Variable Hydrogen Bond Strength in Akaganéite - The Journal of

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ite Variable Hydrogen Bond Strength in Akagane Xiaowei Song and Jean-Franc-ois Boily*
Department of Chemistry, Umea
University, SE-901 87 Umea, Sweden

bS Supporting Information ABSTRACT: Akagan
eite (β-FeOOH) is a chloride-bearing iron oxy-hydroxide with a hollandite-type structure. This high specific surface area mineral has been the object of numerous studies given its high reactivity and involvement in natural and industrial processes. The important ion exchange attributes of this mineral involve ∼0.4  0.4 nm wide channels in which chloride ions are stabilized by hydrogen bonding from bulk OH groups. This work provides further details on the relationship between bulk chloride ion loadings and hydrogen bond strengths. Molecular dynamics calculations were first carried out on chloride-free and bearing lattices to build a conceptual model for possible interactions in the akagan
eite bulk. Experimental work was thereafter carried out on synthetic acicular particles (7  80 to 11  110 nm) reacted to aqueous solutions of HCl, then dried under dry N2(g). These samples were studied by Fourier transform infrared spectroscopy, temperatureprogrammed desorption, X-ray photoelectron spectroscopy, and X-ray powder diffraction as well as transmission electron spectroscopy. Results collectively show that Cl/Fe molar ratios increasing from 0.169 up to 0.442 induce important changes in the hydrogen bonding environment of bulk hydroxyls. This can specifically be seen through shifts in bulk OH stretching frequencies from 3496/3395 to 3470/3350 cm1. These changes are associated with a substantial shortening of particle lengths (97 to 45 nm), expansion of crystallographic lattice size (up to 0.9%), and increases in median thermal dehydroxylation temperatures (260 to 305 °C). Our work thereby highlights important variations in physicochemical attributes of akagan
eite particles reacted with HCl. Such variations should consequently be considered in settings involving submicrometer-sized akagan
eite particles.

1. INTRODUCTION Akagan
eite (β-FeOOH) is an iron oxyhydroxide mineral of importance to various environmental and industrial settings.114 Originally named after the Japanese Akagan
e mine where it was found as a weathering product of pyrrhotite (FeS),15 it is a common corrosion product of steels exposed to chloride rich media. Reports for its occurrences in oceans, groundwater, saline lakes, hot springs, and volcanoes have moreover become more numerous over the years.1517 This mineral notably occurs in various iron, nickel, copper, lead, zinc, and pyrite mines worldwide,18 hydrothermal fields in Atlantic Ocean,19 as well as acidic soils.2022 In the industry, other than being an undesirable corrosion product, it has found varied uses such as in pigments and as an efficient and a low cost contaminant sorbent.26,14 This mineral is furthermore under scrutiny for its potential uses as an environmentally friendly positive electrode in lithiumiron cells23,24 as well as in the fabrication of magnetic nanocapsules engineered for pharmaceutical drug delivery.25 The varied occurrences and uses of akagan
eite have thereby called for an understanding of its structure and reactivity. This is moreover motivated by the need in consolidating our understanding the various structures and reactivities of iron oxyhydroxide minerals. Unlike other FeOOH polymorphs (goethite: α-FeOOH; lepidocrocite: γ-FeOOH), the akagan
eite structure contains Cl ions (Figure 1),15 hence its favored occurrences in chloridebearing media. This mineral can, however, exchange with other r 2011 American Chemical Society

monovalent ions, including F , Br , and OH , through 0.4  0.4 nm channels typically running along the length of needle-like particles (Figure 1).2628 Exchange and adsorption equilibrium and kinetics of a variety of ions have thereby been studied over the years.26,14 Repercussions of these interactions to the uses of akagan
eite in redox as well as acid/base catalytic reactions were notable focal points of such studies.7,29,30 Of these, important changes in particle size and crystallinity31 were highlighted, for example, upon reaction with oxyanions.24 However, an understanding of the phenomenological contributions to these results is still lacking. As such, a fundamental understanding of akagan
eite’s bulk channel structure and anion occupancy is central to follow the reactivity of this mineral in a variety of processes. Our current understanding of the akagan
eite structure and composition lies on a combination of experimental15,23,24,3234 and theoretical35,36 studies. The hollandite structure of this mineral, determined by X-ray diffraction studies,32 consists of double-chained edge-sharing iron octahedra forming 2  2 channels, with atoms organized in body-centered cubic packing.15 Chloride ions lie at the center of channels and accept hydrogen bonds from neighboring hydroxyls 15 (Figure 1). Received: October 10, 2011 Revised: December 19, 2011 Published: December 20, 2011 2303

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Figure 1. TEM (a) and idealized particle morphology (b) of synthetic akagan
eite. Nine unit cells showing spatial arrangement of double chains of iron octahedra, here omitting oxygens and protons, forming chloridebearing 0.4 nm wide channels are shown in (c). MD-optimized molecular structure near one channel showing hydrogen bonding patterns is shown in (d).

X-ray and neutron powder diffraction32,33 studies indicate that chloride sites are surrounded by eight hydroxyls (O1H and O3H) and eight oxygens (O2 and O4) (Figure 1). Because the addition of Cl to β-FeOOH also requires charge balancing by cosorbed protons, a second type of hydroxyl24 or free proton/hydronium23 must be present in the tunnel. Kim and Grey34 recently discussed unpublished calculations by Zhu and Kubicki,35 suggesting protonation to occur on corner-shared bare O atoms (O2 and O4), herein referred as O5. Because typical akagan
eite samples never reach full Cl occupancies but rather about 2/3 occupancy a portion of O1H and O3H groups should not form any hydrogen bonds. Akagan
eite samples of varied chloride occupancy should consequently exhibit variable hydroxyl contents and variable hydrogen bond strengths. Recalling that hydrogen bonding plays a central role on mineral lattice energies and a host of other thermodynamic properties, such as thermal stability,15 effects of variations in chloride occupancy must be resolved. This becomes especially important in monitoring reactivity and mineralogical variations in environmental and industrial systems where akagan
eite is important. It is, moreover, fundamental for building a systematic understanding of properties and inter-relationship of FeOOH minerals altogether. Given that OH stretching and bending vibrations are sensitive to changes in hydrogen bonding strength, vibration spectroscopy can help provide insightful details into these issues. In this study, we probe bulk OH groups of synthetic akagan
eite particles using Fourier transform infrared (FTIR) spectroscopy. This work builds upon a previous study where the same strategy was used to identify OH groups on the surfaces of these particles.36 In that study, variations in HCl loadings produced systematic shifts in vibrational modes of surface OH groups due to protonation and ligand exchange reactions. In the course of those measurements, we also noted substantial changes involving bulk OH groups, in stark contrast with other FeOOH polymorphs (α, γ)37 whose bulk attributes are impervious to changes in solution composition. This study was consequently devised to identify molecular controls affecting changes in bulk OH bond strengths. FTIR measurements of particles exposed to variations

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in acidity, chloride loading, as well as thermal gradients are used to address this issue. Some of these conditions pertain to environmental systems, whereas others pertain to more industrial settings. Taken as a whole the collective evidence from these ranges of conditions allows us to extract useful information. Chemometric methods are notably used to separate uncorrelated spectral intensities into distinct spectral components, akin to molar absorption coefficients. This process moreover facilitated band assignment procedures, as carried out in our previous study on surface hydroxyls of the akagan
eite surface.36 X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), and transmission electron microscopy (TEM) were, respectively used to monitor channel Cl loadings, crystal morphology, as well as particle size and morphology. Molecular dynamics (MD) simulations were also carried out to develop conceptual models for our experimental efforts. The combined set of experimental and theoretical results are used to explain variations in OH hydrogen-bonding patterns in akagan
eite. Implications for these attributes on the thermodynamic stability of this mineral are illustrated in the context of thermal decomposition reactions.

2. MATERIALS AND METHODS 2.1. Materials. Akagan
eite synthesis was initiated at 298 K in aqueous solution by neutralization of a 100 mL 1 M FeCl3 solution with 75 mL of a CO2-free 1 M NaOH solution ([OH]/[Fe] = 0.75).38 The suspension was continuously stirred with a propeller and kept under an atmosphere of N2 (g) (99.996% purity). After 50 h preaging, 20 mL of a 10 M NaOH solution was added dropwise under the same conditions. The resulting suspension was sealed in a polyethylene bottle and transferred to an oven at 343 K for 8 days. Akagan
eite particles were then separated from the mother suspension by centrifugation at 5000 rpm for 15 min and dialyzed for 3 weeks at 298 K. In an effort to minimize contact with atmospheric CO2, all deionized water used for dialysis (18.2 MΩ 3 cm) was degassed first by boiling, then purged with N2 (g) overnight. Dialysis water was replaced repeatedly until the resistivity of the supernatant exceeded 200 kΩ cm for over 7 consecutive days. The residual chloride concentration of the dialyzed akagan
eite suspension, inferred by a Cl ion selective electrode was 2.65 mM, namely, 0.06 mmol/g for our original 44.29 g/L suspension. This concentration corresponds to an equilibrium value arising from the exchange of dissolved chloride with akagan
eite, and is, in our experience, essential to avoid phase transformation to other chloride-free FeOOH phases such as goethite.38 Dialysis of α- and γ-FeOOH leads to, on the other hand, conductivities close to those of deionized water. The resulting akagan
eite suspension was stored in a N2 (g)-filled desiccator. An aliquot was dried at 343 K for 7 days then ground to a fine powder in an agate mortar for dry state analyzes. Specific surface area was determined from a 90 point N2(g) adsorption/desorption isotherm (TriStar, Micrometrics). These measurements were carried out on samples previously dried in situ at 110 °C for 16 h under a stream of dry N2(g). 2.2. FTIR Spectroscopy. Variations of akagan
eite chloride content were induced by reactions with aqueous solutions of HCl (Tables S1 and S2 of the Supporting Information) and NaCl. These reactions were carried out at 298 K under an atmosphere of N2(g) by the addition of standardized HCl, NaOH, or NaCl to aqueous suspensions of akagan
eite (44.29 g/L, initial pH 5.0). 2304

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The Journal of Physical Chemistry C The suspensions were then equilibrated in sealed polyethylene test tubes for 24 h and thereafter purged with N2 (g) for an additional 0.5 h prior centrifugation. The centrifuged wet pastes were transferred to an attenuated total reflectance (ATR) cell (Golden Gate, single-bounce diamond cell) and dried to a thin film in the N2 (g)-filled analysis chamber of the spectrometer. Spectra were collected every 0.5 h during this evaporation procedure to monitor and ensure full removal of free water molecules. FTIR spectra were collected with a Bruker Vertex 70/V FTIR spectrometer, equipped with a DLaTGS detector in a room kept at 298 K. Spectra were taken in the 6004500 cm1 range at a resolution of 2.5 cm1 and at a forward/reverse scanning rate of 10 Hz. These resulted from 1000 coadded spectra for each sample. Blackman-Harris three-term apodization function was used to correct for phase resolution. 2.3. Temperature Programmed Desorption (TPD). The TPD technique was used to trace bulk transformation of akagan
eite particles with controlled thermal gradients. Akagan
eite particles were first reacted in solutions of 0.00, 0.08, 0.22, 0.41, and 2.94 mmol/g HCl for 24 h, dried at 343 K for 7 days, then ground to a fine powder with an agate mortar. Powdered samples (∼4 mg) were pressed onto a fine-tungsten mesh (Unique wire weaving, 0.002” mesh diameter) squeezed into a copper-heating shaft and in direct contact with a K-type thermocouple. They were then heated at a rate of 10 K/min from 318 to 673 K under an operating pressure below 2.5 mTorr, namely, the detection limit of the pressure sensor (MKS, Baratron). These experiments were carried out in a reaction chamber equipped with KBr IR windows and connected to a turbo vacuum pumping station (Pfeiffer Vacuum, HiCube 80 Eco). A mass spectrometer (Preiffer Vacuum, PrismaPlus) was connected to the reaction cell to analyze effluent gases. A background FTIR spectrum was collected in the absence of akagan
eite samples under identical conditions. 2.4. X-ray Photoelectron Spectroscopy (XPS). Ground powders (0.00, 0.08, 0.22, 0.41, and 2.94 mmol/g HCl) were also analyzed by XPS. Survey spectra were collected from 1100 to 0 eV at pass energy of 160 eV. High-resolution spectra for Fe 2p, O 1s, C 1s, and Cl 2p were collected at pass energy of 20 eV. XPS spectra were recorded with Kratos Axis Ultra electron spectrometer equipped with a monochromatic Al Kα X-ray source and a delay line detector. The X-ray beam operated at 150 W provided an analysis area of 0.3  0.7 mm2 for the measurements. Binding energy scale was referenced to the C 1s line of aliphatic carbon, set at 285.00 eV. Processing of the spectra was accomplished with the Kratos software. 2.5. X-ray Powder Diffraction Analysis. X-ray diffraction (XRD) of ground powders (0.00, 0.04, 0.22, and 2.94 mmol/g HCl) were collected with a Bruker AXS d8 Advance diffractometer equipped with a VÅNTEC-1 detector. The instrument worked in θθ mode and with Cu Kα radiation (1.5406 Å). Diffracplus XRD39 was used for setting up the experimental parameters. Data were collected at 0.008° per step with 16 repeats, first refined using then Diffracplus EVA 10.0 program,40 then noise-reduced by singular value decomposition (Section 2.5). Diffraction patterns were modeled in the 1560° (2θ) range using the program CrystalDiffract 5.2. This was carried out by optimizing unit cell size as well as chloride occupancy. 2.6. Transmission Electron Microscopy (TEM). Akagan
eite samples were suspended in a water equilibrium with 0.00, 0.08, and 2.94 mmol/g HCl for 24 h prior TEM imaging. TEM analysis was performed on a Jeol JEM 1230 microscope equipped with a

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digital multiscan camera (Gatan MSC 600CW). Particle size analysis from TEM images was carried out to measure lengths and widths of 3075 particles for each sample. Size distributions were described by Gaussian functions. 2.7. Chemometrics Analyses. FTIR spectra were analyzed by chemometric methods41 under the computational environment of Matlab 7.0 (The Mathworks, Inc.). Reported absorbances are arbitrary units as they were offset to zero absorbance value at 4500 cm1, where absorbances are negligible. Spectral data were baseline-corrected and normalized for area in both bulk OH stretching and bending regions. Single value decomposition (SVD)42 was used to reconstruct noise/error-reduced spectra, in which the number of chemically relevant (eigen)vectors was estimated with the factor indicator function (IND).43 A description of these methods in the context of FTIR studies of FeOOH minerals can be found in Boily and Felmy.44 A multivariate curve resolution (MCR) analysis of these results was then carried out with the MCR-ALS program.45 This analysis was used to obtain linearly independent spectral components (ε) as well as relative fractions (C) by rotating the dominant vectors into real chemical spaces. Finally, MCR components were fitted with Gaussian distributions using the program fityk.46 2.8. Molecular Dynamics. A 4  14  4 akagan
eite supercell containing 1792 FeOOH units was generated using the crystal structure of Post et al.32 Chloride ions were emplaced in the channels to achieve an initial Fe/Cl ratio of 8:1. This was effectively carried out by sequentially concatenating 4  1  4 supercells with two distinct distributions of chloride ions shown in Figure S1 of the Supporting Information. Charge compensation was achieved by protonating O2/O4 sites in the pattern shown in the same Figure. Protonation of these groups was deemed more likely in recent density functional theory
et al.,33 than the initially calculations,35 as well as by Stahl proposed (O1)H 3 3 3 H 3 3 3 Cl linkages of Post et al.32 As the simulations were carried out using the fractional atomic charges of the CLAYFF47 force field, two O2 or O4 were protonated for each chloride ion, instead of one. In this procedure, we substituted two O2/O4 (1.05  2 e/atom) for two O2H/O4H (0.575  2 e/atom) for each added Cl (1.00 e/atom). The resulting charge imbalance of 11.2 e for 224 chlorides original insertions was thereafter counterbalanced by resubstitution of 48 equally spaced hydroxyls for O2/O4 and removal of 14 equally spaced chloride ions. This procedure ensured a neutrally charged Fe1792O1392(OH)2192Cl210 supercell with original crystallographically determined dimensions of 4.235 nm  4.244 nm  4.206 nm. Both Fe1792O1792(OH)1792 and Fe1792O1392(OH)2192Cl210 cells were simulated by classical MD with the CLAYFF47 force field. All parameter values and equations are reported in Table S3 of the Supporting Information (SI). The parameter for Fe3+ was, however, taken from Kerisit48 because it provides more accurate predictions of bulk and surface FeOOH structures than the original CLAYFF parameter. The OH force constant was moreover adjusted to match the experimental OH stretching frequency of 3480 cm1. The cells were repeated infinitely in all three dimensions by periodic boundary condition. An NPT (constant number of particles, constant pressure, and constant temperature) ensemble and a time step of 0.5 fs were used to integrate the equations of motions with the Verlet algorithm.49 The temperature of the whole system (300 K) was coupled to the Nos
eHoover velocity-rescale thermostat at a 0.1 ps relaxation time. Pressure (750.06 Torr) was coupled using an anisotropic 2305

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scaling of the simulation box using the ParrinelloRahman method.50,51 The LINCS algorithm52 was used to treat OH bonds of all hydroxyls. A 0.8 nm cutoff was used for van der Waals interactions, and the particle mesh Ewald method53,54 was used to treat long-range electrostatics. All calculations were carried out with Gromacs (v.4.5.3).55 Simulation cells were first energyminimized (double precision) using a steepest descent algorithm in typically less than 104 steps and equilibrated (single precision) by classical MD for 107 steps at 0.5 fs interval for a total of 5 ns. Additional 5 ns simulations were then generated for production runs to monitor atomic positions, energies, lattice sizes, and to calculate radial distributions functions. All analyses were carried out with Gromacs (v.4.5.3).46 Power spectral densities were generated by fast Fourier transform of the hydrogen velocity autocorrelation function Sx ðf Þ ¼

Z þ∞ ∞

Rx ðτÞe2πif τ

ð1Þ

This problem was solved in the computational environment of Matlab 7.0 by applying a discrete Fourier transform to Rx(τ). Values of Rx(τ) were obtained from separate simulation at a 0.5 as (attosecond) time step starting from the structure equilibrated after 5 ns. This was carried out by turning off the LINCS constraint to all OH bonds and running the simulation for 4  107 steps. The latter 2  10 7 steps were used for the production run.

3. RESULTS AND DISCUSSION 3.1. Sample Characterization. The solid product obtained after dialysis consists entirely of akagan
eite. This was confirmed by X-ray powder diffraction (Figure 2; Figure S2a of the Supporting Information), showing all main reflections of this FeOOH phase (2θ at 17.16, 27.08, 35.53, 39.63, 46.8, 56.25).32,33 FTIR spectra support these findings through characteristic OH stretches (3480, 3390 cm1) and bends (800, 670, 620 cm1) (Figure S2b of the Supporting Information).56,57 No other vibrational modes of any other FeOOH phases were present. XPS retrieved the Fe-normalized FeO0.819(OH)1.218Cl0.169 composition, close to the experimental composition of FeO0.833(OH)1.167Cl0.167
reported by Stahl et al.33 XPS spectra also confirm the absence of trace contaminants, other than the conventional carbon formed in vacuo. Indeed, FTIR measurements of dry particles in N2(g) cannot detect any resolvable CH modes. Particles are acicular in shape and monodisperse in size, with dimensions of 711 nm in the a and c crystallographic directions and 80110 nm in b (Figure 1), all highly comparable with other studies.58,59 On the basis of the crystal habits of the particles, the dominant surfaces were deemed to consist of the isostructural (100) and (001) planes, accounting for 95% of the total area.36 The terminal (010) growth plane accounts for the remaining surface area. Recalling the unit cell size of akagan
eite (a = 1.058 nm, b = 0.303 nm, c = 1.052 nm),32 the (010) face should expose 44109 unit cells and thereby the corresponding number of channels available for ion exchange. The N2(g) BET specific surface area36 (111.2 m2/g) falls on the lower end (102160 m2/g with a median at 125 m2/g) of values calculated from the particle sizes, using a specific gravity of 3.73 g/cm3. The close correspondence of the experimental and calculated surface areas suggests that surfaces are relatively poor in defects. It should nonetheless be emphasized that imperfections (e.g., steps, terraces, kinks, screw dislocations) are inexorably present

Figure 2. XRD diffraction pattern (top) of dry akagan
eite samples equilibrated to aqueous solutions of 0.00, 0.04, 0.22, and 2.94 mmol/g HCl. Theoretical diffraction peaks for 0.00 and 2.94 mmol/g HCl akagan
eite are shown in the bottom (akagan
eite suspension density: 44.29 g/L).

although not sufficiently great enough to impart much surface roughness. In this work, akagan
eite particles are reacted to various levels of HCl and NaCl. Although these reagents first come in contact with surfaces, we show that they ultimately play a considerable role on bulk properties. 3.2. Bulk Structural Changes Induced by Chloride. Akagan
eite particles first reacted in aqueous solutions of HCl of up to 8.47 mmol/g, then dried in N2(g) undergo important physicochemical changes but do not undergo any phase change. This assertion is based on the following observations. HCl induces systematic shifts in the bulk OH stretching and bending modes, as will be discussed in greater detail in Section 3.3.2, (Figures 3 and 4). Powder XRD confirms that all solids retain the akagan
eite structure but that reaction with HCl lowers 2θ values of all main reflections (Figure 2). Analysis of these latter data suggests that the unit cell size expands in both a and c directions with 8 and 11 pm, respectively. The b direction expands by 1.39 pm by Cl occupancy (Cl/Fe) increases from 0.05 to 0.25. TEM imaging (Figure 5) reveals that HCl preferentially etches the particles length-wise, with a decrease in median value from 97 nm in the dialyzed particles at pH 5 to 45 nm in those reacted with 2.94 μmol/g HCl at pH 1.4 (Table 1). Median particle widths are only decreased from 9 to 7 nm. These results thereby point to a preferential dissolution of the (010) plane without, however, any significant change in the density of channels exposed by this plane. The considerable shortening of the particles may thereby be a contributing factor to the increased diffusion of Cl ions into akagan
eite channels with HCl loading. Although specific surface area measurements could not be carried out in these samples, given the small volume of solids that could be produced under our experimental conditions, these changes in dimensions suggest an increase from a median value of 125 to 165 m2/g. XPS data reveal enriched chloride content in samples reacted with HCl, with Cl/Fe molar ratios as high as 0.442. Although the XPS analysis holds for no more than the first top ∼2 nm of the particle surfaces, and therefore two unit cells of the 79 nm thick particles, our results point to a preferential accumulation of chloride to the akagan
eite bulk, one that expands the overall lattice volume and alters the hydrogen bonding environment of 2306

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Figure 3. ATR-FTIR spectra of dry synthetic akagan
eite in N2 (g). Acid concentrations ([H+]-[OH]: mmol/g) in the brackets are derived from [HCl]-[NaOH] values initially present in aqueous suspensions of minerals, prior centrifugation and drying (akagan
eite suspension density: 44.29 g/L). MCR spectral components are color-coded according to their concentration profiles component. Component AI and AII were resolved by Gaussian components. Those with comparable positions are denoted by the same color. Arrows (vV) denote corresponding intensity changes.

hydroxyls. This is in fact manifested in the FTIR spectra showing important shifts in both OH stretching and bending bands. Interestingly, NaCl does not increase chloride loadings to the same extent as HCl (Figure 6). Exposure to solutions as much as 33.87 mmol/g NaCl at pH 5 induce OH stretching and bending shifts of no more than 15 cm1, whereas considerably more dilute HCl solutions induce shifts as large as ∼40 cm1. We consider that this is a strong piece of evidence supporting the importance of proton cosorption in driving the inclusion of chloride to the akagan
eite bulk. In fact, although sodium and chloride have comparable ionic radii, the akagan
eite bulk offers no stable coordination centers for sodium. Protons may diffuse more freely in the akagan
eite bulk and coordinate to oxo groups,
as suggested by Stahl et al.33 The following sections contain detailed accounts of these findings and are organized as follows. We first discuss results of MD simulations of chloride-deficient and -rich akagan
eite lattices and provide clues to possible hydrogen-bonding patterns in the bulk. These results are then discussed to explain how variations in proton and chloride loading affects OH bond strengths of the akagan
eite bulk. Implications of these variations on the thermal stability of akagan
eite are then discussed in the last section of this work. 3.2.1. Structural Modeling. Chloride ions ideally sit at the center of a box defined by 4 O1H and 4 O3H hydroxyl groups of the akagan
eite bulk (Figure 1). Co-sorbed protons would,
et al., bind to according to the crystallographic model of Stahl O5 oxo groups, a claim that found support in unpublished planewave density functional theory calculations.35 Protonation of O5 should then alter local FeO bond lengths and possibly lattice parameters as well. MD simulations were carried out to further test these ideas. The chloride-free β-FeOOH supercell relaxed within 2% of crystallographic values of a chloride-bearing solid. The structure remained stable throughout the course of the

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Figure 4. Bending modes of ATR-FTIR spectra of dry synthetic akagan
eite in N2 (g). Spectrum of original akagan
eite suspension (blue) is shown as a reference. Arrows (vV) denote intensity changes and (r, f) band shifts.

simulation, despite the absence of ions in the channels. FeO2 and FeO4 distances are centered at both 0.196 nm, whereas those of FeO1 and FeO3 are centered at 0.212 nm (Figure 7). The longer FeO1 and FeO3 distances result from weaker FeO bond strengths typical of such sites in double rows of edge-sharing iron octahedra. FeH distances (arrows in Figure 7a) are bimodal in distribution and are typical of FeOOH polymorphs where in-plane and out-of-plane vibrations dictate orientations of protons in the bulk. Addition of chloride also generates lattice parameters again within 2% of the crystallographic values and a predicted specific gravity of 3.74 g/cm3. This value is highly consistent with the experimental value33 of 3.73 g/cm3 for akagan
eite particles of comparable chloride content.33 The unit cell expanded by 0.038 nm in a (1.037 to 1.074 nm) and by 0.034 nm in c (1.037 to 1.071 nm) directions, relative to the chloride-free lattice. It, however, underwent a substantially slighter contraction of 0.005 nm in b (0.309 to 0.304 nm), in contrast with our XRD data pointing to an expansion of 0.014 nm. This discrepancy is hypothesized to arise from possibly different dispositions of bulk chloride ions in the MD model. A more extensive MD study would however necessitate simulations of several supercells of varied Cl contents and distributions. The information currently at hand nonetheless points to a number of noteworthy observations. For instance, the addition of Cl has no appreciable effects on neither FeO2 nor FeO4 distances relative to the chloride-free cell. Those of FeO1 and FeO3 shortened from 0.212 to 0.208 nm. This contraction resulted from hydrogen bonding with chloride, in turn producing a unimodal distribution of FeH distances (Figure 7b), in contrast with the bimodal distribution in chloride-free bulk (Figure 7a). This new unimodal distribution arises from an important reorientation of the FeOH unit toward chloride ions, eliminating different FeH distances. ClH1 and ClH3 distances are centered at 0.220 nm (Figure 7c), whereas those of ClH5 are centered at 0.240 nm. 2307

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Figure 5. TEM images (left) and particles length distributions (right) of akagan
eite reacted in 0.00, 0.08, and 2.94 mmol/g HCl (akagan
eite suspension density: 44.29 g/L). Median particle length of akagan
eite sample decreased from 97 nm in the dialyzed particles to 45 nm in those reacted with 2.94 μmol/g HCl, whereas median particle widths only decreased from 9 to 7 nm. Colored bars in TEM images denote the representative particle length of each sample.

Table 1. Atomic Concentrations (at. %) of the elements of akagan
eite [HCl] (mmol/g) (pH)a atomic concentration

0.00

0.08

0.22

0.41

2.94

(%)

(5.0)

(2.9)

(2.5)

(2.0)

(1.4)

Fe

26.68

30.29

31.05

30.89

27.96

Cl

4.52

6.68

7.85

9.20

12.36

Cl/Fe

0.169

0.220

0.253

0.298

0.442

a

pH of samples reached to total HCl concentrations ([HCl]) were shown in brackets in italic style.

Additional simulations, in which LINCS constraints on OH bond lengths were removed, helped gain further insight into the impact of hydrogen bonding on OH bond strengths. In these simulations (1 as time step, 107 steps) hydrogen bonding of O1H and O3H to chloride elongated OH bonds by 11 pm (from 0.1030 to 0.1041 nm) (Figure 8). O5H hydroxyls have the longest OH bond length (0.1042 nm) despite the weaker hydrogen bond, possibly due to electrostatic and steric effects. Although we stress that absolute values are strong functions of the chosen force field, these results underscore important differences OH bond length and therefore strength. Hydrogen velocity autocorrelation functions of these simulations were used to obtain the power spectra (Figure 8b,c). Both spectra are qualitatively comparable to our FTIR spectrum of dry akagan
eite (Figures 3 and 4), with stretching modes at near 3500 cm1 and in-plane and out-of-plane bending modes below 1000 cm1.

Figure 6. ATR-FTIR spectra of chloride reacted with akagan
eite (7.53, 11.29, 33.87 mmol/g NaCl). Spectrum of original akagan
eite suspension (blue) is shown as a reference. Arrows (vV) denote intensity changes and (r, f) denote band shifts.

Our theoretical bulk OH stretch for a chloride-free lattice is centered at 3509 cm1. Chloride, however, induces a red shift to 3482 cm1. Splitting of this region arises from the coexistence of isolate and hydrogen-bonded OH groups. This can be appreciated in the simulation of the chloride-free cell where the power spectrum produces only one degenerate OH stretch at the highest wavenumber. Interestingly, these shifts are consistent with those detected by FTIR (Figure 3). Bending modes add further information. In the absence of chloride each in-plane and out-of-plane modes display double peaks tentatively ascribed to repulsive motions of vicinal OH groups. Addition of chloride tends to merge each of these peaks into one and results in a blue 2308

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Figure 7. g(r) functions of FeO and FeH distances in (a) chloridefree and (b) chloride-bearing akagan
eites. Functions for ClO, ClH, and ClFe distances for the latter are also shown in panel c. These values were obtained from a 5 ns production run. The arrows in panel a point to two possibly distinct FeH distances.

shift to values consistent with our experimental data (Figure 4). These changes arise from restraining effects of hydrogen bonding of O1H and O3H groups by chloride ions. In summary, incorporation of chloride ions to the bulk lattice of akagan
eite affects both OH stretching modes in systematic ways. These changes occur due to both the reorientation of bulk OH bond toward Cl ions confined in sites bounded by 4 O1H and 4 O3H groups and by proton cosorption to O2 and O4 groups, forming the O5H group. This conceptual model will now be used to interpret our FTIR spectra. 3.2.2. FTIR Spectra. MD simulations and XRD data point to an expansion of the akagan
eite lattice by incorporation of chloride and protons. Systematic changes undergone in OH stretching

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and bending modes (Figures 3 and 4) provide further details of the nature of these changes. Reactions with HCl shift the OH stretching frequency of the original dialyzed solids from 3496 to 3470 cm1 (Figure 3a). A shoulder initially present at 3395 cm1 is also shifted with HCl, producing bands at 3370 cm1 then to 3350 cm1 at the highest HCl loadings. We use MCR to separate overlapping spectral contributions into distinct sets of linearly independent components, akin to molar absorption coefficients. These MCR components group intensities that can be correlated on the basis of their correlated responses with HCl loadings. Our efforts show that only two MCR components are required to explain the data. A linear combination of these components, scaled by their concentration profiles, in fact, explains 99.7% of the variance of the original spectra (Figure 3b,c). Component AI corresponds to a pristine akagan
eite solid, unreacted with HCl, and contains the 3490 and 3380 cm1 bands. Component AII contains the red-shifted 3470 and 3350 cm1 counterparts as well as a broad region associated with HCl concentration exceeding 0.08 mmol/g. Peak-fitting of MCR components AI and AII reveals four common bands centered at 3485, 3450, 3360, and 3285 cm1 but of varying half bandwidths and intensities (Figure 3b). HCl notably promotes a decline in the intensities of the 3485 and 3450 cm1 bands and an increase in those of 3360 and 3285 cm1. Bands at 3485/3450 are consequently assigned to OH stretches in the absence of Cl. This assignment is partially motivated by the dominance of these bands at low HCl concentration as well as higher OH stretching wavenumber resulting from weak or lacking hydroxyl-chloride interactions. Addition of HCl intensifies the 3360 and 3285 cm1 bands. The former undergoes a five-fold increase in intensity with HCl and is assigned to O1H/O3H groups forming hydrogen bonds to Cl. The 3285 cm1 band is also promoted by HCl but undergoes a less dramatic increase than O1H/O3H groups. As it is a fraction (∼1/4) of the area of 3360 cm1 and corresponds to a longer OH bond it is assigned to O5H. Note that the longer OH bond for the isolate O5H was also predicted by MD (Figure 8a). OH bending modes underwent concomitant changes with those of the stretching region. In-plane bends (800 cm1) denote modes taking place along the a-c plane while out-of-plane bends (680 and 615 cm1) are for those along the tunnel of b direction. Reaction with HCl shifts in-plane modes from 800 to 820 cm1 with increasing HCl, then ultimately to both 820 and 850 cm1 as a bifurcated band at the greatest loadings. In contrast, the bifurcated out-of-plane bending mode of at 680 and 615 cm1 in the alkaline samples merge to a single band centered at 637 cm1 under the greatest HCl loadings. MCR analysis of these 15 spectra generated two significant spectral components (Figure 4b) reproducing 99.8% of variance of the data. As also seen in the stretching region, these spectral features are captured in the bending region as well. Component BI consists of bands at 800, 678, and 615 cm1 and represents at least 65% of spectral intensities at HCl concentrations less than 0.075 mmol/g. Component BII is characterized by two intense bands at 637 and 840 cm1 with a shoulder at 820 cm1. Its concentration fractions increase sharply with HCl up to 0.22 mmol/g. Peak-fitting efforts of MCR components of the in-plane bending region retrieve bands at 850, 810, and 790 cm1 (Figure 4b). The 790 cm1 band is assigned to in-plane bending of OH groups that are forming hydrogen bonds with Cl. This assignment is motivated by the dominance of this band at low 2309

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Figure 8. Results of a 5 ps MD simulation at 0.5 as time step in which are LINCS constraints on OH bond strengths were removed. Relaxed OH distances are shown in panel a. Power spectra obtained from the hydrogen velocity autocorrelation function for the OH stretching region are shown in panel b and for the bending region in panel c.

Table 2. Summary of FTIR Band Assignments wavenumber (cm1) 3485 3450

OH speciesα O1H/O3H

3360

O1H/O3H (Cl)

3285

O5H (Cl)

850

O1H/O3H (Cl)

810

O5H (Cl)

790

O1H/O3H

702

X-Cl-X

673 640

Cl-Cl-Cl Cl-X-Cl

609

X-X-X

vibration modes stretching

in-plane bending

out-of-plane bending

α

Species are labeled in Figure 1. ‘(Cl)’ denotes an OH group interacting with chloride. Out-of-plane bends are for OH groups at (1) vacant sites (X) only [X-X-X], (2) vacant site neighbored by two Cl-occupied sites [Cl-X-Cl], (3) Cl-occupied site neighbored by two vacant sites [X-Cl-X], and (4) Cl-occupied sites neighbored by two others that are also occupied [Cl-Cl-Cl].

HCl concentrations only as well as the low wavenumber, suggesting relatively weak energies for bending. The addition of chloride restrains O1H/O3H bends and thereby blue shifts the band to 850 cm1, in turn manifested in component BII. The small intermediate band at 810 cm1 is assigned to O5H given its expected weaker interactions with Cl and the smaller electronegativity of the oxygen. Peak fitting of the out-of-plane MCR bending components resolves bands at 702, 673, 640, and 609 cm1 (Figure 4b). Loss of the highest and lowest wavenumbers (702 and 609 cm1) is strongly correlated to the rise of the intermediate 673 and 640 cm1 bands. Because these modes correspond to bends along the length of the channels, they are not only affected by chloride occupancy alone but also by the sequence of occupancy. To arrive to a plausible band assignment, we considered the following scenarios along any given channel: (1) vacant sites (X) only [X-X-X], (2) vacant site neighbored by two Cl-occupied sites [Cl-X-Cl], (3) Cl-occupied site neighbored by two vacant sites [X-Cl-X], and (4) Cl-occupied sites neighbored by two others that are also occupied [Cl-Cl-Cl]. OH groups in type [X-X-X] and [Cl-X-Cl] channels are expected to have lower out-of-plane bending energies because their bending motions along the b direction are not restrained by hydrogen bonding. Type [X-X-X] hydroxyls should moreover exhibit the lowest values.

Figure 9. Mass spectrometric determination of evolved gases during TPD. Water traces of five dry powder akagan
eite samples are shown. These samples were equilibrated at 0.00, 0.08, 0.22, 0.41, and 2.94 mmol/g HCl concentrations (akagan
eite suspension density: 44.29 g/L) prior drying in N2(g).

Type [X-Cl-X] and [Cl-Cl-Cl] hydroxyls have greater bending energies due to interactions with Cl ions, with the former having even greater values. The expected order of out-of-plane bending energy is consequently [X-X-X] < [Cl-X-Cl] < [Cl-Cl-Cl] < [X-Cl-X]. According to this line of reasoning spectra with the highest (702 cm1) and lowest (609 cm1) out-of-plane energies should correspond to [X-Cl-X] and [X-X-X], respectively. These moreover correspond to channels of low chloride loading, as seen in component BI. Bands with intermediate wavenumbers are, in turn, assigned to channels of greater chloride loading, namely, [Cl-X-Cl] for 640 cm1 and [Cl-Cl-Cl] for 673 cm1. This band assignment also concurs with XPS results retrieving a positive correlation between HCl loading and chloride occupancy (Table 1). In summary, our band assignments fall in line with the concept that HCl addition to akagan
eite induces important changes in the hydrogen-bonding environment (Table 2). Although hydrogen bonding of OH1/OH3 to Cl is attractive in nature, incorporation of the relatively voluminous chloride ion induces an overall expansion in lattice size along all crystallographic directions, as supported by XRD. Overall, our band assignments open a path for tracking variations in hydrogen bonding strengths of the akagan
eite bulk. Implications for these structural changes on its stability are illustrated in the following section. 3.2.3. Implications for Thermal Stability. In an effort to illustrate potential implications on the impact of hydrogen bond strength in akagan
eite reactivity, samples were exposed to temperature gradients using TPD in vacuo. These experiments were carried out to test the hypothesis that akagan
eite samples with stronger hydrogen bonds would display the stronger resilience to thermal decomposition. 2310

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In these experiments akagan
eite converted to hematite (α-Fe 2 O3 )60,61 through the following the reaction 8β-FeOOH f 4α-Fe2 O3 þ 4H2 O

ð2Þ

A concomitant release of Cl also occurs, as notably discussed by Cai et al.28 but will be discussed further in a separate communication. Production of H2O takes place through an associative desorption process, whereby OH groups strip neighboring protons, leaving behind an oxide-rich material. Mass spectrometric determination of released H2O points to an important correlation between dehydroxylation temperature and chloride loadings. Indeed, dialyzed samples give rise to double peaks at 200 and 260 °C, whereas those reacted with HCl are shifted up to 260 and 305 °C, respectively (Figure 9). These shifts are ascribed to the stronger hydrogen bonding environments of the akagan
eite bulk, retarding the associative desorption of H2O. We also note a substantial attenuation of the low-temperature peaks with HCl loading, only producing a faint shoulder in the powders reacted in 2.94 mmol/g HCl. Elevated HCl loadings are thereby proposed to inhibit the desorption of weakly associated water altogether. In summary, Cl occupancy in akagan
eite varies the hydrogenbonding environment in its channel structure and consequently affects its thermal stability.

4. CONCLUSIONS FTIR spectroscopy was used to track bond strength of important OH groups in akagan
eite. Molecular simulations provided theoretical evidence by predicting lattice size, channel OH bond length,and hydrogen bonding strength. FTIR spectra can be decomposed into spectral components consisting of endmember attributes of akagan
eite. Knowledge acquired by MD simulations facilitated band assignment procedures. The results of this study suggest that H+ and Cl cosorption to the akagan
eite bulk is an important process affecting the physicochemical attributes of this material. Greater chloride occupancy leads to stronger hydrogen bonds from bulk hydroxyls and in an overall expansion of crystal lattice size. Not only are such solids more resilient to thermal dehydroxylation but also, given their greater chloride occupancy, we suspect they may be important sources of acidity and chloride once re-exposed to circumneutral solutions. Finally, these observations may open possibilities for further resolving ion exchange properties of akagan
eite, particularly in environments of varied acid content. They should moreover open the door to similar studies on structurally analogous minerals, such as schwertmannite.62 ’ ASSOCIATED CONTENT

bS

Supporting Information. Details of equations, parameters, and supercells used for MD simulations, a synopsis of variations in spectral features, as well as XRD and FTIR characterization results of dry dialyzed akagan
eite. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: +46 90 786 5270.

’ ACKNOWLEDGMENT This work was supported by the Swedish Research Council
no. 2009-3110) as well as the Wallenberg, the (Vetenskapsradet, Carl-Tryggers, and the Kempe Foundation. We thank James Kubicki and Qing Zhu for sharing some of their early results. Kenichi Shimizu and Andrei Shchukarev are thanked for XPS analyses. Nils Skoglund is also thanked for collection of XRD data and useful discussions. All MD calculations were carried on the High Performance Computational Cluster North (HPC2N) at Umea
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