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Silicic Acid Adsorption and Oligomerization at the Ferrihydrite-Water Interface: Interpretation of ATR-IR Spectra Based on a Model Surface Structure Peter J. Swedlund,*,† Gordon M. Miskelly,† and A. James McQuillan‡ †
Department of Chemistry, University of Auckland, Private Bag 92019, Auckland, New Zealand, and ‡ Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand Received August 25, 2009. Revised Manuscript Received December 16, 2009
Oxide surfaces can promote specific lateral interactions between adsorbed species that become concentrated in specific orientations at an interface. In this article, in situ attenuated total reflectance (ATR) IR spectra were collected over time (from 0 to ∼100 h) as the iron oxide ferrihydrite reacted with H4SiO4 (between 0.007 and 1.65 mM) and at a pH of 4, 7, or 10. Under all conditions, the first product formed was a monomeric surface species with distinct bands at 945 and 880 cm-1, and a bidentate 2C complex with SiO4 sharing corners with two edge-linked Fe octahedra was proposed. Once a certain surface concentration (ΓSi) of monomers was reached, a condensed oligomeric surface species with Si-O-Si linkages was observed on the surface with bands at 1005, 917, and 827 cm-1 and one or more bands at >1050 cm-1. This species was observed as a minor surface component at ΓSi that was up to 10 times lower than the calculated density of 2C adsorption sites on ferrihydrite and became the dominant surface species at higher ΓSi. This formation of a specific oligomer is rationalized on the basis of a recent model for the ferrihydrite surface, with the arrangement of 2C adsorption sites on the (021) ferrihydrite face causing adjacent Si monomers to be held in an orientation that is conducive to the formation of a condensed Si species upon insertion of a solution H4SiO4. Therefore, this model predicts that the ferrihydrite surface may act as a template for oligomerization in one dimension forming segments of pyroxene-like structures. The ATR-IR spectra and changes in the surface species’ composition with time are consistent with such a model.
1. Introduction The spatial arrangement of adsorption sites at the metal oxide/ water interface can promote specific lateral interactions, such as condensation reactions, between adsorbed species.1 For example, silicic acid (H4SiO4) condensation produces numerous and diverse products in solution2 whereas the adsorption of H4SiO4 monomers on the surface of iron oxide ferrihydrite leads to the formation of a particular oligomeric species.3 A model for the arrangement of adsorption sites on the ferrihydrite surface that is consistent with the macroscopic properties of ferrihydrite has recently been proposed.4 This model provides an opportunity to rationalize the chemistry of H4SiO4 on ferrihydrite by linking the geometry and structure of surface sites with the reactions between silicate monomers adsorbed on this surface. The interactions of silicates on iron oxide surfaces are important in numerous natural and industrial systems. The presence of H4SiO4 alters the linkages that form between Fe3þ octahedra during ferric hydrolysis5 and *Corresponding author. E-mail:
[email protected]. Fax: þ 64 9 3737 422. (1) Brown, G. E.; Henrich, V. E.; Casey, W. H.; Clark, D. L.; Eggleston, C.; Felmy, A.; Goodman, D. W.; Gratzel, M.; Maciel, G.; McCarthy, M. I.; Nealson, K. H.; Sverjensky, D. A.; Toney, M. F.; Zachara, J. M. Chem. Rev. 1999, 99, 77–174. (2) Sjoberg, S. J. Non-Cryst. Solids 1996, 196, 51–57. (3) Swedlund, P. J.; Miskelly, G. M.; McQuillan, A. J. Geochim. Cosmochim. Acta 2009, 73, 4199–4214. (4) Hiemstra, T.; Riemsdijk, W. H. V. Geochim. Cosmochim. Acta 2009, 73, 4423–4436. (5) Doelsch, E.; Rose, J.; Masion, A.; Bottero, J. Y.; Nahon, D.; Bertsch, P. M. Langmuir 2000, 16, 4726–4731. (6) Jambor, J. L.; Dutrizac, J. E. Chem. Rev. 1998, 98, 2549–2585. (7) Kwon, S. K.; Shinoda, K.; Suzuki, S.; Waseda, Y. Corros. Sci. 2007, 49, 1513–1526. (8) Anderson, P. R.; Benjamin, M. M. Environ. Sci. Technol. 1985, 19, 1048– 1053.
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influences iron oxide mineralogy,6 morphology,7 surface charge,8 and the availability of surface reactive sites.9 Industrial processes where these interactions can be important are as varied as catalytic supports,10 wastewater treatment,11 and silica-coated iron oxide particles used in biomedical applications such as contrast agents in MRI.12 Iron oxides are also important phases influencing the chemistry of many natural systems including soils,6 aquifers,13 surface waters,14 and the atmosphere.15 Silicates attached to iron oxide surfaces can have Si-O-Fe and Si-O-Si linkages. The Si-O-Fe linkage has been observed by infrared spectroscopy (IR),3,16,17 and a bidentate complex involving an SiO4 tetrahedron sharing corners with two adjacent edge-sharing Fe octahedra has been proposed by Pokrovski et al18 on the basis of Fe K-edge extended X-ray absorption fine structure (EXAFS) studies of FeIII hydrolyzed in the presence of H4SiO4. In contrast, crystal truncation rod (CTR) analysis of (9) Swedlund, P. J.; Webster, J. G. Water Res. 1999, 33, 3413–3422. (10) Corrias, A.; Ennas, G.; Mountjoy, G.; Paschina, G. Phys. Chem. Chem. Phys. 2000, 2, 1045–1050. (11) Davis, C. C.; Chen, H.-W.; Edwards, M. Environ. Sci. Technol. 2002, 36, 582–587. (12) Santra, S.; Tapec, R.; Theodoropoulou, N.; Dobson, J.; Hebard, A.; Tan, W. Langmuir 2001, 17, 2900–2906. (13) Postma, D.; Larsen, F.; Hue, N. T. M.; Duc, M. T.; Viet, P. H.; Nhan, P. Q.; Jessen, S. Geochim. Cosmochim. Acta 2007, 71, 5054–5071. (14) Davis, C. C.; Knocke, W. R.; Edwards, M. Environ. Sci. Technol. 2001, 35, 3158–3162. (15) Cwiertny, D. M.; Young, M. A.; Grassian, V. H. Annu. Rev. Phys. Chem. 2008, 59, 27–51. (16) Carlson, L.; Schwertmann, U. Geochim. Cosmochim. Acta 1981, 45, 421–425. (17) Doelsch, E.; Stone, W. E. E.; Petit, S.; Masion, A.; Rose, J.; Bottero, J. Y.; Nahon, D. Langmuir 2001, 17, 1399–1405. (18) Pokrovski, G. S.; Schott, J.; Garges, F.; Hazemann, J. L. Geochim. Cosmochim. Acta 2003, 67, 3559–3573.
Published on Web 01/28/2010
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silicate adsorbed on a hematite surface has suggested the predominance of a monodentate adsorption geometry.19 Silicate species with Si-O-Si linkages have been observed on iron oxide surfaces by 29Si nuclear magnetic resonance,17 IR,3,16,17 and grazing incidence EXAFS.19 Modeling studies of macroscopic properties such as silicate surface concentrations (ΓSi) and surface charge also support the presence of condensed silicate species on iron oxide surfaces.9,11,20 These studies have proposed varying degrees of silicate oligomerization or polymerization upon adsorption but agree that silicate condensation is more prevalent on iron oxide surfaces at higher Si surface concentrations and at lower pH. Doelsch et al.17 measured the IR spectra of the dried solids obtained from FeCl3 hydrolysis in the presence of different concentrations of H4SiO4. The wavenumber of the maximum IR absorbance in the Si-O stretching region increased from 930 to 1080 cm-1 when either the solid-phase Si/Fe mole ratio increased or there was a decrease in pH (at constant Si/Fe). An in situ ATRIR study of H4SiO4 adsorbed on ferrihydrite at pH 4 showed bands in the Si-O stretching region corresponding to only two main species: a monomeric silicate (IR bands at ∼947, 877 (sh), and 815 (sh) cm-1) and an oligomer that was larger than a dimer with the main IR band at 1003 cm-1 and smaller bands at 1140, 1108, 1070, 914, and 825 cm-1.3 A quantitative analysis of IR spectra collected while H4SiO4 was adsorbing onto ferrihydrite showed that initially only monomers were present but oligomer formation occurred once the amount of adsorbed monomer had reached ∼0.01 mol of Si (mol of Fe)-1. As adsorption progressed, the percentage of surface Si present as oligomers increased from 0 to 90% as ΓSi increased from 0.01 to 0.1 mol of Si (mol of Fe)-1.3 The current article uses ATR-IR to examine the interaction of H4SiO4 on ferrihydrite as a function of pH and ΓSi, and spectra are interpreted on the basis of the recently proposed ferrihydrite surface structures.4 Because of the small size of ferrihydrite particles and the lack of long-range order, the actual structure of the ferrihydrite surface is still equivocal. However, the surface structure model proposed by Hiemstra and Riemsdijk4 is consistent with many macroscopic properties of ferrihydrite and provides rationalization of the ATR-IR spectra observed upon silicate adsorption. In addition, we report IR spectra of silicate species on ferrihydrite measured over a wider frequency range than have been previously reported, and these provide further structural constraints on the surface Si structures.
2. Materials and Methods 2.1. Materials. Detailed methods have been reported.3 In brief, all solutions were made from 18 MΩ cm resistivity deionized water that was distilled and then degassed with N2. Solutions were stored in polycarbonate containers that were kept either sealed or under a stream of N2. Adjustments to pH were made with isothermally distilled HCl or low-carbonate NaOH solutions. A stock H4SiO4 solution was prepared by dissolving amorphous silica (1 g) in 50:50 (w/w) NaOH/H2O (8 g) followed by dilution to 1 L. The solution was further diluted to 1.66 mM, the pH was lowered to ∼8 for 1 to 2 days (to allow for depolymerization that is slow at pH 4), and then the pH was lowered to pH 4 and the solution was degassed to produce a stable stock solution of 1.66 mM monomeric silicic acid in 0.01 M NaCl. The rate of color development in the molybdenum yellow method21 was used to (19) Waychunas, G. A.; Jun, Y.-S.; Eng, P. J.; Ghose, S. K.; Trainor, T. P.; Barnett, M. O.; Kent, D. B. In Developments in Earth and Environmental Sciences; Elsevier: 2008; Vol. 7, pp 31-65. (20) Hiemstra, T.; Barnett, M. O.; van Riemsdijk, W. H. J. Colloid Interface Sci. 2007, 310, 8–17. (21) Alexander, G. B. J. Am. Chem. Soc. 1953, 75, 5655–5657.
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determine that, within the limits of detection, monomeric silicic acid was the only species present. Two-line ferrihydrite was prepared by raising the pH of a ferric nitrate and HNO3 solution from 2 to 8.3 The resulting suspension was aged for 1 h, rinsed twice with water (by centrifugation, followed by decanting), and diluted to 10 mM total Fe with water. 2.2. Adsorption Experiments. For all but one experiment, 1 mL of the ferrihydrite suspension was deposited onto a ZnSe ATR crystal with five sample reflections (45°), and the water was removed under vacuum at 25-35 °C. One experiment was performed with a single-bounce 45° diamond ATR crystal to allow measurements down to 500 cm-1. In this case, 0.05 mL of the ferrihydrite suspension was deposited on the crystal and the water was evaporated under a gentle stream of N2. The ZnSe or diamond ATR crystal was placed in a flow cell, and the ferrihydrite film was rinsed for 1 h with 0.01 M NaCl at pH 11 to remove adsorbed carbonate and then equilibrated with 0.01 M NaCl at the desired pH for between 2 and 12 h. After this time, ATR-IR spectra were collected (section 2.3), and when a stable background was obtained, the solution flow was stopped, silicic acid stock solution was added to the electrolyte, the pH was readjusted if necessary, and this solution was circulated through the cell at ca. 1 mL min-1 for the duration of the experiment. Infrared spectra were collected (with the fluid flowing) over time for up to 7 days, after which time there was little change in the collected spectra as a function of time. At this point, the flow cell lid was removed and the ferrihydrite film was rinsed with water and then wiped off of the ATR crystal with a small piece of ashfree filter paper. This ferrihydrite was dissolved off of the filter paper by adding HCl (final pH ∼1), and the Fe and Si contents were determined by flame AAS and ICP MS, respectively, to give the ΓSi at the end of each experiment. All solution H4SiO4 concentrations reported are those measured at the end of the experiment and represent the conditions approaching equilibrium. The surface area of the ferrihydrite film (139 m2 g-1) was measured from a sample dried on glass under vacuum at 35 °C and then scraped off for N2 BET analysis. Surface concentrations (ΓSi) expressed per nm2 were calculated using the BET surface area and the measured ferrihydrite molecular weight of 110 g (mol Fe)-1. BET underestimates ferrihydrite surface areas, so the calculated ΓSi values will represent an upper limit. 2.3. Infrared Spectroscopy. A Nicolet 8700 spectrometer with a DTGS detector and Omnic software was used to obtain ATR-IR spectra (resolution of 4 cm-1) between 4000 cm-1 and either 750 or 500 cm-1 for the ZnSe and diamond ATR crystals, respectively. The spectra were corrected for changes to the water signal that affected the low-wavenumber side of the region of interest (between 1250 and 700 cm-1) by adding or subtracting water spectra and then taking a horizontal baseline. Spectra were ATR corrected using the Omnic advanced ATR correction algorithm. Negative Savitsky-Golay second derivatives were calculated to assist band identification using an order of 3 and with the number of points depending on the signal-to-noise ratio in the original spectra. Multivariate curve resolution with alternating least squares (MCR-ALS) was used in Matlab (Mathworks) to determine the IR spectra for the pure surface species and the amount of each surface species present using the constraints of non-negative concentration and non-negative spectra.22 The data matrix contained 134 rows and 209 columns representing the 134 measured spectra between ∼1250 and 850 cm-1. MCR-ALS analysis was used to decompose the data matrix into the product of two matrices. The “spectra” matrix has 3 rows and 209 columns, representing the absorbance of each of the 3 pure surface species at each wavenumber. The “composition” matrix has 134 rows and 3 columns representing the contribution of each of the 3 surface species to each measured spectrum. The data at pH 4 with 0.044-0.91 mM H4SiO4 are as previously reported3 but have been ATR corrected for this article. Crystal structure diagrams (22) Tauler, R.; Smilde, A.; Kowalski, B. J. Chemom. 1995, 9, 31–58.
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Figure 1. ATR-IR spectra of H4SiO4 adsorbed on ferrihydrite over time at pH 7 in 0.01 M NaCl: (a) 0.015 mM H4SiO4, (b) 0.129 mM H4SiO4, (c) 0.93 mM H4SiO4, and (d) the negative second derivatives (19 points) of the spectra with 0.93 mM H4SiO4.
were created with Vesta software23 using data from the American Mineralogist Crystal Structure Database.24
3. Results and Discussion 3.1. Qualitative Analysis of the ATR-IR Spectra of H4SiO4 Adsorbed on Ferrihydrite. 3.1.1. General Trends. The H4SiO4 adsorption onto ferrihydrite at pH 4, 7, and 10 was monitored over time by ATR-IR spectroscopy. The resulting ATR-IR spectra in the Si-O stretching region of 750 to 1250 cm-1 after ferrihydrite was exposed to H4SiO4 at pH 7 are shown in Figure 1, together with the second derivatives of the spectra obtained at the highest H4SiO4 concentration studied (0.93 mM). Corresponding ATR-IR data obtained at pH 10 are presented in the Supporting Information (Figure SI 1). For all experiments, the area of the Si-O stretching bands in the IR spectra increased approximately linearly with the log of time. This kinetic behavior, which has been observed for other systems in which small molecules are adsorbing onto iron oxides, has been attributed to diffusion (interparticle or intraparticle) or to site heterogeneity.25 The ATR-IR spectrum of monomeric H4SiO4 in solution has one ν3 mode at 937 cm-1, which at a concentration of 0.93 mM would have an absorbance of ∼8 10-5 (after ATR correction).3 The observed absorbance in the Si-O stretching region for H4SiO4 adsorbed on the ferrihydrite surface was e0.10, demonstrating that silicate species were greatly concentrated on the surface of the ferrihydrite. The general features observed in the ATR-IR spectra as H4SiO4 adsorbed onto ferrihydrite at pH 7 and 10 were the same as those previously reported at pH 4.3 The spectra collected immediately after the introduction of H4SiO4, when ΓSi was low, had a maximum at ∼945 cm-1 with a shoulder at ∼880 cm-1 (clearly seen in the second derivative of the spectrum at 0.05 h in Figure 1d). As ΓSi increased over time, there was (23) Momma, K.; Izumi, F. J. Appl. Crystallogr. 2008, 41, 653–658. (24) Downs, R. T.; Hall-Wallace, M. Am. Mineral. 2003, 88, 247–250. (25) Zhang, J.; Stanforth, R. Langmuir 2005, 21, 2895–2901.
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increased IR absorption in the region of 1005 cm-1, and this becomes the dominant spectral feature after 1 h at pH 7 in 0.93 mM H4SiO4 (Figure 1c). The second derivatives of the spectra clearly show the evolution of the band at 1005-1013 cm-1, associated bands at 825 and 917 cm-1, and a band at ∼1120 that broadens and shifts to higher wavenumber at higher ΓSi (Figure 1d). The spectra collected using a diamond ATR crystal allowed measurements to 500 cm-1, but no bands were observed between 750 and 500 cm-1 (data not shown). 3.1.2. Spectra at Low ΓSi. The ATR-IR spectra observed at low ΓSi for pH 4, 7, and 10 have a maximum IR absorbance at 945 cm-1 and no significant bands at higher wavenumbers (e.g., Figure 1a). Orthosilicate minerals with isolated SiO4 units have symmetric and asymmetric Si-O stretching modes (ν1 and ν3, respectively) between 800 and 1000 cm-1 whereas condensed silicate minerals with Si-O-Si bridges have Si-O-Si stretching modes at higher wavenumbers (g1000 cm-1).26,27 Therefore, the spectra indicate that a monomeric silicate is adsorbed at low ΓSi.28 Previous band fitting of this spectral feature indicated that the SiO4 ν3 mode was split into bands at 880 and 946 cm-1, with one band above 946 cm-1 that was not resolved in the second derivatives.3 DFT calculations for H4SiO4 supported the assignment of a small band at 810 cm-1 to the infrared-active ν1 mode.3 This band fitting is consistent with the bidentate attachment of monomeric silicate to ferrihydrite, as has been proposed from EXAFS of ferric ions hydrolyzed in the presence of H4SiO4.18 This contrasts with the monodentate complex proposed from CTR analysis of silicate adsorbed on hematite.19 Whereas different surface complex structures may be formed on different iron oxides,29,30 the relationship between adsorbed ligand-Fe distances and surface complex geometry has recently been questioned31 and it has also been claimed that the structures of protonated surface complexes indicated by ATR-IR are not definitive.32 Therefore, whereas EXAFS18 and ATR-IR spectra indicate a bidentate complex, a monodentate complex cannot be conclusively excluded. Bidentate complexes can involve either H4SiO4 bound to one Fe ion (forming a mononuclear edge-sharing complex, termed 2E) or H4SiO4 bound to two Fe ions (forming a binuclear cornersharing complex, termed 2C). A recent model for ferrihydrite has a surface dominated by singly coordinated oxygens (i.e., bound to one Fe).4 These available oxygens were arranged in pairs either as the edge of a single Fe octahedron (an 2E adsorption site) or as the corners of two edge-linked octahedra (a 2C adsorption site). EXAFS of FeIII hydrolyzed in the presence of H4SiO4 is consistent with a 2C bonding configuration,18 and many spectroscopic studies have indicated the presence of an analogous 2C surface complex for iron oxide adsorbed oxyanions such as phosphate, arsenate, and arsenite. Therefore, a model in which monomeric H4SiO4 is bound to ferrihydrite in a 2C configuration is most consistent with the IR spectra measured in this work and with previous studies. (26) Lazarev, A. N. Vibrational Spectra and Structure of Silicates; Consultants Bureau: New York, 1972. (27) Tarte, P.; Pottier, M. J.; Proces, A. M. Spectrochim. Acta, Part A 1973, 29, 1017–1027. (28) Farmer, V. C. In Infrared Spectra of Minerals; Farmer, V. C., Ed.; Mineralogical Society: London, 1974; Monograph No. 4. (29) Eggleston, C. M.; Hug, S.; Stumm, W.; Sulzberger, B.; Afonso, M. D. Geochim. Cosmochim. Acta 1998, 62, 585–593. (30) Peak, D.; Ford, R. G.; Sparks, D. L. J. Colloid Interface Sci. 1999, 218, 289– 299. (31) Loring, J. S.; Sandstrom, M. H.; Noren, K.; Persson, P. Chem.;Eur. J. 2009, 15, 5063–5072. (32) Catalano, J. G.; Park, C.; Fenter, P.; Zhang, Z. Geochim. Cosmochim. Acta 2008, 72, 1986–2004.
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The adsorbed silicate monomer spectra have the same general shape at pH 4, 7, and 10, with the two main peaks in the secondderivative spectra occurring at the same position. This suggests that either there is no change in the degree of protonation of the adsorbed silicate monomer between pH 4 and 10 or that protonation does not affect the IR spectra. In solution, the IR spectra of H3SiO4- and H4SiO4 are quite distinct with H4SiO4 (pKA1 = 9.9) having a single Si-O stretching band3 at 938 cm-1 whereas H3SiO4- has two Si-O stretching bands33 at 1023 and 885 cm-1. For goethite-adsorbed phosphate, the ATR-IR spectra at pH 8.4 have bands at 1090 and 1048 cm-1 whereas at pH 4.5 (at a similar surface coverage) the bands are at 1120 and 1007 cm-1.34 This change in IR spectra has been attributed to the nonprotonated bidentate-adsorbed PO43- at pH 8.4 being protonated at pH 4.5, with protonation possibly causing a change from bidentate to monodentate attachment.34-36 A fully protonated 2C surface complex of H4SiO4 could be written as (tFe)2O2Si(OH)2 where two of the H4SiO4 protons have been replaced by Fe3þ ions on the ferrihydrite surface (tFe). The IR spectra at pH 4 and 10 suggest that the surface species is not appreciably deprotonated at pH 10, implying that the pKA1 of (tFe)2O2Si(OH)2 is higher than that of H4SiO4. The acidity of protons on oxyacids that are coordinated to metal cations can be assessed from the pKA’s of solution complexes such as [(NH3)4Co 3 μ(NH2,H2PO4) 3 Co(NH3)4]4þ, which has an H2PO4- group bridging two Co3þ cations. The pKA’s of this complex (1.5 and 6.0) were close to the pKA1 and pKA2 of H3PO4, indicating that the effect of two Co3þ ions on the acidity of the remaining OH groups was approximately equivalent to one Hþ.37 This is consistent with the nonprotonated bidentate PO43ligand on goethite being protonated between pH 8.4 and 4.5 (pKA2 H3PO4 = 7.2). Therefore, it is reasonable that the (tFe)2O2Si(OH)2 complex on the ferrihydrite surface would be less acidic than H4SiO4 in solution as indicated by the IR spectra in this work. 3.1.3. Spectra at Intermediate to High ΓSi. As the Si surface coverage on ferrihydrite increases, the ATR-IR band at 1005 cm-1 becomes increasingly dominant (e.g., Figure 1c). Thus, the ratio of the absorbances at 1005 and 945 cm-1 (termed R) increased from about 0.3 up to a maximum of 1.3, 1.5, or 1.2 at pH 4, 7, or 10, respectively. IR spectra with similar R values had a similar overall shape irrespective of pH as is shown for R ≈ 1.2 (Figure 2a). Despite the similarity in the shape of these IR spectra, the second derivatives of the spectra between 1050 and 1150 cm-1 obtained at pH 4 were quite different from those at pH 7 and 10, reflecting a fairly subtle but reproducible difference in the shape of the spectra in this region. Figure 2b shows that the wavenumber and relative intensity of the bands in the second derivative at pH 4 (at ∼827, 918, 1005, 1140, 1110, and 1070 cm-1) did not change from when the bands were just detected as minor components in the spectra until when they were the dominant spectral feature.3 For this reason, the species responsible for these spectral features was considered to be a discrete silicate rather than an ill-defined polymeric phase where the number of bands and their positions would change with ongoing polymerization.26 The oligomeric silicate on the ferrihydrite surface was considered to be larger than a dimer because IR spectra of minerals with dimeric silicate (33) Yang, X.; Roonasi, P.; Holmgren, A. J. Colloid Interface Sci. 2008, 328, 41–47. (34) Tejedor-Tejedor, M. I.; Anderson, M. A. Langmuir 1990, 6, 602–611. (35) Kwon, K. D.; Kubicki, J. D. Langmuir 2004, 20, 9249–9254. (36) Rahnemaie, R.; Hiemstra, T.; van Riemsdijk, W. H. Langmuir 2007, 23, 3680–3689. (37) Edwards, J. D.; Foong, S. W.; Sykes, A. G. J. Chem. Soc., Dalton Trans. 1973, 829–838.
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Figure 2. (a) ATR-IR spectra with an R value (see text) of 1.2 and negative second derivatives (∼0.9 mM H4SiO4). (b) Negative second derivatives of spectra at pH 4 with 0.91 mM H4SiO4. (c) Difference ATR-IR spectra of H4SiO4 adsorbed onto ferrihydrite at pH 7 with 0.93 mM H4SiO4 after subtracting the previously collected spectra. (d) Estimate of the spectrum of a polymerized silica phase on the ferrihydrite surface.
anions have only one bridging Si-O-Si band at >1000 cm-1 and a maximum IR absorbance between 900 and 950 cm-1 due to the stretching of the terminal SiO groups that are more prevalent than the bridging Si-O-Si groups.26 In more condensed silicates, the proportion of bridging Si-O-Si bonds increases at the expense of nonbridging Si-O bonds, the number of Si-O-Si stretching modes (at >1000 cm-1) increases, and the maximum absorbance shifts to higher wavenumber. In general, it is observed that the Si-O stretching frequencies of silicate species increase as the degree of condensation increases.38 Cyclic silicates have been proposed as possible oligomeric species on iron oxide surfaces,3,20 but they have strongly absorbing IR ring deformation modes39 between ∼740 and 600 cm-1 that were not observed in this work. In silicates with polymerization occurring in three dimensions, the Si-O asymmetric stretching bands occur at ∼1100 cm-1.38 For example, quartz and the amorphous SiO2(am) phase have the strongest bands at 1090 and 1110 cm-1, respectively. The oligomeric silicate species on ferrihydrite has the strongest band at ∼1000 cm-1, indicating that polymerization is not occurring in three dimensions. For now, this species is termed an oligomer, and further discussion of its structure is presented in section 3.2. Hiemstra et al20 modeled macroscopic measurements of H4SiO4 adsorption onto goethite and proposed the formation of a tetrameric silicate with a pKA of 6.5-7.1 on surfaces with ΓSi > 0.5-1 nm-2. Therefore, we investigated the possibility that the marked change in the second derivatives of the spectra with pH in Figure 2a was due to changes in the degree of protonation. However, when ferrihydrite was equilibrated with 1 mM H4SiO4 (38) Kieffer, S. W. Rev. Geophys. 1979, 17, 20–34. (39) Sitarz, M.; Mozgawa, W.; Handke, M. J. Mol. Struct. 1997, 404, 193–197.
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at pH 4 for 30 h and then the pH changed to 10, the second derivative still had three distinct bands at 1070, 1107, and 1141 cm-1 (as at pH 4) even after 3 days at pH 10 (data not shown). Similarly, when ferrihydrite was equilibrated at pH 10 for 62 h and then the pH changed to 4, the second derivative maintains the broad band at 1130 cm-1 even after 3 h at pH 4. This shows that the change in the second derivative with pH is not due to a rapid, reversible process such as protonation or a change in surface charge. Given the similarity in the overall shape of the IR spectra at pH 4, 7, and 10 and similarities of the second derivatives from 750 to 1050 cm-1, our interpretation is that the same surface species is present at pH 4, 7, and 10 with a subtle change in structure that is pH-dependent. 3.1.4. Spectra at High ΓSi. At high ΓSi, the ATR-IR spectra had increased intensity between 1050 and 1150 cm-1 attributed to the development of an amorphous silica phase (SiO2(am)) containing SiO4 units with polymerization occurring in three dimensions.3 This high-ΓSi feature made a comparatively small contribution to the measured IR spectra and was evident only from analyzing the difference spectra where the spectrum collected after a given time (ti) is subtracted from a spectrum collected at some later time (tj). At pH 7 and 10, the difference spectra had the same trends as at pH 4. Figure 2c shows the difference spectra at pH 7 for 0.93 mM H4SiO4, where each spectrum has had the previously collected spectrum subtracted from it and the maximum IR absorbance normalized to emphasize the changes in spectral shape. When ΓSi was low, the difference spectra steadily decreased in the region below 1005 cm-1 (though they are similar above 1005 cm-1) because of the decreasing significance of the monomer. The difference spectra at higher ΓSi were similar in the region below 1005 cm-1 but steadily increased between 1005 and 1200 cm-1, indicating the development of a species with a broad IR absorption in this region. Figure 2d shows the average spectrum of this species estimated from the difference spectra at pH 4, 7, and 10. This feature is similar to the IR spectra of amorphous silica and framework silicates that have broad bands centered at ∼1100 cm-1.28 Because the difference spectra have their maximum absorbance at ∼1000 cm-1, the spectral feature shown in Figure 2d may be artificially forced to zero at ∼1000 cm-1 and for this reason can be considered to be only an estimate of the spectrum of the high-ΓSi surface species. This estimate was refined during quantitative modeling (section 3.2). 3.2. Quantitative Analysis of the ATR-IR Spectra of H4SiO4 Adsorbed on Ferrihydrite. The above analysis indicates that the ATR-IR spectra of H4SiO4 adsorbed on ferrihydrite have the same features at pH 4, 7 and 10 and supports the formation of only three surface silicate species: a monomeric silicate at low surface concentration, a discrete oligomeric species that becomes the dominant species at intermediate to high surface coverage, and a small amount of an amorphous silica phase present at high surface coverage. Spectra for these three surface species have previously been estimated from the data at pH 4 using MCR-ALS,3 and each measured ATR-IR spectrum at pH 4 could be described by a linear combination of these three spectra scaled by a factor proportional to the surface concentration of each species.3 The entire set of IR spectra at pH 4, 7, and 10 had the same features and have now been simultaneously analyzed by MCR-ALS22 to determine the contribution of each surface species to each measured spectrum at the same time that the estimates for the spectra of the individual surface species were improved. Data at wavenumbers lower than 850 cm-1 had comparatively high noise because of the strong librational IR absorption of water in this region and so were omitted from the initial statistical analysis. Prior exploratory analysis of the data 3398 DOI: 10.1021/la903160q
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Figure 3. (a) Monomer spectrum and initial estimate and MCRALS-optimized oligomer and polymer spectra. Typical MCR-ALS fitting of measured spectra for a surface with (b) predominantly monomeric species and (c) predominantly oligomeric species.
using PCA gave eigenvalues (Figure SI 2) that are similar to the values reported for just the pH 4 data3 that are consistent with the proposed model of two main surface species and a third minor species. MCR-ALS requires an initial estimate of the spectrum of each component. Because only monomer is present at low ΓSi, the pure monomer spectrum can be measured directly and was fixed to the average spectrum of the spectra measured at low ΓSi where spectral shape was independent of ΓSi. The initial estimate for the pure oligomer spectrum used the spectra optimized from the data at pH 4 with 0.044-0.91 mM H4SiO4.3 The initial estimate for the pure polymerized silica spectrum was set as the difference spectrum shown in Figure 2d. The initial estimates and MCR-ALS-optimized oligomer and polymer spectra together with the monomer spectrum are shown in Figure 3a. The shape of the optimized oligomer spectrum was little changed from the initial estimate. The bands in the optimized polymer spectrum are at lower wavenumber than the initial estimate, mainly because the method used to obtain the initial estimate artificially forced an absorbance difference value of 0 at ∼1000 cm-1. Typical calculated fits to measured spectra for a predominantly monomeric and a predominantly oligomeric surface composition are shown in Figure 3b,c. The optimized model fits the details of the measured ATR-IR spectra very closely, and the largest absorbance residual for any spectrum was on average 3% of the largest absorbance of that spectrum. 3.2.1. Change in Surface Chemistry over Time. The contributions that each surface species made to the area of the measured spectra as H4SiO4 adsorbs onto ferrihydrite over time are shown in Figures 4 and 5. In these Figures, the areas of the Si-O stretching region of the ATR-IR spectra were normalized to be equal to the numerical value of ΓSi on a nm-2 basis. The molar absorption coefficients of the Si-O stretches for the Langmuir 2010, 26(5), 3394–3401
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Figure 4. Surface speciation as a function of time for H4SiO4 adsorbing onto ferrihydrite from 0.01 M NaCl for systems with similar final surface compositions: (, monomer; Δ, oligomer; and O, polymerized silica.
Figure 5. Surface speciation as a function of time for H4SiO4 adsorbing onto ferrihydrite from 0.01 M NaCl for systems with ∼0.9 mM H4SiO4: (, monomer and O, polymerized silica (left-hand axes) and Δ, oligomer (right-hand axes).
monomer and oligomer species (per mole of Si) are considered to be approximately equal3 so that the areas given for each species will approximate the surface concentration of that species (per mol of Si), termed ΓM and ΓO, respectively. In Figure 4, data is shown for H4SiO4 concentrations that yield similar final surface species distributions at pH 4, 7, and 10, and Figure 5 shows the data at each pH with ∼0.9 mM H4SiO4. Data for other H4SiO4 concentrations are presented in Figures SI 3-SI 5. At each pH, there is a similar pattern with the monomeric species adsorbing initially, and then after some lag period, the oligomer species starts to form. The lag phase before oligomer formation is shorter at higher H4SiO4 concentrations. For example, at pH 7 the lag phase decreased from ∼10 to 0.1 h as [H4SiO4] increased from 0.015 to 0.93 mM. At each [H4SiO4], the growth of oligomer was delayed until a certain surface coverage of monomer was obtained corresponding to ΓM ≈ 0.4, 0.6, and 1.0 nm-2 at pH 4, 7, and 10, respectively. The ΓM values of 0.4 to 1.0 nm-2 required before oligomers are formed are up to 10 times lower than the calculated Langmuir 2010, 26(5), 3394–3401
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density of 2C adsorption sites4 (discussed in the following section), which indicates that oligomerization starts to occur on the surface at low coverage. 3.2.2. Surface Chemistry as a Function of ΓSi. Oxide surfaces can be conducive to oligomerization reactions between adsorbed species because adsorbed species become concentrated at the interface.1 The high concentration of H4SiO4 at the ferrihydrite surface is evident from the IR absorbance for the ferrihydrite-adsorbed monomer (up to 0.014), which is ca. 100 times higher than that of a solution with 1.66 mM H4SiO4 (10% below saturation with respect to SiO2(am)) measured under the same conditions (0.00014 after ATR correction).3 Because of the high concentration of adsorbed species that can occur at an iron oxide interface, condensation reactions forming small clusters and ultimately a surface precipitate at high Γ can be observed. For example, CrIII adsorption onto a ferrihydrite surface leads to a monomeric surface species at low ΓCr, but dimers and then higher-order polymers form as ΓCr increases.40 Small CrIII-containing clusters were formed on surfaces even when CrIII occupied only 10% of the available surface sites. The progression from monomeric adsorbates to high-order polymers is dependent on the structure and geometry of both the surface sites and the highorder polymers, and depending on that structural relationship, surfaces can either promote or inhibit polymerization between adsorbed species.1 The only appreciable H4SiO4 species in solution at pH 1000 cm-1 due to Si-O-Si stretching modes. Therefore, it is reasonable that the proposed progression from trimer to pentamer and heptamer at pH 7 and 10 would not greatly change the general shape of the IR spectrum but may change the band structure as revealed by the second derivatives. 3.2.3. Surface Chemistry as a Function of pH. The ΓM required before oligomerization occurred increased from 0.4 to 1 nm-2 as the pH increased from 4 to 10. At each pH, ΓM decreased when ΓO was above ∼2.3 nm-2 and the maximum value that ΓM reached was higher at higher pH (i.e., ΓM reached ∼0.6, 0.9, and 1.5 nm-2 at pH 4, 7, and 10, respectively). In general, it was observed that oligomers were favored at lower pH as has previously been observed.9,17 The distribution of surface species as a function of ΓSi is shown at pH 4-10 in Figure 7a, and the ferrihydrite film H4SiO4 adsorption isotherm is shown in Figure 7b. As the pH increases, the proportion of monomer at a given surface coverage increases. A similar effect of pH on the proportion of monomeric Si adsorbed onto goethite was predicted from modeling studies,20 although spectroscopic data for H4SiO4 adsorption on goethite is not yet available. Langmuir 2010, 26(5), 3394–3401
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species explains the observed lowering of an iron oxide’s PZC caused by silicate adsorption and also the fact that oligomer formation is favored at low pH.9,17
4. Conclusions
Figure 7. (a) Percentage of monomeric Si on the iron oxide surface as a function of the Si surface concentration. Solid symbols are at the end of each ATR-IR experiment; 0 are data at pH 4 over time from the start of each experiment. (b) H4SiO4 adsorption isotherm for the ATR-IR data. In both a and b, the solid-line rings indicate the experiments corresponding to Figure 4 and the dashed-line rings indicate the experiments corresponding to Figure 5.
The acidity of the silanol group (-SiOH) increases as the degree of silicate polymerization increases,2 as is demonstrated by the pKA of H4SiO4 (9.8) and the PZC of silica (∼2). The presence of silicates on an iron oxide surface can lower the PZC of iron oxide20 though the monomeric silicate on ferrihydrite appears to to be less acidic than H4SiO4 in solution and therefore would not contribute significantly to a lowering of the ferrihydrite PZC. The central Si in a trimeric Si species, formed by bridging two surface monomeric Si species, would have two corner oxygen ions shared with Si4þ cations, producing a more acidic species than the adsorbed monomer (which has two corner oxygen ions shared with Fe3þ cations). In addition, on the basis of the acidity of condensed solution species, the central Si in a trimer would be more acidic than solution-phase H4SiO4. The ATR-IR spectra of a ferrihydrite surface dominated by the oligomeric species were very similar at pH 4, 7, and 10. Therefore, either the oligomer species is deprotonated and negatively charged between pH 4 and 10 or protonation of the oligomer does not cause a detectable change in the IR spectrum. A negatively charged oligomeric
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The reactions of H4SiO4 at the ferrihydrite-aqueous interface as monitored by ATR-IR spectroscopy can provide insights into the relationship between surface structures and the nature of reactions between adsorbed monomers. Under all conditions studied, H4SiO4 initially formed a monomeric surface complex on the ferrihydrite surface. A model in which a 2C complex with two corners of the SiO4 tetrahedra shares oxygens with the corners of two adjacent edge-linked octahedra is consistent with the experimental data. Once a certain surface concentration of this monomeric adsorbed species was attained, oligomerization to form a specific type of condensed Si species occurred and this species dominates the surface at intermediate to high surface coverage. The recently proposed model for the ferrihydrite surface4 provides a rationalization for the observed IR Si-O stretching bands. The arrangement of 2C adsorption sites on the model ferrihydrite 021 surface means that adjacent adsorbed Si monomers are held in an orientation that would promote the formation of a condensed trimeric Si species by solution H4SiO4 bridging the adsorbed monomers. Thus, this model predicts how the ferrihydrite surface might act as a template, promoting oligomerization in one direction to form linear trimers and possibly pentamers and heptamers, depending on the number of available adjacent sites on a single surface. This model for the chemistry of H4SiO4 is expected to assist in the understanding of silicate binding to other metal oxide phases and to explain the effects that H4SiO4 interactions on metal oxides have on metal oxide properties. Acknowledgment. We thank Michele Nieuwoudt and Grant McIntosh (University of Auckland) for their help. P.J.S. thanks the NZFRST (grant UOAX0506) and Mighty River Power (New Zealand) for funding. Supporting Information Available: ATR-IR spectra of H4SiO4 adsorbing onto ferrihydrite. Eigenvalues for the data set of spectra. Surface composition as a function of time at pH 4, 7, and 10. This material is available free of charge via the Internet at http://pubs.acs.org.
DOI: 10.1021/la903160q
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