Selenite and selenate uptaken in ettringite: Immobilization mechanisms, coordination chemistry, and insights from structure

Cement and Concrete Research 100 (2017) 166–175

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Selenite and selenate uptaken in ettringite: Immobilization mechanisms, coordination chemistry, and insights from structure

MARK

Binglin Guo, Keiko Sasaki⁎, Tsuyoshi Hirajima Department of Earth Resource Engineering, Kyushu University, Fukuoka 819-0395, Japan

A R T I C L E I N F O

A B S T R A C T

Keywords: Coordination Ettringite Radioactive waste Heavy metals Characterization

Although ettringite is a crucial material in terms of Se immobilization, the immobilization mechanisms, atomic configuration, and intercolumn structure of Se sorbed in ettringite are unclear. The immobilization mechanism of Se oxoanions was evaluated through structural insight into ettringite. It is contrasting between SeO32 − and SeO42 − in chemical property of the solid residues after immobilization. The oxoanion exchange with structural SO42 − is the main mechanism for immobilization of SeO42 −. In contrast, SeO32 − is easily immobilized to form inner-sphere complexes in ettringite. In addition, it is necessary to reveal the SeO32 − complexation sites for understanding the mechanisms in immobilization of SeO32 −. Based on the characterization results with the bond valence theory, the location sites of sorbed SeO32 − in ettringite structure were proposed. The results obtained in this work are relevant to the understanding of Se and its isotopes immobilized in cement or alkaline environments, especially for nuclear waste management.

1. Introduction Selenium (Se) is one of the important micronutrients for human and animal health. At high concentrations, however, selenium toxicity is recognized as a severe environmental and health hazard, because it is toxic to organs. Such an element is existed in nature as selenide minerals and is often associated with sulfide minerals by the substitution of sulfur. Some anthropogenic activity such as various mining and oil refinery activities can cause high concentrations of Se in the effluent (170–6000 μg/L) [1,2]; this could result in Se entering the environment and hydrosphere. In addition, the 79Se isotope as one kind of radioactive contamination is a continuing concern surrounding activities related nuclear waste disposal, reprocessing, and nuclear accidents [3]. It has a long half-life time of approximately 3.27 × 105 years with high mobility in the environment [4]. Because of its high toxicity, the WHO and U.S. established the primary drinking water standard as 0.01 mg/L of selenium [5,6]. Therefore, the removal of Se from aqueous environments is urgently required. The reduced forms of selenium (Se2 −) are mainly immobilized in ores, whereas the oxidized forms (SeO32 − and SeO42 −) represent the much more mobile and toxic species in water and soil systems. The pyramidal oxoanion SeO32 − can exist as H2SeO3 or HSeO3− depending on the pH of the solution (pKa1 = 2.64 and pKa2 = 8.4) [6,7]. The tetrahedral oxoanion SeO42 − behaves as HSeO4− with a pKa = 1.7 [7]. The interactions of these oxoanions with mineral surfaces affect their

⁎

Corresponding author. E-mail address: [email protected] (K. Sasaki).

http://dx.doi.org/10.1016/j.cemconres.2017.07.004 Received 3 April 2017; Received in revised form 29 June 2017; Accepted 10 July 2017 0008-8846/ © 2017 Published by Elsevier Ltd.

solubility and mobility. It has been proven that iron and aluminum oxide minerals and coatings are the most common geosorbents for Se oxoanions due to their relatively high point of zero charge with 6–9.5 [8–10]. However, under high pH conditions, both oxidized forms show a lower adsorption capacity because of the negatively charged surface of most minerals. Ettringite (Ca6Al2(SO4)3(OH)12·26H2O) forms in the natural alkaline environment, associated with other species like portlandite, gypsum or afwillite [11], and also occurs as the early hydration product of Portland cement generated by hydration [12,13]. Furthermore, Portland cement is an important material for the immobilization and storage of various hazardous material and ettringite is assumed to play an important role in immobilization process. Because of high ion exchange ability, many researchers [14–17] have investigated incorporating the oxoanion of Se by substituting SO42 − in ettringite. Some of them also indicated that more efficient immobilization of SeO32 − than that of SeO42 − occurred through the formation of SeO32 −-substituted ettringite [18]. However, the reasons for the difference in immobilization efficiency of these oxoanions and intercolumn structure changing of ettringite after sorption these oxoanions are still unclear. Based on the previously reported structure model of ettringite [19,20], the unit cell of ettringite consists of columnar {Ca6[Al (OH)6]2·24H2O}6 +. In columns, 6 coordinated octahedral Al(OH)63 − is linked to 3 neighbors Ca atoms. Each Ca atom is eight coordinated (square antiprismatic geometry) by 4 H2O and 4 OH− ions, which

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Fig. 1. Structure of ettringite. (a) Perpendicular to c-axis shown columns and channels, (b) projection of ettringite structure, (c) expended the dash line regions in (a) and (b) [1,2].

means Al(OH)63 − octahedra and Ca-O8 square antiprismatic polyhedra share OH− ions. The inter-column spaces are occupied by H2O molecules and SO42 − ions. This holds the columns together through electrostatic force, as described in Fig. 1. Ettringite is particularly well known as an anion-exchanger [15–17,21–24], and the anions are commonly immobilized through substitution for sulfate in ettringite [15]. In addition, arsenate (AsO43 −) sorbed in ettringite could cause the formation of inner-sphere complexes with some functional groups on the ettringite channels [25,29]. Therefore, the immobilization of oxoanions by ettringite is due to the substitution with inter-column oxoanions (SO42 −) or formation of complexes with the channel edge functional groups (≡XeOH and ≡ XeOH2). Ettringite is a crucial material in terms of SeO32 − and SeO42 − immobilization under alkaline conditions [26]; however, the immobilization mechanism of these oxoanions has not yet been well interpreted, and atomic configuration is needed to consider on the columnar parts edges of SeO42 − and SeO32 − sorbed on this mineral. Thus, to understand the SeO32 − and SeO42 − immobilization by ettringite in alkaline environments, revealing the mechanism of SeO42 − and SeO32 − immobilization in ettringite is necessary for the purpose of environmental remediation and nuclear wastes treatment. In this study, the coordination chemistry of SeO42 − and SeO32 − was discussed and the mechanism of these oxoanions' immobilization through co-precipitation with ettringite was elaborated systematically. The combination of extended X-ray adsorption fine structure (EXAFS), thermogravimetric analysis (TG), and Fourier Transform Infrared (FTIR) results, the bond length of atoms, and the detailed analysis of FTIR vibration mode in hydroxyl groups provides evidence to evaluate the coordination chemistry of SeO32 − in the ettringite.

2. Experimental 2.1. Sample preparation A series of SeO42 − and SeO32 − doped ettringite was synthesized through coprecipitation: 100 ml aqueous solutions containing 10 mM Al2(SO4)3·16-18H2O and 0.001–5 mM Na2SeO4 or Na2SeO3. 0.445 g of powdery Ca(OH)2 was introduced into 100 ml of the above solutions to obtain a theoretical (3:1) molar ratio of Ca: Al in ettringite. The mixtures were subsequently covered with parafilm to avoid CO2 contamination and then thoroughly mixed using a magnetic stirrer for 24 h at room temperature. After that, suspensions were taken and filtered through a 0.2 μm membrane filter (Advantec, Japan) to provide the solutions for determining the residual Ca, Al, S, and Se concentrations by using inductively coupled plasma optical emission spectrometry (ICP-OES, Perkin Elmer, Optima 8300, US). Separately, SeO42 − could be totally substituted with sulfate [27], the pure SeO42 −-substituted ettringite was applied to be the typical SeO42 − doped ettringite for analysis. The precipitates were dried in silica gel in desiccators under mild vacuum conditions for one week to remove excess water. Moreover, the synthesized SeO3 and SeO4-doped ettringite were dissolved by 0.5 M HNO3 and the elements were measured by ICP-OES. Based on the water chemistry, the chemical composition of SeO3 and SeO4-doped ettringite were determined. In addition, the crystal water of ettringite was determined by TG analysis.

2.2. Characterization of solid residues After precipitation, solid residues were characterized using powder X-ray diffraction patterns on an Ultima IV (RIGAKU, Akishima, Japan) using Cu Kα radiation (40 kV, 40 mA) at a scanning speed of 2° min− 1 and a scanning step of 0.02°. In addition, using the Miller indices reported on ICDD PDF Card (# 042-0224 and # 041-1451), the d spacings from the XRD analyses of the subjected solid residues, and the equation 167

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2.4. Bond valence analysis

Table 1 Elemental compositions in oxyanion-doped ettringite as a function of initial SeO32 − and SeO42 − concentrations. Se initial concentration (mM) SeO32 − 0.001 0.01 0.1 1 5 SeO42 − 0.001 0. 01 0.1 1 5

Se equilibrium concentration (mM)

Ca mmol/g

Al mmol/g

S mmol/g

– – – 0.004 ± 0.001 0.551 ± 0.007

7.14 7.14 7.14 7.14 7.14

± ± ± ± ±

0.05 0.04 0.06 0.09 0.09

2.44 2.44 2.44 2.45 2.45

± ± ± ± ±

0.02 0.03 0.06 0.01 0.01

3.68 3.68 3.68 3.63 3.13

± ± ± ± ±

0.02 0.04 0.04 0.01 0.01

– – 0.083 ± 0.004 0.415 ± 0.017 3.481 ± 0.024

7.11 7.12 7.12 7.14 7.14

± ± ± ± ±

0.04 0.03 0.06 0.04 0.04

2.43 2.42 2.42 2.41 2.41

± ± ± ± ±

0.07 0.02 0.03 0.03 0.03

3.70 3.69 3.70 3.69 3.55

± ± ± ± ±

0.03 0.03 0.03 0.05 0.05

The bond valence theory is a useful tool for providing a quantitybased description of inorganic bonding. Each bond between atoms i and j should be associated with a bond valence number. Sij Bond valence is defined as [33]:

Sij = exp[(ro − rm − o) 0.37]

(1)

where rm − o is the bond length between two atoms and ro is the empirical constants described by Brown et al. [34]. Furthermore, most inorganic compounds have been described to obey the formula:

∑ Sij ≅ Vi (2)

j

where Vi is the oxidation state of atom i. 3. Results 3.1. Composition of the SeO32 − and SeO42 − doped ettringite

for hexagonal system minerals, the lattice parameter a and c can be obtained. FTIR spectra were recorded with the DRIFT technique using an FTIR spectrometer (JASCO 670 Plus, Japan). The SeO32 − and SeO42 − solutions were examined using the attenuated total reflection Fourier Transform infrared spectroscopy (ATR-FTIR) using ZnSe crystal. The curve-fitting program PeakFit ver. 4.12 with the mixed Gaussian and Lorentzian function [28,29] determined the overlapped peaks, and the second derivative method was used for curve resolution. The water content in samples was examined using a 2000 SA thermal balance (Bruker, Germany), with an air flow rate of 80 mL/min.

Based on the results of water chemistry and the XRD patterns of solid residues, precipitations were identified as ettringite. In addition, the compositions of oxoanion-substituted ettringite as a function of initial SeO32 − and SeO42 − concentrations are shown in Table 1. Analysis of SeO42 − and SeO32 − doped ettringite yielded a Ca:Al: (SO4 + SeOx) ratio of 5.8:1.9:2.9, which is in good agreement with the theoretical ratio of 6:2:3 in pure ettringite. Approximately 95% SeO32 − was sorbed from solution (SeO32 −≤5 mM) through coprecipitation. In comparison to SeO32 − sorption, immobilized SeO42 − in ettringite was much lower. At high concentrations (SeO42 −≥ 1 mM), only approximately 50% could be immobilized. SeO32 − is preferentially incorporated in ettringite over SeO42 −, as reported previously [18].

2.3. EXAFS analysis X-ray absorption fine structure (XAFS) spectra of Ca K-edge and Se K-edge for SeO32 − and SeO42 − doped ettringite were collected in the fluorescence/transmission mode at room temperature on a BL 06 at the SAGA-LS (Saga, Japan) with the storage ring operating at energy of 1.4 GeV. The energy range of this light source (bending magnet) is 2.1–23 keV. A silicon (111) double-crystal monochromator was used to obtain the incident X-ray beam. The typical photon flux is 1010 photons/s. The intensities of the fluorescence/transmission X-rays were monitored with a silicon drift detector and ionization chamber, respectively. The absorption edge of each Se K-edge spectrum was calibrated by selenium (Se0) powder (Wako, special grade, Osaka, Japan) to 12,675 eV. Ca K-edge spectrum was calibrated by Ca(OH)2 (Wako, special grade, Osaka, Japan) to 4038 eV. CaSeO3 (Wako, special grade, Osaka, Japan), CaSeO4 (Wako, special grade, Osaka, Japan), and pure sulfate ettringite (synthesized) [27] were used as standard substances. All powder samples were diluted with BN (Wako, special grade, Osaka, Japan) to adjust to 2 wt% of Se and then pressed into a tablet with a diameter of 1 cm. Data processing was performed using the IFEFFIT software package ver. 0.9.25 (ATHENA and ARTEMIS) via the following procedures [30–32]. The spectra were averaged after energy calibration. The χ(k) function was Fourier transformed through k3 weighing and filtered using “HANNING” before Fourier transmission. The shell fit was operated in R-space on the first and second shells within the data range R + ΔR = 1–4 Å using the single scattering model. The theoretical backscattering path was calculated by FEFF using ettringite, CaSeO4, and CaSeO3 as standards. Because the Debye–Waller factor (σ) is correlated highly with CNs, CNs values for pure ettringite, CaSeO3, and CaSeO4 were fixed to their known values. Errors are within 0.01 Å for the Rvalue of the first shell and within 0.05 Å for the additional shells. The coordination number errors are ± 20% for the first shells and ± 50% for the additional shells.

3.2. XRD patterns The XRD patterns of SeO32 − and SeO42 − doped ettringite are shown in Fig. 2. There are no significant changes in the XRD patterns as the results of various concentrations of SeO32 − and SeO42 − incorporated in ettringite. Moreover, no new phase was created after the uptake of SeO32 − and SeO42 − in ettringite, and only the ettringite can be characterized (ICDD PDF # 00-041-1451), indicating that the reduced SeO32 − and SeO42 − ions were incorporated in the structure of ettringite. Based on the results of XRD patterns, the lattice parameters of ettringite were plotted against the initial SeO32 − and SeO42 − concentrations after calculation, as shown in Fig. 3. It was observed that the lattice parameters of ettringite changed differently after immobilizing different amounts of SeO32 − and SeO42 −. By increasing the incorporated SeO32 − concentration in ettringite, both lattice parameters of a and c decreased. In contrast, incorporated SeO42 − can increase the lattice parameter a of ettringite, keeping the lattice parameter c constant (Fig. 3). As reported by the former researcher, intercolumn free water will not affect the crystal structure and lattice parameters of ettringite [27,35]. The lattice parameter a increasing in SeO42 − doped ettringite could result from SeO42 −, which has larger ionic radius than SeO42 −. When SeO42 − incorporated in ettringite, the intercolumn space and lattice parameter a will be increased. 3.3. FTIR analysis Fig. 4 shows the FTIR spectra of the ettringite sorbed with varying amounts of SeO32 − and SeO42 −. The completely overlapping bands at 3000 to 3650 cm− 1 were attributed to the stretching vibration mode of channel water, crystal water, and OH groups in ettringite. The OeH bending vibration mode of the interchannel H2O molecules is also presented at 1620 cm− 1. The peak at 872 cm− 1 is due to a bending vibration mode of Al–OH and that at 547 cm− 1 to a bending vibration 168

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(a) SeO2-3

-Ettringite

(b) SeO2-4

-Ettringite

5 mM (0.551 mM)

0.1 mM (n.d.)

5 mM (3.481 mM)

1 mM (0.451 mM)

Intensity/ a.u.

Intensity/ a.u.

1 mM (0.004 mM)

0.1 mM (0.083 mM)

0.01 mM (n.d.)

0.01 mM (n.d.)

0.001 mM (n.d.)

0.001 mM (n.d.)

5 10 15 20 25 30 35 40 45 50 55 60 65 70

5 10 15 20 25 30 35 40 45 50 55 60 65 70

Diffraction angle,2 [Cu K ]/ degree

Diffraction angle,2 [Cu K ]/ degree

11.40 (a)

Fig. 2. XRD patterns of solid residues after coprecipitation of (a) SeO32 − and (b) SeO42 − with ettringite at various initial concentrations. Numbers in brackets indicate the equilibrated Se concentrations.

(a)

Pure Selenate Ettringite

Se(IV)-O stretching

OH-stretching

5 mM SeO2-3 OH-bending

(33.19)

11.38

S-O stretching 23

1 mM SeO

11.32 2-

SeO3

11.30

2-

SeO4

11.28 11.26 11.24 11.22 11.20

(n.d. mM) (n.d. mM) (30.48) (29.61) (n.d. mM)

(n.d. mM)

(30.46)

(29.54)

21.49

21.48

(3.481 mM) (32.08)

(0.083 mM)

(0.415 mM)

(30.08)

(31.84)

(n.d. mM) (28.47)

(0.004 mM)

(0.551 mM)

(27.98)

(26.14)

0.1 mM SeO2-3 0.01 mM SeO2-3 0.001 mM SeO2-3 Pure Sulfate Ettringite

4000

1 5 0.1 0.01 Initial Se concentration (mM)

0.001

Lattice parameter c ( )

Transmittance/ a.u.

11.34

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber/ cm

(b)

OH-stretching

(b)

Se(VI)-O stretching

Pure Selenate Ettringite

(n.d. mM) (n.d. mM) (30.46) (29.54) (n.d. mM) (30.48)

21.47

(n.d. mM) (29.61)

(n.d. mM) (0.415 mM) (28.47) (31.84) (0.083 mM) (30.08)

(3.481 mM) (32.08)

OH-bending S-O stretching

(33.19)

5 mM SeO2-4

Transmittance/ a.u.

Lattice parameter a ( ))

11.36

Pure Selenate Ettringite

(0.004 mM) (27.98)

21.46

1 mM SeO2-4 0.1 mM SeO2-4 0.01 mM SeO2-4 0.001 mM SeO2-4

21.45

2-

SeO3

2-

SeO4

Pure Sulfate Ettringite

(0.551 mM) (26.14)

4000

21.44 0.001

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber/ cm

5 0.01 0.1 1 Initial Se concentration (mM)

Fig. 4. FTIR vibration spectra of ettringite reacted with (a) SeO32 − and (b) SeO42 − at various initial concentrations.

2−

Fig. 3. Changes of lattice parameters (a) a and (b) c of ettringite reacted with SeO3 and SeO42 − at various initial concentrations. Numbers in brackets indicate the equilibrated Se concentrations and whole water molecules.

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Fig. 5. TG curves of ettringite reacted with (a) SeO32 − and (b) SeO42 − at various initial concentrations.

100 2-

[SeO3 ]0/ mM Mass loss/ wt.%

Weight/ %

90

0.001 0.01 0.1 1 5

80 70

48.43% 47.80% 46.81% 46.26% 44.06%

60 50

(a) Se(IV)-doped ettringite

100

200

300

400

500

600

700

800

Temperature/ C

100 2-

[SeO4 ]0/ mM Mass loss/ Wt.%

Weight/ %

90

0.001 0.01 0.1 1 5

80 70

48.42% 47.75% 48.20% 49.52% 49.67%

60 50

(b) Se(VI)-doped ettringite

100

200

300

400

500

600

700

800

Temperature/ C mode of Al–OH. In addition, the peak at 1442 cm− 1 could be assigned to the anti-symmetric vibration mode of CO32– (Table S1, Fig. S1). It is worth noting that after ettringite reacted with different initial concentrations of SeO32 −, the intensity of vibration spectra in the region of 3000 to 3650 cm− 1 gradually decreased. In terms of the spectra after ettringite reacted with different initial concentrations of SeO42 −, however, the intensity of broad bandings at 3000 to 3650 cm− 1 exhibit constant.

progressive weight loss (~ 15%) until the end of the experiment (800 °C). As shown in Fig. 5 (a), with increasing the contents of SeO32 −, the mass loss of SeO32 −-doped ettringite decreased. The total weight loss of SeO32 −-doped ettringite (5 mM) is approximately 44%, which is much lower than other SeO32 −-doped ettringite samples (0.001–1 mM), indicating that the numbers of H2O in ettringite decreased with increasing the incorporated SeO32 − amount. In contrast, the water content of SeO42 −-doped ettringite does not change significantly, and even the H2O content slightly increased after increasing the incorporated SeO42 − content in ettringite. As shown in Fig. 5 (b), all of the SeO42 −-doped ettringite samples exhibit approximately 47% of total weight loss until the end of the experiment. Specifically, the SeO42 −-doped ettringite (5 mM) showed more weight loss than other samples. This is due to the SeO42 − incorporated in ettringite and enlarged the intercolumn spaces, which could more water molecules enter in intercolumn spaces [27].

3.4. TG analysis Fig. 5 (a) and (b) illustrates the TG curves of ettringite reacted with different initial concentrations of SeO32 − and SeO42 −. In the previous report, the mass loss of pure ettringite should be attributed to water loss when the temperatures are lower than 800 °C [27,36]. For the pure ettringite, most of the mass loss occurred from 90 to 108 °C. Approximately 33% mass loss corresponded to H2O evaporation. As the temperature increased, the mass loss gradually decreased, approximately 46% mass was eventually lost until 800 °C [36]. Although the TG curves obtained for SeO32 − and SeO42 − doped ettringite exhibit a similar profile, a detailed study of the TG curves indicate differences in the dehydration behavior of SeO32 − and SeO42 − doped ettringite samples. The SeO32 − and SeO42 − doped ettringite dehydrated in two separated stages (Fig. 5). The first stage occurs between 25 and 130 °C, with a total loss of approximately ~33–34%. After the first stage, both the TG curves of SeO32 − and SeO42 − doped ettringite (Fig. 5) show a

3.5. EXAFS analysis of Se k-edge of Se(IV) and Se(VI) doped ettringite The X-ray absorption near edge structure (XANES) spectra for the SeO32 − and SeO42 −-doped ettringite and reference compounds are shown in Fig. S2. The XANES spectra are highly characteristic of the Se oxidation states. The edge positions clearly correspond to the expected Se oxidation states (Se(IV) and Se(VI)). The result suggests that the Se oxidation states are stable under the experimental conditions and that the SeO32 − and SeO42 − entities are preserved upon uptaken by 170

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Fourier transform amplitude

(a) Se(VI) O

(b)

2-

pure SeO4 -ettringite

Se(VI) Ca 2pure SeO4 -ettringite

Fig. 6. (a) k3-weighted Se K-edge EXAFS data of SeO32 − and SeO42 − doped ettringite and reference compounds. (b) Corresponding Fourier transforms (not corrected for phase shift) showing both raw (solid lines) and fitted data (dash lines).

k (k)

CaSeO4

3

CaSeO4 Se(IV) O

2-

pure SeO4 -ettringite 2-

SeO3 -doped ettringite CaSeO3

CaSeO3

0

1

2

3

4

5

6

4

6

8

R( )

10 k(

Standards CaSeO3 CaSeO4 Se sorption samples Se(IV) – ettringite Pure selenate ettringite

CN

R(Å)

ơ2

Rf(%)

SeeO SeeO SeeCa

3a 4a 4a

1.69 1.64 3.39

0.002 0.002 0.012

3.44 2.03 –

SeeO SeeO

2.97 3.67

1.69 1.64

0.002 0.002

1.55 1.03

a

∑in= 1 (k3χexp − k3χtheo )2 ∑in= 1 (k3χexp )2

)

Ca K-edge EXAFS spectra of the SeO32 − and SeO42 −-doped ettringite and standard compound are shown in Fig. 7. As to all of the samples, the Fourier transform is dominant by the first shell of backscattering from oxygen. For the pure ettringite, every Ca atom is coordinated by 8 O atoms, 4 of which are provided by OH while the others are from H2O molecules at approximately 2.4 Å. In addition, approximately 2 Al atoms were fitted at 3.46 Å. The interatomic distance between Ca and O and Al deduced in fitting agrees with data from the literature [19]. The SeO32 −-doped ettringite showed that Ca atoms coordinate with approximately 8 O atoms. Four of them are from OH and the remainders are from H2O molecules. The interatomic distance and coordinated number of Ca and Al did not change after uptaking SeO32 −. For SeO42 −-doped ettringite, the first shell of Ca can be fitted with 8 O atoms near 2.5 Å. Four O atoms are provided by OH and the remaining 4 are from H2O molecules. Approximately 2 Al atoms were fitted at 3.47 Å, similar to that of the pure ettringite. 4. Discussion 4.1. SeO32 − and SeO42 − incorporation by ettringite It is well known that ettringite immobilizes oxoanions through electrostatic force in intercolumn spaces. The ionic radius of SO42 − is much smaller than that of SeO42 − because the SeeO bond length of SeO42 − and SeO32 − (Table 2) is longer than the SeO bond length of SO42 − (1.49 Å) [41]. Thus, the lattice parameter a should increase more significantly after incorporating SeO32 − and SeO42 − than pure sulfate ettringite. However, after sorption of SeO32 − in ettringite, the lattice parameters a and c decreased (Fig. 3). This could be derived from the fact that the immobilization of SeO32 − in ettringite occurs not only through electrostatic forces but also the surface complexation of SeO32 − on the intercolumn edge sites of ettringite. The eOH in gibbsite (Al(OH)3) and portlandite (Ca(OH)2) presented peaks at 3519, 3618, and 3459, and 3387 and 3643 cm− 1, respectively (Table S1, Fig. S1). As previously reported, H2O coordinated with Ca in clay or calcium carbonate produces eOH of stretching bands at 3540 and 3247 cm− 1 [37,38], and the strong peak in ettringite in this range should be assigned to the stretching vibration mode of OH in H2O bonded to Ca [29]. In addition, after background substitution for H2O, aqueous SeO32 − and SeO42 − exhibited bands at 2900 to 3500 and

Rf residual factor indicates the quality of fitting results and is expressed by following formula:

Rf =

16

3.6. EXAFS analysis of Ca k-edge of Se(IV) and Se(VI) doped ettringite

Table 2 Se K-edge EXAFS fitting results of SeO32 − and SeO42 − doped ettringite and standards. Coordination number (CN), interatomic distance (R), Debye-Waller factor (ơ2), and residual factor (Rf). Shell

14

doped ettringite. Because the Fourier transforms spectra showed no further peaks at a longer distance, the structural information obtained from EXAFS data analysis is limited to the first coordination shell.

ettringite. This is consistent with the previous report that both Se(IV) and Se(VI) oxidation states are found to be stable under the cementitious systems [39]. The Se K-edge EXAFS spectra of raw and fitted k3-weighted χ functions for SeO32 − and SeO42 − doped ettringite are shown in Fig. 6. For the Fourier transform in SeO32 − immobilized samples, the backscattering from an oxygen first shell is dominant, as indicated by the vertical dashed line at 1.3 Å (not corrected for phase shift). In addition, the Fourier transform of SeO32 −-doped ettringite and reference compound (CaSeO3) did not present an addition peak, indicating that Ca atom exhibits weak backscattering in SeO32 − bearing compounds. Thus, EXAFS result could not provide direct evidence for revealing the SeO32 − formation of inner-sphere or outer-sphere complexes in ettringite. In contrast, the Se K-edge EXAFS data for SeO42 −-doped ettringite and CaSeO4 are dominated by the backscattering resulted from an oxygen shell approximately 1.2 Å in the Fourier transform. The CaSeO4 showed an additional peak at approximately 2.9 Å in the Fourier transform. However, it was not observed for the SeO42 −-doped ettringite. The analyzed results of EXAFS fitting of SeO32 − and SeO42 − doped ettringite and related compounds are summarized in Table 2. The first shell of SeO32 −-doped ettringite can be fitted with 3 O atoms at 1.69 Å, similar to that of CaSeO3, which is characteristic of Se in pyramidal coordination. For SeO42 −-doped ettringite, the first shell of Se can be fitted approximately 4 O atoms at 1.64 Å, similar to that of CaSeO4, which is characteristic of the interatomic distance between Se and O in SeO42 − tetrahedron [39,40]. Unlike CaSeO4, no further peaks above the noise level were found near 3 Å in the Fourier transform of SeO42 −-

Sample

12

× 100

Value was fixed during the fitting procedure.

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(a)

Fig. 7. (a) k3-weighted Ca K-edge EXAFS data of SeO32 − and SeO42 − doped ettringite and a reference compound. (b) Corresponding Fourier transforms (not corrected for phase shift) showing both raw (solid lines) and fitted data (dash lines).

(b)

Ca O

2-

pure SeO4 -ettringite

Fourier transform amplitude

Ca Al 2-

pure SeO4 -ettringite 2-

3

k (k)

SeO3 -doped ettringite

2-

SeO3 -doped ettringite

2-

pure SO4 -ettringite

2-

pure SO4 -ettringite

0

1

2

3

4

5

2

6

4

6

R( )

8

k(

2-

)

Orginal sulfate ettringite Calculated

(a) Pure SO4 -ettringite

2

Absorbance/ a.u.

R 0.9987

Al

Ca

4000

Ca

3410.4 3466.5 channel 3534.8 3351.5 3631.6 3285.2 3204.4 3631.7

3800

3600

3400

water forms H-bonds with sufate

3200

3000

2800

-1

Wavenumber/ cm 2-

(b) 15% SeO3 doped ettringite

Orginal selenite ettringite Calculated 2

Absorbance/ a.u.

R 0.9983 Ca

Al 3540.5 3605

Ca

3473

3410.2 channel 3345.6 3276.5 3201

3605.1

4000

3800

water forms H-bonds with selenite 3119.3

3600

3400

3200

3029.4

2923.8

3000

2800

-1

Wavenumber/ cm 2-

Orginal selenate ettringite Calculated

(c) Pure SeO4 -ettringite

2

Absorbance/ a.u.

R 0.9973 Ca

Al

3377.6 3525.8 3452.6

Ca

3308.7 channel

3629.3

4000

3800

3600

water forms H-bonds with selenate

3226.5

3629.4

3118.4

3400

3200

3000

-1

Wavenumber/ cm

172

10

2800

Fig. 8. Curve fitted FTIR spectra of the 2750–4000 cm− 1 region for (a) pure sulfate ettringite; (b) 15% selenite-doped ettringite; (c) pure selenate ettringite.

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Fig. 5(b), increasing the incorporated SeO42 − slightly increased the water content in ettringite. This may be attributed to the increased lattice parameter a (Fig. 3(a)), which means the intercolumn space was enlarged after incorporating SeO42 − and more free H2O molecules could be retained in the intercolumn space. Thus, the water content of SeO42 − doped ettringite slightly increased (Fig. 5(b)).

Table 3 Ca K-edge EXAFS fitting results of SeO32 − and SeO42 − doped ettringite and standards. Coordination number (CN), interatomic distance (R), Debye-Waller factor (ơ2), and residual factor (Rf). Sample Standards Pure sulfate ettringite

Se sorption samples Se(IV)–ettringite

Pure selenate ettringite

Shell

CN

R(Å)

ơ2

Rf(%)

CaeOH CaeOH2 CaeAl

4a 4a 2a

2.37 2.50 3.47

0.005 0.005 0.004

1.11 – –

CaeOH CaeOH2 CaeAl CaeOH CaeOH2 CaeAl

3.76 3.76 1.88 3.86 3.86 1.93

2.34 2.48 3.46 2.37 2.51 3.47

0.007 0.007 0.004 0.005 0.005 0.004

1.59 – – 1.61 – –

4.2. Coordination chemistry analysis The Ca K-edge EXAFS spectra fitting result suggests that no coordination change occurred when SeO32 − was incorporated into the structure of ettringite. However, uptaking SeO32 − ions will result in H2O molecules decreasing in ettringite (Fig. 4(a) and 5(a)). In addition, based on the ettringite structure, the unit cell consists of column parts {Ca6[Al(OH)6]2·24H2O]}6 +, the columns based on the line of octahedral [Al(OH)6]3 − in the (001) plane, that link with three Ca atoms. Each Ca atom is coordinated by 4 OH and 4 H2O. Each Ca2 + shares 2 OH with the adjacent octahedral [Al(OH)6]3 − [19]. Therefore, there are two kinds of sites in these columns surface, namely, AlCa2eOH and CaeOH2. The CaeOH2 is the dominant species on the surface of column parts. SeO32 − can interact with these eOH or eOH2 surface sites through ligand exchange during immobilization, and AlCa2eOSeO2 and CaeOSeO2 complexes can be formed after the sorption of SeO32 −. Pauling's electrostatic valence principle can be used to predict the reactivity of ions on the mineral interface [29,42,43]. Combined with bond length (Tables 2 and 3) between different atoms and bond valence theory, it can predict and quantify whether the ions are coordinately saturated, unsaturated or oversaturated. The bond length of Se(IV)eO is approximately 1.69 (Table 2); CaeOH is approximately 2.45 Å (Table 3); and AleOH is approximately 1.92 Å [19]. Moreover, the valence unit of Se(IV)eO, CaeOH, and AleOH is 1.39, 0.26, and 0.48 v.u., respectively. If AlCa2eOSeO2 could be formed, the ∑j Sij of valence unit from different atoms linked with O will be approximately 2.47 v.u. ∑j Sij ˃2 suggests that coordination of SeO32 − with AlCa2eOH site will lead to this site being coordinatively oversaturated, and then this kind of bonding complex is predicted to be unstable and difficult to form. Thus, only the CaeOH2 site could be an ideal site for forming complexes through ligand exchange. Based on the above description and bond valence theory, the surface complexes of SeO32 − on ettringite are schematically illustrated in Fig. 9. Compared with EXAFS spectra of Se K-edge for CaSeO4 and SeO42 −doped ettringite, no distinct peak was observed approximately 3 Å in Se K-edge for SeO42 −-doped ettringite. Moreover, the structure information obtained from EXAFS spectra fitting is limited in the first shell, which is that the Se shell was fitted by around 4 O atoms close to 1.64 Å. In addition, uptaking of SeO42 − will not decrease the structural H2O molecules (Fig. 4(b) and 5(b)). These results also suggest that SeO42 − is immobilized by ettringite through electrostatic force or the formation of H-bonds with H2O molecules that existed in the intercolumn space. The surface structure of ettringite is maintained after incorporating SeO42 −.

Rf is the same as in Table 2. a Value was fixed during the fitting procedure.

3122 cm− 1, respectively (Fig. S1), indicating that interchannel water could form H-bonds with SeO32 − and SeO42 −. Based on these reference spectra, the vibration spectra in the region of 3000 to 3650 cm− 1 of pure SO42 − ettringite, SeO32 −-doped ettringite (5 mM), and SeO42 −doped ettringite were separated into the several components and then fitted with the original spectra, as shown in Fig. 8 (a), (b), and (c). In the region from 2800 to 3700 cm− 1 of pure ettringite, the peaks at 3631, 3534, 3467, 3352, and 3632 cm− 1 should be assigned to the OH group of AleOH and CaeOH vibration. The highest peak at 3410 and 3285 cm− 1 could be assigned to the OH group from CaeOH2. Moreover, the peak at 3204 cm− 1 is due to the vibration mode of channel water forming H-bonds with SO42 − [29] (Fig. 8(a)). After incorporating SeO32 −, the peaks related to OH vibrations were modified and shifted. Specifically, the intensities of the peaks at 3410 and 3276 cm− 1 of the OH groups from CaeOH2 vibration decreased, and the band at 3201 cm− 1 increased. This could result from the fact that incorporating SeO32 − removes Ca-coordinated H2O molecules and directly coordinates with Ca atoms. Furthermore, several new peaks from 2900 to 3200 cm− 1 appeared after the sorption of SeO32 − (Fig. 8(b)), since immobilized SeO32 − could interact with channel water (Fig. S1). The peak separation of the eOH vibration region of SeO42 −-doped ettringite is shown in Fig. 8(c). The eOH vibration spectra are similar to that of pure SO42 − ettringite Fig. 8(a). When all of the SO42 − was substituted with SeO42 −, however, the lattice parameters a of ettringite increased (Fig. 3(a)). This indicates that the crystal unit cell of ettringite was expanded and the bond length of atoms was expected to be longer. Therefore, the eOH vibrations of AleOH, CaeOH, and CaeOH2 shifted to lower wavenumbers than pure SO42 − ettringite. Although SeO42 − substitutes all of the SO42 − ions, the peak intensity of OH vibrations of CaeOH2 did not decrease (Fig. 4(b) and 8(c)). This also indicates that SeO42 − in ettringite was immobilized by electrostatic force and formed H-bonds with crystal H2O molecules in intercolumns. This agrees with the EXAFS fitting results of SeO42 − ettringite, which shows the absence of any contributions from second neighbors (Fig. 6). Based on the structure of ettringite, the H2O molecules are mostly coordinated with Ca atoms. When SeO32 − is immobilized in ettringite, the changes in the FTIR and TG results are likely to correspond to the H2O molecules, which are coordinated with Ca in the structure of ettringite and removed after immobilizing SeO32 − (Fig. 8(b)). Thus, with the amount of immobilized SeO32 − ions increasing in ettringite, the numbers of crystal H2O molecules decreased. This is consistent with the results that the peak intensities of eOH band in FTIR spectra and water contents in TG decreasing with increasing incorporated SeO32 − ions amounts (Fig. 4(a) and 5(a)). In contrast, increasing incorporated SeO42 − concentrations in ettringite did not decrease the intensity of eOH vibrations of CaeOH2 in FTIR spectra (Fig. 8(c)). As shown in

5. Conclusions Understanding of coordination chemistry and immobilization mechanisms of Se oxoanions in ettringite columnar parts edges is particularly important in environment remediation, because ettringite is proved to play an important role in toxic ions immobilization in hydration cement. In particular, spent radioactive species are usually stabilized and enclosed in cement. Based on EXAFS, FTIR, and TG results, SeO32 − is easily immobilized to form inner-sphere complexes with CaeOH2 on the channel edges of ettringite. Meanwhile SeO42 − is immobilized through outer-sphere complexation via anion exchange with SO42 − in ettringite. This suggests that SeO32 − in ettringite is more stable than SeO42 − because inner-sphere complexes are more resistant to remobilization than outer-sphere complexes. This implies that 173

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Fig. 9. Schematic illustration of coordination of SeO32 − in ettringite. Numbers indicate the sum of bond valence values of different atoms.

ettringite in artificial cement and/or nature can be an absorbent of SeO42 − to reduce the mobility of SeO42 − in the cement-based repository. However, some coexisting oxoanions e.g. B(OH)4−, which have higher affinity with ettringite than SeO42 − [15], might exclude SeO42 − from ettringite, because SeO42 − only can form outer-sphere complexes in ettringite. Thus, this study sounds the warning of the rediffusion risk of SeO42 − from ettringite when ettringite in the matrix of cement is exposed to the competing oxoanion in environments. However, the SeO32 − is much more stable in ettringite of the matrix of cement.

[16] [17]

[18]

[19] [20]

Acknowledgement

[21]

Financial support was provided to KS by the Japan Society for the Promotion of Science (JSPS) KAKENHI (A) (No. JP16H02435). The EXAFS experiments were performed at Kyushu University Beamline (SAGA-LS/BL06) with the proposal No. 2016IIK002.

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