Journal of Colloid and Interface Science 415 (2014) 13–17
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
An EXAFS study on the adsorption structure of phenyl-substituted organoarsenic compounds on ferrihydrite Masato Tanaka a, Yoko S. Togo b, Noriko Yamaguchi c, Yoshio Takahashi a,⇑ a
Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science & Technology (AIST), 1-1-1, Higashi, Tsukuba, Ibaraki 305-8567, Japan c National Institute for Agro-Environmental Sciences, 3-1-3, Kan-nondai, Tsukuba, Ibaraki 305-8505, Japan b
a r t i c l e
i n f o
Article history: Received 23 June 2013 Accepted 10 October 2013 Available online 20 October 2013 Keywords: Organic arsenic Phenylarsonic acid (PAA) Diphenylarsinic acid (DPAA) Adsorption structure Ferrihydrite Extended X-ray absorption fine structure (EXAFS) Quantum chemical calculation
a b s t r a c t Adsorption structures of mono- and diphenyl substituted organoarsenic compounds (PAA and DPAA, respectively) on ferrihydrite were analyzed by extended X-ray absorption fine structure (EXAFS), which suggested that PAA and DPAA form inner-sphere complexes with ferrihydrite regardless of their bulky functional groups. In addition, coexistence of two types of inner-sphere complex modes, i.e. bidentate– binuclear and monodentate surface complexes was suggested by EXAFS fitting with two type of the AsAFe shell. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Water pollution by arsenic has been a serious problem around the world, and its predominant source is naturally-derived inorganic arsenic compounds. In addition, organoarsenic species such as methyl and phenyl arsenic compounds were also found, although organoarsenic compounds are thought to be less toxic than inorganic ones [1,2]. It has been reported that these organoarsenic compounds are distributed in environment. Methyl-substituted arsenic species are produced as microbial metabolites in marine and terrestrial systems [3,4] and also have been used as pesticides and herbicides [2,5]. Phenyl-substituted, on the other hand, that has been also used as pesticides and herbicides are exclusively anthropogenic compounds [2]. Furthermore, phenyl arsenic species were considered as degradation products of chemical warfare agents used during the World Wars I and II [6]. After the World War II, such kind of chemical weapons including these agents were abandoned by dumping in the sea or by burying in the geosphere [7]. Even in the present day, these materials and their derivatives can be found in environment [6–8]. As a direct casualty of phenyl derivative arsenic compounds, well water contaminated by diphenylarsinic acid (DPAA) posed
⇑ Corresponding author. E-mail address: [email protected] (Y. Takahashi). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.006
health hazards to the residents at Kamisu Town in Japan [6]. In addition, Nakamiya et al. and Harada et al. reported biodegradation of DPAA to phenylarsonic acid (PAA) in environment [9,10]. These studies indicated that organoarsenic compounds can be gradually converted to inorganic forms by microbial activities, and such anthropogenic organoarsenic compounds have a potential to be a secondary source of more toxic inorganic arsenics. Recently, the effect of phenyl-substituted arsenic compounds in the agricultural environment such as uptake of these arsenic compounds from polluted soil to rice has been investigated [11–14], which revealed a growing concern of these phenyl-substituted arsenics in environment. However, there have been few studies on the physicochemical processes of PAA and DPAA in natural soil–water or in sediment–water systems such as their adsorption on iron (oxyhydr)oxides. The adsorption of inorganic (arsenite and arsenate) and methylated arsenics has been widely studied by several methods [15–17]. For example, adsorption amount of arsenite, arsenate, and mono-methylarsonic acid (MMA) on ferrihydrite have been reported by adsorption experiments at various pH and ionic strength [15,17]. In addition, extended X-ray absorption fine structure (EXAFS) analysis has been employed to obtain their adsorption structures, which are related to the stability of adsorbed species, or distribution between water and soil. Therefore, it is important to study the adsorption structures of arsenic compounds on iron (oxyhydr)oxides to understand its migration processes in
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M. Tanaka et al. / Journal of Colloid and Interface Science 415 (2014) 13–17
soils. Ferrihydrite is one of the most important iron (oxyhydr)oxides in natural systems due to its large surface area that can be a strong adsorbent of anionic species such as inorganic arsenic species in environment [18]. Thus, ferrihydrite may control migration of arsenic as its scavenger in soils. Adsorption structures of methyl-substituted arsenic compounds such as MMA and dimethylarsinic acid (DMA) on several minerals as well as goethite have been investigated also by EXAFS, since the adsorption on minerals can be an important process for the migration of methyl arsenic species in terrestrial environment [19–21]. However, those of phenyl-substituted arsenics had not been investigated yet except for our preliminary study on the adsorption structures of PAA and DPAA on ferrihydrite by EXAFS [22]. Since phenylsubstituted arsenic compounds have many carbon atoms in the phenyl groups, the EXAFS spectra can be complicated and difficult to interpret. Hence, the objective of this study is to reveal the adsorption structure of phenyl organoarsenic compounds, i.e. PAA and DPAA onto ferrihydrite by EXAFS spectra with the aid of quantum chemical calculations based on the density functional theory (DFT) to obtain structural parameters. 2. Materials and methods 2.1. Materials Two-line ferrihydrite was synthesized in the laboratory as described by Schwertmann and Cornell [23]. The mineralogy of the 2-line ferrihydrite was confirmed by powder X-ray diffraction. Aqueous solutions of organoarsenic species were prepared for PAA (C6H5AsO(OH)2; Wako) or DPAA ((C6H5)2AsO(OH); Wako) dissolved in Milli-Q water. Adsorption samples for EXAFS were prepared by mixing 15 mL solution of PAA or DPAA (concentration: 82.9 mg As/L) and 30 mg ferrihydrite. The pH was adjusted by adding a HNO3 or NaOH solution to pH 4, and the ionic strength was fixed at 0.10 M by NaNO3. The samples were shaken by a reciprocation shaker for 24 h at 25 °C and at 130 rpm, and then filtered by a cellulose membrane filter (0.45 lm). The amounts of PAA and DPAA adsorbed on ferrihydrite were determined by the concentration of arsenic in the initial and the solutions after reaction using ICP–MS and the adsorption densities were 1.76 lmol/m2 and 1.15 lmol/m2 for PAA and DPAA, respectively, using a surface area of ferrihydrite, 330 m2/g, reported in our previous study [24]. 2.2. EXAFS analysis Arsenic K-edge EXAFS spectra of PAA and DPAA dissolved in water or adsorbed on ferrihydrite were collected at beamline BL01B1 in SPring-8 (Hyogo, Japan) or at BL-12C in Photon Factory (Tsukuba, Japan). A Si(1 1 1) double-crystal monochromator was used in the beamlines. EXAFS spectra for arsenic adsorbed on ferrihydrite were measured in fluorescence mode using a 19-element germanium semiconductor detector, whereas spectra of reference solution samples were measured in transmission mode. X-ray energy was calibrated by defining a single peak of KAsO2 at 11,865 eV. The measurement was carried out at room temperature under ambient air condition. EXAFS data were analyzed by REX2000 Version 2.5.9 (Rigaku Co., Ltd.). Fourier transformation of the k3-weighted v(k) EXAFS oscillation from k (Å 1) space to R (Å) space was performed in a range of k = 2.4–13.7 Å 1 to obtain a radial structural function (RSF) in R space. In the EXAFS fitting, the coordination numbers (CNs) of the shells related to the molecule of the organoarsenic compounds such as AsAO and AsAC (AsAC1 to AsAC4) were fixed, since it was suggested that the bondings with first neighboring atoms of PAA and DPAA were kept unchanged during the adsorption reaction based on the fact that XANES spectra of their solution and adsorption samples were same (Fig. 1).
Fig. 1. As K-edge XANES spectra (a) and their first derivative (b) of solution (sol.) and adsorption on ferrihydrite (ads.) for PAA and DPAA.
Under this experimental condition, an As-bearing precipitation was not found in the solution in the absence of ferrihydrite. In addition, in the crystal structure of PAA [25] and DPAA ([AsPh2 (O)OHAsPh2(S)OH]) [26], the nearest interatomic distances of As and As are 4.6 Å and 4.9 Å for PAA and DPAA, respectively. Thus, the contribution of AsAAs shell itself can be small and negligible. In fact, using AsAAs shell does not improve the fitting results of EXAFS spectra.
2.3. Quantum chemical calculation For the DFT calculations, geometry optimization and normal mode analysis were performed with B3LYP/6-311+G* [27,28] level using Gaussian 09 program [29] for mono- and bidentate complex models of PAA and DPAA on ferrihydrite to confirm the validity of structural parameters obtained by curve fittings of EXAFS. The electronic state of the system was assumed with high spin 11-et state. Geometrical parameter for Fe-(oxyhydr)oxide model was referred to goethite structure [30] instead of ferrihydrite due to the amorphous nature of the mineral. In the Fe-(oxyhydr)oxide model, four H2O molecules were located at the surface to take account of the interaction between arsenic compounds and neighboring OH groups around binding site (Fig. 2). The geometry of Fe-(oxyhydr)oxide model except for four H atoms in neighboring OH groups was fixed during geometry optimization. Optimized structures and structural parameters obtained were shown in Fig. 2 and Table 1, respectively. For phenyl-substituted compounds, four types of C atoms (C1–C4) were defined according to the distance from central arsenic atom (Fig. 2).
3. Results and discussion The EXAFS spectrum of DPAA in k space (Fig. 3) is more complex than that of PAA. Contribution of each shell suggested that the complexity in DPAA spectrum is caused by contributions of various AsAC shells originated from two phenyl groups (Fig. 3). The k3v(k) spectra of PAA and DPAA adsorbed on ferrihydrite are different from those of the solution samples. The difference in the spectra can be explained by the contribution of AsAFe shell. In the spectra for PAA, for example, flat shape from k = 6.5–7.5 Å 1 is caused by the two peaks of the AsAFe shell at k = 6.6 and 7.7 Å 1, which is different from the spectrum of PAA solution possibly due to the presence of Fe in the second shell. A similar trend is found in the spectra for DPAA. These facts indicate that both PAA and DPAA adsorb on ferrihydrite as inner-sphere complex.
M. Tanaka et al. / Journal of Colloid and Interface Science 415 (2014) 13–17
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Fig. 2. Optimized structures of PAA and DPAA adsorption complex models by DFT calculation (B3LYP/6-311+G*). (a) PAA bidentate model, (b) PAA monodentate model, (c) DPAA bidentate model, (d) DPAA monodentate model. Yellow-colored O atom is O atom in the modeled neighbor OH group. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Interatomic distances (Å) in PAA and DPAA obtained with B3LYP/6-311+G*. PAA
As@O AsAOH AsAC1 AsAC2 AsAC3 AsAC4 AsAFe a
DPAA
Monodentate
Bidentate
Monodentate
Bidentate
1.72 (1.64)a 1.79 1.94 2.90 4.20 4.72 3.52
1.71 1.75 1.91 2.87 4.17 4.69 3.25
1.73 (1.65)a – 1.96 2.91 4.23 4.75 3.52
1.72 – 1.94 2.90 4.20 4.72 3.29
As@O bond length in non-bonding to Fe.
In radial structural function (RSF; phase shift uncorrected) in Fig. 4, the first shell consists of oxygen and nearest carbon atom in the phenyl group (C1): AsAO and AsAC1 distances for solution samples obtained by the fitting were ca. 1.68 and 1.86 Å for PAA and 1.67 and 1.89 Å for DPAA, respectively (Table 2). Corresponding distances of adsorbed samples (Table 2) are similar to these distances. This fact indicates that geometries of PAA and DPAA did not change by the adsorption on ferrihydrite. The results are similar to previous EXAFS study for methyl-substituted arsenic compounds adsorbed on goethite [20]. In RSF, a peak appears for adsorbed samples around R + DR = 2.9 Å which was not observed in the solution samples for PAA and
Fig. 3. k3-Weighted As K-edge EXAFS spectra of PAA and DPAA dissolved in water (sol.) and adsorbed on ferrihydrite (ads.). Solid line: observed spectra; dashed line: fitted spectra; dotted line: contribution of each shell.
DPAA (Fig. 4). It is suggested that this peak corresponds to the AsAFe shells. For the curve fitting, structural parameters obtained by DFT calculations were considered (Table 1). Three models were examined here for the AsAFe shell fitting: (i) single site model
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M. Tanaka et al. / Journal of Colloid and Interface Science 415 (2014) 13–17
Fig. 4. Radial structural functions for PAA and DPAA dissolved in water (sol.) and adsorbed on ferrihydrite (ads.). Solid line: observed spectra; dashed line: fitted spectra.
Table 2 Structural parameters obtained by curve fitting of EXAFS spectra for PAA and DPAA dissolved in water (sol.) and adsorbed on ferrihydrite (ads.). Shell
PAA
DPAA
Sol.
Ads.
Sol.
Ads.
AsAO
CN R /Å r2 /Å2
3.0 1.69(0) 0.0031
3.0 1.68(0) 0.0020
2.0 1.68(0) 0.0013
2.0 1.68(0) 0.0012
AsAC1
CN R /Å r2 /Å2
1.0 1.84(1) 0.0021
1.0 1.82(2) 0.0023
2.0 1.90(1) 0.0027
2.0 1.89(1) 0.0020
AsAC2
CN R /Å r2 /Å2
2.0 2.82(3) 0.0042
2.0 2.81(3) 0.0046
4.0 2.87(1) 0.0045
4.0 2.86(1) 0.0026
AsAC3
CN R /Å r2 /Å2
2.0 4.31(9) 0.0057
2.0 4.35(8) 0.0054
4.0 4.38(3) 0.0040
4.0 4.36(4) 0.0048
AsAC4
CN R /Å r2 /Å2
1.0 4.72(9) 0.0029
1.0 4.73(8) 0.0031
2.0 4.79(6) 0.0028
2.0 4.75(7) 0.0027
AsAFe1
CN R /Å r2 /Å2
2.3(4) 3.25(1) 0.0076
2.1(3) 3.27(1) 0.0076
AsAFe2
CN R /Å r2 /Å2 DE0 /eV Res /%
1.0(4) 3.44(3) 0.0076 3.2 2.5
0.8(3) 3.46(3) 0.0076 4.8 6.0
4.1 4.9
4.8 6.5
CN: coordination number; R: interatomic distance; r: Debye–Waller factor; DE0:E0 shift; Res: residual in the fitting; Numbers in Italics were fixed during fitting. For all cases, reduction factor (S02) was fixed to 1.0.
considering monodentate complex, (ii) single site model considering bidentate complex, and (iii) two-site model considering both complexes. The fitting residual assuming two-site model was lower than that by single site cases. The AsAFe distances (RAsAFe) and its coordination numbers (CNs) of two shells were estimated to be (i) (RAsAFe, CN) = (3.24 Å and 2.3) and (3.44 Å and 1.0) for PAA and (ii) (RAsAFe, CN) = (3.27 Å and 2.1) and (3.46 Å and 0.8) for DPAA, respectively. The two AsAFe distances of bi- and monodentate complexes calculated by DFT were (i) 3.25 Å and 3.52 Å and (ii) 3.29 Å and 3.52 Å for PAA and DPAA, respectively, while CNs 2.0 and 1.0 for shorter and longer AsAFe distance shells, respectively. These DFT results are consistent with EXAFS results assuming the bi- and monodentate complexes, showing the coexistence
of bidentate and monodentate structures in PAA and DPAA adsorption on ferrihydrite. Guo et al. reported that bidentate–binuclear and monodentate– mononuclear complexes coexist for arsenate species adsorbed on goethite, where the AsAFe distances and CNs were (i) 3.34 Å and 2.1 and (ii) 3.45 Å and 1.1 for the bidentate–binuclear and monodentate–mononuclear complexes, respectively, by EXAFS analysis [31]. Similar results on the coexistence of bi- and monodentate complexes were reported for arsenate adsorption on ruthenium oxide by Luxton et al. using EXAFS, which was also confirmed by the pressure-jump relaxation spectroscopy [32]. These studies support our multi-site EXAFS fitting results. In addition, previous ATRFTIR studies indicated that MMA and DMA adsorbed on aluminum oxides [21] and goethite [33] with both inner-(bi- or monodentate) and outer-sphere complexation. On the other hand, bidentate complexation for MMA and DMA adsorbed on goethite was suggested by the solo-site EXAFS fitting result of the AsAFe distances and its CNs were (i) 3.31 Å and 1.8 for MMA and (ii) 3.30 Å and 1.9 for DMA, respectively [20]. Depalma et al. also suggested that p-arsanilic acid (p-NH2C6H4AsO(OH)2), which is an amino-substituted PAA compound, forms inner-sphere monodentate or outer-sphere complex on iron (oxyhydr)oxides at pH 7 from the ATR-FTIR study [34]. These results also suggest that coexistence of several type of adsorption structure in organoarsenic compounds on mineral surface. It is worth noting that obtained interatomic distances with optimized geometry by DFT calculations are in good agreement with EXAFS results, for example, the distances were slightly longer than those of PAA, which can be caused by steric hindrance of two phenyl groups (Table 1 and Table 2). This fact shows atomic distances of optimized structure by DFT calculation is helpful to confirm the parameters determined by EXAFS spectra, especially for the complex species. 4. Conclusions The adsorption structures of the mono- and diphenyl substituted organoarsenic compounds, i.e. PAA and DPAA, on ferrihydrite were analyzed by XAFS measurements. The EXAFS spectra suggested that PAA and DPAA form inner-sphere complexes with ferrihydrite regardless their bulky functional groups. In addition, coexistence of two types of inner-sphere complex models, i.e. bidentate–binuclear and monodentate structures was suggested by EXAFS fitting with two type of the AsAFe shell. Surface complexation modeling (SCM) [35,36] can estimate adsorption behavior of ions on minerals based on the adsorption structures, which can be employed widely for inorganic arsenic species and other toxic ions. In the recent SCM [36], information about adsorption structure is important to interpret the adsorption behavior. Thus, our results on the adsorption structure can contribute to the precise modeling of the behavior of anthropogenic organoarsenic compounds in the natural system based on the SCM. Acknowledgments This study has been performed with the approval of KEK (2009G655) and SPring-8 (2011B1742 and 2012A1299). References [1] [2] [3] [4]
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[25] N.C. Lloyd, H.W. Morgan, B.K. Nicholson, R.S. Ronimus, J. Organomet. Chem. 693 (2008) 2443. [26] L. Silaghi-Dumitrescu, M.N. Gibbons, I. Silaghi-Dumitrescu, J. ZukermanSchpector, I. Haiduc, D.B. Sowerby, J. Organomet. Chem. 517 (1996) 101. [27] A.D. Becke, Phys. Rev. A 38 (1988) 3098; A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [28] R. Krishnan, J.S. Binkley, R. Seeger, J.A. Pople, J. Chem. Phys. 72 (1980) 650; L.A. Curtiss, M.P. McGrath, J.-P. Blaudeau, N.E. Davis, R.C. Binning Jr., L. Radom, J. Chem. Phys. 103 (1995) 6104; B. Roos, A. Veillard, G. Vinot, Theoret. Chim. Acta 20 (1971) 1; P.J. Hay, J. Chem. Phys. 66 (1977) 4377; K. Raghavachari, G.W. Trucks, J. Chem. Phys. 91 (1989) 1062. [29] M.J. Frisch, et al., Gaussian 09 Revision C.01, Gaussian Inc., Wallingford, CT, 2010. [30] H. Yang, R. Lu, R.T. Downs, G. Costin, Acta Cryst. E62 (2006) i250. [31] H. Guo, Y. Ren, Q. Liu, K. Zhao, Y. Li, Environ. Sci. Technol. 47 (2013) 1009. [32] T.P. Luxton, M.J. Eick, K.G. Scheckel, J. Colloid Interface Sci. 325 (2008) 23. [33] A. Adamescu, W. Mitchell, I.P. Hamilton, H.A. Al-Abadleh, Environ. Sci. Technol. 44 (2010) 7802. [34] S. Depalma, S. Cowen, T. Hoang, H.A. Al-Abadleh, Environ. Sci. Technol. 42 (2008) 1922. [35] D.A. Dzombak, F.M.M. Morel, Surface Complexation Modeling: Hydrous Ferric Oxide, Wiley-Interscience, New York, 1990. [36] K. Fukushi, D.A. Sverjensky, Geochim. Cosmochim. Acta 71 (2007) 3717.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
An EXAFS study on the adsorption structure of phenyl-substituted organoarsenic compounds on ferrihydrite Masato Tanaka a, Yoko S. Togo b, Noriko Yamaguchi c, Yoshio Takahashi a,⇑ a
Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science & Technology (AIST), 1-1-1, Higashi, Tsukuba, Ibaraki 305-8567, Japan c National Institute for Agro-Environmental Sciences, 3-1-3, Kan-nondai, Tsukuba, Ibaraki 305-8505, Japan b
a r t i c l e
i n f o
Article history: Received 23 June 2013 Accepted 10 October 2013 Available online 20 October 2013 Keywords: Organic arsenic Phenylarsonic acid (PAA) Diphenylarsinic acid (DPAA) Adsorption structure Ferrihydrite Extended X-ray absorption fine structure (EXAFS) Quantum chemical calculation
a b s t r a c t Adsorption structures of mono- and diphenyl substituted organoarsenic compounds (PAA and DPAA, respectively) on ferrihydrite were analyzed by extended X-ray absorption fine structure (EXAFS), which suggested that PAA and DPAA form inner-sphere complexes with ferrihydrite regardless of their bulky functional groups. In addition, coexistence of two types of inner-sphere complex modes, i.e. bidentate– binuclear and monodentate surface complexes was suggested by EXAFS fitting with two type of the AsAFe shell. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Water pollution by arsenic has been a serious problem around the world, and its predominant source is naturally-derived inorganic arsenic compounds. In addition, organoarsenic species such as methyl and phenyl arsenic compounds were also found, although organoarsenic compounds are thought to be less toxic than inorganic ones [1,2]. It has been reported that these organoarsenic compounds are distributed in environment. Methyl-substituted arsenic species are produced as microbial metabolites in marine and terrestrial systems [3,4] and also have been used as pesticides and herbicides [2,5]. Phenyl-substituted, on the other hand, that has been also used as pesticides and herbicides are exclusively anthropogenic compounds [2]. Furthermore, phenyl arsenic species were considered as degradation products of chemical warfare agents used during the World Wars I and II [6]. After the World War II, such kind of chemical weapons including these agents were abandoned by dumping in the sea or by burying in the geosphere [7]. Even in the present day, these materials and their derivatives can be found in environment [6–8]. As a direct casualty of phenyl derivative arsenic compounds, well water contaminated by diphenylarsinic acid (DPAA) posed
⇑ Corresponding author. E-mail address: [email protected] (Y. Takahashi). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.006
health hazards to the residents at Kamisu Town in Japan [6]. In addition, Nakamiya et al. and Harada et al. reported biodegradation of DPAA to phenylarsonic acid (PAA) in environment [9,10]. These studies indicated that organoarsenic compounds can be gradually converted to inorganic forms by microbial activities, and such anthropogenic organoarsenic compounds have a potential to be a secondary source of more toxic inorganic arsenics. Recently, the effect of phenyl-substituted arsenic compounds in the agricultural environment such as uptake of these arsenic compounds from polluted soil to rice has been investigated [11–14], which revealed a growing concern of these phenyl-substituted arsenics in environment. However, there have been few studies on the physicochemical processes of PAA and DPAA in natural soil–water or in sediment–water systems such as their adsorption on iron (oxyhydr)oxides. The adsorption of inorganic (arsenite and arsenate) and methylated arsenics has been widely studied by several methods [15–17]. For example, adsorption amount of arsenite, arsenate, and mono-methylarsonic acid (MMA) on ferrihydrite have been reported by adsorption experiments at various pH and ionic strength [15,17]. In addition, extended X-ray absorption fine structure (EXAFS) analysis has been employed to obtain their adsorption structures, which are related to the stability of adsorbed species, or distribution between water and soil. Therefore, it is important to study the adsorption structures of arsenic compounds on iron (oxyhydr)oxides to understand its migration processes in
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M. Tanaka et al. / Journal of Colloid and Interface Science 415 (2014) 13–17
soils. Ferrihydrite is one of the most important iron (oxyhydr)oxides in natural systems due to its large surface area that can be a strong adsorbent of anionic species such as inorganic arsenic species in environment [18]. Thus, ferrihydrite may control migration of arsenic as its scavenger in soils. Adsorption structures of methyl-substituted arsenic compounds such as MMA and dimethylarsinic acid (DMA) on several minerals as well as goethite have been investigated also by EXAFS, since the adsorption on minerals can be an important process for the migration of methyl arsenic species in terrestrial environment [19–21]. However, those of phenyl-substituted arsenics had not been investigated yet except for our preliminary study on the adsorption structures of PAA and DPAA on ferrihydrite by EXAFS [22]. Since phenylsubstituted arsenic compounds have many carbon atoms in the phenyl groups, the EXAFS spectra can be complicated and difficult to interpret. Hence, the objective of this study is to reveal the adsorption structure of phenyl organoarsenic compounds, i.e. PAA and DPAA onto ferrihydrite by EXAFS spectra with the aid of quantum chemical calculations based on the density functional theory (DFT) to obtain structural parameters. 2. Materials and methods 2.1. Materials Two-line ferrihydrite was synthesized in the laboratory as described by Schwertmann and Cornell [23]. The mineralogy of the 2-line ferrihydrite was confirmed by powder X-ray diffraction. Aqueous solutions of organoarsenic species were prepared for PAA (C6H5AsO(OH)2; Wako) or DPAA ((C6H5)2AsO(OH); Wako) dissolved in Milli-Q water. Adsorption samples for EXAFS were prepared by mixing 15 mL solution of PAA or DPAA (concentration: 82.9 mg As/L) and 30 mg ferrihydrite. The pH was adjusted by adding a HNO3 or NaOH solution to pH 4, and the ionic strength was fixed at 0.10 M by NaNO3. The samples were shaken by a reciprocation shaker for 24 h at 25 °C and at 130 rpm, and then filtered by a cellulose membrane filter (0.45 lm). The amounts of PAA and DPAA adsorbed on ferrihydrite were determined by the concentration of arsenic in the initial and the solutions after reaction using ICP–MS and the adsorption densities were 1.76 lmol/m2 and 1.15 lmol/m2 for PAA and DPAA, respectively, using a surface area of ferrihydrite, 330 m2/g, reported in our previous study [24]. 2.2. EXAFS analysis Arsenic K-edge EXAFS spectra of PAA and DPAA dissolved in water or adsorbed on ferrihydrite were collected at beamline BL01B1 in SPring-8 (Hyogo, Japan) or at BL-12C in Photon Factory (Tsukuba, Japan). A Si(1 1 1) double-crystal monochromator was used in the beamlines. EXAFS spectra for arsenic adsorbed on ferrihydrite were measured in fluorescence mode using a 19-element germanium semiconductor detector, whereas spectra of reference solution samples were measured in transmission mode. X-ray energy was calibrated by defining a single peak of KAsO2 at 11,865 eV. The measurement was carried out at room temperature under ambient air condition. EXAFS data were analyzed by REX2000 Version 2.5.9 (Rigaku Co., Ltd.). Fourier transformation of the k3-weighted v(k) EXAFS oscillation from k (Å 1) space to R (Å) space was performed in a range of k = 2.4–13.7 Å 1 to obtain a radial structural function (RSF) in R space. In the EXAFS fitting, the coordination numbers (CNs) of the shells related to the molecule of the organoarsenic compounds such as AsAO and AsAC (AsAC1 to AsAC4) were fixed, since it was suggested that the bondings with first neighboring atoms of PAA and DPAA were kept unchanged during the adsorption reaction based on the fact that XANES spectra of their solution and adsorption samples were same (Fig. 1).
Fig. 1. As K-edge XANES spectra (a) and their first derivative (b) of solution (sol.) and adsorption on ferrihydrite (ads.) for PAA and DPAA.
Under this experimental condition, an As-bearing precipitation was not found in the solution in the absence of ferrihydrite. In addition, in the crystal structure of PAA [25] and DPAA ([AsPh2 (O)OHAsPh2(S)OH]) [26], the nearest interatomic distances of As and As are 4.6 Å and 4.9 Å for PAA and DPAA, respectively. Thus, the contribution of AsAAs shell itself can be small and negligible. In fact, using AsAAs shell does not improve the fitting results of EXAFS spectra.
2.3. Quantum chemical calculation For the DFT calculations, geometry optimization and normal mode analysis were performed with B3LYP/6-311+G* [27,28] level using Gaussian 09 program [29] for mono- and bidentate complex models of PAA and DPAA on ferrihydrite to confirm the validity of structural parameters obtained by curve fittings of EXAFS. The electronic state of the system was assumed with high spin 11-et state. Geometrical parameter for Fe-(oxyhydr)oxide model was referred to goethite structure [30] instead of ferrihydrite due to the amorphous nature of the mineral. In the Fe-(oxyhydr)oxide model, four H2O molecules were located at the surface to take account of the interaction between arsenic compounds and neighboring OH groups around binding site (Fig. 2). The geometry of Fe-(oxyhydr)oxide model except for four H atoms in neighboring OH groups was fixed during geometry optimization. Optimized structures and structural parameters obtained were shown in Fig. 2 and Table 1, respectively. For phenyl-substituted compounds, four types of C atoms (C1–C4) were defined according to the distance from central arsenic atom (Fig. 2).
3. Results and discussion The EXAFS spectrum of DPAA in k space (Fig. 3) is more complex than that of PAA. Contribution of each shell suggested that the complexity in DPAA spectrum is caused by contributions of various AsAC shells originated from two phenyl groups (Fig. 3). The k3v(k) spectra of PAA and DPAA adsorbed on ferrihydrite are different from those of the solution samples. The difference in the spectra can be explained by the contribution of AsAFe shell. In the spectra for PAA, for example, flat shape from k = 6.5–7.5 Å 1 is caused by the two peaks of the AsAFe shell at k = 6.6 and 7.7 Å 1, which is different from the spectrum of PAA solution possibly due to the presence of Fe in the second shell. A similar trend is found in the spectra for DPAA. These facts indicate that both PAA and DPAA adsorb on ferrihydrite as inner-sphere complex.
M. Tanaka et al. / Journal of Colloid and Interface Science 415 (2014) 13–17
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Fig. 2. Optimized structures of PAA and DPAA adsorption complex models by DFT calculation (B3LYP/6-311+G*). (a) PAA bidentate model, (b) PAA monodentate model, (c) DPAA bidentate model, (d) DPAA monodentate model. Yellow-colored O atom is O atom in the modeled neighbor OH group. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Interatomic distances (Å) in PAA and DPAA obtained with B3LYP/6-311+G*. PAA
As@O AsAOH AsAC1 AsAC2 AsAC3 AsAC4 AsAFe a
DPAA
Monodentate
Bidentate
Monodentate
Bidentate
1.72 (1.64)a 1.79 1.94 2.90 4.20 4.72 3.52
1.71 1.75 1.91 2.87 4.17 4.69 3.25
1.73 (1.65)a – 1.96 2.91 4.23 4.75 3.52
1.72 – 1.94 2.90 4.20 4.72 3.29
As@O bond length in non-bonding to Fe.
In radial structural function (RSF; phase shift uncorrected) in Fig. 4, the first shell consists of oxygen and nearest carbon atom in the phenyl group (C1): AsAO and AsAC1 distances for solution samples obtained by the fitting were ca. 1.68 and 1.86 Å for PAA and 1.67 and 1.89 Å for DPAA, respectively (Table 2). Corresponding distances of adsorbed samples (Table 2) are similar to these distances. This fact indicates that geometries of PAA and DPAA did not change by the adsorption on ferrihydrite. The results are similar to previous EXAFS study for methyl-substituted arsenic compounds adsorbed on goethite [20]. In RSF, a peak appears for adsorbed samples around R + DR = 2.9 Å which was not observed in the solution samples for PAA and
Fig. 3. k3-Weighted As K-edge EXAFS spectra of PAA and DPAA dissolved in water (sol.) and adsorbed on ferrihydrite (ads.). Solid line: observed spectra; dashed line: fitted spectra; dotted line: contribution of each shell.
DPAA (Fig. 4). It is suggested that this peak corresponds to the AsAFe shells. For the curve fitting, structural parameters obtained by DFT calculations were considered (Table 1). Three models were examined here for the AsAFe shell fitting: (i) single site model
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M. Tanaka et al. / Journal of Colloid and Interface Science 415 (2014) 13–17
Fig. 4. Radial structural functions for PAA and DPAA dissolved in water (sol.) and adsorbed on ferrihydrite (ads.). Solid line: observed spectra; dashed line: fitted spectra.
Table 2 Structural parameters obtained by curve fitting of EXAFS spectra for PAA and DPAA dissolved in water (sol.) and adsorbed on ferrihydrite (ads.). Shell
PAA
DPAA
Sol.
Ads.
Sol.
Ads.
AsAO
CN R /Å r2 /Å2
3.0 1.69(0) 0.0031
3.0 1.68(0) 0.0020
2.0 1.68(0) 0.0013
2.0 1.68(0) 0.0012
AsAC1
CN R /Å r2 /Å2
1.0 1.84(1) 0.0021
1.0 1.82(2) 0.0023
2.0 1.90(1) 0.0027
2.0 1.89(1) 0.0020
AsAC2
CN R /Å r2 /Å2
2.0 2.82(3) 0.0042
2.0 2.81(3) 0.0046
4.0 2.87(1) 0.0045
4.0 2.86(1) 0.0026
AsAC3
CN R /Å r2 /Å2
2.0 4.31(9) 0.0057
2.0 4.35(8) 0.0054
4.0 4.38(3) 0.0040
4.0 4.36(4) 0.0048
AsAC4
CN R /Å r2 /Å2
1.0 4.72(9) 0.0029
1.0 4.73(8) 0.0031
2.0 4.79(6) 0.0028
2.0 4.75(7) 0.0027
AsAFe1
CN R /Å r2 /Å2
2.3(4) 3.25(1) 0.0076
2.1(3) 3.27(1) 0.0076
AsAFe2
CN R /Å r2 /Å2 DE0 /eV Res /%
1.0(4) 3.44(3) 0.0076 3.2 2.5
0.8(3) 3.46(3) 0.0076 4.8 6.0
4.1 4.9
4.8 6.5
CN: coordination number; R: interatomic distance; r: Debye–Waller factor; DE0:E0 shift; Res: residual in the fitting; Numbers in Italics were fixed during fitting. For all cases, reduction factor (S02) was fixed to 1.0.
considering monodentate complex, (ii) single site model considering bidentate complex, and (iii) two-site model considering both complexes. The fitting residual assuming two-site model was lower than that by single site cases. The AsAFe distances (RAsAFe) and its coordination numbers (CNs) of two shells were estimated to be (i) (RAsAFe, CN) = (3.24 Å and 2.3) and (3.44 Å and 1.0) for PAA and (ii) (RAsAFe, CN) = (3.27 Å and 2.1) and (3.46 Å and 0.8) for DPAA, respectively. The two AsAFe distances of bi- and monodentate complexes calculated by DFT were (i) 3.25 Å and 3.52 Å and (ii) 3.29 Å and 3.52 Å for PAA and DPAA, respectively, while CNs 2.0 and 1.0 for shorter and longer AsAFe distance shells, respectively. These DFT results are consistent with EXAFS results assuming the bi- and monodentate complexes, showing the coexistence
of bidentate and monodentate structures in PAA and DPAA adsorption on ferrihydrite. Guo et al. reported that bidentate–binuclear and monodentate– mononuclear complexes coexist for arsenate species adsorbed on goethite, where the AsAFe distances and CNs were (i) 3.34 Å and 2.1 and (ii) 3.45 Å and 1.1 for the bidentate–binuclear and monodentate–mononuclear complexes, respectively, by EXAFS analysis [31]. Similar results on the coexistence of bi- and monodentate complexes were reported for arsenate adsorption on ruthenium oxide by Luxton et al. using EXAFS, which was also confirmed by the pressure-jump relaxation spectroscopy [32]. These studies support our multi-site EXAFS fitting results. In addition, previous ATRFTIR studies indicated that MMA and DMA adsorbed on aluminum oxides [21] and goethite [33] with both inner-(bi- or monodentate) and outer-sphere complexation. On the other hand, bidentate complexation for MMA and DMA adsorbed on goethite was suggested by the solo-site EXAFS fitting result of the AsAFe distances and its CNs were (i) 3.31 Å and 1.8 for MMA and (ii) 3.30 Å and 1.9 for DMA, respectively [20]. Depalma et al. also suggested that p-arsanilic acid (p-NH2C6H4AsO(OH)2), which is an amino-substituted PAA compound, forms inner-sphere monodentate or outer-sphere complex on iron (oxyhydr)oxides at pH 7 from the ATR-FTIR study [34]. These results also suggest that coexistence of several type of adsorption structure in organoarsenic compounds on mineral surface. It is worth noting that obtained interatomic distances with optimized geometry by DFT calculations are in good agreement with EXAFS results, for example, the distances were slightly longer than those of PAA, which can be caused by steric hindrance of two phenyl groups (Table 1 and Table 2). This fact shows atomic distances of optimized structure by DFT calculation is helpful to confirm the parameters determined by EXAFS spectra, especially for the complex species. 4. Conclusions The adsorption structures of the mono- and diphenyl substituted organoarsenic compounds, i.e. PAA and DPAA, on ferrihydrite were analyzed by XAFS measurements. The EXAFS spectra suggested that PAA and DPAA form inner-sphere complexes with ferrihydrite regardless their bulky functional groups. In addition, coexistence of two types of inner-sphere complex models, i.e. bidentate–binuclear and monodentate structures was suggested by EXAFS fitting with two type of the AsAFe shell. Surface complexation modeling (SCM) [35,36] can estimate adsorption behavior of ions on minerals based on the adsorption structures, which can be employed widely for inorganic arsenic species and other toxic ions. In the recent SCM [36], information about adsorption structure is important to interpret the adsorption behavior. Thus, our results on the adsorption structure can contribute to the precise modeling of the behavior of anthropogenic organoarsenic compounds in the natural system based on the SCM. Acknowledgments This study has been performed with the approval of KEK (2009G655) and SPring-8 (2011B1742 and 2012A1299). References [1] [2] [3] [4]
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