Spectroscopic and DFT study on the species and local structure of arsenate incorporated in gypsum lattice

Accepted Manuscript Spectroscopic and DFT study on the species and local structure of arsenate incorporated in gypsum lattice

Shaofeng Wang, Danni Zhang, Xu Ma, Guoqing Zhang, Yongfeng Jia PII: DOI: Reference:

S0009-2541(17)30205-X doi: 10.1016/j.chemgeo.2017.04.011 CHEMGE 18314

To appear in:

Chemical Geology

Received date: Revised date: Accepted date:

7 November 2016 8 April 2017 11 April 2017

Please cite this article as: Shaofeng Wang, Danni Zhang, Xu Ma, Guoqing Zhang, Yongfeng Jia , Spectroscopic and DFT study on the species and local structure of arsenate incorporated in gypsum lattice. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Chemge(2017), doi: 10.1016/ j.chemgeo.2017.04.011

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ACCEPTED MANUSCRIPT Spectroscopic and DFT study on the Species and Local Structure of Arsenate Incorporated in Gypsum Lattice

Key Laboratory of Pollution Ecology and Environmental Engineering, Institute of

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Shaofeng Wang,*,a Danni Zhang,a Xu Ma,a Guoqing Zhang,a Yongfeng Jia*,a,b

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Applied Ecology, Chinese Academy of Sciences, Shenyang, China, 110016 Institute of Environmental Protection, Shenyang University of Chemical Technology,

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Shenyang, China, 110142

* Corresponding author: Dr. Shaofeng Wang, Email: [email protected], Tel:

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+86 24 83970502, Fax: +86 24 83970503; Prof. Yongfeng Jia, Email:

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[email protected], Tel: +86 24 83970503, Fax: +86 24 83970503

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ACCEPTED MANUSCRIPT ABSTRACT The incorporation of arsenate (As(V)) in the gypsum structure is an important process for arsenic (As) fixation during industrial effluent treatment and may influence the mobility and bioavailability of As in surface environment. However,

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spectroscopic evidence is still lacking for its species and local structure. The species

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and local structure of As(V) in a gypsum lattice were investigated using Fourier

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transform infrared (FTIR) spectroscopy, density functional theory (DFT) modelling, and full-potential multiple scattering (FPMS) simulations. Ascorbic acid-treated

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As(V)-gypsum co-precipitates were used to avoid the influence of amorphous calcium

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arsenate on the characterization. The lack of the FTIR band in the range of 750 – 860 cm-1 was an indicative that no AsO43- species was incorporated into the gypsum

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structure. DFT calculations proved that the incorporation of AsO43- was energetically

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much harder than HAsO42- species. The FPMS structural refinement yielded the optimal As-O interatomic distances of 1.77, 1.67, 1.65, and 1.66 Å, with an average of

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1.69 ± 0.057 Å, in agreement with the DFT and EXAFS results. Our work

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conclusively showed that HAsO42- dominated as the species of As(V) incorporated into the gypsum lattice, with the H atom in the HAsO42- group adjacent to water layer, regardless of pH.

Keywords: arsenate; gypsum; incorporation; species; full potential multiple scattering

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ACCEPTED MANUSCRIPT 1. INTRODUCTION Arsenic (As), in close association with base metals and precious metal ores, can be liberated by natural weathering and anthropogenic processes (e.g., pyrometallurgical and hydrometallurgical operations). Tens of thousands of tons of As are liberated

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every year by the non-ferrous metal industry around the world. Gypsum is generated

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as a principal by-product during industrial As removal processes involving the

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neutralization of acidic sulfate-rich solutions with lime as a base. Appreciable amounts of As(V) can be incorporated into the lattice structure of gypsum via the

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isomorphic substitution of sulfate (Fernandez-Martinez et al., 2006, 2008; Jia et al.,

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2010; Zhang et al., 2015; Fujita et al., 2009). For instance, the concentration of As in gypsum sludge generated by the milling processes of borate deposits in Turkey

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reached ~2000 ppm (Lin et al., 2013). Fujita et al. (2009) found that the gypsum still

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contained 433 ppm of As after 5 times washing with pH 1 H2SO4. Because the annual production of gypsum by the non-ferrous smelting industry is enormous, the amount

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of As incorporated into the bulk structure of gypsum and its impact on local

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environments cannot be ignored. Therefore, it is important to understand clearly the species and atomic structure of As contained within the gypsum structure. The species and local structure of As(V) incorporated into gypsum have been investigated by using neutron and X-ray diffraction, extended X-ray absorption fine structure (EXAFS) spectroscopy, DFT calculations, and single-crystal electron paramagnetic resonance spectroscopy (EPR) (Fernandez-Martinez et al., 2006, 2008; Jia et al., 2010; Lin et al., 2013; Zhang et al., 2015) . However, there is still no clear 3

ACCEPTED MANUSCRIPT spectroscopic evidence for the local structure of As(V) in gypsum lattices. Fernandez-Martinez et al. thought, based on the Mulliken population analyses and charge equilibration, that the dominant species of As(V) incorporated into gypsum is most likely HAsO42- (Fernandez-Martinez et al., 2008). Lin et al. found that a

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considerable fraction of As(V) (26–60%) was reduced to As(III) during its

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incorporation into gypsum using As K-edge X-ray absorption near edge spectra

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(XANES) (Lin et al., 2013). This seems to be impossible because limited amount of As(III) can be incorporated into gypsum (Zhang et al., 2015), and As(V) in the

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structure of gypsum is hindered in contacting the aqueous reducing agent and bacteria

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under oxidizing conditions. It has been discovered that calcium (hydrogen) arsenate species (e.g., pharmacolite) can be formed during the interaction of gypsum and

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aqueous As(V) at a higher pH and greater initial As(V) concentrations (Jia et al., 2010;

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Rodrfguez-Blanco et al., 2007, 2008; Zhang et al., 2015). Chemical analysis and LCF of Ca K-edge XANES spectra showed that approximately 50% of As(V) occurred as

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amorphous calcium arsenate when gypsum was co-precipitated with a 5 g L-1 As(V)

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solution at pH 7 (Jia et al., 2010; Zhang et al., 2015). The samples of gypsum in the As(V) solution were synthesized at pH 4, 7.5, and 9 by Fernandez-Martinez et al. (2006), and at pH 7.5 with an initial As(V) concentration of 0.02 M by Lin et al. (2013). Therefore, the samples synthesized at higher pH (pH > 7) by both of them very likely contained appreciable amounts of calcium arsenate, which may have affected the interpretations of the EXAFS data. In this work, pure As(V)-doped gypsum was obtained by treating the coprecipitates 4

ACCEPTED MANUSCRIPT with ascorbic acid to remove the calcium arsenate species that might precipitate. Samples precipitated from a lead-zinc smelting effluent containing approximately 6.6 g L-1 As, and a variety of other constituents (e.g., Cd, Pb, etc.) were also prepared to investigate incorporation of As(V) in gypsum during real wastewater treatment. The

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species and structure of As(V) incorporated in the gypsum lattice were investigated by

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the joint application of FTIR spectroscopy, DFT modelling, FPMS simulations, and

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EXAFS fitting. 2. MATERIALS AND METHODS

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2.1 Synthesis of As-free and As(V)-doped Gypsum

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As-free gypsum was precipitated as the reference material by mixing a calcium solution (from Ca(NO3)24H2O, 20,000 mg L-1) with a solution of sulfate (from

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Na2SO4, 48,000 mg L-1) at pH 7. As(V)-doped gypsum was synthesized by adding a

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calcium solution to a solution of sulfate and As(V) at pH 3-10, with As(V) concentrations of 5,000 mg L-1. Slaked lime was used as a base to adjust and control

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the pH. An industrial effluent (IE) (See Table S1 for solution composition) collected

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from a lead-zinc smelter was also used to precipitate As(V)-doped gypsum samples in the same manner at pH 5 (designated as IE-1) and pH 7 (designated as IE-2). The As(V)-gypsum co-precipitates synthesized at higher pH values (pH≥7), and IE samples were treated two times with ascorbic acid solutions for one hour to remove the calcium arsenate and other metal arsenate phases that might have precipitated during the co-precipitation process. All of the solids were separated by filtration, rinsed five times with DI-water, dried in a vacuum oven at 60 C overnight, and 5

ACCEPTED MANUSCRIPT stored at room temperature in a desiccator for further analysis. The concentrations of Cd, Pb, and Zn were lower than 50 mg kg-1 in the IE samples treated with ascorbic acid (Table S2 in the supporting information). XRD patterns confirmed that the resulting solids occurred as gypsum without other crystalline phases detected (Fig. S1

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in the supporting information).

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2.2 Elemental Analysis

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The solid samples were dissolved in a 1 mol L-1 HCl solution and diluted for the analysis of As and Na concentrations. The concentrations of As in the digests were

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determined on an atomic fluorescence spectrophotometer coupled with a hydride

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generator (HG-AFS) with a detection limit of 0.1 µg L-1. The concentration of sodium was analyzed by a Dionex (Thermo Fisher, USA) ICS-5000 ion chromatograph. The

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concentrations of SO42- was measured with inductively coupled plasma—atomic

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emission spectroscopy (ICP-AES, Thermo-6300) with the detection limit of 0.03 mg L-1.

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2.3 Infrared Analysis

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The FTIR spectra of the powdered samples were collected on a Thermo Nicolet 6700 Fourier transform IR spectrometer. Before characterization, the KBr/sample discs were prepared by mixing 0.5% of finely ground samples in KBr. The samples were scanned in the mid-IR range (400 to 4000 cm-1) at a resolution of 4 cm-1. The FTIR spectra were analyzed by comparison with previously reported spectra. 2.4 DFT Modelling The structures of pure and As(V)-doped gypsum (2×2×2 supercell) were optimized 6

ACCEPTED MANUSCRIPT using computational codes (pw.x) available in the QUANTUM-ESPRESSO package (Version 6.0), which solves the Kohn-Sham equations based on density function theory, plane waves, and pseudopotentials (Giannozzi et al., 2009). The crystallographic data of gypsum measured at 0.0001 GPa was used as the initial

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model (Comodi et al. 2008). The exchange and correlation functions were

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approximated through the generalized gradient approximation using the PW91

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functional (Fernandez-Martinez et al., 2008; Giacomazzi and Scandolo, 2010). The geometric optimization of the lattice and internal parameters was carried out using

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variable-cell relaxation (vc-relax) in a periodic system. Optimized Norm-Conserving

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Vanderbilt (ONCV) norm-conserving pseudopotentials (Schlipf and Gygi, 2015) were used for calculations. To balance the computation cost and efficiency, supercell

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calculations were carried out at the Γ-point of the Brillouin zone, while denser

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k-meshes were used for other bulk minerals. Convergence tests showed that the use of plane-wave kinetic energy cut-off at 80 Ry for wave functions and 320 Ry for charge

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density and potential could give a satisfactory convergence of the total energy (Etot) (<

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0.2 mRy atom-1) (Fig. S2 in the supporting information). The optimization procedures were finished when the convergence thresholds of Etot and forces reached 10-4 Ry and 10-3 Ry/a.u., respectively. The semiempirical dispersion correction at D2 level (DFT-D2) (Grimme, 2006) was applied to the structural optimization. 2.5 X-Ray Absorption Spectroscopy (XAS) Measurements To collect the XAS spectra, approximately 0.1 g of the powdered solids were pressed into discs of approximately 1 cm in diameter. The As K-edge XAS spectra 7

ACCEPTED MANUSCRIPT were obtained on the XAFS beamline at the Beijing Synchrotron Radiation Facility (BSRF). The storage ring was operated at 2.5 GeV and the ring current was 250 mA. The white beam was monochromatized using a fixed double crystal monochromator with Si(111) crystals. The absolute position of the monochromator was calibrated to

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11919 eV by setting the first inflection point in the LIII-edge absorption spectrum of an

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Au foil. The absorption spectra of As(V)-doped gypsum formed at pH 4, 6, 8, and 9

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(in fluorescence mode) and reference materials (Na3AsO4, Na3AsO3, and BaHAsO4) (in transmission mode), were collected in the energy range of 11660-12680 eV at 77

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K in order to minimize the photo-reduction of As(V) under the X-ray (Muehe et al.,

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2016). The scan steps were set to 5 eV/step for the pre-edge, 0.5 eV/step for the XANES, and 3 eV/step for the EXAFS regions. Each individual sample was measured

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one to three times depending on the spectral quality, and the averaged results were

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reported.

2.6 Data Pre-processing and EXAFS Analysis

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The pre-processing of the spectra was performed using the Athena program in

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the Demeter package (version, 0.9.20) (Ravel and Newville, 2005). The pre-edge of spectrum was fitted with a straight line in the range of 150–50 eV below the absorption edge. The post-edge absorption background was subtracted by fitting a cubic polynomial spline to the data in the energy range of 150 – 750 eV above the absorption edge. EXAFS data analysis was carried out by fitting the Fourier transformed experimental data with the theoretical amplitude and phase functions calculated by 8

ACCEPTED MANUSCRIPT FEFF code. The crystallographic data of the DFT optimized structure was used as the FEFF input. The least-square fitting of the k3-weighted EXAFS spectra was carried out in the k range of 2.8 – 12.5 Å-1 and the R range of 0.9 – 3.6 Å for the As K-edge spectra using the Artemis code available in the Demeter package with a Kaiser−Bessel

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window function (Ravel and Newville, 2005). The amplitude reduction factor (S02),

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which was set to 0.95, was obtained from the fitting result of the BaHAsO4 standard

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material (Fig. S3 and Table S3 in the supporting information). 2.7 FPMS Calculations

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The As K-edge XANES spectra from 20 eV below the absorption edge to 150 eV

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above the absorption edge were calculated by FPMS calculations using the recently developed full-potential MXAN (FPMXAN) software (Hatada et al., 2007, 2009,

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2010). The analysis procedures have been described in previously published literature

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(Benfatto et al., 2001; Benfatto and Della Longa, 2001; Chillemi et al., 2007; D'Angelo et al., 2002; Monesi et al., 2005; Sarangi et al., 2005; Wang et al., 2016).

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The real part of the self-energy was calculated using the Hedin-Lunqvist (H-L)

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potential. The program used the Monte-Carlo search to optimize the structure and non-structure parameters alternatively until the global minimum was achieved. The fitting quality was evaluated using the goodness-of-fit parameter, square residue function (S2), which is defined by the following equation (Benfatto et al., 2001; Benfatto and Della Longa, 2001):

w i y ith  i m

S2  n

 

 y iexp  i1

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1

m

wi

i 1

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ACCEPTED MANUSCRIPT where n is the number of independent parameters, m is the number of data points, yith and yiexp are the theoretical and experimental values of the absorption, respectively, εi is the error in each point of the experimental data set, and wi is a statistical weight.

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3.1 Arsenic and Sodium Concentrations in the Solid Phase

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3. RESULTS AND DISCUSSION

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Table 1 shows the concentrations of As and Na in the ascorbic acid-treated As-gypsum co-precipitates formed at different pHs and the aqueous As concentrations.

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The As concentrations in the separated gypsum were in the range of 530–19,700 mg

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kg-1. At pH < 8, the As content in the gypsum increased as the pH increased, while it decreased to 7,640 mg kg-1 over pH 8-10. This variation trend is different from the

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results reported by Fernandez-Matinez et al. (2008) because the amorphous calcium

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arsenate species formed at high pH was not removed in their study. The IE-1 and IE-2 samples after acid treatment contained approximately 6,000 and 13,280 mg kg-1 of As,

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respectively.

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Sodium may be a potential charge compensator if the AsO43- species is the dominant species incorporated in the gypsum lattice. Our results showed that the concentration of Na in the ascorbic acid-treated gypsum was in the range of 2,850 – 3,460 mg kg-1. Although the concentration of Na was higher than the concentration of As in the solid phase at most pH values, it was not enough to compensate for the charge at pH 6 and 7 if only the AsO43- species was incorporated. This implied that Na+ might not be a charge compensator, at least not exclusively. 10

ACCEPTED MANUSCRIPT Based on the total As(V) concentration in the final solution (Table 1), the equilibrium species of As(V) in the aqueous phase at 25 °C was calculated (Fig. 1a). The relationships between As content in gypsum and various aqueous As(V) species were plotted in Fig. 1b. Although H2AsO4- and H3AsO40 are the main aqueous As(V)

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species over the pH 2 – 6.9, poor correlations between them and As contents in

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gypsum (R2 = -0.02, p > 0.05 for H2AsO4- and R2 = 0.24, p > 0.05 for H3AsO40)

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indicated that these species are not likely to be incorporated into the gypsum structure. The content of As in gypsum correlated significantly with the both aqueous HAsO42-

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(R2 = 0.85, p < 0.01) and AsO43- (R2 = 0.72, p < 0.01) (Fig. 1b). However, the aqueous

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HAsO42- concentration is 1 – 5 orders of magnitude higher than aqueous AsO43- at pH 3 – 10, implying that aqueous HAsO42- species may have stronger influence on the

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3.2 Infrared Analysis

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incorporation of As(V) in gypsum than AsO43-.

The FTIR spectra of As-free and As-doped gypsum are presented in Fig. 2. In

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general, all samples appeared to have the characteristic bands of gypsum. The two

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strong bands peaking at 1,119 cm-1 and 1,145 cm-1 on the FTIR spectra are assigned to SO4 symmetric stretching vibrations (1) and asymmetric stretching vibrations (3), respectively, whereas the SO4 in-plane bending vibrations (4) appeared at 602 cm-1 and 669 cm-1 (Giacomazzi and Scandolo, 2010; Knittle et al., 2001). The bands in the range of 3,100-3,700 cm-1 and 1,550-1,750 cm-1 corresponded to the symmetric and antisymmetric water stretching and bending vibration regions, respectively (Giacomazzi and Scandolo, 2010; Knittle et al., 2001). It was proposed that sulfate 11

ACCEPTED MANUSCRIPT may have been substituted with unprotonated AsO43- ion via the formation of H3O+ molecules to balance the charge (Fernandez-Martinez et al., 2008). The formation of H3O+ molecules should induce the stretching vibration (H3O+) bands appeared in the range of 2621 – 2990 cm-1 and the bending vibrations in the range of 1718 – 1843

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cm-1 (Clavier et al., 2016). However, we did not observe a band shift in the O-H

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vibration s of As-doped gypsum compared to the As-free sample (Fig. 2b), indicating

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that no or very little H3O+ was formed.

All As-doped gypsum samples displayed the same As-O bands at 872, 903, and 928

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cm-1 and a shoulder band at 863 cm-1 on the FTIR spectra (Fig. 2c and Fig. S4). This

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showed that the presence of cationic ions (Cd2+, Pb2+, and Zn2+) in industrial effluents have no effect on the structure of As(V) in gypsum. The species of As(V) in solid

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phase strongly influence the As-O vibrations. Vibrational spectroscopy of AsO4

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tetrahedra in metal arsenate crystals has been reported by many previous works (Frost et al., 2010a, 2010b, 2012; Gomez et al., 2010a, 2010b, 2011; Miller and Wilkins,

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1952; Myneni et al., 1998; Sejkora et al., 2010). For complexation with Ca2+, the

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FTIR spectra of sainfeldite (Ca5(HAsO4)2(AsO4)2·4H2O) include the symmetric (νs) and antisymmetric (νas) stretching of HAsO42- at 869 and 895 cm-1, the νs and νas stretching of AsO43- at 824 and 852 cm-1, and the As-OH stretching at 713 cm-1, respectively (Myneni et al., 1998). For HAsO42- species in pharmacolite and haidingerite (CaHAsO4·2H2O), the infrared bands at 864 cm-1 (the ν1 symmetric stretching mode), at 903, 893, 840 cm-1 (the ν3 (AsO3) antisymmetric stretching mode), and at 711 cm-1 (the ν As-OH stretching) were observed (Frost et al., 2010b). 12

ACCEPTED MANUSCRIPT According to these results, the FTIR νs and νas stretching of AsO43- complexed with Ca2+ are located lower than 860 cm-1, while the νs and νas vibrations of HAsO42appear at positions higher than 840 cm-1 for calcium (hydrogen) arsenate minerals (Myneni et al., 1998; Frost et al., 2010a, b). Therefore, the lack of bands in the range

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of 750 - 860 cm-1 on the FTIR spectra indicates that no AsO43- was incorporated into

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the As-doped gypsum. In combination of correlations between As contents in gypsum

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and the aqueous As(V) species, it can be concluded that the incorporated As(V) species in the gypsum lattice ought to be solely HAsO42-.

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The bands at 863, 872, 903, and 928 cm-1 in As-doped gypsum were assigned to the

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ν1 symmetric and/or ν3 antisymmetric stretching vibrations of unprotonated As-O bonds. The 928 cm-1 band arose from unprotonated As-O bonds in the HAsO42- groups

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(Myneni et al., 1998). A very weak feature was observed at ~708 cm-1 on the FTIR

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spectra of the solids formed at pH 6, 7, and 8, as well as the IE samples. This band was consistent with the As-OH vibrations centered at 707 cm-1 in pharmacolite

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(CaHAsO42H2O) and haidingerite (CaHAsO42H2O) (Frost et al., 2010a, b), which

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dominantly form at Ca/As molar ratio = 1 and pH 3-7 (Swash and Monhemius, 1996), representing the As-OH symmetric stretching vibrations. The As-OH stretching vibration band was not obvious at pH 3, 4, 5, 9, or 10, due to the low As content in the solid phase. 3.3 DFT Calculation Results Based on the aqueous As(V) species analysis and the FTIR results, HAsO42- were the most probable species incorporated into the gypsum lattice. Since two different 13

ACCEPTED MANUSCRIPT types of O sites (O1 and O2) exist in SO4 groups within gypsum (Fig. S5 in the supporting information), three configurations for the As(V) incorporated into gypsum were proposed: SO42- substituted by (C1) HAsO42- with a protonated O1, (C2) HAsO42- with a protonated O2, and (C3) AsO43- with the charge balanced by Na+ for

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comparison. After preliminary evaluations, 2×2×2 supercells with and without As

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substitution were constructed as the starting models. The As concentration of 13418

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mg kg-1 in the theoretical models was at the same order of magnitude with the highest As content in the pH 7 sample (19,700 mg kg-1) (Table 2).

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The calculated lattice constants of As-free gypsum agreed well with the theoretical

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values reported by Giacomazzi and Scandolo (2010) and with the experimental values reported by Comodi et al. (2008), with slight decrease of approximately 2.9% in the

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cell volume (Table 2). The mean S-O bond length in the DFT-optimized supercells of

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different As-doped gypsums was equal to that of the As-free gypsum supercell, indicating that the incorporation of As(V) within the gypsum lattice had little effect on

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the bond length of S-O. This result agreed with the FTIR spectra showing that the SO4

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vibration bands of all As-doped gypsums were at the same location as those of the As-free gypsum. The DFT optimized local structure and whole supercells of As-doped gypsum are shown in Fig. 3 and Fig. S6, respectively. The mean calculated bond length of As-O ranged from 1.68 to 1.69 Å, with As-OH at 1.77 Å for C1 and at 1.79 Å for C2, in agreement with previously reported DFT-calculated values (1.69 – 1.72 Å) (Fernandez-Martinez et al., 2008). Due to the larger size of the AsO4 tetrahedra (protonated and/or unprotonated arsenate ions) compared to that of the SO4 tetrahedra 14

ACCEPTED MANUSCRIPT in gypsum, the incorporation of As induced expansions of 0.12%, 0.21%, and 0.47% in the supercell volumes of configurations C1, C2, and C3, respectively, relative to the calculated values for the pure gypsum supercell. The slight distortion of the gypsum lattice was also observed. The lattice constant β increased from 115.45° in the As-free

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gypsum supercell to 115.73° (C1), 115.55° (C2), 115.56° (C3). These increases in cell

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parameters were consistent with a previous report in which Rietveld refinements of

concentration in gypsum (Zhang et al., 2015).

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X-ray diffraction showed that the lattice constants a, b, c, and β increased with As

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Based on the Etot of each component (Table 2), the incorporation energy (Einc) of

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each configurations was calculated to be 0.0153, 0.0326, and 0.0666 Ry for configuration C1, C2, and C3, respectively according to the reactions listed in Table 3.

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The lowest Einc of configuration C1 indicated that the HAsO42- species was

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energetically favorable to be incorporated into the gypsum lattice with the HO group of HAsO42- toward the water layer rather than toward the Ca atoms. The high Einc of

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configuration C3, more than four times that of C1, implied that the incorporation of

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the AsO43- species with charge compensation by Na+ was energetically much harder than HAsO42- species. This agreed with the geochemical modelling and FTIR results that HAsO42- was the dominant species in gypsum. 3.4 FPMS Results The As K-edge XANES spectra of As-doped gypsum obtained at pH 4, 6, 8, and 9 as well as the standard materials (Na3AsO4 and Na3AsO3) clearly suggested that As occurs exclusively as As(V) in gypsum after treatment with ascorbic acid (Fig. 4a). 15

ACCEPTED MANUSCRIPT The XANES spectra of As-doped gypsum at different pH values were closely similar, indicating that the species of As(V) incorporated into the gypsum structure was similar and independent of pH values. This was consistent with the results of FTIR spectroscopy presented above.

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To further probe the As(V) species incorporated in the gypsum structure,

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non-structural FPMS simulations were carried out based on the DFT optimized

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structures by optimizing the non-structural parameters including Fermi energy, energy shift, normalization factor, energy independent broadening, and energy dependent

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broadening, etc. Since the hydrogen atoms in water molecules have a negligible effect

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on the best fit accuracy (Benfatto and Della Longa, 2001), only the H atom in the HAsO42- group was considered during the theoretical calculations. The cluster radius

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of 6 Å, which included 56 atoms, was chosen to calculate the theoretical XANES

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spectra. Due to the negligible differences among the samples, the averaged As K-edge XANES spectrum of four samples was used for comparison with the theoretical data

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(Fig. 4b). There are five features (A-E) in the As K-edge spectra of As-doped gypsum

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(See Fig. 4b). Features A and B were mainly influenced by the O atoms of water molecules and feature C was attributed to the multiple scattering of all atoms. The configuration C1 best simulated all five features with the smallest goodness-of-fit (S2 = 3.1). However, the features D and E, which were influenced by Ca atoms, was not well reproduced by C2 (S2 = 3.8) and C3 (S2 = 4.9). The results indicated that the HAsO42- species was incorporated in the gypsum structure with the O1 atom in the HAsO42- group protonated, instead of AsO3(O2H)2- and AsO43- species. This result 16

ACCEPTED MANUSCRIPT was in agreement with the incorporation energy results discussed above, but disagreed with the conclusion that the O2 atom in HAsO42- was protonated, as proposed by Fernandez-Martinez et al (2008). Our result is more reasonable because the protonated O was always found to be adjacent to the water layer rather than to metal

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ions in most hydrated metal hydrogen arsenate minerals, such as BaHAsO4·H2O

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(Jimenez et al., 2004), pharmacolite (CaHAsO4·2H2O) (Ferraris et al., 1972), and

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Na2HAsO4·7H2O (Baur and Khan, 1970).

A shell-by-shell structural refinement was carried out to determine the optimal

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interatomic distance in the As-doped gypsum cluster. During the refinement, the

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groups of atoms, i.e., 4 O atoms around the central absorber, two SO4 groups, and 4 Ca atoms, were refined together at each step. The structurally refined theoretical

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XANES spectra matched well with the experimental data, with an S2 of 2.5 (Fig. 5a).

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The optimized bond lengths were 1.77, 1.67, 1.65, and 1.66 Å for the As-O1H, As-O2, As-O3, and As-O4 bonds, respectively, with an average of 1.69 ± 0.07 Å (Fig. 5b).

CE

The refined As-Ca interatomic distances were 3.30, 3.13, 3.69, and 3.62 Å, averaging

AC

at 3.44 ± 0.27 Å. The two near Ca atoms were bidentate mononuclear (edge sharing) complexed to HAsO42- and the two far Ca atoms were monodentate mononuclear (corner sharing) complexed to HAsO42-, similar to sulfate groups. 3.5 EXAFS Analysis Results Based on the DFT optimized structure of configuration C1, the As K-edge EXAFS spectra were also fitted (Fig. 6 and Table 4) for comparison. The best-fit interatomic distances were ~1.66 Å for As-O (unprotonated) and ~1.73 Å for As-OH, with an 17

ACCEPTED MANUSCRIPT average of 1.68 ± 0.057 Å. The averaged As-O bond length agreed well with our DFT and

FPMS

results

and

with

previously

reported

values

(~

1.68

Å)

(Fernandez-Martinez et al., 2008), suggesting that C1 is a reliable configuration for the local structure of HAsO42- incorporated into gypsum. The averaged interatomic

PT

distances of As-Ca were 3.15 ± 0.07 Å for the two nearer Ca atoms (edge-sharing)

RI

and 3.67± 0.07 Å for the other two Ca atoms (corner-sharing). These interatomic

SC

distances were also consistent with those derived from FPMS structural refinement. 4. CONCLUSIONS

NU

We jointly used FTIR, DFT, FPMS, and EXAFS to investigate the species and local

MA

structure of the As(V) substituting sulfate ions in gypsum. The lack of FTIR bands in the range of 750 – 860 cm-1 indicated that no AsO43- species occurred in the gypsum

D

structure. The calculated incorporation energy suggested that the incorporation of

PT E

HAsO42- species with the H atom toward water layer is energetically more favorable, whereas the AsO43- species is much harder to be incorporated in the gypsum structure.

CE

This result was validated by FPMS simulations and EXAFS analysis. The FPMS

AC

structural refinements delivered the optimized bond lengths of 1.77, 1.67, 1.65, and 1.66 Å for the As-O1H, As-O2, As-O3, and As-O4 bonds, respectively, with an average of 1.69 ± 0.07 Å, consistent with the EXAFS results. Our results suggested that HAsO42- was the sole As(V) species incorporated in the gypsum lattice independent of pH, with the protonated O atom oriented towards the water layer. A chemical formula of Ca(SO4)x(HAsO4)1-x·2H2O for the As-doped gypsum could be proposed based on our results. 18

ACCEPTED MANUSCRIPT

ACKNOWLEDGEMENTS Financial support was provided by the National Natural Science Foundation of China (Nos. 41530643, 41303088, and 41473111) and the Strategic Priority Research

PT

Program of the Chinese Academy of Sciences (No. XDB14020203). The As K-edge

RI

XAS spectra measurements were conducted on the XAFS beamline at the Beijing

SC

Synchrotron Radiation Facility. The DFT calculation and FPMS simulation were

MA

Appendix A. Supplementary data

NU

performed at the Beijing Super-computing Center of Chinese Academy of Sciences.

Additional materials include the composition of the industrial effluent, DFT

D

convergence tests, unit cell of gypsum, theoretical structures of As(V)-doped gypsum,

PT E

and EXAFS fitting parameters of BaHAsO4.

CE

References

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2016. Arsenic(V) Incorporation in Vivianite during Microbial Reduction of Biogenic

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Density

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Zhang, D., Yuan, Z., Wang, S., Jia, Y., Demopoulos, G.P., 2015. Incorporation of

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arsenic into gypsum: Relevant to arsenic removal and immobilization process 300,

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in hydrometallurgical industry. Journal of Hazardous Materials

AC

CE

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D

MA

NU

272-280.

26

ACCEPTED MANUSCRIPT Table 1 The concentration of As in the solution after coprecipitation, the contents of As and Na, and the S/As molar ratio in ascorbic acid-treated gypsum precipitated at

As in solution (mg L-1)

As in gypsum (mg kg-1)

Na in gypsum (mg kg-1)

S/As ratio

3a

4890

530

2850

823

4

a

4680

810

3140

538

5

a

4530

3430

3000

127

6

a

3440

13160

3060

33

7

a

3420

19700

3200

8

a

1620

9290

3360

9

a

302

7610

2940

57

SC

22 47

50

7640

3460

57

IE-1(pH5)

b

-

6000

-

73

IE-2(pH7)

b

-

13280

-

33

The initial concentration of As in the synthetic solution was 5000 mg L-1.

b

The

MA

a

NU

10

a

molar

PT

pH

RI

pH 3-10.

AC

CE

PT E

D

initial concentration of As in the industrial effluent (IE) was 6600 mg L-1.

27

ACCEPTED MANUSCRIPT

Table 2. Experimental and DFT calculated cell parameters, interatomic distances of As-O and S-O for pure gypsum, different types of As-doped gypsum, and relavent minerals used for incorporation energy calculation. The optimized structures of different As-doped gypsum configurations are shown in Fig. 3 and Fig. S6 in the supporting information. Formula

Description

As content

Ca4(SO4)4·8H2O

a

Ca32(SO4)32·64H2O

b

NaCa32(SO4)31(AsO4)·64H2O (C3)

2-

3-

1 SO4 substituted by 1 AsO4

and 1

D E

Na Ca2(HAsO4)2·4H2O

Ca10(HAsO4)4(AsO4)4·8H2O

Ca8(HAsO4)8·8H2O

Ca8(OH)16 Na8(OH)8·8H2O a

PT

Primitive cell of pharmacolite

E C

Primitive cell of sainfeldite

C A

Unit cell of Ca(HAsO4)·H2O

Supercell of portlandite Supercell of NaOH·H2O

13418 N.D.b

N.D.

b

N.D.

b

N.D.

b

N.D.

b

1.47 ± 0.005

N.D.b

-9275.0133

90, 115.73, 90

3836.54

1.47 ± 0.005

1.68 ± 0.060

-9268.5624

11.28

29.75

protonated)

3831.87

11.27

12.69

(C2)

90, 115.45, 90

90, 114.11, 90

13418

13418

N.D.b

5.67

C S U

2×2×2 gypsum supercell with:

1 SO42- substituted by 1 HAsO42- (O2 is

N.D.b

15.18 29.73

Ca32(SO4)31(AsO3O2H)·64H2O

1.47 ± 0.000

6.28 12.67

protonated)

493.34

α,β,γ (°)

N.D.b

(C1)

As-O

c (Å)

2×2×2 gypsum supercell

N A

12.70

M

29.74

11.27

Etot (Ry)

S-O

b (Å)

N.D.

1 SO42- substituted by 1 HAsO42- (O1 is

I R

Bond length (Å)

Volume (Å3)

a (Å) Observed unit cell of gypsum

Ca32(SO4)31(AsO3O1H)·64H2O

T P

Cell parameters

(mg kg-1)

90, 115.55, 90

(As-OH, 1.77) 3839.82

1.47 ± 0.006

1.69 ± 0.071

-9268.5445

(As-OH, 1.79)

12.70

29.79

11.28

90, 115.56, 90

3849.70

1.47 ± 0.005

1.68 ± 0.017

-9352.691

8.26

8.26

6.12

76.9, 76.9, 133.0

251.06

N.D.b

1.68 ± 0.041

-566.6814

892.81

N.D.

b

N.D.

b

N.D.

b

N.D.b

-1119.9743

N.D.

b

b

-1219.7794

(As-OH, 1.75) 10.41

10.41

10.11

95.7, 95.7, 55.2

1.68 ± 0.02

-2137.1704

(As-OH, 1.73) 6.73

7.81

16.70

90, 90, 90

877.08

1.68 ± 0.060

-1993.4580

(As-OH, 1.78) 7.10 11.04

7.10 6.15

9.36 5.83

90, 90, 120 90, 90, 90

408.48 395.76

N.D.

b

Data from crystallographic data measured at P=0.0001 GPa by Comodi et al. (2008). N.D. means no data.

28

ACCEPTED MANUSCRIPT Table 3. Reactions and calculated incorporation energies (EInc) for each configuration Configuration

Reactions

EInc (Ry)

Ca32(SO4)32·64H2O + CaHAsO4·2H2O = Ca32(SO4)31(AsO3O1H)·64H2O

+

C1

0.0153 CaSO4·2H2O

C2

+

1/2

RI

CaSO4·2H2O

Ca32(SO4)32·64H2O

+

PT

Ca32(SO4)32·64H2O + CaHAsO4·2H2O = Ca32(SO4)31(AsO3O2H)·64H2O

Ca5(HAsO4)2(AsO4)2·4H2O

SC

C3

+

NaOH·H2O

0.0326

= 0.0666

AC

CE

PT E

D

MA

NU

NaCa32(SO4)31(AsO4)·64H2O + CaSO4·2H2O + CaHAsO4·H2O + 1/2 Ca(OH)2

29

ACCEPTED MANUSCRIPT Table 4. EXAFS fitting parameters determined by shell-fit analysis of As K-edge EXAFS spectra of As incorporated gypsum samples precipitated at pH 4, 6, 8, and 9.a Fit uncertainties in parenthesis are given for the last significant figure. CN

pH4

pH6

pH8

R (Å)

σ2 (Å2)

R (Å)

σ2 (Å2)

R (Å)

pH9 σ2 (Å2)

PT

Path

R (Å)

σ2 (Å2)

3

1.67(0)

0.002(0)

1.66(0)

0.002(0)

1.66(1)

0.002(0)

1.66(1)

0.002(0)

As-OH

1

1.74(0)

0.002(0)

1.73(0)

0.002(0)

1.73(1)

0.002(0)

1.74(0)

0.002(0)

As-O-O

12

3.04(5)

0.003b

3.03(6)

0.003 b

3.02(6)

0.003 b

2.99(6)

0.002 b

As-O-As-O

4

3.35c

0.004d

3.35c

0.004d

3.34c

0.003d

3.37c

0.003d

As-Ca1

2

3.15(7)

0.018(11)

3.15(7)

0.018(10)

3.15(6)

0.015(10)

3.15(5)

0.012(7)

As-Ca2

2

3.68(7)

0.018(11)

3.68(7)

0.018(10)

3.67(6)

0.015(10)

3.67(5)

0.012(7)

As-O2

7

3.54(5)

0.004e

3.54(5)

0.004e

3.54(5)

0.004e

3.54(4)

0.003 e

Red. χ2

62

R-factor

0.012

SC

NU

5.1±1.5

5.5±1.5

6.3±1.5

65

84

87

0.016

0.016

0.016

The amplitude reduction factor, S02, was set to 0.95 according to the fitting result of

PT E

a

MA

5.6±1.3

D

ΔE

RI

As-O1

BaHAsO4. The fit ranges of R and k space were set to 0.9-4.0 Å and 2.8-12.5 Å-1,

b 2

σ

2 As-O-O=1.7*σ As-O1.

c

ΔRAs-O-As-O=2*ΔRAs-O1.

d 2

σ

2 As-O-As-O=2*σ As-O1.

AC

respectively.

CE

respectively. The numbers of independent points and variables were 22 and 14,

e 2 σ As-O2=2*σ2As-O1.

30

ACCEPTED MANUSCRIPT

4

H2AsO4

-

H3AsO4

0

6 pH

(b) 0.0

2-

8

PT

HAsO4

2

10

2

-

R =-0.02, H2AsO4

-1

2

-0.5

0

R =0.24, H3AsO4

-1.0 2

-1.5

NU

Log [As in gypsum] (mol kg )

3-

3-

R =0.72, AsO4

-2.0 2

2-

R =0.85, HAsO4

D

-12 -8 -4 0 -1 Log [As species] (mol L )

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-2.5 -16

RI

AsO4

SC

0 -2 -4 -6 -8 -10 -12 -14 -16

MA

-1

Log [As species] (mol L )

(a)

Fig. 1 (a) The distribution of the aqueous As(V) species with pH. (b) The relationships

AC

CE

between the As content in gypusm and the various aqueous As(V) species.

31

ACCEPTED MANUSCRIPT

(a)

(b) pH10

(c) 928

3548 3405 1621 1686

pH10

pH9

903

872 863

pH9

708

pH8

pH9

pH8

pH10

pH7

pH7

pH7

pH6

pH6

pH5

PT

pH6

pH5

pH5

pH4

pH4 pH4

pH3

RI

pH3

pH3 IE-2

IE-2

IE-2 IE-1

IE-1 Gyp

IE-1

Gyp

Gyp

-1

1600 -1

Wavenumber (cm )

950

900

850720 -1

Wavenumber (cm )

MA

Wavenumber (cm )

3800 3600 3400

NU

4000 3000 2000 1000

SC

Absorbance

pH8

Fig. 2 FTIR spectra of As-free and As-doped gypsum precipitated from calcium and

D

sulfate solutions (with an initial As concentration of 5000 mg L-1) at various pH

PT E

values and from the industrial effluent (IE, with an As concentration of ~ 6600 mg L-1). ‘Gyp’ stands for As-free gypsum. (a) 400 – 4000 cm-1, (b) 1550 – 1750 cm-1 and

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3237 – 3800 cm-1, and (c) 680 – 970 cm-1.

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Fig. 3 DFT optimized local structures of the As-doped gypsum for configuration C1

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(a), C2 (b), and C3 (c).

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ACCEPTED MANUSCRIPT (a)

(b) Na3AsO3

pH4 pH6 pH8

2

Absorbance

Absorbance

Na3AsO4 S = 4.9

C3

2

S = 3.8

C2

2

S = 3.1

C1

B A C D E

0

Energy (eV)

50

100

Energy (eV)

150

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11850 11900 11950 12000

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pH9

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Fig. 4. (a) Normalized As K-edge XANES spectra of Na3AsO3, Na3AsO4, and

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As-doped gypsum formed at pH 4, 6, 8, and 9. (b) Comparison between theoretical (red dots) and experimental As K-edge XANES spectra (black lines) based on the

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DFT optimized structures.

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Absorbance

(a)

2

S = 2.5 Residue 0

30

60

90

120

150

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Energy (eV)

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Fig. 5. (a) Comparison of As K-edge XANES spectra between theoretical (red dots)

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cluster with FPMS.

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and experimental data (black line). (b) The structurally refined As(V)-doped gypsum

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(a)

(b) pH9

pH9 -4

|(R)| (Å )

-3

k |(k)| (Å )

pH8

pH8

pH6

pH4

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3

pH6

4

6

8

10

12

0

1

-1

2

3

4

R (Å)

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k (Å )

5

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pH4

Fig. 6. As K-edge EXAFS data for As(V)-doped gypsum precipitated at pH 4, 6, 8,

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and 9. (a) k3-weighed χ(k) EXAFS plot and (b) corresponding radial distribution

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functions. Experimental and fitted spectra are displayed as black solid and red dotted

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lines, respectively.

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ACCEPTED MANUSCRIPT Supporting information to “Spectroscopic and DFT study on the Species and Local Structure of Arsenate Incorporated in Gypsum Lattice”

Key Laboratory of Pollution Ecology and Environmental Engineering, Institute of

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a

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Shaofeng Wang,*,a Danni Zhang,a Xu Ma,a Guoqing Zhang,a Yongfeng Jia*,a,b

b

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Applied Ecology, Chinese Academy of Sciences, Shenyang, China, 110016 Institute of Environmental Protection, Shenyang University of Chemical Technology,

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Shenyang, China, 110142

* Corrsponding author: Dr. Shaofeng Wang, Email: [email protected], Tel:

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+86 24 83970502, Fax: +86 24 83970503; prof. Yongfeng Jia, Email:

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[email protected], Tel: +86 24 83970503, Fax: +86 24 83970503

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This material contains 2 tables and 6 figures.

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