Identification of Fluoride and Phosphate Binding Sites at FeOOH

Article pubs.acs.org/JPCC

Identification of Fluoride and Phosphate Binding Sites at FeOOH Surfaces Xiangbin Ding, Xiaowei Song, and Jean-François Boily* Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden ABSTRACT: Iron oxyhydroxide minerals occur widely in nature and play important roles in environmental and industrial processes. Owing to their high reactivity, these minerals can act as sinks and/or transformation centers for a variety of inorganic and organic ions. Interfacial reactions are often mediated by surface (hydr)oxo groups. These groups can be singly, doubly, or triply coordinated with respect to underlying Fe atoms. In order to investigate the reactivity of these differently coordinated groups, Fourier transform infrared (FTIR) spectroscopy was used to examine adsorption products formed on iron oxyhydroxide surfaces. The absence of water was required to probe the O−H stretching region after initial reactions in aqueous media. This work was specifically focused on synthetic, submicrometer-sized lepidocrocite and goethite particles reacted with aqueous solutions of sodium fluoride and monosodium phosphate. Langmuir−Freundlich adsorption isotherms were calibrated on adsorption data in aqueous media at various pH values to obtain the maximum sorption densities for these ions under these conditions. FTIR measurements of the resulting solids dried under N2(g) show that fluoride and phosphate ions preferentially exchange with singly coordinated hydroxyls. Doubly coordinated groups can, however, be exchanged with fluoride ions at relatively high loading densities. Triply coordinated groups remain, in contrast, resilient to exchange. They may, however, stabilize phosphate species by hydrogen bonding. These findings add further constraints to our understanding of adsorption reactions and to the formulation of molecularly adequate thermodynamic models.

1. INTRODUCTION Iron, as the fourth most abundant element in Earth’s crust, plays essential roles in the planet’s biogeochemical cycles, ecosystem functioning, agriculture, and industry.1 It forms up to 16 different iron (hydro)oxide phases, many of which are widely distributed in the global system and participate in a variety of biogeochemical processes in the lithosphere, atmosphere, hydrosphere, and biosphere.2 Inorganic and organic ligand adsorption to iron oxide surfaces affects contaminant and nutrient transport in ecosystems, as well as mineral weathering. In industry, adsorption reactions impact water pollution control as well as anticorrosion processes. Iron oxyhydroxide (FeOOH) phases have, in particular, received much attention owing to their widespread abundance in nature and high surface reactivity. The goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) phases are of especial interest given their importance in soils and aquatic systems and the roles they play in a global biogeochemical cycle of iron. FeOOH surface reactivity under environmental conditions is generally determined by the ability of surface hydroxyl (OH) groups to interact or exchange with solutes and water. Surface OH groups can be differentiated on the basis of the number of coordinating lattice Fe atoms as singly (≡FeOH, −OH), doubly (≡Fe2OH, μ−OH), or triply coordinated (≡Fe3OH, μ3−OH)3,4 hydroxyls. These groups occur with different densities and distributions on different crystallographic faces of FeOOH minerals (Figure 1). In aqueous environments, they can be sources for Brönsted acidity and can interact or exchange with a variety of solutes, including (i) metal ions (e.g., Cd2+, Pb2+, Zn2+, Hg2+, and UO22+),5−7 (ii) organic ions8,9 (e.g., © 2012 American Chemical Society

Figure 1. Transmission electron microscopy (TEM) images (scale bars = 50 nm), particle morphology, and structures of dominant planes of LL, RL, and G (Fe = yellow, O = red, H = black, hydrogen bonds = dashed lines).

acetate,10 oxalate,11,12 EDTA,13 malonate,14 and citrate15), and (iii) inorganic ions (e.g., fluoride,16 sulfate,17 phosphate,18 arsenate,19,20 and selenite21,22). Their reactivity is, however, Received: August 23, 2012 Revised: September 25, 2012 Published: September 25, 2012 21939

dx.doi.org/10.1021/jp3083776 | J. Phys. Chem. C 2012, 116, 21939−21947

The Journal of Physical Chemistry C

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strongly controlled by the number of coordinating Fe atoms, by hydrogen-bonding patterns with neighboring OH groups,4,23 as well as by interfacial water layers. In this work, we identify which OH groups are responsible for the coordination of fluoride and phosphate species to FeOOH mineral surfaces. Both ions are of considerable interest because of their strong exchange abilities with surface OH groups and of their importance in environmental studies. Fluoride can, for instance, accumulate in plants and animals and may also form toxic inorganic and organic compounds such as sulfuryl fluoride and perfluoro-n-alkanes, both of which are also fluoride-bearing greenhouse gases.24,25 Phosphorus can be the main cause of eutrophication in aquatic systems,26−28 especially in those affected by the widespread use of phosphate fertilizers. It also acts as a growth-limiting nutrient in ecosystems.29,30 Our approach, as in our previous studies on the protonation of surface (hydr)oxo groups,4,23,31,32 was to monitor adsorption reactions on synthetic FeOOH mineral particles with distinct morphologies and thereby distinct OH population densities and distributions. In those studies, we resolved these populations using Fourier transform infrared (FTIR) spectroscopic measurements of dry particles of lath- (LL) and rodshaped (RL) lepidocrocite and of needle-shaped goethite (G) (Figure 1; Table 1). The characteristic O−H stretching

Figure 2. FTIR spectral components of dry G, LL, and RL from Song and Boily.23

on spectral profiles and (2) using modern and objective modelfree chemometric analyses40 in treating FTIR spectra. These developments are now calling for renewed interpretations of FTIR spectra of dry FeOOH surfaces reacted with fluoride and phosphate species. In this work, sorption capacities of LL, RL, and G particles for aqueous fluoride and phosphate ions are first reported and modeled with Langmuir−Freundlich isotherms. Adsorption maxima achieved through these experiments are then compared with site populations predicted by the particles’ specific morphologies. We thereafter turn to FTIR spectroscopy to identify which OH groups form complexes with phosphate and fluoride species. Some of these latter efforts are also assisted by ab initio calculations (Figure 3) to understand possible changes in hydrogen-bonding patterns along rows of −OH groups common to both lepidocrocite and goethite surfaces. Measurements of samples dried under inert conditions are required to eliminate strong spectral contributions for liquid water and to reveal O−H stretching vibrations of mineral surface OH groups. Although speciation changes may occur during drying, optimal experimental conditions (e.g., low surface loadings, pH) were chosen to minimize these effects. The results of this work provide evidence supporting the predominance of −OH groups in accounting for fluoride and phosphate adsorption reactions. Doubly coordinated groups will, however, only participate at high fluoride loadings. Although triply coordinated groups are resilient to ligand exchange, they can stabilize phosphate species already bound to −OH groups by hydrogen bonding.

Table 1. Synthetic FeOOH Particle Characteristics characteristics

LL

RL

G

length (nm)a width (nm)a BET (m2/g)b pore size (nm)c major crystal planed major crystal planed minor crystal planed −OH (sites/nm2)e,f μ-OH (sites/nm2)e μ3-OH (sites/nm2)e

95−210 25−35 81.6 5.6 71−83% (010) 23−13% (001) 3−5% (100) 1.28−2.20 7.24−7.73 0.84−1.43

180−250 8−12 52.9 5.4 49−65% (010) 48−33% (010) 2−3% (100) 2.02−3.01 6.64−7.22 1.77−2.63

80−100 8−10 55.6 5.7 97% (110)/(100) 3% (021) 3.13 3.13 5.93

a

Particle dimensions were estimated from TEM imaging (Figure 1). Determined from BET analysis. cDetermined from BJH analysis. d Crystal plane percentage were estimated assuming particle thickness in the range of 5−10 nm. eTotal surface site densities over all surface area. fEach η-OH2 site of the (100) plane of LL and RL are counted as two −OH groups in site-density calculation and modeling. b

vibration bands of these distinct surface OH groups, based on our recent findings, are shown in Table 2 and Figure 2. Those findings expanded upon several previous similar efforts12,33−39 made since the 1970s by, however, (1) recognizing the importance of proton coadsorption reaction Table 2. Summary of Band Assignments site −OH μ-OH μ3OH

wavenumber (cm−1)

mineral

crystal plane

site density (sites/nm2)

3661 3667 3648 3626 3648 (μ3,II-OH) 3490 (μ3,I-OH) 3532 3546, 3552

G LL(minor), RL G LL, RL G G LL, RL LL(minor), RL

(110) (001) (110) (010) (110) (110) (100) (001)

3.03 5.20 3.03 8.40 3.03 3.03 4.12 5.20

2. MATERIALS AND METHODS 2.1. Mineral Synthesis and Characterization. All solutions were made from doubly distilled deionized (DI) water (18.2 MΩ·cm), which was boiled and then purged from CO2 overnight with N2(g). LL was synthesized in water at 298 K by oxidation of a carbonate-free solution of 0.02 M FeCl2 in the presence of 0.2 M NaCl.33 This solution was first filtered (0.2 μm nitrocellulose) to remove any adventitious akaganéite, then adjusted to pH 6.0 with a CO2-free 1 M NaOH solution under an atmosphere of dry N2(g). Compressed air was then passed into the solution for 2.5 h to induce lepidocrocite 21940



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adsorption effects, especially in the dry samples used for FTIR (section 2.4). The total starting ionic strengths of these suspensions were controlled by the concentrations of the dissolved ligands, which were less than 10 mM for the fluoridebearing systems and 1 mM for the phosphate-bearing systems. Aliquots of all three FeOOH suspensions were diluted to 400 m2/L under an atmosphere of humidified N2(g). The 0.2 M NaF stock solution was diluted with DI to solutions of 0.02, 0.2, 2, and 20 mM. The 2 mM NaH2PO4 stock solution was diluted to solutions of 0.04, 0.1, 0.2, 1, and 2 mM. Each of these working solutions were then mixed with mineral suspensions in a 1:1 ratio. The resulting suspensions contained 200 m2/L of mineral and had half the total salt concentrations of the initial salt solutions. pH values were then adjusted to 3.0 and 7.5 for fluoride-bearing solutions and to 9.0 for phosphate-bearing solutions, with negligible volumes of 1.0 M HCl or NaOH. These suspensions were equilibrated on an end-to-end rotator for 65 h in a thermostatted room at 298 K. Preliminary experiments showed that this time was sufficient to achieve equilibrium with respect to ligand adsorption. The suspensions were thereafter centrifuged twice (4200 rpm, 10 min), and the supernatants were collected for determination of residual fluoride and phosphate concentrations. These values were used to determine the amount of ligand associated to the mineral surface. The centrifuged mineral pastes were analyzed by FTIR as described in section 2.4. 2.3. Fluoride and Phosphate Analysis. Aqueous concentrations of fluoride were determined at 293 K with an ion-selective electrode (ISE). A total ionic strength adjuster and buffer (TISAB) solution was used to obtain matrix-match solutions. The TISAB solution was made from 1 M NaCl (Merck, p.a.), 1 M acetic acid (AnalaR, p.a.), and 1 M Nacitrate (AnalaR, p.a.), and adjusted to pH 5.3 by the addition of 5 M NaOH (Merck, p.a.). Standard calibration solutions (pF of 6, 5, 4, 3, and 2) were made by mixing NaF (0.02 to 200 mM), TISAB, and 15% v/v acetic acid in the ratios 1:9:10. The samples were measured using the same procedure as in the calibration steps. Replicated experiments showed measurements to be within 5% error. Phosphate analysis was carried out with the phosphorus molybdenum blue spectrophotometric method46 using a sample/mixed reagent ratio of 10:1. The mixed reagent consisted of a 30 g/L ammonium molybdate (AnalaR, p.a.) solution with 17% v/v sulfuric acid, 54 g/L ascorbic acid (AnalaR, p.a.), and potassium antimony tartrate (Merck, p.a.). Samples were equilibrated for at least 5 min prior to UV−vis spectrophotometric analysis at 720 nm (Shimadzu UV-2100). Absorbance measurements were all completed well within the 2 h stability period of these solutions.46 The procedure was carried out using standard solutions of 0.005, 0.010, 0.015, 0.020, and 0.025 mM. The measurement error of this method is no more than 3%. 2.4. Attenuated Total Reflectance (ATR)−FTIR Analysis. The centrifuged wet pastes from the adsorption experiments were applied onto the ATR cell (Golden Gate, single-bounce diamond cell) of a FTIR spectrometer, then dried under dry N2(g). Spectra were collected every 0.5 h during this evaporation procedure until all O−H stretching and bending modes of free water disappeared. Spectra were collected with a Bruker Vertex 70/V FTIR spectrometer, equipped with a DLaTGS detector in a room kept at 298 K. Measurements were carried out in the 600−4500 cm−1 range at a resolution of 2.5 cm−1 and at a forward/reverse scanning rate

Figure 3. Al30O92H94 cluster used in ab initio calculation. Top shows octahedral representation with populations of (hydr)oxo groups on the (110) surface. Bottom shows map view of surface O only and Al atoms along with hydrogen-bonding patterns developed by −OH and adjacent μ3,I-OH groups.

precipitation. RL was also synthesized with this method except that the procedure was carried out at pH 7.0 and in the absence of dissolved NaCl.2 G was prepared at 298 K in polyethylene bottles by dropwise addition of a 2.5 M NaOH solution to a 0.15 M Fe(NO3)3 solution until pH 12.0.4 The precipitates were then aged at 358 K for 48 h. All suspensions were afterward washed and dialyzed with preboiled and N2(g) degassed deionized water for 2 weeks. Mineral particles were dried at 313 K for 7 days in a N2(g) filled oven. All dry synthetic minerals were characterized by Xray powder diffraction (XRD, Bruker d8 Advance working in θ−θ mode with Cu Kα radiation) and showed no traces of mineralogical (e.g., hematite) impurities. X-ray photoelectron spectroscopy (Kratos Axis Ultra electron spectrometer with monochromated Al Kα source at 150 W) showed no elemental impurities other than aliphatic carbon typically associated to in vacuo measurements. Transmission electron microscopy (TEM, JEM-1230 JEOL instrument) was used to investigate particle size and geometry. Specific surface area (Brunauer− Emmett−Teller41 [BET]) and porosity (Barret−Joyer−Halenda [BJH]) were determined from a 90-point adsorption/ desorption N2(g) isotherm (TriStar, Micrometrics). These latter measurements were carried out on samples predegassed in situ at 110 °C in N2(g) for 16 h. The isoelectric points/point of zero charge of LL, RL (7.7),42 and G (9.4)43−45 were measured by a Zen3600 Malvern Zetasizer instrument in aqueous suspensions of 3, 10, and 100 mM NaCl. 2.2. Fluoride and Phosphate Adsorption Isotherms. All adsorption experiments were carried out in the absence of a background electrolyte in order to minimize any competitive 21941



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molecular standpoint, LL surfaces are ideally dominated by μOH sites of the (010) plane. The −OH, μ-OH, and μ3-OH sites of the edge (001) plane are, in contrast, in greater abundance in RL. The minor terminal plane (100) of LL and RL (not shown) contains geminal η-OH2 sites consisting of two singly coordinated OH sites bound to the same surface Fe. G particles are predominantly terminated by the highly comparable (110) and (100) planes, as well as minor end plane (021). The (110) plane displays a mixture of −OH, μ-OH, μ3,IOH, μ3,II-OH, and μ3-O sites. In contrast to the sites of the (001) plane of RL, −OH sites and one-third of the μ3−OH sites (μ3,I-OH) form a network of hydrogen bonds (Figures 1 and 3). The (021) plane (not shown) displays, however, −OH and μ-OH groups that are strongly hydrogen-bonded to one another, as previously suggested by molecular dynamics.23 In the following sections, both adsorption and FTIR data will be discussed by taking these different surface configurations into consideration. 3.2. Fluoride and Phosphate Adsorption. Fluoride adsorption results were obtained from solutions equilibrated at pH 3.0 and 7.5. These conditions were chosen to highlight differences in surface OH group speciation in OH−/F− exchange, as discussed in previous work.16 Phosphate adsorption experiments were, however, limited to pH 9.0 in an attempt to (1) minimize the diversity of species formed on the surface and (2) to avoid ferric-phosphate precipitation. Note that phosphate adsorption occurs under conditions where LL and RL are slightly negatively charged (point of zero charge 7.742) and where G is slightly positively charged (point of zero charge 9.443−45). The adsorption data revealed important differences in the minerals’ ability to adsorb fluoride and phosphate, which mostly increased in the order LL < RL < G (Figure 4). This order was

of 10 Hz, resulting in 1000 coadded spectra for each sample. The Blackman−Harris 3-term apodization function was used to correct phase resolution. 2.5. Ab Initio Cluster Calculations. A diaspore (αAlOOH) cluster (Figure 3) oriented along the (110) plane (Pbmn notation of goethite) was used to emulate the corresponding plane of goethite for ab initio calculations. This Al-isomorph of goethite was chosen for the sake of computational efficiency in that it avoids the costly treatment of the d-orbital electrons of Fe in our relatively large clusters. These calculations were carried out to guide our interpretation of the FTIR spectra and particularly to see how F− and PO43− exchange to an −OH site would disrupt or promote the hydrogen-bonding environment in rows of −OH groups. Calculations of the (110) plane of the diaspore structure moreover enabled us to follow the impact of this exchange on the hydrogen bonds between −OH groups and adjacent μ3,IOH groups of goethite. As the spacing between two adjacent −OH groups in the (110) plane is 0.28 nm for AlOOH, in contrast to 0.30 nm for FeOOH, hydrogen bonding may be slightly favored in AlOOH. The results should thereby provide a qualitative overview of possible configurations adopted along rows of −OH groups. Calculations were carried out on neutrally charged Al30O92H94, Al30O91H93F, and Al30O91H95PO4 clusters, all consisting of three rows of double-chain Al octahedra each containing 10 Al atoms (Figure 3). Both sides of each cluster were protonated in accordance with the configuration of a neutrally charged goethite surface, with the following positions of −OH0.5−/μ−OH0/2μ3−OH0.5+/μ3−O0.5−. All octahedral Al atoms were hexacoordinated with respect to oxygens while all edge oxygens were coordinated to either one or two protons. Ab initio calculations were carried out using the ONIOM47 methodology. Three consecutive sites along the −OH rows and one μ3,I-OH (adjacent to the sorption center) were treated at the HF/6-31G level of theory, while all other ions were treated at the lower HF/3-21G level of theory. All Al and O atoms treated at the lower level of theory were frozen in Cartesian space to preserve the structure of the cluster, while the positions of remaining O, H, P, and F atoms were geometryoptimized. Wave functions obtained from single-point energy calculations of the resulting structures were used to map the electronic density of the clusters using the method of the electron localization function.48 These calculations showed that no fictitious basins of electrons had formed at the peripheries of the cluster. Other sets of calculations on clusters where edge O atoms had not been saturated with respect to H showed, however, considerable leakage of electrons to these areas and were therefore abandoned.

3. RESULTS AND DISCUSSION 3.1. Particle Properties. The particles (LL, RL, and G) considered in this work exhibit considerable differences in −OH, μ-OH, and μ3-OH densities and distributions (Table 1, Figure 1). These properties are listed in Table 1 and can be summarized as follows. Lepidocrocite particles are terminated by the (010), (001), and (100) planes. Assuming particle thickness in the 5−10 nm range, LL is dominated by the (010) plane that represents about 71−83% total surface area. On RL, the (010) plane represents 49−65% of the total area, while the remaining areas are predominantly represented by the edge (001) plane and, to a lesser extent (2−3%), by the terminal (100) plane. From a

Figure 4. Adsorption data for LL, RL, and G. Fluoride adsorption (a) and Phosphate adsorption (b). Lines are models generated from eq 1 and parameters of Table 3. 21942



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Table 3. Modeling Parameters of One-Site Langmuir−Freundlich Isotherm adsorption type LL

RL

G

a

−

F (pH 3.0) F− (pH 7.5) PO43− (pH 9.0) F− (pH 3.0) F− (pH 7.5) PO43− (pH 9.0) F− (pH 3.0) F− (pH 7.5) PO43− (pH 9.0)

Γmax (μmol/m2)

Γmax (site/nm2)

log β

m

r2a

6.62 1.23 0.57 7.82 1.85 0.83 9.74 3.54 1.38

3.99 0.74 0.35 4.71 1.11 0.50 5.87 2.04 0.83

0.32 0.5 2.25 0.88 −0.20 2.16 0.38 −0.70 1.32

0.78 0.92 0.91 1.00 1.00 1.00 1.00 1.00 1.00

0.99 0.99 0.99 0.96 0.97 0.95 0.99 0.99 0.66

Correlation coefficient of model fit to data.

Figure 5. FTIR spectra of dry LL (a,d), RL (b,e), and G (c,f) particles pre-equilibrated in fluoride-bearing aqueous solutions at pH 3.0 and 7.5, then dried in a N2(g) atmosphere. Loadings of adsorbed fluoride are reported in μmol/m2.

mostly directly correlated with the density of −OH sites of each mineral; these densities are 1.28−2.20 sites/nm2 for LL, 2.02− 3.01 sites/nm2 for RL, and 3.13 sites/nm2 for G (Table 1). The surface loadings of fluoride at pH 7.5 and phosphate at pH 9.0 were within the expected ranges of these densities. However, the adsorption of fluoride at pH 3.0 exceeded the total −OH densities (Table 3). This excess adsorption can be explained from the involvement of μ-OH groups under high loadings, as will be later shown with FTIR spectroscopy. The possibility for FeF3 precipitation can be dismissed given previous experimental results16 that showed it to occur only under greater ionic strengths. It is also noteworthy to mention that fluoride loadings achieved for G are comparable to those of Hiemstra and van Riemsdijk16 obtained with the same particle types. Loadings achieved by phosphate at pH 9.0 are also comparable to other studies on synthetic goethite.49−51 Adsorption curves were first fitted using a one-site Langmuir−Freundlich (L−F) isotherm with co-optimized site

density parameters (Γi,max), affinity constants (βi), and nonideality coefficients (mi): n

ΓM =

∑ i=1

Γi·max(βi Maq)mi 1 + (βi Maq)mi

(1)

where n = 1 for a one-site reaction involving site i = 1 only. For RL and G, the m values converged to unity, suggesting discretelike adsorption processes. Values of m for LL (0.78−0.92; see Table 3), however, pointed to a broader range of chemical affinities for both fluoride and phosphate. Other parameters obtained by assuming m = 1 for LL (not shown) remained nonetheless close to those obtained from the full L−F model. The adsorption results for LL (Figure 4) yielded Γmax values of 3.99 (pH 3.0, F−), 0.74 (pH 7.5, F−), and 0.35 (pH 9.0, PO43−) sites/nm2. As mentioned above, the Γmax value of 3.99 sites/nm2 exceeded the total density of singly coordinated sites. The Γmax values of RL were larger than those of LL, being 4.71 (pH 3.0, F−), 1.11 (pH 7.5, F−), and 0.50 (pH 9.0, PO43−) 21943



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Figure 6. FTIR spectra of dry LL (a), RL (b), and G (c) pre-equilibrated in phosphate-bearing aqueous solutions at pH 9.0, then dried in a N2(g) atmosphere. Loadings of adsorbed phosphate are reported in μmol/m2.

sites/nm2. This falls in line with the observation52,53 that phosphate adsorption is favored on particles containing less LL (010) plane-like terminations. It is also notable that, even though RL had about two times more −OH (2.07−3.08 sites/ nm2) groups than LL, its Γmax values were only 25−50% larger, pointing to electrostatic contributions to adsorption maxima. We also note that Torrent et al.54 found preferential phosphate adsorption on multidomainic goethite particles exhibiting predominant (110) and (100) planes rather than monodomainic with exhibiting more of the terminal (021) plane. Planes exhibiting rows of −OH groups are thereby highly important binding sites for these ions. Finally, the Γmax values for the one-site L−F model of G were the largest of these three minerals, being 5.87 (pH 3.0, F−), 2.04 (pH 7.5, F−), and 0.83 (pH 9.0, PO43−) sites/nm2, even though the crystallographic density of −OH groups (3.13 sites/nm2; see Table 1) was comparable to that of RL (2.02−3.01 sites/nm2). These larger adsorption values for G point to the involvement of other surface OH groups in these sorption reactions. F− adsorption curves obtained at pH 3.0 gave the largest Γmax values, and as stated previously, this likely arises from the involvement of μ-OH groups.16 In an effort to resolve this issue further, a two-site model (i = 2 in eq 1) was derived from this adsorption data set, and −OH and μ-OH were treated separately. Values for Γi,max were, in this case, fixed to crystallographic values, and m was set to 1 to reduce the number of adjustable parameters. In all cases (not shown), the models ascribe a 2−4 order of magnitude stronger affinity to −OH groups than to μ-OH groups, the latter groups only gaining importance at the greatest loadings. This concept will be now tested against the results of the FTIR spectra. 3.3. FTIR Spectra. Adsorption isotherms indicated important differences in the fluoride and phosphate surface loadings of the three particles considered in this study. In this section, FTIR spectra of dry solids containing these ligands are presented in order to identify the OH functional groups involved in the adsorption reactions. FTIR spectral components (Figure 2) and band assignments (Table 2) from our previous work23 provide all the information necessary to identify the OH groups affected by fluoride and phosphate adsorption (Figures 5 and 6). We note that these band assignments are for discrete-like OH groups of the (010) and (001) planes of LL and RL and of the (110)/(100) planes of G. Few bands can be specifically assigned to groups of particle terminations, namely, (100) of LL and RL and (021) of G, as the OH groups are strongly hydrogen bonded to one another.23 Therefore, although adsorption reactions can also proceed on these terminations, this article is focused on elucidating reactions involving sites displaying discrete O−H stretching

frequencies, namely, those of the dominant crystallographic planes of the minerals under study. The band corresponding to −OH groups on the (001) plane of LL and RL is present at 3667 cm−1, while μ-OH groups on the (010) plane are indicated by the 3626 cm−1 band. As previously described in Song and Boily,23 the −OH band is more readily consumed through protonation reactions than are the other bands. This discrete-like band typically subsided and red-shifted to a broader range of values upon exposure to anions, representing the formation of interconnected −OH2 groups. The μ-OH band also lost intensity upon exposure to F− (Figure 5), but this does not, in our view, arise from the formation of μ-OH2. Elevated acid concentrations largely exceeding the values considered in this work would be required for this to occur. Finally, the presence of μ3-OH on (001) and (100) planes is revealed through the bands at 3552/3546 and 3532 cm−1, respectively. For G, the sharp band at 3661 cm−1 is due to −OH groups. A shoulder of this sharp band (3648 cm−1) arises from isolated μ-OH and μ3,II-OH groups. The μ3,I-OH group, which donates a hydrogen bond to adjacent −OH (3661 cm−1) groups, produces a red-shifted band at 3490 cm−1 (Figure 1). This group represents one-third of the total density of μ3-OH groups (cf. Figure 3). As will be shown in the following sections, these surface O−H stretching bands undergo systematic changes with both fluoride and phosphate surface loadings. In an effort to focus the discussion on shifts arising from ligand adsorption, as opposed to those arising from the proton cosorption reactions revealed in our previous work,23 we also report spectra of ligand-free solids pre-equilibrated at pH 3.0, 7.5, and 9.0. 3.3.1. Fluoride Adsorption. Spectra of LL (Figure 5a,d) and RL (Figure 5b,e) samples after exposure to fluoride at pH 3.0 and 7.5 reveal a preferential uptake of −OH groups over μ-OH. As expected from the isotherm data, the effect of fluoride adsorption on band intensities were greater at pH 3.0 than at pH 7.5. For example, the intensity of the 3667 cm−1 band of −OH is lower at pH 3.0 than at pH 7.5. This band becomes also red-shifted to about 3654 cm−1. To explain this shift, we point to our earlier work23 showing that −OH groups of the (001) plane are present as rows of hydrogen-bonded sites and that 25% of these sites donate or accept hydrogen bonds as [−OH···−OH −OH −OH]. A comparable pattern was also retrieved in the ab initio cluster generated for this study (Figure 3). Substitution of a H-bond-accepting −OH group by the more electronegative F−, [−OH···−F −OH −OH] as depicted in Figure 7a, and the subsequent formation of a stronger hydrogen bonds with adjacent −OH groups are responsible for this shift. 21944



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coadsorption reactions, which weaken the μ3,I-OH···O− hydrogen bond and result in a shift to 3568 cm−1. Bands corresponding to μ-OH (3626 cm−1) and μ3-OH (3532 cm−1) of LL (Figure 6a) and RL (Figure 6b) were, in contrast, unaffected by the presence of phosphate. Bands of the corresponding groups of G (Figure 6c) are, however, not as clearly resolved as they lie in the 3661−3648 cm−1 region. As high phosphate loadings appear to reveal residual bands likely arising from these groups, we conclude that these groups are not involved in phosphate adsorption. Overall, these observations are in line with the Γmax values at pH 9.0 (Table 3) suggesting that −OH sites are predominantly responsible for phosphate adsorption. The P−O stretching regions of the three minerals displaying bands are centered at 1175, 1110, 1048, and 1000 cm−1 (Figure 8). Sample prepared at high phosphate loadings did give rise to

Figure 7. Rows of μ3,II-OH (gray), −OH (red), and μ3,I-OH (blue) groups along the (110) plane of the diaspore structure. Optimized structures of F− (a) and PO43− (b) bound to this plane were obtained from ab initio calculations of the cluster type shown in Figure 3.

The effect of fluoride sorption on μ-OH groups of LL and RL is seen in the reduction in intensity of the 3626 cm−1 band. However, μ3 -OH groups (3532 cm −1 ) were generally unaffected by exposure to fluoride. Consistent with these findings, our estimated Γmax value of 3.99 sites/nm2 for LL (Table 3) is larger than the expected density of −OH groups (1.28−2.20 sites/nm2, Table 1). The same can be said for RL, while maximal loadings at pH 7.5 are lower than the crystallographic site densities. This evidence is consistent with the idea that μ-OH groups on the (010) plane are responsible for fluoride uptake at high loadings. These findings thereby agree with the idea that fluoride will first effectively bind with reactive (high affinity) −OH groups, then substitute with the more recalcitrant (low affinity) μ-OH groups at higher fluoride loadings. Fluoride adsorption also consumes −OH sites of G (Figure 5c,f), which is in this case manifested in the 3661 cm−1 band. Note that the strong hydrogen bond between −OH and μ3,IOH is responsible for the 6 cm−1 lower stretching frequency of −OH in G compared to RL and to the low 3490 cm−1 frequency of μ3,I-OH (Figure 2). A direct consequence to −OH/F exchange is manifested in the 3490 cm−1 band, which underwent a concomitant attenuation and blueshift to 3576/ 3568 cm−1, as can particularly be seen at pH 3.0 (Figure 5f). These changes can again be understood by the hydrogen bonding patterns formed along −OH groups on the (110) plane, [−OH···−OH−OH···−OH], where every other group donates H-bonds. Substitution of a H-bond donating group would reduce the intensity of the 3661 cm−1 band. Substitution of an accepting group would, however, induce the redshift seen for this band. In addition to this, the H-bonding strength from adjacent μ3-OH species would be increased, thereby producing a redshift of the 3490 cm−1 band. This can be seen through a slight shift and band broadening at high fluoride loadings. This does not, however, explain the more important blueshift to 3576/3568 cm−1. This shift is rather a consequence of coprotonation reactions16 involving −OH groups, which occur in response to fluoride adsorption. Finally, no clear changes could be identified for the minor bands of the isolate μOH group (3648 cm−1), as its vibrational intensities are already small. As our isotherm-derived Γmax value of 5.87 sites/nm2 (pH 3.0) is well within the available density of −OH and μ-OH groups on G surfaces (6.26 sites/nm2), both −OH groups are the dominant reactive centers for F− on G. 3.3.2. Phosphate Adsorption. Phosphate adsorption, as in the case of fluoride, attenuates the 3667 cm−1 band of LL/RL and the 3661 and 3490 cm−1 bands of G (Figure 6). Attenuation of the latter band with phosphate loading is, as in the case for fluoride (Figure 5), ascribed to proton

Figure 8. P−O stretching region of phosphate bound to goethite. Samples were first equilibrated in aqueous solutions at pH 9.0, then dried in a N2(g) atmosphere. Loadings of adsorbed phosphate are reported in μmol/m2.

precipitated phosphate phases, as evidenced by the broad bands (not shown) at 1200−950 cm−1 and centered at 1050 cm−1, akin to those reported in Bengtsson et al.55 Those were consequently discarded from the current work in order to focus on monomeric forms of mineral-bound phosphate. The narrow 1175, 1110, 1048, and 1000 cm−1 bands, in turn obtained from the lower phosphate loadings, are highly comparable to those obtained by Persson et al.56 in wet samples equilibrated at pH 3.1. As our samples were initially equilibrated at pH 9.0, but thereafter dried in N2(g), our surface species could have undergone protonation reactions, hence the similarity with the wet samples equilibrated at pH 3.1.56 Interestingly, our frequencies are also highly comparable with frequency calculations of Kwon and Kubicki,57 which, in turn, suggest singly and/or doubly protonated monodentate phosphate species. These frequencies would also fall in line with doubly protonated bidendate species proposed by the same authors,57 an observation that could be consistent with previous FTIR studies of goethite-bound phosphate species in the dry state35−37 as well as in the wet state.58 We, however, stress that elucidation of coordination geometries from the P−O stretching region alone can, as has been done in these previous efforts,35−37,58 be prone to numerous pitfalls and have led to disagreements in the literature. Band shifts can notably be affected by strong hydrogen bonding with interfacial water molecules and surface hydroxo groups, as well as local electric fields.59 Recent efforts to resolve these issues include a study involving arsenate adsorption on goethite.20 This study, which combines FTIR and X-ray spectroscopic methods, in fact, 21945



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4. CONCLUSIONS This study demonstrates the selectivity of fluoride and phosphate ions for exchange with −OH groups of G, LL, and RL over any other surface group. Although such results have been largely assumed in the development of thermodynamic adsorption models, they have generally lacked direct experimental proof. Our work, therefore, supports the general idea that (i) −OH groups are the primary adsorption centers for ligands, (ii) μ-OH groups can be exchanged at substantially higher loadings, and (iii) triply coordinated groups are largely resilient to any ligand-exchange reaction. These experimentally confirmed findings hence solidify the basis of new thermodynamic adsorption models for such systems. They should, moreover, be considered when including hydrogen-bonding patterns in such models. Formulation of a full surface complexation model45 to include proton silent groups is thereby encouraged to account for such surface species. This can, for instance, be carried out by invoking multidentate-type species (e.g., [−OPO3(μ3,II−OH)2]1.5−) accounting for direct −OH phosphate oxo group exchange and hydrogen bonding to adjacent μ3,II-OH species or by the coexistence of compounds bound to two distinct sites (e.g., high affinity −OH and low affinity μ-OH). In summary, consideration of these findings is recommended for improving the molecular relevance of such models altogether.

largely favors monodentate complexes irrespective of pH. A recent nuclear magnetic resonance (NMR) study60 supports, in contrast, bidentate coordination in dry samples. The conclusions of that NMR60 work were, however, obtained by calibrating chemical shifts with a phosphonate-bearing model compound, which, in our view, is an inappropriate proxy for the P−O bonds of phosphate. Most importantly, as maximal surface loadings achieved in that study60 are to the level of crystallographically available −OH densities, the complexes must rather either truly be in monodentate coordination, or involve exchange with μ-OH and/or μ3-OH groups. As we do not find any evidence for interactions with these two latter surface sites, our findings favor monodentate coordination as the predominant phosphate species on dry goethite. This being said, bidentate coordination may possibly coexist as mononuclear (Fe) binding sites on the minor (021) plane of goethite and (100) plane of lepidocrocite, as well as on surface defects. These species, if present, are, however, not sufficiently abundant to be clearly manifested in the data on hand. The impact of such complexes on the hydrogen bonding environment of mineral surfaces monodentate complexes was followed by ab initio modeling of a diapore slab containing one phosphate ion (Figures 3 and 7b). Substitution of one −OH group for a phosphate oxo group breaks local hydrogen bonds from [−OH···−OH −OH···−OH] to [−OH···−OPO3 −OH···−OH] (Figure 7b). This, we believe, not only consumes the dominant 3667 (RL, LL) and 3661 (G) cm−1 bands (Figure 6) but also leaves isolate −OH and islands of H-bonded −OH···−OH groups in proportions that can be correlated to phosphate loadings. These changes are in fact manifested in a number of residual blue- and red-shifted bands in the 3650−3680 cm−1 region. Calculations also show that hydrogen bonds are formed between two phosphate oxo and two neighboring μ3,II-OH groups, as illustrated in Figure 7b. Conformations of this type were notably inferred in an early paper by Russell et al.37 Our calculations also show that this complex is tilted to an ≡Al−O−P angle of 126°. This value is highly comparable to the 130° value found in a structurally analogous cobalt-arsenate solid,20 which was in turn used as a proxy for studying arsenate complexes on goethite. We repeated these calculations with arsenate, instead of phosphate, and obtained a highly comparable result (124°). In another set of calculations, we modeled a −OPO3H complex and obtained the same configuration as in −OPO3, with the exception that the hydroxo group forms a hydrogen bond with aneighboring −OH group (Figure 7b). We thereby caution that shifts in the 3660−3680 cm−1 region could be caused by such interactions as well. Finally, it is important to note that this configuration was also achieved by geometry optimization of the slab but with starting with a −OH2 group adjacent to a −OPO3 species. The energetically favorable proton transfer reaction, leading to hydrogen-bonded −OH and −OPO3H species, instead of H2O desorption from the slab from −OH2, adds further supports to the concept for protonated phosphate as a stable surface species in dry environments. In summary, the spectra do not reveal any evidence for direct substitution of either μ3,II-OH or μ-OH groups for phosphate groups. −OH groups are thereby the sole reaction centers for phosphate adsorption on LL, RL, and G surfaces. Participation of μ3,II-OH in the stabilization of phosphate oxo groups in G may, however, be invoked in predicting the formation of these species.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +46 (0)90 786 5270. E-mail: jean-francois.boily@chem. umu.se. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Per Persson for insightful discussions. This work was supported by the Swedish Research Council (Vetenskapsrådet, #2009-3110), the Carl Tryggers Foundation, and the Kempe Foundation.

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REFERENCES

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Identification of Fluoride and Phosphate Binding Sites at FeOOH

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