Arsenate adsorption on an Fe–Ce bimetal oxide adsorbent: EXAFS study and surface complexation modeling

Colloids and Surfaces A: Physicochem. Eng. Aspects 379 (2011) 109–115

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Arsenate adsorption on an Fe–Ce bimetal oxide adsorbent: EXAFS study and surface complexation modeling Xiaomin Dou a,b,∗∗ , Yu Zhang b,∗ , Bei Zhao b , Xiaomei Wu b , Ziyu Wu c , Min Yang b a b c

College of Environmental Science and Engineering, Beijing Forestry University, 100083, Beijing, China State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, 100085, Beijing, China Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, 100049, Beijing, China

a r t i c l e

i n f o

Article history: Received 30 June 2010 Received in revised form 17 October 2010 Accepted 18 November 2010 Available online 25 November 2010 Keywords: Arsenate Adsorption Fe–Ce bimetal oxide Surface complexes EXAFS CD-MUSIC model

a b s t r a c t The mechanism of arsenate (As(V)) adsorption on an Fe–Ce bimetal (hydrous) oxide (Fe–Ce) was investigated using complementary analysis techniques including extended X-ray absorption fine structure (EXAFS) and surface complexation modeling. The As K-edge EXAFS spectra showed that the second peak of Fe–Ce after As(V) adsorption was the As–Fe shell, which supported the finding that As(V) adsorption occurred mainly at the Fe surface active sites. Two As–Fe distances of 3.30 A˚ and 3.55 A˚ were observed from the EXAFS spectra of As(V) adsorbed samples at three pH levels (5.0, 7.6, and 9.0) and two initial surface loadings of 70 and 130 mg/g, which indicated that monodentate mononuclear and bidentate binuclear As surface complexes coexisted. When compared with the reported dominant species of bidentate binuclear complex for As existing on iron (hydro)oxides, the existence of Ce atoms in the bimetal oxide and the high surface loading were the likely reasons for the existence of the monodentate complex. A Charge Distribution-Multi-site Sites Complexation (CD-MUSIC) model showed that protonated monodentate (MH) and deprotonated bidentate (B) complexes preferred to exist on the Fe–Ce surface in a high surface loading range ( = 5.11–14.4 ␮mol/m2 ). The MH complex was shown to be dominant at pH < 8. Based on the results from EXAFS analysis and the CD-MUSIC model, the adsorptive behavior of As(V) on Fe–Ce with high surface loadings was satisfactorily interpreted and understood. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Arsenic (As) occurs widely in groundwater through natural or anthropogenic sources, and people in many parts of the world are exposed to As via drinking water supplies [1]. Due to the increased awareness of the health risks of As, a great deal of effort has been devoted to the development of adsorbents for the removal of As from water [2]. For example, the use of iron-, manganese-, alumina, titanium-oxide, as well as their composite oxides, have been investigated extensively for the removal of As from water [2–8]. Among these, ferric oxide-based materials have received the greatest attention because they have promising binding affinity for As and are inexpensive [2]. The mechanism of As(V) adsorption on iron (hydro)oxides has been extensively investigated using the modern surface and structure analytical techniques, such as extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), (T-, DR-, ATR-) Fourier-transform infra-red spectroscopy (FTIR), or

∗ Corresponding author. Tel.: +86 10 6292 3475; fax: +86 10 6292 3541. ∗∗ Co-corresponding author. Tel.: +86 10 62336615; fax: +86 10 62336596. E-mail addresses: [email protected] (X. Dou), [email protected] (Y. Zhang). 0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.11.043

their combinations [3,7,8]. Among them, EXAFS is a powerful in-situ technique that can provide local atomic structural information such as the coordination number, inter-atomic distance and the nature of the neighboring atoms [3,9–11]. It has been used to directly identify species of As adsorbed onto iron (hydro)oxides containing minerals and adsorbents [12–18]. The presence of a bidentate binuclear complex (2 C) with an As–Fe inter-atomic distance of about 3.2 A˚ under relatively high surface loading conditions has been reported in most studies conducted to date, including goethite [13,15,19], ferrihydrite [13] and lepidocrocite [13,18]. These findings have been supported by density functional theory calculations [13]. In addition, 2 C was observed on other oxides such as gibbsite [20], synthetic birnessite [6] and titanium dioxide [3]. The other two complex forms of bidentate-mononuclear complex (2 E) and monodentate complex (1 V), in which the anticipated As–Fe inter˚ respectively, have also been atomic distances are 2.85 and 3.60 A, suggested, but controversy remains regarding their possible existence [10,13,16,19,21,22]. Recently, a study based on IR and EXAFS reported that monodentate coordination was, in fact, the predominant geometry of arsenate-goethite surface complexes, and that there is no evidence for bridging-bidentate coordination [23]. Additionally, the thermodynamically based surface complexation model (SCM), based on spectroscopic observations, is a powerful tool for simulation of the surface reaction behavior and

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for providing the protonation state of the adsorbed species [24]. The most widely used SCM is the one-pK/multi-site model, which considers both the charge distribution (CD) over the surface ligands and the diverse and heterogeneous surface sites. This model uses the Pauling concept of the CD and is an extension of the multi-site complexation (MUSIC) approach [14]. Based on EXAFS analysis of As(V)/As(III) adsorption on goethite, the protonation states of surface complexes and their distribution were simulated using the CD-MUSIC model [16,25]. Similarly, monomethylarsonic acid (MMA)/dimethylarsinic acid (DMA) adsorption on TiO2 was investigated, and non-protonated bidentate and monodentate complexes were suggested, respectively [11]. As discussed above, most studies conducted to evaluate As adsorption mechanisms have focused on single metal oxides such as ferric-, alumina-, manganese-oxides or TiO2 [3–7,16,26,27]. However, there is little information available regarding bimetal oxide adsorbents such as Fe–Mn, Fe–Zr, and Ce–Ti oxides, which have been reported for As adsorption [28–30]. Recently, an iron–cerium bimetal oxide (Fe–Ce) adsorbent was successfully developed for As(V) removal [31–33], which showed a significantly higher As(V) adsorption capacity than the referenced cerium and ferric oxides (CeO2 and Fe3 O4 ) prepared by same procedure and some other adsorbents reported recently [31]. Also, it has good performance characteristic of both cerium oxide (e.g. high affinity) and ferric oxide (e.g. resistance to acids and bases, low solubility and cheap). Additionally, batch and long-term column studies of arsenate adsorption performance using groundwater samples from Inner Mongolia and the suburbs of Beijing, respectively, have shown that the Fe–Ce adsorbent was promising [32]. Moreover, XPS results showed the existence of abundant metal hydroxyl groups on the surface of Fe–Ce, among which Fe-OH mainly reacted with As. The monodentate species was assumed to be formed on Fe–Ce at pH 5 after As(V) absorption based on the triplet splitting of the As-O 3 vibration in T-FTIR and XPS analysis [31]. However, direct insitu evidence explicating the adsorption mechanism has yet to be realized for the Fe–Ce bimetal adsorbent. In the present study, a combination of EXAFS and the CD-MUSIC model was used to investigate the As(V) adsorption mechanism on the Fe–Ce surface as a function of pH and surface loading. Specifically, the objectives of this study were as follows: (1) to investigate the forms of surface complex of As(V) on Fe–Ce and their relative distribution at different pH and surface loadings by using a combination of EXAFS and the CD-MUSIC model; and (2) to simulate the adsorption behavior of As(V) on Fe–Ce using the CD-MUSIC model. 2. Materials and methods 2.1. Materials All chemicals used were of analytical reagent grade. 1000 mg L−1 As(V) stock solutions were prepared by dissolving 4.1653 g Na2 HAsO4 ·7H2 O in 1 L of distilled water. As(V)-bearing solution was prepared by diluting As(V) stock solution to the given concentrations with distilled water. Fe–Ce bimetal oxide adsorbents were prepared using a previously described co-precipitation method [33]. 2.2. Methods 2.2.1. Zeta potential measurements The zeta potential () was measured for a 0.05 g/L Fe–Ce suspension with 0.1 and 1.0 mM As(V), and without As(V) in the pH range 3 to 10 using a Zetasizer 2000 (Malvern Instruments Inc., United Kingdom), with 0.01 M NaClO4 added as the background electrolyte. Samples for testing were prepared according to the fol-

lowing procedures. Fe–Ce suspensions with or without As(V) at the desired pH and ion strength were shaken at 25 ◦ C and 180 rpm for 24 h. The equilibrium pH was then measured and the suspension was injected into the electrophoretic cell for Zeta potential measurement in triplicate, after which the average reading was recorded. The pH at the point of zero charge (PZC) was obtained by interpolating the zeta potential data to the zero potential. 2.2.2. EXAFS data collection and analysis Six Fe–Ce samples were prepared as a function of surface loadings (70 and 130 mg As(V)-adsorbed/g-Fe–Ce) and pH values (5.0, 7.6, and 9.0). The samples were prepared by shaking the As(V) and Fe–Ce suspension at predefined conditions, after which the suspension was centrifuged. The soluble As in the filtrate was analysed, and the wet paste was used to take the spectra. In addition to the Fe–Ce samples, scorodite from the Geological Museum of China (GMC) and synthesized CeAsO4 were also scanned as reference As-containing compounds. Transmission XAS spectra of As K-edge for arsenate adsorbed Fe–Ce and references were collected at the 4W1B beamline of the Beijing Synchrotron Radiation Facility (BSRF). The storage ring was operated at 2.2 GeV with a beam current of 80 mA. A Si(1 1 1) double crystal monochromator was used to provide an energy resolution of 1.5 eV. To suppress the unwanted harmonics, the monochromatic crystal faces were detuned, reducing the incident beam by 30%. EXAFS data reduction and analysis were performed with WinXAS 2.3 [34] using the following procedures: (1) Two scans per sample were aligned and then averaged; (2) First- and secondorder polynomial functions were used to fit the pre-edge region for background removal and the post-edge region for normalization, respectively; (3) The spectra were then converted to photoelectron wave vector (k) space with respect to E0 determined from the second derivative of the raw spectra; (4) ␹(k) spectra for Fe–Ce after arsenate adsorption and the reference samples (CeAsO4 and FeAsO4 ·2H2 O) were extracted using a cubic spline function consisting of ≤7 knots over the range k = 3.5–14.7 A˚ −1 . Fourier transformation (FT) of the raw k3 (k) function was conducted over a consistent region in K space (3.5–14.7 A˚ −1 ) to obtain the radial structure function (RSF) using a Bessel window function and a smoothing parameter (ˇ) of 3 to minimize the effects from truncation in the RSFs; (5) The experimental spectra were fitted with single-scattering theoretical phase-shift and amplitude functions calculated with the ab initio computer code FEFF 7 [35] using atomic clusters generated from the crystal structure of gasparite–(Ce) (CeAsO4 ) for synthesized CeAsO4 , and using scorodite (FeAsO4 ·2H2 O) for As(V) adsorbed Fe–Ce and reference As compounds, respectively. The final nonlinear least squares fit was conducted on the raw k3 weighted (k) function. The manybody amplitude reduction factor (S20 ) was fixed at 0.9. For the As K-edge, each spectrum was first fit roughly to estimate E0 , which is the difference in threshold energy between theory and the experiment, by fixing coordination numbers (CN) and the Debye-Waller parameter (o’2 ) as the same values as those of the related reference model oxide (e.g. butlerite, scorodite), after which E0 was fixed according to the best fit. Finally, the spectrum was fitted using estimated values for CN, R, and o’2 as starting values. The fitting results were then evaluated based on the residual value, with a good fit being considered as having a residual value of less than 20. Error ˚ and estimates of the fitted parameters were CN, ±20%; R, ±0.02 A; o’2 , ±20–30%. 2.2.3. Surface complexation modeling The CD-MUSIC model with the triple plane option was used to describe As(V) adsorptive behavior on Fe–Ce. The basic principles of the model have been well documented elsewhere [36–38]. Model calculation was conducted using the FITEQL 4.0 program [39], with

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Table 1 Structural parameters for As(V) adsorbed Fe–Ce samples prepared at pH values of 5.0, 7.6 and 9.0 and two As(V) surface loadings of 70 mg As(V)/g Fe–Ce and 130 mg As(V)/g Fe–Ce at each pH value. CN

R

o’

CN

R

o’

E0

pH 5.0  = 70 mg g−1

4.7

1.69

0.00238

2.4 1.2

3.31 3.54

0.00830 0.00705

9.64

pH 5.0  = 135 mg g−1

4.8

1.68

0.00298

2.3 1.1

3.32 3.55

0.00982 0.00851

9.64

pH 7.6  = 70 mg g−1

4.7

1.69

0.00206

2.1 1.1

3.32 3.56

0.00939 0.00579

9.64

pH 7.6  = 135 mg g−1

4.6

1.69

0.00230

2.2 1.2

3.31 3.54

0.00955 0.00753

9.64

pH 9.0  = 70 mg g−1

4.6

1.69

0.00307

2.3 1.2

3.32 3.54

0.00911 0.00655

9.64

pH 9.0  = 135 mg g−1

4.6

1.69

0.020405

2.1 1.1

3.31 3.56

0.00991 0.00752

9.64

Samples

As-O shell

modified executable files from Gustafsson [40]. A description of the CD-MUSIC formulation for phosphate adsorption on goethite using a modified version of the FITEQL program was presented by Tadanier and Eick [41], and for molybdate and tungstate adsorption on ferrihydrite by Gustafsson [40]. For Fe–Ce, since the Fe–OH site was probably the main active site for As(V), three types of active surface oxygens on Fe–Ce may be distinguished as singly (SOH1/2 )− , doubly (S2 OH0 ) and triply (S3 O1/2− ) coordinated. Based on Pauling’s rules and bond valence, S2 OH0 was deduced as inert because it cannot become either protonated or deprotonated in a normal pH range. Thus, only the singly and triply coordinated surface groups were considered in surface complexation reactions (Table 1). The triply coordinated surface group could not form inner sphere complexes [25]; therefore, only its protonation reactions and ion-pair reactions were listed (Eqs. (5) and (8)–(9) in Table 2). The proton affinity constants of SOH1/2− and S3 O1/2− were set to be the same and equal to the pHpzc value of Fe–Ce for symmetrical adsorption based on previous reports [11,36]. The site densities of the SOH1/2− and S3 O1/2− groups were optimized as 18.01 sites/nm2 and 5.2 sites/nm2 , respectively, by fitting the surface titration data

As–Fe shell

of Fe–Ce (Fig. S1); further details of the surface titration and data reduction are provided in the supplementary materials. In addition, the outer layer capacitance C2 was set as 5.0 F/m2 , which was the same as that used for goethite [36]. The electrolyte affinity constants, KClO4− and KNa+ , and the inner layer capacitance, C1 , were determined by fitting the surface titration data for Fe–Ce (Fig. S1 and Table 2). An important aspect of the CD model is that adsorbed ions should not be treated as point charges on the scale of the compact portion of the interface. Inner sphere surface complexes of oxyanions as well as cations have a spatial distribution of charge that can be attributed to two different electrostatic planes. A certain fraction f (CD factor) of the charge of the central ions in the complex will be attributed to the surface plane (0-plane), while the remaining part (1 − f) is attributed to other ligands in the complex that are located in the 1-plane. For the species listed in Table 2, when the charge of central As ions was assumed to be symmetrically neutralized by all surrounding ligands in accordance with Pauling’s rules, values of f = 0.25 and f = 0.50 were gained for monodentate and bidentate species, respectively. It should be noted that

Table 2 Surface reactions and related parameters used in the CD-MUSIC model for As(V) adsorption on Fe–Ce. Reactions

Equilibrium expressions

log K

Arsenic acid dissociation reactions (1) (2) (3)

H+ + AsO4 − = HAsO4 2− 2H+ + AsO4 − = H2 AsO4 − 3H+ + AsO4 − = H3 AsO4 −

11.50* 18.46* 20.70*

Surface protonation (4) (5)

SOH1/2− + H+ = SOH2 1/2+ S3 O1/2− + H+ = S3 OH1/2+

5.8 5.8

Ion-pair reactions (6) (7) (8) (9)

SOH1/2− + Na+ = SOH1/2− · · ·Na+ SOH1/2− + H+ + ClO4 − = SOH2 1/2+ · · ·ClO4 − S3 O1/2− + Na+ = S3 OH1/2− · · ·Na+ S3 O1/2− + H+ + ClO4 − = S3 OH1/2+ · · ·ClO4 −

Oxyanion (AsO4 3− ) surface reactions

Option I (10) (11) Option II (12) (13) (14) * **

−1.2 4.6 −1.2 4.6 AsO4 3− log K**

f

WSOS/DF

SOH1/2− + 2H+ + AsO4 3− = SOAsO3 H1.5− + H2 O 2 SOH1/2− + 2H+ + AsO4 3− = S2 O2 AsO2 2− + 2H2 O

31.6 34.1

0.25 0.50

2.72 ± 0.2

SOH1/2− + 2H+ + AsO4 3− = SOAsO3 H1.5− + H2 O 2 SOH1/2− + 2H+ + AsO4 3− = S2 O2 AsO2 2− + 2H2 O 2 SOH1/2− + 3H+ + AsO4 3− = S2 O2 AsO2 H− + 2H2 O

31.6 34.1 37.3

0.25 0.50 0.60

2.52 ± 0.2

From Dzo mbak and Morel [50]. This study.

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3.2. Arsenic K-edge EXAFS analysis

Fig. 1. Zeta potential of Fe–Ce suspensions reacted with 0, 0.1 and 1.0 mM As(V) in 0.01 M NaClO4 solution.

nonsymmetrical neutralization of the central ion charge is possible, for instance, in response to a shift in electron density in the adsorbed species caused by protonation of an oxygen atom, which would lead to a change in the f value. These findings indicate that the use of the Pauling concept to estimate the CD value of surface species may be insufficiently accurate [36,42]. Recently, CD values derived from geometries optimized with molecular orbital calculations using density functional theory (MO/DFT) were used in modeling As(III)/As(V) [25], carbonate [43] and silicate [42] adsorption on goethite. Because the Fe–Ce was amorphous, the exact structural molecular cluster could not be extracted from model ferric oxides; therefore, the Pauling concept was used to calculate the CD value in this study. For the modified FITEQL program developed by Gustafsson [40], the f value was restricted to being only manually adjustable. The f values for species SOAsO3 2.5− , SOAsO3 H1.5− and SOAsO(OH)2 0.5− were set as 0.25 following Antelo et al. [44]. In addition, preselected parameter sets of f values of 0.5 and 0.5 (as in the phosphate complex on goethite [41]), 0.5 and 0.6 [36], and 0.35 and 0.65 [44] for species S2 O2 AsO2 2− and S2 O2 AsO2 H− were attempted in the fitting process. The fitting was initiated by assuming that only monobentate and bidentate species existed, after which a trial and error procedure was pursued. If the fitting results were poor, combinations of two or more species were tried. The procedures were repeated until a good fit to the experimental data was achieved. 3. Results and discussion 3.1. Changes in Zeta potential before and after As loading The zeta potential results of the Fe–Ce adsorbent in the absence and presence of As(V) under different concentrations as a function of pH are shown in Fig. 1. pHpzc of Fe–Ce occurred at pH 5.8 for Fe–Ce in 0.01 M NaClO4 . In the presence of 0.1 mM and 1 mM As(V), pHpzc shifted to low pH values (about 4.8 and 3.6, respectively), and the zeta potential became more negative in the pH range of 3–10. These results indicated that the higher the anion concentration, the lower the zeta potential and corresponding values of pHpzc, which was similar to results obtained for As(V) and P adsorption on ferric oxides [44,45]. Shifts in the pHpzc and reversal trends of the zeta potential values with increasing ion concentrations were used as evidence of strong specific ion adsorption and inner-sphere surface complex formation [45]. Therefore, it was assumed that inner-sphere complexes formed on Fe–Ce after As(V) adsorption.

EXAFS spectra were employed to further determine the local coordination environments of the As adsorbed complexes. Fig. 2(a) and (b) showed the k3 weighted As K-edge EXAFS spectra and the RSFs as Fourier transforms (FT) versus radial distance for the As adsorbed Fe–Ce and reference As compounds (CeAsO4 and FeAsO4 ·2H2 O). The resolved structural parameters obtained by fitting the theoretical paths to the experimental spectra for As adsorption on Fe–Ce are shown in Table 1. FT of the (k) function isolates the contributions of different coordination shells, in which the peak position corresponds to the interatomic distances. These peak positions in Fig. 2(b) are uncorrected for the phase shift, so ˚ they deviate from the true distance by 0.3–0.5 A. Under different As(V) loads (70 and 135 mg/g) and pH values (5.0, 7.6, and 9.0), the RSF spectra of As(V) adsorbed Fe–Ce yielded patterns very similar to those of the first and second shells centered at about 1.40 and 2.99 A˚ (Fig. 2(b)), respectively. As shown in Fig. 2(b) and Table 1, the first peak in the RSF was the result of backscattering from the nearest neighbor As–O shell for Fe–Ce ˚ after As(V) adsorption. The average As–O distance was about 1.68 A, which is in good agreement with the results of previous studies [14,17,46]. The average CN of O was calculated to be 4.8. The position of the second shell of Fe–Ce was consistent with that of scorodite, and relatively lower than that of CeAsO4 (Fig. 2(b)). In resolving the structural parameters of the surface complexes, the theoretical paths of As–Fe, As-Ce or a combination of As–Fe and AsCe were attempted when fitting the raw k3 weighted (k) function in the data reduction process. The fits were not successful when AsCe or a combination of As-Ce and As–Fe were used. Consequently, As–Fe was finally adopted and the best fit results are shown in Table 1. Based on these findings, it was inferred that the second shell was contributed by As–Fe and that As(V) might primarily react with Fe active sites. Combined with the results of XPS [31], these findings suggested that Ce sites were not the main active sites involved in the reaction with As(V) for Fe–Ce bimetal oxide. However, the introduction of Ce into Fe oxide might play an important role in the modification of the surface characterization of the Fe oxide. The main roles of Ce were to break the magnetite structure of the Fe(II)/Fe(III) system through oxidation of Fe2+ , and to activate the Fe atoms to acquire more abundant Fe-OH on the surface of Fe–Ce [31]. The As atom was coordinated by 1.8–2.25 Fe atoms with ˚ RAs–Fe = 3.30–3.32 A˚ and 0.8–1.25 Fe atoms at RAs–Fe = 3.58–3.65 A, respectively (Table 1). The As–Fe distance of 3.30–3.32 A˚ is in good agreement with the distance of 3.31 A˚ for As(V) on goethite, lepidocrocite, maghematite, hematite, and ferrihydrite lepidocrocite [13,15,18], 3.30 A˚ for As(V) on goethite [17], 3.37 A˚ for As(V) on argonaut mine [14], and 3.35 A˚ for As(V) on magnetite and ferrihydrite [12,16]. It has been reported that the distance RAs–Fe = 3.20–3.32 A˚ indicated the existence of bidentate binuclear complex on adsorbed ferric oxides [10,13,19,21]. In the present study, the distance RAs–Fe = 3.58–3.65 A˚ indicated that the monodentate mononuclear complex was also formed on the surface of Fe–Ce, which is consistent with the distance of about 3.6 A˚ for As(V) on ferrihydrite [10,21], goethite [19], iron incorporated mine tailings [14] and corrosion products of zero valent iron [18]. The above direct in-situ EXAFS results further supported our previously speculation that bidentate binuclear (C2 point group) and monodentate mononuclear complexes (C1 point group) existed on Fe–Ce under a high surface load ( = 22.2 ␮mol/m2 ) at pH 5.0 [31]. This speculation was based on the ex-situ techniques of triplet splitting of As–O 3 (As–O FTIR peak) vibration via Gaussian profile fitting and XPS [31]. Most previous EXAFS studies have suggested that the dominant species of As existing on iron oxides as well as oxyhydroxides

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Fig. 2. (a) Raw (solid line) and fitted (dotted line) k3 weighted (k) spectra for As(V) adsorbed Fe–Ce, (b) Corresponding radial structure functions (RSFs).

were bidentate binuclear complexes (2 C) in a surface loading range of 0.04–4.2 ␮mol/m2 [16–18,25,47]. It has also been suggested that the existence of monodentate mononuclear (1 V) complex was favorable under low surface loading conditions. For example, increased amounts of 1 V complex were observed for As(V) on goethite, lepidocrocite and maghemite under low surface loading conditions (∼0.08 ␮mol/m2 ) [18], and also increased amounts of 1 V were reported on goethite as surface loading decreased from 2.0 to 1.2 ␮mol/m2 [19]. However, an opposite trend was also reported in which increasing surface loading of As(V) from 0.51 to 1.70 ␮mol/m2 was associated with increased amounts of protonated monodentate complex based on EXAFS results and CDMUSIC modeling [25]. Most importantly, a recent rethinking report pointed out that As(V) coordinates at the water-goethite interface in a predominately monodentate fashion under surface loadings of 0.9–1.8 ␮mol/m2 and pH values of 3.3–8.3, and there was no evidence of bridging-bidentate coordination under these conditions [23]. In the present study, the 1 V complex was considered to coexist with 2 C under extremely high surface loading conditions ( = 5.11–14.4 ␮mol/m2 ). The existence of Ce atoms on the surface of bimetal oxides was probably one of the reasons for the existence of monodentate. The pHpzc of ferric (hydro)oxides were commonly observed to be near 9.0 [48]; however, the pHpzc of Fe–Ce biometal oxide decreased to 5.8 [31]. In our previous study, more amorphous structure, higher contents of hydroxyl groups and lower pHpzc for Fe–Ce oxide were observed in comparison with the ferric oxide without Ce doping [31]. Therefore, the doping of Ce might influence the structure of the iron (hydro)oxides. Accordingly, when compared with iron (hydro)oxides, the surface complexes on Fe–Ce differed because of the chemical and physical modifications of the iron oxides. On the other hand, the much higher surface loading of As(V) on the composite adsorbent

( = 5.11–14.4 ␮mol/m2 ) than on naturally or synthesized environmental samples ( = 0.04–4.2 ␮mol/m2 ) might also be responsible for the differences in these complexes [16]. When compared with other complex forms (e.g. 2 C and 2 E), a linear, monodentate

Fig. 3. Experimental and simulated adsorption edge with initial As(V) concentration of 34.6 ␮M ( = 5.54–10.5 ␮mol/m2* , squares), 54.2 ␮M ( = 5.11–12.0 ␮mol/m2* , circles), and 106.7 ␮M ( = 6.86–14.4 ␮mol/m2* , triangles). Fe–Ce addition was 0.03 g/L. Solid lines show the fitting assuming two surface species, MH and B (option I); dashed lines show the fitting assuming three surface species, MH, B and BH (option II). (*) Note, the values of surface loading were calculated from the adsorbed amounts of As(V) on Fe–Ce in the investigated pH range. The adsorbent dose was 0.03 g/L in this study and the BET surface area was 90 m2 /g.

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Fig. 4. The surface speciation of As(V) calculated using option I. The symbols stand for three initial As(V) concentrations, 34.6 ␮M ( = 5.54–10.5 ␮mol/m2 , squares), 54.2 ␮M ( = 5.11–12.0 ␮mol/m2 , circles), and 106.7 ␮M ( = 6.86–14.4 ␮mol/m2 , triangles), in 0.05 M NaClO4 solutions. The dose of Fe–Ce was 0.03 g/L.

arrangement was thought to save surface space and minimize steric hindrance when there was too much arsenate molecular binding on the Fe–Ce surface to reach the extremely high surface loading level of 5.11–14.4 ␮mol/m2 . As discussed above, the EXAFS results showed that As(V) primarily reacted with Fe active sites and that the 2 C and 1 V complexes co-existed on Fe–Ce under the two surface loadings at the three pH values investigated in this study.

3.3. As(V) adsorption edge modeling To describe the adsorption behavior and determine the protonation state of the adsorbed species of As(V) on Fe–Ce, the As(V) adsorption edge was simulated using the CD-MUSIC model (Fig. 3). Based on the species obtained from EXAFS, the possible surface reactions were constructed and further resolved (Table 2). The results showed that the adsorption edge could be well modeled using two options (Table 2): protonated monodentate mononuclear (MH, Fig. S2(a)) and bidentate binuclear (B, Fig. S2(b)) (option I) or MH, B, and protonated bidentate binuclear (BH, Fig. S2(C)) (option II). The findings also demonstrated that the two surface species, B and MH (option I), described the adsorption edge well with a WSOS/DF value of 2.72 ± 0.2. WSOS/DF means the weighted sum of squares divided by degrees of freedom and is an indicator of the goodness of fit (a value falling in the range of 0.1 to 20 indicates a reasonably good fit) [49]. When combined with another species of BH in option II, the quality of the fit remained good (WSOS/DF = 2.52 ± 0.2), but no significant improvement was achieved. Accordingly, both options I and II were equally good, therefore, we were not able to distinguish between the two model options. Option I was preferred because the minimum number of adjustable parameters were used in the model simulation. The abundance of the adsorbed surface species vs. pH based on the parameter sets of option I are shown in Fig. 4, while those based on option II are shown in supplementary Fig. S3. Similar findings for option I (Fig. 4) and II (Fig. S3) were observed in that both MH and B were the predominant species. MH was abundant at pH values below 8, while B was abundant at pH values above 8. Moreover, MH showed an increased trend with surface loading of As increasing from 5.11 to 14.4 ␮mol/m2 , but for B, an inverse trend was observed (Fig. 4 and Fig. S3). These findings indicated that the higher the surface loading of As(V), the more MH became dominant at neutral and acid pH (<8), which contradicted the previous conclu-

sions that monodentate species were only present under relatively low As surface loading conditions[19,21]. It has been reported that two similar options for surface complexes, MH and B (option I) or MH, B and BH (option II) were used to model As(V) on goethite ( = 0.5–1.7 ␮mol/m2 ), and that option II was preferred. These findings were similar to the finding that MH was present at lower pH values and shown to increase as As(V) surface loading increased from 0.5 to 1.7 ␮mol/m2 [25]. The model results presented here further supported the results of EXAFS, which showed that two types of surface complexes (monodentate mononuclear and bidentate binuclear complex) coexisted on adsorbed Fe–Ce. 4. Conclusions In this study, the adsorption mechanism of As(V) on a Fe–Ce bimetal oxide adsorbent was investigated by using a combination of EXAFS observation and the CD-MUSIC model. The resolved results from the As K-edge EXAFS spectra of As(V) adsorbed Fe–Ce under three pH levels and two initial surface loadings showed that As(V) primarily reacted with Fe-OH sites and monodentate mononuclear and bidentate binuclear As surface complexes coexisted. The existence of monodentate complex might be due to the existence of Ce atoms in the bimetal oxide and the high surface loading observed in this investigation. Further, the As(V) adsorption edge was well simulated using the CD-MUSIC model with the TPM option. These results showed that protonated monodentate complex (MH) and deprotonated bidentate complex (B) preferred to co-exist at the Fe–Ce surface under a significantly high surface loading range ( = 5.11–14.4 ␮mol/m2 ). MH was found to be dominant at pH values below 8, while B was dominant at pH values above 8. As As(V) surface loading increased, MH increased. Based on the above results, the adsorptive behavior of As(V) on Fe–Ce was satisfactorily interpreted and understood. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 50508006, 50921064), the National High Technology Research and Development Program of China (No. 2007AA06Z319), the Beijing Nova Program (No.2008A33) and the Fundamental Research Funds for the Central Universities (YX201033). The authors are also thankful to Dr. K. Tanaka, honorary professor of Tokyo University, Japan, for in-depth discussions.

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