Environ. Sci. Technol. 2006, 40, 3248-3253
Observation of Surface Precipitation of Arsenate on Ferrihydrite Y O N G F E N G J I A , * ,† L I Y I N G X U , † ZHEN FANG,† AND G E O R G E P . D E M O P O U L O S * ,‡ Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang, China, 110016 and Department of Mining, Metals and Materials Engineering, McGill University, Montreal, Quebec, H3A 2B2, Canada
X-ray diffraction and Raman spectroscopy were used in this study to characterize arsenate phases in the arsenateferrihydrite sorption system. Evidence has been obtained for surface precipitation of ferric arsenate on synthetic ferrihydrite at acidic pH (3-5) under the following experimental conditions: sorption density of As/Fe ∼0.125-0.49 and arsenic equilibrium concentration of 3). Ferrihydrite is produced in this process due to the hydrolysis of excess amount of iron. There have been numerous studies on the mechanism of interaction between arsenate and ferrihydrite involving multiple experimental methodologies and techniques. Current understanding favors the surface complexation mode of retention of arsenate anions by ferrihydrite. Harrison and * Address correspondence to either author. Phone: +86 (24) 83970503 (Y.J.); (514) 398-2046 (G.P.D.). E-mail:
[email protected] (Y. J.);
[email protected] (G.P.D.). † Chinese Academy of Sciences. ‡ McGill University. 3248
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Berkheiser proposed the bonding mechanism for the adsorption of arsenate on freshly prepared hydrous ferric oxide (HFO) to be via bidentate bridging complexation (1). This proposal was based on infrared analysis and comparison to the bonding structure of other oxyanions with HFO, such as sulfate and phosphate. Waychunas et al. used EXAFS to characterize both coprecipitated and adsorbed arsenate on ferrihydrite samples and concluded that arsenate adsorbs on ferrihydrite mainly as an inner-sphere bidentate complex sharing apical oxygens of two adjacent edge-sharing Fe oxyhydroxyl octahedra (2). Bidentate inner-sphere complexation has been confirmed to be the major interaction mode between arsenate and ferrihydrite or goethite by other studies as well using EXAFS (3-9), wide-angle X-ray scattering (WAXS) (10), and vibrational spectroscopy (11-14). Surface complexation reactions involving ferrihydrite (hydrous ferric oxide) (15) or precipitation and dissolution of ferric arsenate compounds (16, 17) are all pH-dependent processes. It is interesting to note that the role of pH in previous arsenate-ferrihydrite adsorption studies has not been adequately assessed. Thus, the bidentate complexation mechanism, that is taken to represent this system, was proposed based on examining arsenate-ferrihydrite materials synthesized at pH 6.5-8 (1, 2, 4, 6, 8-10). This mechanism may not be applicable to the adsorption systems at acidic pH. Carlson et al. recently proposed that poorly crystalline ferric hydroxyarsenate (FeOHAs) surface precipitate formed when arsenate was adsorbed on schwertmannite at pH 3 (18). Stanforth suggested the possibility of surface precipitation of arsenate on ferrihydrite (19). His lab also reported the evidence of surface precipitation of phosphate on goethite (20, 21). When other arsenate complexing cations are present, surface precipitation of the corresponding metal arsenate may also occur. For example, the surface precipitate [Zn2(AsO4)OH] was reported to have formed during adsorption of arsenate on goethite in the presence of Zn2+ (22). In particular, surface precipitation is expected to be favored in solutions rich in arsenic, as are those encountered in the mining industry. The objective of this study is to characterize arsenateferrihydrite sorption solids synthesized at pH 3-8 by using X-ray diffraction (XRD) and Raman spectroscopy in an attempt to find evidence of surface precipitation of ferric arsenate on ferrihydrite. Poorly crystalline ferric arsenate FeAsO4‚2.4H2O and ferrihydrite were used as reference materials. 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.
Experimental Section All materials used in this study, i.e., ferrihydrite, poorly crystalline ferric arsenate, and arsenate-ferrihydrite sorption solids, were synthesized at 21 °C. Synthesis of Two-Line Ferrihydrite. Two-line ferrihydrite was synthesized using a procedure adapted from ref 23. Ferric sulfate, Fe2(SO4)3‚5H2O, was used as the source of ferric ion. Briefly, the iron(III) solution was prepared by dissolving Fe2(SO4)3‚5H2O in deionized water. The pH of the solution was raised to ∼7.5 in about 5 min using a 1 M NaOH solution and maintained at that pH for 1 h with the slurry vigorously mechanically agitated. The solid was washed four times with deionized water and used as slurry in adsorption tests right after synthesis. Synthesis of Arsenate-Ferrihydrite Sorption Samples. For adsorption experiments, the prepared ferrihydrite slurry 10.1021/es051872+ CCC: $33.50
2006 American Chemical Society Published on Web 04/13/2006
was adjusted to a different pH between 3 and 8 with NaOH and HNO3 and allowed to equilibrate for 1 h. Arsenate stock solution (10 g/L As) was prepared by dissolving Na2HAsO4‚ 7H2O in deionized water. Arsenate solution was introduced into ferrihydrite slurry from a buret in 10 min with the slurry mechanically stirred at a moderate level. The system was controlled at constant pH by addition of NaOH and/or HNO3 solution and allowed to equilibrate for 2 weeks. The volume of adsorption slurry was 500 mL for all experiments, and the concentration of iron in the slurry system was 4 g/L. At each pH, three initial Fe/As molar ratios (i.e., Fe/As ) 2, 4, 8) were applied. The slurry was sampled using a 10-mL syringe and filtered with a 0.2 µm syringe filter. The collected solid sample was washed and digested with HCl. The concentration of residual arsenic in solution and the sorption density (i.e., As/Fe molar ratio of the solid) were determined by ICP-AES analysis of the filtrate and the digested solid. The synthesized arsenate-ferrihydrite products were filtered, washed, and air-dried. The effect of equilibration time on the mineralogical characteristics of the arsenate-ferrihydrite sorption solid was examined by sampling at 0, 6, and 24 h for the pH 3, Fe/As ) 2 sorption system. For time 0, the sample was taken immediately upon the arsenate was introduced into the ferrihydrite slurry. The samples were filtered, washed with HNO3-acidified water (pH 3) and acetone, and dried at 60 °C. Synthesis of Poorly Crystalline Ferric Arsenate. A mixture of 0.02 M As(V) (Na2HAsO4‚7H2O) and 0.02 M Fe(III) (Fe2(SO4)3‚5H2O) was adjusted from initial pH 1.3 to pH 1.8 with NaOH solution and maintained at that pH for 1 h. The solid product was separated by filtration, washed with HNO3acidified water (pH 2), and vacuum-dried at 60 °C. X-ray Diffraction (XRD) Measurements. The powder XRD patterns were obtained on a Philips PW1710 diffractometer equipped with a copper target (Cu KR1 radiation, λ ) 1.54060 Å), a crystal graphite monochromator and a scintillation detector. The equipment was run at 40 kV and 20 mA by step-scanning from 10° to 100° 2θ with increments of 0.1° 2θ and a counting time of 0.3 s at each step. Raman Spectroscopy. Raman spectra were obtained on a Renishaw 3000 Raman microprobe (20× objective), using the 514.5-nm line of an Ar+-ion laser for excitation (5-100 mW laser power, 1 cm-1 spectral slit-width; 5s exposure time; 10 accumulation). The band resolution is 1 cm-1.
Results and Discussion Sorption Density. Freshly synthesized ferrihydrite shows large uptake capacity for arsenate. The sorption density of arsenate on ferrihydrite is largely governed by the initial Fe/ As molar ratio and the pH of adsorption medium (Figure 1). Nearly all added arsenate was sorbed by ferrihydrite for the initial Fe/As molar ratio of 8 and 4. The sorption density is equal to the nominal As/Fe molar ratio. For the initial Fe/As molar ratio of 2, the sorption density decreases from 0.49 to 0.3 as the pH increases from 3 to 8. High sorption density of arsenate on synthetic ferrihydrite was also reported in previous studies, i.e., ∼0.25 mole-As/mole-Fe (24-26), 0.38 mole-As/mole-Fe (18). Such high sorption density is unusual for surface adsorption (19). It is rather a process of ferrihydrite dissolving and ferric arsenate precipitating than a twodimension surface adsorption process as proposed by Stanforth (19). X-ray Diffraction (XRD). The arsenate-ferrihydrite sorption products are poorly crystalline in nature, which makes it difficult to characterize the arsenate phases using XRD. In this study, we used synthetic two-line ferrihydrite and poorly crystalline ferric arsenate as reference materials. Poorly crystalline ferric arsenate is not a well-defined system and often loosely termed as “amorphous” ferric arsenate (17) or “amorphous” scorodite (27) due to its structural likeness to
FIGURE 1. Sorption density and arsenic equilibrium concentration as a function of pH for arsenate-ferrihydrite sorption systems with initial Fe/As molar ratio of 2, 4, and 8 and equilibration time of two weeks (corresponding to initial CAs ) 2800, 1400, 700 mg/L respectively).
FIGURE 2. Comparison of XRD patterns of pH 3-arsenate-ferrihydrite sorption materials with reference materials (ferrihydrite, poorly crystalline ferric arsenate, and scorodite). The sorption materials were synthesized at pH 3 with different initial Fe/As molar ratios and equilibration time of two weeks. scorodite (FeAsO4‚2H2O) (28). It is an unstable arsenate phase with increasing pH (17). The poorly crystalline ferric arsenate was synthesized in an acidic medium (i.e., pH 1.8) to avoid the formation of ferrihydrite. The formula has been determined to be Fe1.02AsO4‚2.4H2O. The XRD spectrum of the two-line ferrihydrite shows two broad bands at ∼34° and ∼61° 2θ respectively as indicated by its name (see Figure 2). The poorly crystalline ferric arsenate also shows two broad XRD bands but located at lower positions. The first band peaks at ∼28° and the second one peaks at ∼58° 2θ. In addition, the second band is weaker in relative intensity compared to ferrihydrite. These features are used to differentiate between ferric arsenate and arsenateadsorbed ferrihydrite (29). The pH 3-arsenate-ferrihydrite sorption materials with different initial Fe/As molar ratios show two XRD bands lying between the characteristic bands of ferrihydrite and poorly crystalline ferric arsenate. With increasing initial Fe/As molar ratio, the first peak shifts noticeably to higher degrees 2θ. VOL. 40, NO. 10, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Comparison of XRD patterns of pH 4-arsenate-ferrihydrite sorption materials with reference materials (ferrihydrite, poorly crystalline ferric arsenate). The sorption materials were synthesized at pH 4 with different initial Fe/As molar ratios and equilibration time of two weeks.
FIGURE 4. Changes of mineralogical characteristics of the arsenate-ferrihydrite sorption solids over equilibration time at pH 3 and initial molar ratio Fe/As ) 2.
The shape of both bands also changes appreciably. For the Fe/As ) 2 sample, the first peak is located at ∼28° 2θ with a shoulder at ∼34° 2θ. The second peak is located at ∼58° 2θ and is weak in relative intensity. They are clear XRD features of poorly crystalline ferric arsenate. The shoulder at ∼34° 2θ is the characteristic XRD band for ferrihydrite. This suggests that both poorly crystalline ferric arsenate and ferrihydrite are present in the Fe/As ) 2 arsenate-ferrihydrite sorption materials. The relative intensities of the bands indicate that ferric arsenate is the dominant phase in the solid. When initial Fe/As molar ratio increases from 2 to 4, the first peak shifts to ∼34° 2θ with a shoulder at ∼28° 2θ. The second peak also shifts to a higher position (∼61° 2θ) and increases in relative intensity. In this case, ferrihydrite is a major phase in the sample. However, the shoulder at ∼28° 2θ is indicative of the presence of the poorly crystalline ferric arsenate phase. When the initial Fe/As molar ratio is increased further to 8, both peaks shift to the same positions as ferrihydrite. The feature of poorly crystalline ferric arsenate on the first band is not as obvious as that of Fe/As ) 2 and 4 samples. However, a weak shoulder at ∼28° 2θ still indicates the presence of the ferric arsenate phase. Ferrihydrite is the dominant phase in the product with such a high Fe/As molar ratio. Hence, the features of the ferric arsenate phase (sorption density only ∼0.125) on the XRD spectrum are obscured by the abundant ferrihydrite phase. In this case, the XRD pattern resembles that of ferrihydrite. The arsenate-ferrihydrite sorption samples synthesized at pH 4 show similar XRD results to those synthesized at pH 3 (see Figure 3). The poorly crystalline ferric arsenate phase is present in all three samples as indicated by the feature at ∼28° 2θ on the first band. For the Fe/As ) 2 sample, the first peak splits into two peaks at ∼28° and ∼34° 2θ, respectively, compared to only one peak at ∼28° 2θ for the corresponding pH 3 sample. From the above observations, we conclude that equilibration of arsenate-rich solution with ferrihydrite at acidic pH for two weeks leads to the formation of ferric arsenate surface precipitate. Changes of mineralogy of the arsenateferrihydrite sorption solids may reveal the mechanism of the formation of ferric arsenate. Figure 4 shows the effect of equilibration time on the XRD patterns of the arsenate-
FIGURE 5. Comparison of XRD patterns of pH 8-arsenate-ferrihydrite sorption materials with reference materials (ferrihydrite, poorly crystalline ferric arsenate). The sorption materials were synthesized at pH 8 with different initial Fe/As molar ratios and equilibration time of two weeks.
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ferrihydrite sorption products. It is interesting to note that after the arsenate solution was brought to contact with ferrihydrite (i.e., equilibration time 0 h), the XRD spectrum of the solid is dominated by ferrihydrite features, i.e., the bands at 34° and 61° 2θ. After the system was equilibrated for 6 h, a new band emerges at 28° 2θ, indicating the development of ferric arsenate phase. This phase becomes more pronounced with increasing equilibration time. After equilibration for two weeks, the XRD spectrum of the solid is dominated by the features of a poorly crystalline ferric arsenate. The arsenate-ferrihydrite sorption materials synthesized at pH 8 show different XRD patterns compared with those synthesized at acidic pH (i.e., pH 3 and 4) (see Figure 5). For the Fe/As ) 4, and 8 arsenate-ferrihydrite sorption products, the XRD patterns are almost identical to that of ferrihydrite. Both bands are located at the same position as ferrihydrite, i.e., at ∼34° and ∼61° 2θ respectively, indicating the materials
FIGURE 6. Effect of pH on the XRD patterns of arsenate-ferrihydrite sorption materials with initial molar ratio Fe/As ) 2 and equilibration time of two weeks. are predominantly ferrihydrite. Arsenate is adsorbed apparently in this case on the surface of ferrihydrite by innersphere complexation (2). For the Fe/As ) 2 material, both XRD peaks are located at the same positions as ferrihydrite and the XRD pattern basically resembles that of ferrihydrite. However, the presence of a ferric arsenate phase cannot be ruled out. A clear view of the changing arsenate phases with increasing pH in the Fe/As ) 2 arsenate-ferrihydrite sorption system is shown in Figure 6. High arsenate loading materials (i.e., initial molar ratio Fe/As ) 2) are chosen in order to clearly see the differences. With increasing pH, the features of ferric arsenate on XRD spectrum are decreasing. The first band shifts gradually from ∼28° to ∼34° 2θ and the second band from ∼58° to ∼61° 2θ when pH increases from 3 to 8, indicating that the arsenate phase changes from poorly crystalline ferric arsenate to surface adsorption on ferrihydrite. At pH 3, ferric arsenate is the dominant phase, whereas at pH 4 and 5, both ferric arsenate and ferrihydrite are equally important phases as indicated by the two components (∼28° and ∼34° 2θ) on the first band. When the pH further increases to 8, the ferric arsenate features fade away and only ferrihydrite bands are evident on the XRD spectrum. It is proposed, therefore, on the basis of the presented XRD data that ferric arsenate surface precipitate forms when arsenate is sorbed on ferrihydrite at acidic pH, whereas at mildly alkaline pH, surface complexation dominates the process. Raman Analysis. Raman spectroscopy has been employed to identify the arsenate phases in the arsenate-ferrihydrite sorption materials synthesized at both acidic and mildly alkaline pH. In this study we have focused on the As-O stretching vibration region of the Raman spectra to differentiate among the various arsenate-iron(III) coordination modes. Ferrihydrite shows three strong well-resolved Raman bands at 222, 289, and 407 cm-1 and a weak band at 606 cm-1 (see Figure 7). These bands are characteristic of iron oxyhydroxides (30, 31). The Raman spectrum of scorodite (i.e., crystalline ferric arsenate FeAsO4‚2H2O) shows two very strong sharp peaks at 803 and 890 cm-1. These are characteristic As-O stretching vibration bands for scorodite (13). The As-O stretching vibration is sensitive to its chemical environment and is used here to identify different arsenate
FIGURE 7. Raman spectra of arsenate-ferrihydrite sorption materials and reference materials (scorodite, poorly crystalline ferric arsenate, and ferrihydrite). The sorption materials were synthesized at pH 3 and 8 with initial molar ratio Fe/As ) 4 and equilibration time of two weeks. phases. Poorly crystalline ferric arsenate shows a strong band at 825 cm-1 and a shoulder at 910 cm-1. The arsenateferrihydrite sorption material synthesized at pH 3 and initial molar ratio of Fe/As ) 4 also exhibits a strong band at 825 cm-1 on its Raman spectrum. This band is at the same position as the As-O stretching vibration of poorly crystalline ferric arsenate, indicating the formation of the ferric arsenate phase. This constitutes further supportive evidence (in addition to the XRD data) that surface precipitation of ferric arsenate on ferrihydrite occurred at acidic pH. The Raman spectrum of pH 8-arsenate-ferrihydrite sorption product shows a strong peak at 845 cm-1 (see Figure 7), which obviously is different from the Raman features of pH 3-sorption product. It has been well established that arsenate is adsorbed on ferrihydrite at pH 8 via bidentate complexation. Hence, this band is due apparently to As-O stretching vibration of the bidentate-complexed arsenate on the surface of ferrihydrite. From the XRD and Raman data there is clear evidence for surface precipitation of poorly crystalline ferric arsenate when high concentration arsenate is sorbed on synthetic ferrihydrite in acidic media (pH 3-5), whereas at alkaline pH arsenate is sorbed via surface complexation. Carlson et al. also observed differences in XRD patterns after adsorption of arsenate on schwertmannite at pH 3 (18). The authors attributed the change to the formation of poorly crystalline ferric hydroxyarsenate (FeOHAs) surface precipitate. This phase is not a well-defined one to compare with the surface precipitate found to form in the current work. Poorly crystalline ferric arsenate is a scorodite-like mineral (28) and can be considered as a single phase (Fe/As ∼1). Hence, under the conditions used in this study, it is more appropriate to describe the surface precipitate formed at acidic pH as poorly crystalline ferric arsenate. Solution pH is an important factor controlling the type of arsenate phase in arsenate-ferrihydrite sorption system. A poorly crystalline ferric arsenate phase develops on ferrihydrite at acidic pH. The saturation state with respect to poorly crystalline ferric arsenate at acidic pH and various equilibrium concentrations of arsenic is estimated (see Table 1). For the Fe/As ) 2 sorption system, the log IAP is similar VOL. 40, NO. 10, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Estimation of Saturation State with Respect to Poorly Crystalline Ferric Arsenate As equilibrium concentration (mol/L)a pH
Fe/As
Total As
AsO43-
3
2 4 8 2 4 8 2 4 8
2.54 × 10-03 1.27 × 10-05 2.67 × 10-07 4.12 × 10-03 2.00 × 10-05 2.27 × 10-06 5.84 × 10-03 3.60 × 10-05 2.53 × 10-06
8.45 × 10-16 4.20 × 10-18 8.84 × 10-20 1.60 × 10-13 7.80 × 10-16 8.84 × 10-17 2.29 × 10-11 1.41 × 10-13 9.94 × 10-15
4 5
calculated [Fe3+] (mol/L)b 1 × 10-06 1 × 10-09 1 × 10-12
log IAP ([Fe3+][AsO43-]) -22 -24 -26 -22 -25 -26 -23 -25 -27
log Ksp (amorphous scorodite)c
-22.89
a The concentration of AsO 3- is calculated based on pK ) 2.3, pK ) 6.8, and pK )11.6 for H AsO (32) and is treated as [AsO 3-]. 4 1 2 3 3 4 4 calculated based on pKsp ) 39 for ferrihydrite (33). c Log Ksp of amorphous scorodite is obtained from ref 34.
to the log Ksp of poorly crystalline ferric arsenate (i.e., amorphous scorodite). Since FeH2AsO42+, FeHAsO4+, and ferric sulfate complexes are not considered, the actual log IAP is probably lower than log Ksp. For the Fe/As ) 4, and 8 sorption systems, the log IAP ([Fe3+][AsO43-]) is obviously lower than log Ksp, indicating the system is undersaturated with respect to poorly crystalline ferric arsenate. Ler and Stanforth also suggested that surface precipitation started at concentrations well below saturation for the phosphategoethite system (21). However, if a lower pKsp value for ferrihydrite (ferrihydrite can exhibit a range of values depending on the conditions of preparation) is assumed, say 36 (27) then log IAP significantly exceeds log Ksp (amorphous scorodite) for the Fe/As ) 2, 4 systems, which means that the system favors (supersatured) amorphous scorodite precipitation. Moreover, it is conceivable that at the surface of ferrihydrite the solution properties are different (local supersaturation) from the bulk phase where pH and As concentration are measured. Quantitative analysis is further hindered by the fact that no activity coefficients for the surface species are known or taken into account. It is likely that surface precipitation of ferric arsenate on ferrihydrite involves initial surface complexation of arsenate ions, followed by a partial dissolution of ferrihydrite and the formation of surface precipitate by the reaction of the released iron cations and the sorbed arsenate as proposed by Carlson et al. (18) and Stanforth (19, 21) for analogous systems. As further discussed by Dzombak and Morel (15), surface precipitation is favored with increasing sorbate concentration as is the system studied in this work. From Figure 4 we can clearly see the slow evolution of surface complex to surface precipitate. It was also suggested previously that the development of surface precipitate is generally a slow process (19-21, 35). This may explain why adsorption of arsenate on ferrihydrite showed slow kinetics (25, 36). Compared to twodimension surface complexation, the formation of ferric arsenate surface precipitate allows the buildup of threedimension Fe/As ∼1 arsenate phase, thereby providing maximized site density for arsenate sorption. This is probably the reason for the unusually high arsenate sorption density on ferrihydrite and schwertmannite (18, 24-26). Since ferrihydrite was precipitated from sulfate-rich solution, a significant amount of sulfate ions was incorporated (e.g., mol SO42-/mol Fe ∼ 0.2 at pH 3) (36). Adsorption of arsenate on ferrihydrite involved ligand exchange partly displacing the previously incorporated sulfate ions (36). It was also reported that adsorption of arsenate on schwertmannite involved substitution of sulfate ions (18, 37). Hence the finding that ferric arsenate surface precipitate develops at acidic pH during the sorption of arsenate on ferrihydrite has an implication not only for the arsenate-iron(III) coprecipitation process where ferrihydrite is generated, but 3252
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b
[Fe3+] is
for the acid mine drainage system as well, where schwertmannite is reported to form.
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Received for review September 21, 2005. Revised manuscript received March 8, 2006. Accepted March 21, 2006. ES051872+
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