Infrared spectroscopic and X-ray diffraction characterization of the nature of adsorbed arsenate on ferrihydrite

Geochimica et Cosmochimica Acta 71 (2007) 1643–1654 www.elsevier.com/locate/gca

Infrared spectroscopic and X-ray diffraction characterization of the nature of adsorbed arsenate on ferrihydrite Yongfeng Jia a

a,*

, Liying Xu a, Xin Wang a, George P. Demopoulos

b,*

Institute of Applied Ecology, Key Laboratory of Terrestrial Ecological Process, Chinese Academy of Sciences, Shenyang 110016, China b Department of Mining, Metals and Materials Engineering, McGill University, Montreal, QC, Canada H3A 2B2 Received 30 March 2006; accepted in revised form 14 December 2006; available online 30 January 2007

Abstract Fourier transformed infrared (FTIR) spectroscopy was used to characterize arsenate–ferrihydrite sorption solids synthesized at pH 3–8. The speciation of sorbed arsenate was determined based on the As–O stretching vibration bands located at 650–950 cm1 and O–H stretching vibration bands at 3000–3500 cm1. The positions of the As–O and O–H stretching vibration bands changed with pH indicating that the nature of surface arsenate species on ferrihydrite was strongly pH dependent. Sorption density and synthesis media (sulfate vs. nitrate) had no appreciable effect. At acidic pH (3, 4), ferric arsenate surface precipitate formed on ferrihydrite and constituted the predominant surface arsenate species. X-ray diffraction (XRD) analyses of he sorption solids synthesized at elevated temperature (75 °C), pH 3 clearly showed the development of crystalline ferric arsenate (i.e. scorodite). In neutral and alkaline media (pH 7, 8), arsenate sorbed as a bidentate surface complex (in both protonated BFeO2 AsðOÞðOHÞ and unprotonated BFeO2 AsðOÞ2 2 forms). For the sorption systems in slightly acidic media (pH 5, 6), both ferric arsenate and surface complex were probably present on ferrihydrite. It was further determined that the incorporated sulfate in ferrihydrite during synthesis was substituted by arsenate and was more easily exchangeable with increasing pH. Ó 2007 Elsevier Ltd. All rights reserved.

1. INTRODUCTION Ferrihydrite is a poorly ordered hydrous iron oxide commonly present in low-temperature geochemical processes. It is widely occurring in surface environments, e.g. in soils, lake and river sediments and water columns (Waychunas et al., 1993; Cornell and Schwertmann, 1996; Jambor and Dutrizac, 1998). Arsenate is an important form of arsenic in both natural water systems and mineral processing tailings. Ferrihydrite shows strong affinity for arsenate at mineral–water interfaces. Adsorption on ferrihydrite is an important factor controlling transport, fate and bioavailability of arsenic in soils, groundwater

*

Corresponding authors. Fax: +86 24 8397 0436 (Y. Jia), +1 514 398 4492 (G.P. Demopoulos). E-mail addresses: [email protected] (Y. Jia), george. [email protected] (G.P. Demopoulos). 0016-7037/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2006.12.021

and surface water systems (Smedley and Kinniburgh, 2002). Ferrihydrite is also a common product in many hydrometallurgical operations such as the coprecipitation process of iron with arsenate as well as in acid mine drainage (Jambor and Dutrizac, 1998; Carlson et al., 2002). Hence the adsorption of arsenate on ferrihydrite plays an important role in the removal and immobilization of arsenic from industrial effluents as well as the fate of arsenate in tailings impoundment. Adsorption of arsenate on ferrihydrite involves ligand exchange with H2O and/or OH on the substrate surface (Jain et al., 1999). Factors influencing adsorption of arsenate on ferrihydrite include medium pH, type and concentration of co-ions, initial Fe/As molar ratios etc. pH is one of the most important factors that control aqueous arsenate speciation and surface functional groups of hydrous oxides, consequently influencing the macroscopic characteristics of adsorption process and the microscopic characteristics such as bonding modes and the nature of

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adsorbed arsenate on the surface of ferrihydrite (Masscheleyn et al., 1991; Hsia et al., 1992; Fuller et al., 1993; Bowell, 1994; Wilkie and Hering, 1996; Raven et al., 1998; Jain et al., 1999; Meng et al., 2000; Grafe et al., 2002; Dixit and Hering, 2003). The degree of protonation of arsenate anion in aqueous solution is a function of pH with pKa1 = 2.3, pKa2 = 6.8 and pKa3 = 11.6 (Goldberg and Johnston, 2001), resulting in arsenate species varying from H3AsO4, H2 AsO4  , HAsO4 2 , to AsO4 3 when pH increases from acidic region to alkaline region (Myneni et al., 1998; Raven et al., 1998; Goldberg and Johnston, 2001). On the other hand, the presence and the density of surface groups of ferrihydrite, i.e. H2O, OH, are also strongly pH dependent. The point of zero charge (PZC) is approximately 8.5 (Jain et al., 1999; Goldberg and Johnston, 2001). Hence, the modes of complexation of arsenate anions with ferrihydrite by replacing surface hydroxyl groups and/or waters are largely controlled by the pH of reaction medium. Ligand exchange in the mode of bidentate binuclear inner-sphere complexation is the widely accepted mechanism of the adsorption of arsenate on iron oxides. It has been proposed based on infrared (Harrison and Berkheiser, 1982) and extended X-ray absorption fine structure (EXAFS) (Waychunas et al., 1993) analyses and confirmed to be the dominant interaction mode of arsenate–ferrihydrite and arsenate–goethite systems (Lumsdon et al., 1984; Manceau, 1995; Sun and Doner, 1996; Waychunas et al., 1996; Fendorf et al., 1997; Foster et al., 1998; Myneni et al., 1998; O’Reilly et al., 2001; Roddick-Lanzilotta et al., 2002; Sherman and Randall, 2003; Arai et al., 2004; Cance`s et al., 2005; Waychunas et al., 2005). However, most of the studies were conducted at neutral to alkaline pH. The applicability of the conclusions to the acidic arsenate–ferrihydrite adsorption system may be questionable. Moreover, most of the studies have dealt mainly with characterizing the bonding mechanism between arsenate anions and surface iron polyhedra without identifying arsenate species on the surface of iron oxide. Based on macroscopic measurements of the adsorption process, arsenate was adsorbed as BFeO2 AsðOHÞ2 ; BFeO2 AsðOÞðOHÞ ; BFeO2 AsðOÞ2 2 on the surface of ferrihydrite at mildly acidic and alkaline pH (Jain et al., 1999). In a recent work on direct characterization of arsenate coordination on mineral (portlandite, gibbsite, ettringite, Fe-oxyhydroxides) surfaces using FTIR, both protonated and unprotonated arsenate species (i.e. HAsO4 2 and AsO4 3 were present on the goethite surface at alkaline pH (Myneni et al., 1998). When arsenate was adsorbed on schwertmannite and ferrihydrite at acidic pH (i.e. pH 3), surface precipitates were proposed to form and were termed as ferric hydroxyarsenate (FeOHAs) (Carlson et al., 2002). Evidence for surface precipitation of phosphate on goethite has been observed (Ler and Stanforth, 2003). Similarly, surface precipitation of ferric arsenate on ferrihydrite is likely to occur in addition to bidentate binuclear complexation according to the XRD and Raman spectroscopic evidence we reported recently (Jia et al., 2006). The objective of this paper was to provide further evidence of surface precipitation of ferric arsenate on ferrihydrite. This was done via characterization of the interactions

between arsenate and ferrihydrite in terms of bonding modes and surface arsenate species as a function of pH and coverage density by Fourier transformed infrared spectroscopy (FTIR) and evolution of crystallinity at elevated temperature (75 °C) by X-ray diffraction (XRD) analysis. The effect of ferrihydrite synthesis media (NO3  vs. SO4 2 ) was also evaluated since it significantly influenced arsenate adsorption capacity (Jia and Demopoulos, 2005). Poorly crystalline ferric arsenate was used as reference material in the study. Moreover, relatively high arsenic concentration solutions were used in this study since this is the case in important hydrometallurgical operations where arsenic removal is practiced. 2. MATERIALS AND METHODS 2.1. Synthesis of poorly crystalline ferric arsenate Poorly crystalline ferric arsenate was synthesized at 21 °C by adjusting a 0.02 M As(V)/0.02 M Fe(III) solution (as sodium arsenate and ferric sulfate) from initial pH 1.3 to 1.8 with NaOH solution and maintained at that pH for 1 h (Jia et al., 2006). The resultant solid was separated by filtration, washed with de-ionized water (pH 2) and vacuumdried at 60 °C. The chemical formula (Fe1.02AsO4Æ2.4H2O) was determined by digestion with hydrochloric acid followed by ICP-AES analysis. 2.2. Synthesis of arsenate–ferrihydrite sorption samples Two-line ferrihydrite samples were synthesized at 21 °C using a slightly modified procedure from that reported in the literature (Schwertmann and Cornell, 1991). Both nitrate (Fe(NO3)3Æ9H2O) and sulfate (Fe2(SO4)3Æ5H2O) salts were used as the sources of ferric iron [Fe(III)]. Briefly, the Fe(III) solution was prepared by dissolving ferric nitrate or ferric sulfate in de-ionized water. The pH of the solution was raised to 7.5 in about 5 min using 1 M NaOH solution and maintained at that pH for 1 h with the slurry mechanically agitated vigorously. The ferrihydrite samples synthesized from sulfate and nitrate media were termed as ‘‘sulfate–ferrihydrite’’ and ‘‘nitrate–ferrihydrite’’, respectively, for simplicity. The prepared ferrihydrite slurry was adjusted to different pH between 3 and 8 with NaOH and HNO3 and allowed to equilibrate for 1 h. Arsenate solution was introduced into the ferrihydrite slurry from a burette over a 10-min period with the slurry mechanically stirred moderately. The pH was controlled by addition of NaOH and/or HNO3 solution and allowed to equilibrate at 21 °C for 2 weeks. The volume of adsorption slurry was 500 mL for all experiments and the concentration of Fe(III) in the slurry system was 4 g/L. At each pH, three initial Fe/As molar ratios (i.e. Fe/As = 2, 4 and 8) were applied. Arsenate–ferrihydrite sorption samples were also synthesized at 75 °C, Fe/As molar ratio of 2 and 4. The pH of the slurry was controlled constant at pH 3 throughout the sorption reaction. Samples were taken at 1 day, 3 day, 1 week, 2 weeks and 2 months of reaction time. The synthesized arsenate–ferrihydrite sorption products were filtered, DI water-rinsed

The nature of adsorbed arsenate on ferrihydrite

and vacuum-dried at 60 °C. The equilibrium concentration of arsenic was determined by ICP-AES analysis. 2.3. FTIR analysis The infrared spectra of the samples were obtained on a Bio-Rad FTS60 Fourier Transformed Infrared Spectrometer with a MCT liquid nitrogen cooled detector. The KBr/ sample discs were prepared by mixing 0.5% of finely ground samples in KBr. The sample chamber was purged by N2 gas for 10 min before scans were started. The measurement resolution was set at 4 cm1 and the spectra were collected in the range of 400–4000 cm1 with 200 co-added scans. 2.4. X-ray diffraction (XRD) analysis The powder XRD patterns were collected on a Rigaku D/Max 2500 PC X-ray diffractometer with graphite monochromated CuKa1 radiation. The powder samples were scanned from 10 to 90° 2h with increments of 0.02° 2h. 3. RESULTS AND DISCUSSION 3.1. Effect of pH on the nature of adsorbed arsenate Aqueous arsenate species have no direct bearing on the surface arsenate species adsorbed on mineral surfaces (Myneni et al., 1998). However, the effect of complexation of arsenate ions on oxide surfaces is similar to that of protonation of aqueous arsenate species. A brief discussion on infrared absorption of aqueous arsenate can assist with understanding the infrared characteristics of surface species (Myneni et al., 1998; Goldberg and Johnston, 2001; Roddick-Lanzilotta et al., 2002). The free arsenate anion, AsO4 3 , is present in highly alkaline (pKa3 = 11.6) aqueous solution and belongs to Td symmetry. Only m3 and m4 fundamental bands are infrared active in this form. The infrared

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spectrum of an AsO4 3 dominated solution exhibits a major band at 792 cm1 (Roddick-Lanzilotta et al., 2002). Upon protonation or complexation with metal cations, the symmetry decreases and splitting of the m3 band occurs (Harrison and Berkheiser, 1982; Myneni et al., 1998). Aqueous HAsO4 2 species belong to the C3v symmetry. Its infrared spectrum shows two broad bands at 859 and 689 cm1, the latter was assigned to stretching vibration of As–OH (Myneni et al., 1998). Curve fitting of the former band gave two bands at 865 and 846 cm1, which were assigned to asymmetric and symmetric stretching vibration of uncomplexed As–O, respectively (Myneni et al., 1998). Poorly crystalline ferric arsenate was used as reference material in this study to identify the possible occurrence of a surface precipitate of arsenate on ferrihydrite. This compound is not well defined and often termed loosely as amorphous ferric arsenate (Krause and Ettel, 1989) or amorphous scorodite (Langmuir et al., 1999), because it possesses similar bonding structures to crystalline ferric arsenate, i.e. scorodite (FeAsO4Æ2H2O). It is an unstable arsenate phase with increasing pH and tends to convert to ferrihydrite (Krause and Ettel, 1989). The poorly crystalline ferric arsenate synthesized in this work was determined to have the formula Fe1.02AsO4Æ2.4H2O. Fig. 1 shows the effect of pH on the infrared spectra of the sorption samples of arsenate on sulfate–ferrihydrite (initial molar ratio of Fe/As = 2). Both detailed display of the As–O stretching vibration region (500–1000 cm1) and the whole range of the scanning (400–4000 cm1) are shown in the figure. Poorly crystalline ferric arsenate shows a strong well-resolved band at 838 cm1. Within the crystalline ferric arsenate (i.e. scorodite) structure, AsO4 tetrahedra and FeO4(OH2)2 octahedra connect alternately at vertices (Kitahama et al., 1975). The arsenate is coordinated with four iron ˚ . The octahedra with an average As–O bond length of 1.68 A band at 838 cm1 was attributed to the stretching vibration of As–O coordinating to iron atom, i.e. As–O–Fe. The weak

pH 8

pH 8 pH 7

pH 7 pH 6

Absorbance

Absorbance

pH 6 pH 5

833 pH 4

pH 5

pH 4

838 pH 3

pH 3 1625

AMSC

500

600

700 800 900 -1 Wave number (cm )

1000

3190

3370

AMSC

500 1000 1500 2000 2500 3000 3500 4000 -1

Wave number (cm )

Fig. 1. Effect of pH on the infrared spectra of arsenate adsorbed on sulfate–ferrihydrite at initial molar ratio of Fe/As = 2, pH 3–8 (AMSC represents poorly crystalline ferric arsenate).

750

800

700

750

800

700

750

800

pH 6

878 850

900

950

850

900

950

878 850

1000

pH 7

878

808

700

806

shoulder at 750 cm1 was probably caused by the hydrogen bonding between H2O and AsO4 since the H-bonding resulted in increased bond length and a red shift of the wave number (Myneni et al., 1998). The 1625 cm1 band was due to water O–H bending mode whereas the stretching vibration bands of O–H were located at 3194 and 3373 cm1. The bands between 950 and 1250 cm1 were assigned to structural SO4 2 ions, which were incorporated into the poorly crystalline ferric arsenate by substitution of AsO4 3 ions during synthesis from sulfate medium. Sulfate ions were incorporated into crystalline scorodite synthesized from sulfate solution (Singhania et al., 2005). The infrared spectra of pH 3 and 4 sorption samples also exhibited a strong, well-resolved band in the As–O stretching vibration region at similar position (833 cm1) to that of poorly crystalline ferric arsenate, indicating similarities of the arsenate bonding structures between sorption samples and poorly crystalline ferric arsenate. This suggested the formation of a ferric arsenate surface precipitate in the arsenate– ferrihydrite sorption samples synthesized in acidic media. However, the weak shoulder at 750 cm1 on the FTIR spectrum of the poorly crystalline ferric arsenate was missing for the sorption solids for some unknown reasons. The band at 833 cm1 for the pH 3 and 4 sorption samples was assigned to As–O stretching vibration of the As–O–Fe coordination of ferric arsenate precipitate on ferrihydrite. The formation of ferric arsenate phase in pH 3 and 4 sorption samples was also supported by the O–H stretching vibration band at 3190 cm1. All samples showed a strong broad O–H band at 3370, but only the acidic sorption samples displayed the 3190 cm1 O–H stretching vibration band like the case of poorly crystalline ferric arsenate. This characteristic O–H band of poorly crystalline ferric arsenate at 3190 cm1 was fading out with increasing pH, indicating the disappearance of ferric arsenate surface precipitate at neutral and alkaline pH. As pH increased from 3 to 8, the As–O stretching vibration band shifted gradually from 833 cm1 down to 806 cm1. At the same time, a new band emerged at higher frequency (870–880 cm1) and its intensity was more pronounced with increasing pH. At pH 8, we could clearly see the splitting of the single band into two bands. Peak deconvolution and curve fitting of the band produced two peaks at 806–810 and 878 cm1 (Fig. 2). It is well established that at mildly alkaline pH, arsenate is adsorbed on ferrihydrite via bidentate binuclear complexation with surface iron polyhedra (Harrison and Berkheiser, 1982; Waychunas et al., 1993). The band at 878 cm1 of the pH 6–8 sorption samples was assigned to uncomplexed/ unprotonated As–O, whereas the 806–808 cm1 arose from the two As–O–Fe complexed to ferrihydrite surface. Two infrared bands at 824/861 and 817/854 cm1 were observed for the arsenate adsorbed on amorphous iron oxide at pH 5 and 9, respectively (Goldberg and Johnston, 2001). The lower frequency bands at 817 and 824 cm1 were assigned to the stretching vibration of As–O–Fe and the higher frequency bands at 854 and 861 cm1 were attributed to ‘‘non-surface-complexed’’ As–O bonds of the adsorbed arsenate species (Goldberg and Johnston, 2001). It is interesting to note that the lower frequency band increased from

810

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Absorbance

1646

1000

pH 8

900

950

1000

-1

Wave number (cm ) Fig. 2. Curve fitting of the As–O stretching vibration of pH 6–8 sorption solids as shown in Fig. 1.

817 and 824 cm1 as pH increased from 5 to 9. This observation and the band assignments are in good agreement with present work. Roddick-Lanzilotta et al. (2002) also reported that the As–O stretching vibration band of the adsorbed arsenate on ferrihydrite shifted from 825 to 800 cm1 as pH increased from 2.6 to 8. In the case of bidentate binuclear complexation, two of the four As–O bonding structures are complexed to iron atoms (i.e. As–O–Fe) and the remaining two are present either both as unprotonated As–O or one as unprotonated As–O and the other one as protonated As–O–H. In comparison, arsenate ions are coordinated to four iron atoms in ferric arsenate. According to Myneni et al. (1998), the force constant of the As–OM bond increases with coordination number and decreases compared to uncomplexed As–O. Hence, for the bidentate adsorbed arsenate ion, the force constant of the two coordinated As–O–Fe is lower than that of the As–O–Fe in ferric arsenate, whereas the uncomplexed/unprotonated As–O bond has larger force constant compared to ferric arsenate. Consequently, the stretching vibration frequency of the uncomplexed/unprotonated As–O is located at higher position while the frequency of the complexed As–O–Fe band is located at lower position. The increase and decrease of the As–O force constant for the bidentate binuclear complexed arsenate compared to ferric arsenate is supported by the As–O bond ˚ and the other two at 1.71 A ˚, length (two at 1.62, 1.67 A ˚ compared to 1.68 A of ferric arsenate) (Sherman and Randall, 2003). The shorter bond distance results in a stronger force constant and consequently higher infrared frequency.

The nature of adsorbed arsenate on ferrihydrite

A very weak band was observed at 700 cm1 on the infrared spectrum of pH 8 arsenate–ferrihydrite sorption sample (see Fig. 1). This band was reasonably assigned to protonated As–O–H bond of the adsorbed arsenate species, which was located at similar position to that of aqueous protonated arsenate species (Myneni et al., 1998). Complexation with metals cannot give such a low As–O stretching vibration frequency. The presence of protonated arsenate species was also proposed for the adsorption of arsenate on freshly prepared hydrous iron oxide and goethite (Myneni et al., 1998). It was noted that the weak As–O–H band at 700 cm1 was absent at acidic pH indicating the absence of protonated adsorbed arsenate species on ferrihydrite at acidic pH. In a previous study using a dispersion infrared instrument (Harrison and Berkheiser, 1982), three bands at 875, 805 and 700 cm1 were observed for the adsorbed arsenate on hydrous ferric oxide (HFO) at pH 6.5. They are very similar to the infrared bands of the pH 8 sorption samples of this work (878, 806 and 700 cm1). pH 3 and 8 are the extreme cases for the adsorption of arsenate on ferrihydrite in this work. At pH 3, a surface precipitate developed and the adsorbed arsenate species were present mainly as poorly crystalline ferric arsenate. The possibility of surface precipitation of arsenate on ferrihydrite was also suggested previously by Stanforth (1999) and Carlson et al. (2002). According to the latter research, a poorly crystalline ferric hydroxyarsenate (FeOHAs) surface precipitate was found to form during adsorption of arsenate on schwertmannite and ferrihydrite at pH 3 (Carlson et al., 2002). Similarly, surface precipitation of phosphate on goethite has been proposed in recent studies (Zhao and Stanforth, 2001; Ler and Stanforth, 2003). It was suggested that the adsorption reaction consisted of two phases: the first phase of rapid surface complexation followed by the second phase of

1647

slow buildup of a surface precipitate (Zhao and Stanforth, 2001). At the other extreme (i.e. pH 8) arsenate was adsorbed via bidentate binuclear complexation with surface iron atoms in the form of unprotonated and probably protonated arsenate species as well (i.e. BFeO2 AsðOÞ2 2 and BFeO2 AsðOÞðOHÞ , where BFe represents the surface of ferrihydrite). This is in good agreement with the model proposed previously by Myneni et al. (1998), Jain et al. (1999) and Goldberg and Johnston (2001). Myneni et al. (1998) suggested that both protonated and unprotonated arsenate species were present as surface arsenate species adsorbed on ferrihydrite at alkaline pH. The surface arsenate species were proposed to be XH2AsO4, XHAsO4  , XAsO4 2 (X was Al or Fe) (Jain et al., 1999; Goldberg and Johnston, 2001). As pH increased from 3 to 8, surface arsenate species shifted from poorly crystalline ferric arsenate precipitates to bidentate surface complexes. This is reasonable since poorly crystalline ferric arsenate is stable only at acidic pH and decomposes with increasing pH (Krause and Ettel, 1989). For the sorption systems whose media pHs lay between 3 and 8, both types of arsenate species were probably present on the surface of ferrihydrite, with the feature of poorly crystalline ferric arsenate being more pronounced at lower pH and the feature of bidentate complexes being more pronounced at higher pH. Similar to the Fe/As = 2 systems, the single As–O stretching vibration band shifted down gradually and split into two bands as pH increased from 3 to 8 for the Fe/As = 4 sorption samples (Fig. 3). The single band at acidic pH was assigned to As–O–Fe of ferric arsenate surface precipitate. The presence of 3190 cm1 O–H stretching vibration band also indicated the development of ferric arsenate in the sorption solids at acidic pH. This band was absent on

pH 8 pH 8

pH 7

pH 6

pH 6

pH 5

pH 5

Absorbance

Absorbance

pH 7

pH 4 833 pH 3

pH 4

pH 3

838

3190 3370

AMSC 500

600

700

AMSC

800

900 -1

Wave number (cm )

1000

500 1000 1500 2000 2500 3000 3500 4000 -1

Wave number (cm )

Fig. 3. Infrared spectra of arsenate adsorbed on sulfate–ferrihydrite at initial molar ratio of Fe/As = 4, pH 3–8 (AMSC represents poorly crystalline ferric arsenate).

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systems of Fe/As = 2, no sulfate band was observed for all pH samples (see Fig. 1). Ferrihydrite synthesized from sulfate media was found to adsorb significantly more arsenate than that from nitrate media (Jia and Demopoulos, 2005), which could not be explained by the difference in BET surface areas. Therefore, it was of interest to compare the surface arsenate species of the two types of arsenate–ferrihydrite sorption samples. Fig. 4 shows the effect of pH on the infrared spectra of arsenate adsorbed on nitrate–ferrihydrite. Apparently the spectra are similar to those of arsenate adsorbed on sulfate–ferrihydrite (Fig. 1). Both are dominated by the strong As–O stretching vibration bands at 700–950 cm1 and strong O–H stretching vibration bands at 3000–3500 cm1. In the As–O stretching vibration region (700– 950 cm1), the infrared spectra of acidic sorption samples (pH 3, 4) exhibited a strong, well-resolved single band at similar position to that of poorly crystalline ferric arsenate. This was indicative that a ferric arsenate surface precipitate formed when arsenate adsorbed on nitrate–ferrihydrite at acidic pH, similar to As(V) adsorption on

the infrared spectra of sorption samples at alkaline pH. The two bands (808 and 878 cm1) at alkaline pH are attributed to As–O–Fe bidentate–binuclear coordinating to ferrihydrite and the uncomplexed/unprotonated As–O bond, respectively. The presence of protonated As–O–H bond could not be ruled out, since there appeared to be a weak feature at 700 cm1. It was interesting to note that all sorption samples from pH 3 to 8 had very similar sorption density (i.e. As/Fe 0.25, see Table 1), but the surface arsenate species varied from poorly crystalline ferric arsenate to bidentate–binuclear surface complexes as pH increased. This was indicative that the nature of surface arsenate species strongly depended on the pH of the sorption media. The spectra of pH 3 and 4 sorption samples showed strong sulfate bands between 950 and 1250 cm1. Sulfate ions were apparently adsorbed onto the ferrihydrite during synthesis from sulfate medium and displaced by arsenate during adsorption as discussed elsewhere (Jia and Demopoulos, 2005). At higher pH (i.e. 5–8), the sulfate ions were substituted by arsenate ions as indicated by the absence of sulfate bands on the infrared spectra. For the adsorption

Table 1 Arsenic equilibrium concentration (mg/L) and sorption density (mol-As/mol-Fe) for the adsorption of arsenate on ferrihydrite synthesized from sulfate and nitrate medium at pH 3–8 and initial Fe/As molar ratio of 2, 4 and 8 pH

3 4 5 6 7 8

Fe/As = 2

Fe/As = 4

Fe/As = 8

Sulfate

Nitrate

Sulfate

Nitrate

Sulfate

198 321 440 498 702 1178

318 471 615 788 1070 1439

1.0 (0.25) 1.9 (0.25) 3.6 (0.25) 5.0 (0.25) 15.8 (0.25) 141 (0.24)

1.9 (0.25) 3.4 (0.25) 14.8 (0.25) 29.6 (0.25) 100 (0.24) 178 (0.23)

<0.02 (0.125) 0.2 (0.125) 0.2 (0.125) 0.2 (0.125) 0.6 (0.125) 2.1 (0.125)

(0.49) (0.46) (0.44) (0.43) (0.40) (0.30)

(0.46) (0.43) (0.41) (0.38) (0.32) (0.25)

The numbers in bracket are sorption densities, i.e. As/Fe molar ratio of the solids.

pH 8 pH 8

pH 7 pH 7

pH 6

pH 5

Absorbance

Absorbance

pH 6

833

pH 4

838

pH 3

3190

3370

AMSC HFO

AMSC 600

pH 4 pH 3

HFO

500

pH 5

700

800

900 -1

Wave number (cm )

1000

500 1000 1500 2000 2500 3000 3500 4000 -1

Wave number (cm )

Fig. 4. Infrared spectra of arsenate adsorbed on nitrate–ferrihydrite at initial molar ratio of Fe/As = 2, pH 3–8 (HFO represents hydrous ferric oxide, i.e. ferrihydrite; AMSC represents poorly crystalline ferric arsenate).

The nature of adsorbed arsenate on ferrihydrite

sulfate–ferrihydrite. As pH increased, the single As–O stretching vibration band split into two strong bands at 806–810 cm1 and 878 cm1 as observed for the sulfate–ferrihydrite samples (see Fig. 1) and were attributed to the As–O stretching vibration of bidentate binuclear complexed arsenate ions. The formation of a ferric arsenate surface precipitate at acidic pH was also evidenced by the change of the 3190 cm1 O–H band with pH. The spectrum of ferrihydrite displayed strong NO3  bands at 1250–1500 cm1. These bands disappeared after adsorption of arsenate indicating that nitrate ions previously adsorbed during ferrihydrite synthesis were displaced by arsenate ions.

838

1649

3.2. Effect of coverage density on the nature of adsorbed arsenate The effect of coverage density on the nature of adsorbed arsenate on ferrihydrite was evaluated. At acidic pH (i.e. pH 3, 4), the As–O stretching vibration region (700– 950 cm1) was dominated by a strong well-resolved band (Fig. 5). As the Fe/As molar ratio increased from 2 to 8 (i.e. the adsorption density As/Fe decreasing from 0.49 to 0.125) for the pH 3 sorption systems (see Table 1), the intensity of the As–O stretching vibration band decreased markedly. The As–O stretching vibration peak was located at similar position. The pH 4 arsenate–ferrihydrite sorption

838

pH 3

pH 4

832

833

AMSC

AMSC

827 Fe/As=2 (0.47)

Absorbance

Fe/As=2 (0.49)

828

Fe/As=4 (0.25)

Absorbance

827

Fe/As=8 (0.125)

Fe/As=8 (0.125)

HFO

HFO

400

600

800

Fe/As=4 (0.25)

826

1000

1200

400

600

800

1000

1200

-1

-1

Wave number (cm )

Wave number (cm )

838

838

pH 6

pH 8 AMSC

823

AMSC

808 878

822

Fe/As=4 (0.25)

822

Fe/As=2 (0.31)

Absorbance

Absorbance

Fe/As=2 (0.43)

Fe/As=4 (0.24)

Fe/As=8 (0.125)

Fe/As=8 (0.125) HFO

HFO 400

600

800

1000 -1

Wave number (cm )

1200

400

600

800

1000

1200

-1

Wave number (cm )

Fig. 5. Infrared spectra of arsenate adsorbed on sulfate–ferrihydrite: effect of coverage density (HFO represents hydrous ferric oxide, i.e. ferrihydrite; AMSC represents ‘‘amorphous scorodite’’, i.e. poorly crystalline ferric arsenate; the numbers in brackets are coverage density, i.e. As/Fe of the sorption solids).

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samples with various arsenate sorption densities exhibited very similar infrared spectra to that of pH 3 samples. Poorly crystalline ferric arsenate was the major surface arsenate species. The nature of surface arsenate species was not significantly influenced by arsenate sorption density and equilibrium concentration. At pH 6, the major As–O stretching vibration peak was located at 822 cm1 for all the initial Fe/As molar ratios used, i.e. 2, 4 and 8. A visible shoulder was also emerging at 878 cm1 on the infrared spectra. A bidentate complex of arsenate was becoming detectable in addition to the major surface species of ferric arsenate. When pH increased to 8, all infrared spectra exhibited two bands at 808 and 878 cm1 irrespective of the initial Fe/As molar ratios indicating that bidentate complexes were the dominant surface arsenate species. The equilibrium concentration of arsenic ranged from 0.028 to 15.8 mM for the pH 8 sorption systems and the sorption density (As/Fe) ranged from 0.125 to 0.31. Waychunas et al. (1993) conducted EXAFS analysis on samples with sorption density of 0.001–0.1 and concluded that arsenate was sorbed via bidentate surface complexation. Goldberg and Johnston (2001) used FTIR to characterize the arsenate–iron oxide sorption solids synthesized at pH 5 and 9 with equilibrium concentration of 0.1– 1.0 mM and suspension density of 4 g/L (8 g/L for the present work). The obtained results showed similar As–O band to the present study, i.e. two bands located at 817–824 and 854–861 cm1. The infrared spectrum obtained by Roddick-Lanzilotta et al. (2002) for the sorption system pH 2.6 and arsenic equilibrium concentration of 0.5 mM displayed a well-resolved band 825, which is similar to the single As–O band of the acidic samples in this work. Carlson et al. (2002) proposed the formation of ferric hydroxyarsenate in Fe/As = 2.6–3.2 sorption solids prepared from solutions of pH 3. Therefore, by comparing the present study with other works, it is proposed that the pH plays the most important role in controlling the nature of the surface arsenate species sorbed on ferrihydrite, whereas other conditions (e.g. coverage density, suspension density etc.) play less important factors. The fate of sulfate in synthetic ferrihydrite before and after adsorption of arsenate can also be monitored on the infrared spectra. As indicated by the bands between 950 and 1250 cm1, more sulfate was incorporated into the ferrihydrite at acidic pH (3, 4) than neutral to alkaline pH (6, 8) (Fig. 5). After adsorption of arsenate at pH 3, 4 and Fe/As = 4, 8, there was still measurable amount of sulfate remaining in the ferrihydrite, while at Fe/As = 2, all sulfate has been desorbed. It was proposed that sulfate ions were adsorbed on goethite as both inner-sphere and outersphere surface complexes at acidic pH (Peak et al., 1999). Adsorption of arsenate on the ferrihydrite involved ligand exchange with previously adsorbed sulfate ions. With increasing pH, the adsorbed sulfate ions on ferrihydrite were more easily displaceable by arsenate. At pH 6, sulfate bands were visible only on the Fe/As = 8 infrared spectrum while no sulfate was detectable by FTIR in the pH 8 samples. This is in good agreement with a previous study that

reported the quantitative analysis of the sorption solids (Jia and Demopoulos, 2005). It was suggested that surface precipitation of phosphate and arsenate on goethite occurred almost simultaneously (Zhao and Stanforth, 2001; Ler and Stanforth, 2003). The process of surface precipitation may involve slow dissolution of ferrihydrite, ternary complexation of Fe3+ and subsequent precipitating of arsenate (Ler and Stanforth, 2003). The process was depicted schematically in Fig. 6. It was not surprising that arsenate surface precipitate formed at such a high initial Fe/As molar ratio of 8 where arsenic equilibrium concentration is below the detection limit (<0.02 mg/L) in this work. There possibly also existed tridentate complex structure that formed initially upon contacting of the arsenate ions with ferrihydrite given the amorphous nature of the latter. The saturation state with respect to poorly crystalline ferric arsenate in acidic media was estimated in a previous study (Jia et al., 2006). When the pKsp(ferrihydrite) = 39 (Langmuir, 1997) and pKsp(ferric arsenate) = 22.89 (Mahoney, 2002) were taken in the calculation, the log IAPð½Fe3þ ½AsO4 3  was obviously lower than log Ksp (ferric arsenate) for the pH 3, Fe/As = 4 and Fe/As = 8 sorption systems, indicating that the systems were undersaturated with respect to poorly crystalline ferric arsenate. For

Fig. 6. Schematic diagram of evolution of bidentate surface complex of As(V)–ferrihydrite to surface precipitation of ferric arsenate.

The nature of adsorbed arsenate on ferrihydrite

scorodite (FeAsO4.2H2O)

adsorption o 75 C, 2 months

adsorption o 75 C, 2 weeks

adsorption o 75 C, 1 week

Relative intensity

the Fe/As = 2 system, the log IAP was similar to log Ksp(ferric arsenate). The surface of ferrihydrite consists of both labile and non-labile sites (Rea et al., 1994; Poulson et al., 2005). The labile sites are the end sites of the dioctahedral chain structure of ferrihydrite hence having fewer neighboring iron octahedra and binding less strongly to the ferrihydrite structure (Rea et al., 1994). The solution properties at the water–ferrihydrite interface were different from the bulk solution phase where the concentration of Fe and As were measured. The concentration of arsenic and iron at the liquid–solid interface likely exceeded that in bulk solution. This might result in an initial local supersaturation on the surface of ferrihydrite. However, quantitative analysis was difficult due to the fact that the activity coefficients for the surface species were unknown. Adsorption of arsenate decreased the number of labile sites and stabilized the surface structure of ferrihydrite. Blocking of the labile sites and reducing of the distortion may slow the process of iron release from the substrate. This may explain the observed slow evolution of surface species to ferric arsenate (Jia et al., 2006) and the slow adsorption kinetics of arsenate on ferrihydrite (Fuller et al., 1993; Jia and Demopoulos, 2005).

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adsorption o 75 C, 3 days

adsorption o 75 C, 1 day

adsorption, o 21 C, 2 weeks

3.3. X-ray diffraction (XRD) characterization The arsenate–ferrihydrite sorption solids synthesized at room temperature were poorly crystalline in nature, hence it was difficult to differentiate between arsenate and ferric oxide phases using powder XRD analysis. In a previous study, poorly crystalline ferric arsenate phase was observed by using XRD when arsenate was sorbed onto ferrihydrite at 21 °C and acidic pH (Jia et al., 2006). Adsorption experiments were conducted at elevated temperature (75 °C) in the present work to force the poorly crystalline ferric arsenate surface species into a more crystalline state. Poorly crystalline ferric arsenate exhibited two broad characteristic XRD bands at 28° and 58° 2h, whereas the characteristic XRD bands of ferrihydrite were located at 34° and 61° 2h (Fig. 7). The XRD patterns of the Fe/As = 2 sorption solid synthesized at 21 °C, pH 3 and equilibration time of two weeks was dominated by two broad bands, the first peak at 28° 2h and the second peak at 58° 2h. This indicated that poorly crystalline ferric arsenate was the dominant arsenate phase in the sorption solid. When the Fe/As = 2, pH 3 sorption system was equilibrated at elevated temperature (75 °C) from one day to one week, the first band split into two peaks at 28° and 34° 2h, the characteristic band for poorly crystalline ferric arsenate and ferrihydrite, respectively. The poorly crystalline ferric arsenate converted to the crystalline state of ferric arsenate (i.e. scorodite FeAsO4Æ2H2O) after the sorption system was equilibrated at 75 °C for 2 weeks (Fig. 7). After two months of reaction, the XRD spectrum of the resultant sorption solid resembled basically that of scorodite. The Fe/As = 4 arsenate–ferrihydrite sorption solid displayed single XRD band at lower position similar to that of ferrihydrite after the system was equilibrated at room temperature (21 °C), pH 3 for two weeks (Fig. 8). The

58

poorly crystalline ferric arsenate

ferrihydrite 28 34 0

10

20

30

61 40

50

60

70

80

90

100

2θ

Fig. 7. Comparison of XRD patterns of Fe/As = 2 arsenate– ferrihydrite sorption solids at pH 3, 75 °C with those at room temperature and reference materials (ferrihydrite, poorly crystalline ferric arsenate and scorodite).

shoulder at 28° suggested the presence of poorly crystalline ferric arsenate. When the reaction temperature was raised to 75 °C, the lower position band clearly split into two bands at 28° and 34° 2h, which were the characteristic XRD bands for poorly crystalline ferric arsenate and ferrihydrite. Unlike the Fe/As = 2 system, no crystalline phase was developed after two months of retention at 75 °C. The broad band was attributed to the microcrystallite of ferric arsenate. For the Fe/As  4 sorption system, the equilibrium concentration of arsenic was <1 mg/L and the surface of ferrihydrite was not saturated by arsenate ions. Fewer arsenate ions were available in the solution for the buildup of three-dimension ferric arsenate. Hence it was difficult for the ferric arsenate microcrystallites to grow into larger-size crystal grains even at elevated temperature.

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adsorption o 75 C, 2 months

adsorption o 75 C, 2 weeks

Relative intensity

adsorption o 75 C, 1 week

adsorption o 75 C, 1 day

adsorption 21 oC, 2 weeks

58

poorly crystalline ferric arsenate

0

10

20

30

61 40

50

60

70

80

90

4. CONCLUSIONS The Fourier transformed infrared spectroscopy and X-ray diffraction were employed for the speciation of arsenate sorbed on the surface ferrihydrite. The results indicated that the media pH was the important factor controlling the nature of surface arsenate species whereas the effect of coverage density was negligible. Surface precipitation of poorly crystalline ferric arsenate was observed for the arsenate–ferrihydrite sorption solids synthesized at acidic pH (3, 4) and room temperature, while bidentate surface complex was the dominant mode of arsenate–ferrihydrite interaction at neutral to alkaline pH. Adsorption reaction at elevated temperature (75 °C), pH 3 gradually forced the arsenate species into more crystalline phase of ferric arsenate. There was no difference between the nature of sorbed arsenate species on ferrihydrites precipitated from nitrate and sulfate media. Sorption of arsenate on ferrihydrite precipitated from sulfate media involved displacement of sulfate ions from the surface as clearly found on the infrared spectra. ACKNOWLEDGMENTS

ferrihydrite 28 34

work, poorly crystalline ferric arsenate is likely the major arsenic species in acid mine drainage precipitates and hydrometallurgical coprecipitates. Actually, a poorly crystalline ferric hydroxyarsenate (FeOHAs) has been identified from an acid mine drainage precipitate (Carlson et al., 2002). Poorly crystalline ferric arsenate was reported to be present in iron(III)–arsenate solids coprecipitated at pH 4 (Jia et al., 2003).

100

2θ Fig. 8. Comparison of XRD patterns of Fe/As = 4 arsenate– ferrihydrite sorption solids at pH 3, 75 °C with those at room temperature and reference materials (ferrihydrite, poorly crystalline ferric arsenate and scorodite).

3.4. Environmental relevancy of the findings Arsenate–ferrihydrite sorption reaction is an important process controlling arsenic mobility and is widely present in natural and engineered systems, e.g. acid mine drainage and hydrometallurgical coprecipitation arsenic removal process. Ferrihydrite and schwertmannite are generated in acid mine drainage which thereafter adsorb aqueous arsenate species from acidic media. Coprecipitation of arsenate with iron(III) is widely used in hydrometallurgical industry for the removal and immobilization of arsenic from mineral processing solutions. Ferrihydrite is generated duo to the use of excess amount of iron (Fe/ As  4). A two-stage neutralization process is practiced in some hydrometallurgical operations (e.g. McClean Lake Operation, Northern Saskatchewan) to remove arsenic at pH 4 and to precipitate co-existent heavy metals at pH 8. According to the findings of the present

We thank National Natural Science Foundation of China (NSFC, No. 40673079) and the Natural Sciences and Engineering Research Council (NSERC) of Canada for the support of this work. We also thank Associate Editor Dr. Donald Sparks and three anonymous reviewers for the constructive comments and suggestions.

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Zhao H., and Stanforth R. (2001) Competitive adsorption of phosphate and arsenate on goethite. Environ. Sci. Technol. 35, 4753–4757. Associate editor: Donald L. Sparks