.A. SO42− + Na+ ⇌ NaSO4− + − N.A. N.A. −13.997 N.A. H2O ⇌ H + OH a Molar concentration based equilibrium constants, as input in MINEQL. These constants for surface reactions are corresponding to the site density (N = 22 sites nm-2), specific surface area (A = 75 m2 g-1) and hematite concentration (Cs = 3 g L-1). Capacitance C1 = 90 µF/cm2, C2 = 20 µF/cm2. All those parameters are from the database in Sahai and Severjensky43. b Site occupancy based equilibrium constants (Kθ) calculated based on the correction established by Sverjensky52. Note that the molar concentration-based equilibrium 23



Page 24 of 31

constants as input in MINEQL are dependent on solid concentration for bidentate surface complexation reactions. See Wang and Giammar53 about how to apply the present model to other conditions with different ferrihydrite concentrations, site densities and specific surface areas. c The present model followed the ETLM formulation, in which the water dipole leaving the charged surface experiences a change in potential that is equal to –n(ψ0−ψβ) where n is the number of desorbed waters per reaction. For this inner-sphere adsorption reaction, n = 2 so the ETLM gives ∆ψr = 2ψ0 − 2ψβ – 2(ψ0− ψβ) = 0. Note that although the sulfate is placed on the β-plane, this reaction still represents inner-sphere surface complexes54. Fukushi and Sverjensky described the details about how this electrostatic factor is determined.19 d For outer-sphere surface complexes, we continue to express the electrostatic terms in the traditional way for β-plane complexes in the triple-layer model55, as it is done for the ETLM formulation. 389

The model framework used in this study is the same as those in our recent sulfate-ferrihydrite

390

model5 except that the outer-sphere surface complex on hematite used is (≡FeOH2+)2--SO42−, which

391

gives better fitting than using ≡FeOH2+--HSO4− as for the ferrihydrite system 5. The outer-sphere

392

species (≡FeOH2+)2--SO42− is suggested by Fukushi and Sverjensky19 in sulfate adsorption on

393

goethite surfaces. The present model found that the combination of (≡FeO)2SO2 and

394

(≡FeOH2+)2--SO42− gives good fits to all results including the macroscopic adsorption percentage

395

and the respective fractions of each surface complexes. The model successfully predicted the

396

suppression of outer-sphere surface complexes and the persistence of inner-sphere surface complex

397

at higher ionic strength. Overall, the present sulfate-hematite surface complexation model could

398

simulate a wide range of water chemistry conditions and maintain high degree of consistence with

399

spectroscopically determined fractions of inner- and outer-sphere surface complexes.

400

Our previous study of sulfate adsorption on ferrihydrite used comparable sorbate/sorbent mass

401

ratio and ionic strength, and it was evident that hematite was a weaker sorbent for sulfate from the

402

adsorption envelope results. After accounting for the variations of specific surface area and site

403

density, the derived BB inner-sphere surface complex has a larger logK value for ferrihydrite

404

(20.7)56 than for hematite (17.0), indicating that the stronger inner-sphere adsorption of sulfate to 24


Page 25 of 31

405

ferrihydrite than to hematite is thermodynamically intrinsic beyond the difference in specific

406

surface area (600 vs. 75 m2/g, respectively).

407

The stoichiometry of the outer-sphere surface complexes, i.e., ≡ FeOH2+--HSO4− for

408

ferrihydrite5 and (≡FeOH2+)2--SO42− for hematite, was different for the two models so that a direct

409

comparison of their logK values is invalid. The difference in the outer-sphere complexes is also

410

suggested by their distinct ATR-FTIR spectra. The MCR-derived outer-sphere component spectra of

411

sulfate adsorbed on hematite surfaces split (Figure 2b), in contrast to the symmetric peaks for

412

outer-sphere sulfate complexes on ferrihydrite and goethite surfaces reported previously14,21.

413

However, we do not believe that the IR peak splitting suggests inner-sphere complexation. In fact, a

414

similar degree of peak splitting was also observed in an Fe(III)-SO4 solution4, which was attributed

415

to H-bonding between sulfate and Fe(III) within the outer-sphere complex. Therefore, the sulfate

416

outer-sphere complexation in the present study likely involves significant H-bonding between

417

sulfate and surface OH or H2O molecules. The difference in the IR spectra of the outer-sphere

418

complexes between hematite and ferrihydrite probably contributes to the spectral difference of

419

overall sulfate sorption (Figure 7). (a)

(b) Ferrihydrite Hematite

Ferrihydrite Hematite

Abs (a.u.)

Sulfate

Abs (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4000

420

3500

3000

2500

2000

1500

1000

500

1200

1150

-1

Wavenumber (cm )

1100

1050

1000

950

-1

Wavenumber (cm )

421

Figure 7. The ATR-FTIR spectra of sulfate adsorption on hematite with 2 mM sulfate in 0.05 M

422

NaNO3 at pH 5, and on ferrihydrite5 with 1 mM sulfate in 0.1 M NaNO3 at pH 5 in the range of 25



Page 26 of 31

423

4000 – 400 cm-1 (a) and 1220 – 950 cm-1 (b). The sulfate adsorption loadings are 460 µmol·g-1 for

424

ferrihydrite and 85 µmol·g-1 for hematite. The background of the spectra is removed and the spectra

425

are then normalized to the same intensity.

426

It remains unclear what specific surface structural and chemical properties of ferrihydrite and

427

hematite lead to the different outer-sphere complexes on the two oxides. We believe that the

428

difference is partially attributed to the different H-bonding environment caused by different surface

429

H2O structure of ferrihydrite and hematite, as suggested by the different shape of their water

430

vibration bands at ~ 3400 cm-1 (Figure 7a). This is not surprising because the two minerals have

431

different bulk structure and ferrihydrite surfaces also contain abundant structural defects57,58.

432

SUMMARY AND CONCLUSIONS

433

It was reported that sulfate adsorbs on hematite surfaces only via inner-sphere complexation

434

(see references in Table 1). However, the present study identifies co-existing outer-sphere

435

complexes strongly evidenced from both the ionic strength tests and S K-edge XANES

436

spectroscopy. Although there are ambiguities in interpreting the ATR-FTIR spectra regarding the

437

types of surface complexes, the ATR-FTIR data are generally consistent with this conclusion.

438

Sample hydration (drying) and pH both strongly affect the relative proportions of outer- and

439

inner-sphere complexes, which is similar to sulfate adsorption on goethite and ferrihydrite

440

surfaces5,59. The bidentate-binuclear type of the inner-sphere complexes on hematite is also similar

441

to that on ferrihydrite surfaces. The tendency to form both inner- and outer-sphere complexes on

442

diverse minerals suggests that the types of sulfate complexes are controlled by the physiochemical

443

properties of sulfate, likely the relative magnitude of the sulfate hydration and binding energies5,

444

involved in sulfate adsorption, more than the properties of mineral surfaces. The co-existence of the 26


Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


445

inner- and outer-sphere complexes can profoundly affect sulfate overall adsorption kinetics on

446

hematite surfaces60, as the two have different adsorption behavior. The abundance of sulfate

447

outer-sphere complexes suggests binding on hematite surfaces might not limit sulfate availability to

448

plants in tropical and subtropical soils. It also indicates that sulfate is unable to significantly affect

449

adsorption of phosphate and arsenate on hematite surfaces.

450

In addition, the spectroscopic quantification of the respective outer- and inner-sphere

451

adsorption loadings as a function of ionic strength in the present and our previous study5 allows us

452

to conclude that the simple macroscopic ionic strength test correlates well with outer-sphere

453

complexes although it cannot exclude possible co-existence of inner-sphere complexes.

454

ACKNOWLEDGMENTS

455

This work was supported by the Wyoming Agricultural Experimental Station Competitive

456

Grants Program. X.W. also thanks National Natural Science Foundation of China (No. 41601228)

457

and the China Postdoctoral Science Foundation (No. 2016M590700) for its support. Z.W.

458

acknowledges support of Louisiana Board of Regents under contract LEQSF(2017-20)-RD-A-07.

459

This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE)

460

Office of Science User Facility operated for the DOE Office of Science by Argonne National

461

Laboratory under Contract No. DE-AC02-06CH11357. Use of the Stanford Synchrotron Radiation

462

Light source, SLAC National Accelerator Laboratory, is supported by the U.S. Department of

463

Energy,

464

DE-AC02-76SF00515. Use of the Advanced Photon Source, Argonne National Laboratory,

465

supported by U.S. DOE-BES under Contract DE-AC02-06CH11357.

466

SUPPORTING INFORMATION

Office

of

Science,

Office

of

Basic

Energy

Sciences

under

Contract

No.

27



Page 28 of 31

467

The supporting information is available free of charge via the internet at http://pubs.acs.org.,

468

including (1) detailed methods for hematite characterization, (2) pH and HNO3 addition curves of

469

adsorption kinetics, (3) the variations of OHrelease/SO42-ads ratios with reaction time and sulfate

470

adsorption data under different conditions, (4) the S K-edge XANES spectra of inner and outer-

471

sphere standards for schwertmannite and ferrihydrite, (5) the ATR-FTIR spectra of the sulfate

472

adsorption envelope samples, (6) the pair distribution functions of the pristine hematite and

473

hematite with sulfate adsorbed, and (7) the Gaussian peak fitting of the ATR-FTIR spectra.

474

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475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502

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Quantification of Coexisting Inner- and Outer-Sphere Complexation of

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