Mechanisms of Chromate, Selenate, and Sulfate Adsorption on Al

Article pubs.acs.org/est

Mechanisms of Chromate, Selenate, and Sulfate Adsorption on AlSubstituted Ferrihydrite: Implications for Ferrihydrite Surface Structure and Reactivity Chad P. Johnston*,† and Maria Chrysochoou‡ †

Engineering Science Program, Loyola University Chicago, Chicago, Illinois 60660, United States Department of Civil and Environmental Engineering, University of Connecticut, Storrs, Connecticut 06269, United States

‡

S Supporting Information *

ABSTRACT: Ferrihydrite is a nanocrystalline Fe (hydr)oxide and important sink for environmental contaminants. Although Fe (hydr)oxides are rarely pure in natural systems, little is known about the effects of structural impurities such as Al on the surface properties and reactivity of ferrihydrite. In this study, we characterized the adsorption mechanisms of chromate, selenate, and sulfate on Al-substituted ferrihydrite (0, 6, 12, 18, and 24 mol % Al) using in situ attenuated total reflection Fourier transform infrared spectroscopy. Spectral data sets recorded as a function of pH were processed using a multivariate curve resolution technique to identify which types of surface species form and to generate their concentration profiles as a function of pH and Al content. Results show a significant increase in relative fraction of outer-sphere complexes for all three oxyanions with increasing Al substitution. In addition, the effect of Al substitution is found to be mechanism-specific in the case of chromate, with bidentate complexes disproportionately suppressed over monodentate complexes at higher Al contents. Overall, our findings have important implications for the fate of chromate, selenate, and sulfate in subsurface environments and offer new insight into the surface reactivity of Al−ferrihydrite.

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INTRODUCTION Ferrihydrite is a nanocrystalline Fe (hydr)oxide commonly found in soils, sediments, and mine-drainage channels.1,2 Because of its high surface-area and density of reactive surface-sites, ferrihydrite is among the most important mineral sorbents affecting the mobility of nutrients, trace elements, and contaminants in aquatic environments.2−5 Ferrihydrite is also used commercially for removing heavy-metal contaminants from industrial wastewater.2,4,5 In natural systems, however, ferrihydrite seldom exists as a pure phase and shows substantial variability in its reactivity and chemical composition on account of structural impurities.3,6−11 Aluminum is among the most common substitutive elements in naturally occurring Fe (hydr)oxides, reaching saturation levels as high as 20−30 mol % Al in ferrihydrite before the precipitation of secondary Al (hydr)oxide phases.3,11−23 Aluminum has been shown to alter the physicochemical properties of ferrihydrite, particularly with respect to solubility,5,18,24 redox,19,21,23 bioavailability,19,20,25 and secondary mineralization to more thermodynamically stable Fe (hydr)oxide phases like goethite and hematite.16,18−20 Structural Al is also known to affect the retention of aqueous contaminants such as arsenic,5,17,22 phosphorus, 10 and uranium.9 However, the effects of Al on the interactions of ferrihydrite with many important trace metals and contaminants remain unknown. © XXXX American Chemical Society

In this study, we characterize the effects of Al substitution on the adsorption mechanisms of sulfate, selenate, and chromate on ferrihydrite. Sulfate is a ubiquitous component of natural waters and is known to influence the cycling of trace metals and pollutants through competition for (hydr)oxide surface sites.26 Selenate is also common in oxidized soil environments and becomes toxic at elevated concentrations.27,28 Chromate is a known carcinogen, acutely toxic in low concentrations, and is associated with industrial releases and geochemical cycling of Cr-bearing minerals.29,30 Spectroscopic studies indicate that chromate forms inner-sphere complexes on Fe (hydr)oxides such as goethite,31 hematite,32 and ferrihydrite,33,34 with monodentate and bidentate configurations favored at high and low pH, respectively. In contrast, recent studies suggest chromate adsorbs primarily as a weak outer-sphere complex on Al (hydr)oxides such as boehmite35 and γ-alumina.36 Sulfate and selenate exhibit similar behavior with respect to adsorption, and have been observed to form both outer-sphere and innersphere complexes on Al and Fe (hydr)oxides, with the latter mechanism being dominant below pH 5−6.27,28,37−43 Similar to chromate, selenate and sulfate have been observed to form Received: November 10, 2015 Revised: January 25, 2016 Accepted: February 22, 2016

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DOI: 10.1021/acs.est.5b05529 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology

MCR model and checking for convergence failure or redundant peaks in the resulting pure spectra. Spectral data sets for all Al contents were combined and evaluated simultaneously for each oxyanion in order to maintain a consistent interpretation of the effect of Al content on surface speciation. To ensure convergence in the fitting procedure, spectra were normalized to peak height prior to analysis and non-negativity constraints were imposed on both concentration and spectra.

more outer-sphere complexes with Al (hydr)oxides than with Fe (hydr)oxides.42,43 Overall, these studies show that the interactions of these oxyanions with soil (hydr)oxide surfaces strongly depend on Al and Fe content, although we are unaware of any such mechanistic studies involving Al− ferrihydrite. Accordingly, the objective of this study was to determine the adsorption mechanisms of chromate, selenate, and sulfate on Al−ferrihydrite as a function of pH and Al-content. To this end, the coordination environments of the oxyanions were characterized using in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). In addition, ATR-FTIR spectra were processed using multivariate curveresolution with alternating least-squares (MCR) to quantify the effects of pH and Al-content on the distribution of oxyanion surface species. On the basis of these analyses, surface complexation mechanisms are proposed and discussed in the context of ferrihydrite surface reactivity.

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RESULTS AND DISCUSSION Mechanisms of Chromate, Selenate, and Sulfate Adsorption on Al-Substituted Ferrihydrite. Spectral pHedges of chromate, selenate, and sulfate adsorbed on 0% and 24% Al-substituted ferrihydrite (Figure 1) are first characterized

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MATERIALS AND METHODS Al-substituted ferrihydrite was synthesized according to a modified version of the procedure of Schwertmann and Cornell.44 Briefly, stock solutions of FeCl3·9H2O and AlCl3 (ACS grade, Fisher Scientific) were combined to obtain 500 mL solutions with a total metals concentration ([Fe] + [Al]) of 0.2 M in proportions corresponding to 0, 6, 12, 18, and 24 mol % Al, and then rapidly hydrolyzed to pH 7.5 with 1 M KOH. The resulting precipitate was centrifuged and dialyzed to remove excess electrolytes. The absence of separate Al (hydr)oxide phases was confirmed by ATR-FTIR. Precipitates prepared with ≥30 mol % Al were found to contain a separate Al-(hydr)oxide phase identified as gibbsite by X-ray diffraction and ATR-FTIR. In situ ATR-FTIR measurements were collected using a Bruker alpha spectrometer equipped with a diamond ATR accessory and a flow-cell apparatus, using an experimental design developed previously for monitoring reactions at the mineral−water interface.26,32,34,35 Briefly, thin mineral films were prepared by drying 20 μL of a 5 g/L mineral suspension on the diamond ATR element under nitrogen gas. A recirculating peristaltic pump was used to equilibrate an N2purged solution of 0.01 M NaCl with the thin mineral-film to obtain a background spectrum. Spectral data sets corresponding to pH-edges were collected by introducing 100 μM concentrations of sulfate, chromate, and selenate into the reaction vessel and incrementally decreasing the pH from 8 to 3 with HCl. Several scans were recorded at each pH level to monitor changes in spectral intensity between successive scans. Spectra typically stabilized within 1 h, at which point the system was assumed to be in equilibrium. Equilibrium spectra were taken as the average of 1200 scans at a resolution of 4 cm−1. Spectral pH-edges were further analyzed using the multivariate curve resolution (MCR) with alternating least-squares (ALS) technique45 to identify the pure component spectra of individual surface complexes and their fractional concentration profiles as a function of pH. All MCR analyses were performed using the MCR-ALS Toolbox46,47 in Matlab (MathWorks). Negative second derivatives of the unprocessed spectra were used to corroborate the pure-component spectra resolved by the MCR procedure, and the number of chemically significant components in each data set was first estimated using singular value decomposition (SVD).48,49 The final number of species was determined by varying the number of components in the

Figure 1. ATR-FTIR spectra corresponding to pH edges of chromate, selenate, and sulfate adsorbed on pure and 24% Al−ferrihydrite.

as representative end points of the mechanistic effect of Alsubstitution. All spectra exhibit the characteristic inverse relationship between infrared intensity (i.e., oxyanion surface coverage) and pH expected for oxyanion sorption on metal (hydr)oxide surfaces, regardless of Al content (Supporting Information, Figure SI-1). The locations and relative intensities of oxyanion peaks vary within each envelope on account of the pH-dependence of oxyanion surface speciation. The presence of multiple stretching peaks indicates a deviation from the single infrared-active band of free tetrahedral oxyanions, consistent with the formation of low-symmetry inner-sphere complexes. However, depending on the Al content of ferrihydrite, spectral pH-edges are quite different, indicating a B


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Figure 2. MCR-resolved spectra (colored lines) and the negative of their second derivatives (black lines) for surface complexes formed by chromate, sulfate, and selenate on Al−ferrihydrite. SVD indicated that the chosen number of components accounts for 99.67%, 99.05%, and 96.38% of the variance in the spectral datasets for chromate, selenate, and sulfate, respectively.

consistent with infrared peaks attributed to outer-sphere chromate on boehmite.35 Spectra of adsorbed selenate are similar for both pure and 24% Al−ferrihydrite (Figure 1). In both cases, the dominant features in the raw spectra are Se−O stretching vibrations near 870 and 825 cm−1 that closely resemble aqueous selenate vibrations.28,43 This is suggestive of significant outer-sphere complexation on ferrihydrite at high pH values regardless of Al substitution. However, second-derivative plots (Supporting Information, Figure SI-1) for pure ferrihydrite reveal an additional Se−O vibration near 910 cm−1 that is less apparent for Al−ferrihydrite. This observation is consistent with an additional inner-sphere selenate complex on ferrihydrite at acidic pH that is suppressed by the presence of Al. MCR results also point to the existence of two significant selenate complexes with pH-dependent concentrations (Figure 2). The frequencies of the high pH selenate species (870 and 825 cm−1) closely match vibrations previously reported for outer-sphere selenate complexes on goethite (Table 1).42,43 The second MCR-resolved selenate complex appears at low pH and produces four Se−O vibrations (910, 880, 850, and 825 cm−1), consistent with an inner-sphere bidentate complex or protonated or strongly H-bonded monodentate complex. The 910 cm−1 vibration is characteristic of bidentate selenate complexes with cobalt,41,43,52 and has been attributed to an inner-sphere bidentate mechanism for selenate adsorption on goethite,41 hydrous ferric oxide (HFO),41 and Al oxide.43 In contrast, this assignment disagrees with other infrared studies that point to a predominantly monodentate adsorption mechanism on Fe (hydr)oxides.27,28 However, it is possible that distinguishing multiple inner-sphere complexes by infrared spectroscopy is complicated by the dominating signal of the outer-sphere complex and the overlapping nature of selenate vibrations. Sulfate complexes on ferrihydrite exhibit the characteristic stretching vibration of uncoordinated sulfate at pH > 6, regardless of Al content. As with chromate and selenate, peaksplitting occurs at lower pH values, but is less intense for 24% Al−ferrihydrite. The MCR analyses indicate the coexistence of two distinct sulfate complexes, consistent with the expected similarity to selenate speciation. The dominant sulfate species at pH > 6 produces peaks (1100 and 980 cm−1) corresponding to the intact asymmetric and symmetric stretches, respectively, of outer-sphere sulfate,26 in agreement with earlier infrared studies.37,53 In contrast, peaks corresponding to the low-pH complex (1170, 1120, 1050, and 980 cm−1) are consistent with an inner-sphere adsorption mechanism. The exact structure of this complex has been controversial, with similar peaks being attributed to both

change in the average oxyanion bonding environment at the ferrihydrite surface. As surface coverage increases at lower pH values, larger shifts in oxyanion stretching modes are observed in the case of pure ferrihydrite, suggesting that Al decreases the relative proportion of inner-sphere complexes. In the case of chromate adsorbed on pure ferrihydrite, the asymmetric Cr−O stretching frequency splits into multiple components in the 950−750 cm−1 region that to shift to higher and lower wavenumbers as the pH decreases from 8 to 3. MCR results show these trends are primarily accounted for by two distinct chromate complexes (Figure 2, Table 1). Above pH 5, Table 1. Stretching Vibrations of Chromate, Selenate, And Sulfate Surface Complexes on Al-Ferrihydrite As Resolved by MCR (Figure 2) oxyanion

surface complex

chromate

outer-sphere monodentate bidentate outer-sphere inner-sphere outer-sphere inner-sphere

selenate sulfate

stretching vibrations 875 910 955 870 910 1105 1170

830 873 930 825 880 980 1120

800 880

830

850

825

1050

980

765

three Cr−O stretching modes are seen (910, 873, and 800 cm−1) that are characteristic of a monodentate chromate complex with C3v symmetry.34 Below pH 5, chromate speciation is dominated by a lower-symmetry complex with five Cr−O stretching vibrations (955, 930, 880, 830, and 765 cm−1 ) observed previously for chromate adsorbed on ferrihydrite,33,34 attributed to a bidentate complex that dominates at acidic pH. This assignment is also consistent with previous quantum mechanical calculations34 and spectroscopic results for hematite32 and goethite.31 Unlike for pure ferrihydrite, stretching vibrations of chromate adsorbed on 24% Al−ferrihydrite appear in a more constrained region with less extensive peak-splitting. MCR results indicate that this effect is associated with a third type of chromate complex with a characteristic Cr−O stretching band centered around 875 cm−1 (Figure 2, Table 1). The location of this feature closely matches the asymmetric stretch of aqueous chromate,50,51 and the lack of peak-splitting is indicative of tetrahedral symmetry.51 Therefore, this species is most consistent with an outer-sphere chromate complex that is favored in the presence of Al at higher pH values. Although spectroscopic evidence of outer-sphere chromate complexes on Fe (hydr)oxides has not been reported, this assignment is C


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Figure 3. MCR-derived concentration-pH profiles of chromate, selenate, and sulfate adsorbed on Al- substituted ferrihydrite (black = total, red = outer-sphere. Monodentate and bidentate chromate complexes are distinguished by blue and purple lines, respectively).

monodentate and bidentate configurations.27,37−40,53 Our results are most consistent with the recent structural analysis by Zhu et al.,53 who observed a bidentate inner-sphere complex on ferrihydrite using X-ray absorption, pair distribution techniques, and infrared spectroscopy. Although the adsorption samples were dried in that study, the reported infrared spectrum is nearly identical to the MCR spectrum of the inner-sphere complex isolated in this work, suggesting that the same bidentate complex accounts for both data sets despite differences in experimental conditions. Our peak assignments also agree closely with quantum mechanical calculations of bidentate sulfate frequencies.39 However, this interpretation differs from a recent ATR-FTIR study54 in which the relative peak intensities of sulfate adsorbed on ferrihydrite were found to be independent of ionic strength, which was attributed to the absence of weakly bound outersphere complexes. Despite the quantitative decrease in sulfate surface coverage at higher ionic strength reported in that study, it was suggested that the peak-shifts of sulfate adsorbed on ferrihydrite are better explained by a single inner-sphere complex responding to a pH-dependent electrostatic effect. Although electrostatic effects are known to distort infrared spectra,55 they are not expected to result in the high degree of peak-splitting observed for adsorbed sulfate.37 Moreover, if it

assumed that only outer-sphere complexes are affected by ionic strength, it is unclear why the overall adsorption of sulfate would decrease if only inner-sphere complexes were present. Another possibility for the apparent lack of ionic strength effect is that the concentration of outer-sphere sulfate is substantially higher on ferrihydrite, such that the relative change in surface speciation is less intense with respect to other iron (hydr)oxides. In an earlier ATR-FTIR study, Peak et al.37 showed that the spectra of sulfate adsorbed on ferrihydrite contain a significantly larger contribution from outer-sphere complexes compared to goethite and hematite under similar pH and ionic strength conditions. Thus, it is likely that sulfate peak positions are influenced more so by changes to the relative abundance of outer-sphere complexes in the case of ferrihydrite. It is also possible that inner-sphere sulfate is suppressed by ionic strength, as has been observed for chromate32 and selenate.41 To summarize our mechanistic assignments, we find that chromate, selenate, and sulfate form both outer-sphere and inner-sphere complexes on Al−ferrihydrite. Second, chromate forms both monodentate and bidentate inner-sphere complexes, whereas sulfate and selenate inner-sphere complexation is attributed primarily to a bidentate configuration. Lastly, the presence of Al in the ferrihydrite structure suppresses the D


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Figure 4. Effect of Al substitution on the fractional speciation of oxyanions at the ferrihydrite−water interface at pH 3, 5, and 7, as derived from the MCR fitting procedure.

complexes (Figure 4). Comparison of the surface speciation of chromate at pH 3 shows that 24% Al-substitution results in a nearly 50% loss in the fraction of bidentate complexes, but has little effect on the amount of monodentate complexes (Figure 3, 4). To explain these phenomena, we consider possible modes of Al incorporation within the ferrihydrite structure. Although the nanocrystalline nature of ferrihydrite complicates structural analysis, previous studies have shown that ferrihydrite can readily incorporate 20−30 mol % Al in its structure without the appreciable formation of secondary phases.5,6,17 Ferrihydrite is also predicted to accommodate Al substitution with limited energy increase compared to other common Fe (hydr)oxides.12 What is less certain, however, is the extent to which Al is dispersed or clustered within the ferrihydrite bulk or surface structure.12,59,60 The possibilities for Al incorporation that are consistent with the increase in outer-sphere complexes include the formation of isolated Al-rich clusters or a more dispersed distribution of Al-substituted surface-sites. In this study, the disproportionate suppression of innersphere bidentate chromate complexes is suggestive of a more dispersed configuration of surface Al, such that the concentration of adjacent Al-octahedra is minimized. According to the recent surface-depletion model of Hiemstra,57,58 the ferrihydrite surface is dominated by two types of reactive surface sites. Both sites consist of singly coordinated hydroxyl groups, but differ in that only one has the sequence of adjacent edge-sharing octahedra conducive to corner-sharing (binuclear) bidentate complexes expected for oxyanions. Assuming oxyanions preferentially form inner-sphere complexes with surface Fe atoms, the disproportionate decrease in bidentate chromate complexes is then suggestive of a decrease in adjacent singly

formation of inner-sphere complexes of all three oxyanions at all pH values. Implications for Ferrihydrite Surface Structure and Reactivity. The dominant effect of Al substitution on ferrihydrite reactivity is an increase in outer-sphere complexation for all oxyanions studied, suggesting that Al incorporation suppresses the availability of inner-sphere binding sites (Figures 3 and 4). MCR-resolved pH edges (Figure 3) show how the concentrations of each of the surface complexes change with pH. The pH corresponding to approximately 50% adsorption increases with increasing Al content for all oxyanions, suggesting a corresponding shift in the point of zero charge.5 When comparing the fractional speciation as a function of Al content at constant pH, a clear downward trend is observed in the relative proportion of inner-sphere complexes of all three oxyanions with increasing Al substitution. This effect is most pronounced for chromate, which does not form a significant quantity of outer-sphere complexes on pure ferrihydrite. For example, the relative abundance of outer-sphere chromate at pH 7 increases from approximately 6% to nearly 50% when the Al content increases from 0 to 24%. Peak (2006) suggested a structure-based model for Al and Fe (hydr)oxide reactivity, in which the oxyanion adsorption mechanism depends on the mineral-dependent orientation more so than the type of metal cation. The outer-sphere enhancement observed here highlights the importance of cation properties within the context of individual (hydr)oxide phases, consistent with previous spectroscopic studies43 and macroscopic modeling results.56 In addition to enhancing outer-sphere adsorption, the effect of Al on inner-sphere chromate appears to be mechanismspecific for chromate, with bidentate complexes being disproportionately suppressed with respect to monodentate E



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ENVIRONMENTAL IMPLICATIONS This study provides molecular-level insight into the effects of Al substitution in ferrihydrite on the adsorption mechanisms of chromate, selenate, and sulfate. Given the widespread occurrence of Al-substituted Fe (hydr)oxides in nature, this new information has several key implications for the environmental fate of these oxyanions, as well as for other ions that may compete for or otherwise react with ferrihydrite surface sites. First, the substantial increase in the relative proportion of outer-sphere surface complexes indicates that Al weakens the chemical interactions of oxyanions with ferrihydrite. Consequently, the mobility of Cr, S, and Se will likely be enhanced in oxidized environments with an abundance of mixed Al−Fe (hydr)oxide phases. Second, the disruption of inner-sphere complexation trends by Al offers clues about the surfacestructure and reactivity of Al−ferrihydrite. The disproportionate suppression of bidentate and monodentate chromate complexes suggests that Al at low concentrations (

Mechanisms of Chromate, Selenate, and Sulfate Adsorption on Al

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