Electrolyte Ion Binding at Iron Oxyhydroxide Mineral Surfaces

Article pubs.acs.org/Langmuir

Electrolyte Ion Binding at Iron Oxyhydroxide Mineral Surfaces Philipp A. Kozin,* Andrey Shchukarev, and Jean-François Boily Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden ABSTRACT: Electrolyte ion loadings at the surfaces of synthetic goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) particles that were pre-equilibrated in aqueous solutions of 10 mM NaCl and NaClO4 at 25 °C were investigated by cryogenic X-ray photoelectron spectroscopy (XPS). Atomic concentrations of Cl−, ClO4−, and Na+ were correlated to potential determining ion (pdi; H+, OH−) loadings obtained by potentiometric titrations. While Cl− promoted more pdi adsorption than ClO4−, due to its greater charge-to-size ratio, both ions followed the same loading dependence on pdi adsorption, in contrast to previous studies supporting the concept for negligible perchlorate adorption. Lepidocrocite particles exhibited a stronger response of electrolyte adsorption to pdi loadings due electrolyte ion adsorption on the proton inactive (010) plane. These particles also acquired greater sodium loadings than goethite. These loadings were moreover considerably enhanced by perchlorate adsorption, possibly due to a thickening of the interfacial region in NaClO4 on the (010) plane. Finally, goethite particles with rougher surfaces acquired greater pdi and ion loadings than on those with smoother surfaces. No strong differences could be discerned between Cl− and ClO4− loadings on these materials. This work thus identified key aspects underpinning the relationship between pdi and electrolyte loadings at FeOOH mineral surfaces of environmental and technological importance.

1. INTRODUCTION Adsorption reactions at iron (oxy)hydroxide particle surfaces are of great importance to both natural and industrial processes.1−9 The widespread abundance of these minerals has prompted numerous studies focused on surface structures and reactivity.10−12 Electric double layer (EDL) composition and structure are equally important focal points for this research area, given the typically substantial contributions of electrostatics to adsorption free energies.13−15 Surface charge development, one that namely takes place through the adsorption of potential determining ions (pdi H+, OH−), is particularly impacted by the charge-neutralizing ability of electrolyte counterions. This ability is, in turn, determined by counterion charge and size, interfacial water structures, as well as surface site type, density, and distribution. Although electrolyte ion adsorption reactions are usually intractable by traditional analytical solution-based methods, a number of surface-sensitive X-ray-based techniques have been considerably successful in this regard.16−18 A recent X-ray reflectivity study by Farquhar et al.,17 for example, resolved altered layers formed at oligoclase surfaces contacted with aqueous solutions of nitric acid and water. The study, which included measurements of thickness, density, and surface roughness, revealed ion exchange of protons for cations in depths of up to 3.0 nm. Another example includes a study by Fenter et al.18 resolving density profiles of Rb+ and Sr2+ ions at the mica/water interface. These efforts uncovered monolayer coverages of inner-sphere Rb+ (6-fold coordinated) but two distinct layers of inner- and outer-sphere Sr2+ (8-fold coordinated) ions. Theoretical studies have also contributed to this area.19−22 Kerisit et al.,19 for example, presented atomistic simulations of iron (oxy)hydroxide surfaces contacted with water and © 2013 American Chemical Society

solutions of NaCl. These simulations revealed multiple layers of ions and underscored their relationship to electrostatic potentials. Another study by Wang et al.20 on brucite, gibbsite, hydrotalcite, muscovite, and talc demonstrated the relationship between mineral surface structure and interfacial water structure. The first water layer formed at these surfaces was notably shown to adopt mineral-specific structures that remain unrelated to those of ice. Surface charge distribution, hydrophobicity, and hydrogen-bonding patterns were also affected by these configurations. In another study, Bourg and Sposito21 discussed EDL properties of a smectite surface contacted with mixed electrolytes of NaCl and CaCl2 in terms of surface-controlled positions of inner-sphere and outer-sphere complexes. They show that charge inversion occurred in the diffuse layer, while ionic positions were not affected by ionic strength. It was also noted that the diffusivity of electrolyte ions were considerably reduced at interface. A common thread linking these and many other studies23,24 dedicated to this issue lies in the importance of mineral surface structure on adsorption reactions. Although these studies made extensive use of oriented single surfaces, nanosized materials with well-defined crystal habits are also of considerable importance to consider surfaces grown and ripened in aqueous systems. These are also primordial materials to consider effects of surface imperfections as well as coexisting and intersecting crystallographic terminations. This becomes especially relevant considering the importance of these materials, with their inherent surface defects and sizes, in natural and technological processes. Received: April 9, 2013 Revised: September 4, 2013 Published: September 6, 2013 12129

dx.doi.org/10.1021/la401318t | Langmuir 2013, 29, 12129−12137

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Figure 1. Idealized crystal morphology and TEM images of FeOOH synthetic particles. dialyzed. The dialysis water was changed on a daily basis until its conductivity was comparable to that of DI water. The synthesis of goethite particles with a specific surface area of 122 m2/g (GT122) also involved amorphous iron (oxy)hydroxide (ferrihydrite) as a precursor, however in the presence of carbonate.31 The amorphous precursor was made by dropwise addition of a 1.0 L solution of 0.5 M NaHCO3 to a continuously stirred 1.0 L solution of 0.4 M Fe(NO3)3·9H2O (Fisher, ACS grade). During this procedure, the solution color turned from orange to brownish red with no visible precipitates. The resulting solid suspension was sonicated for 60 s to improve particle homogeneity and dispersity and then cooled to 15 °C to inhibit conversion to other undesirable iron (oxy)hydroxide phases. The suspension was afterward dialyzed for 3 days at 25 °C with three changes of dialysis water per day. The suspension pH was thereafter adjusted to pH 12 by dropwise addition of 5 M NaOH, resulting in a color change from brownish red to brown. It was then heated at 90 °C for 24 h, during which time a light orange precipitate formed. The particles were thereafter subjected to a second and final dialysis procedure, where they acquired a typical goethite yellow color. The color of GT122 was slightly darker than that of GT69 due to its smaller particle size. Rod-shaped lepidocrocite (RL) was synthesized in 1000 mL of a filtered (0.2 μm) 0.06 M FeCl2·4H2O solution.30 During filtration, the solution color changed from brownish yellow to light green due to the removal of adventitiously formed minerals, such as akaganéite. Solution pH was thereafter adjusted to pH 7 with 1 M NaOH. Purified air was thereafter flushed to the resulting solution to induce RL formation by oxidation of ferrous iron. The continuously stirred suspension was kept at pH 7 throughout the course of the synthesis by episodic additions of small volumes of 1 M NaOH. The resulting dark greenish-gray precipitate turned to the bright orange RL within 3 h of reaction time. The resulting product was then dialyzed. Synthesis of lath-shaped lepidocrocite (LL) was made from 0.02 M FeCl2·4H2O solutions containing 0.2 M NaCl.32 Strong concentrations of chloride are, in this method, required to hinder growth along the [010] direction, thereby promoting the LL morphology. The solutions were then filtered to remove precipitates and neutralized with 1 M NaOH to pH = 6 under N2(g). Purified air was thereafter flushed through continuously stirred solutions to induce LL formation, again by oxidation of ferrous iron. Removal of CO2 was also crucial to this procedure to eliminate possible formation of goethite.30 The solids where thereafter dialyzed. This procedure was carried out on in four different batches of 200 mL solution, as larger (e.g., 1000 mL) volumes gave particles of various shapes due to uncontrollable differences in iron oxidation rates. The dialyzed particles obtained from these different batches were then merged into one single suspension. 2.2. Sample Characterization. Portions of all resulting stock suspensions were dried in an oven at 40 °C for 24 h. The resulting powders were ground in an agate mortar and used for structure and phase confirmation using powder X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and XPS. Particle size and shape were determined by transmission electron microscopy (TEM),

In this study we contribute to this area by showing how variations in (hydr)oxo populations at surfaces of nanosized FeOOH mineral particle affect charge development and electrolyte ion loadings. Specifically, this work is focused on (1) two types of goethite (α-FeOOH) particles with highly comparable morphologies but of different sizes and surface roughness and (2) two distinct types of lepidocrocite (γFeOOH) particles exhibiting the same crystallographic planes but in different proportions (Figure 1). All of these surfaces are fully covered by hydroxyl groups as they have been permanently contacted with aqueous solutions. NaCl and NaClO4 background electrolytes were used to resolve effects of anionic charge-to-size ratio in achieving surface charge neutralization. As electrolyte loadings cannot be determined from classical analytical chemical methods, we turned to cryogenic X-ray photoelectron spectroscopy (XPS). This technique preserves the composition of electrolyte ions and most water molecules stabilized near the mineral/water interfacial region by rapidly freezing centrifuged wet mineral pastes to −155 °C. It thereby provides a means to evaluate effects of variations in electrolyte composition and pH on elemental compositions and ratios stabilized by mineral surfaces. The cryogenic XPS technique has already been used on several different minerals (α-Fe2O3,25 α-FeOOH,26 γMnOOH, SiO2, α-Al(OH)3, CaCO327,28). In this work comparison of ion loadings achieved by different FeOOH minerals and different salts (NaCl and NaClO4) will notably show that electrolyte ion loadings are highly correlated to pdi loadings and depend on such factors as mineral particle morphology, surface irregularities29 (e.g., porosity), and counteranion charge-to-size ratios.

2. MATERIALS AND METHODS 2.1. Mineral Synthesis. All mineral particles were prepared with preboiled doubly distilled deionized (DI) water and from carbonatefree solutions in an atmosphere of N2(g). All suspensions were made in polyethylene bottles from aqueous solutions that were continuously stirred with a propeller. Magnetic stirrers were not used to prevent crushing of mineral particles. All dialysis procedures involved Milli-Q preboiled water and were carried out until the suspension conductivity reached values comparable to that to Milli-Q water (18 MΩ·cm). Finally, all final products synthesized for this work were stored as aqueous suspensions in polyethylene bottles. A suspension of goethite particles with a specific surface area of 69 m2/g (GT69) was prepared by dropwise addition of 2.5 M NaOH to a continuously stirred 1000 mL 0.15 M Fe(NO3)3·9H2O solution until the suspension reached a pH of 12.30 The resulting amorphous solids were converted to goethite in an oven at 50 °C for a 24 h period. The final product was then washed repeatedly with DI water and then 12130


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Table 1. Physical Characteristics of Synthetic FeOOH Particles characteristics

LL

RL

GT69

GT122

lengtha (nm) widtha (nm) BETb (m2/g) BJH pore diamb (nm) BJH pore volb (cm3/g) t-plot micropore areab (m2/g) major crystal planec major crystal planec minor crystal planec −OHd (sites/nm2) μ-OHd (sites/nm2) μ3-OHd (sites/nm2)

120−224 (164) 29−51 (35) 71 19 0.38 7.68 60−73% (010) 32−21% (001) 8−6% (100) 2.65−1.83 7.12−7.22 1.98−1.33

145−296 (188) 6.5−21 (10) 77 17 0.37 6.83 48% (010) 48% (001) 4% (100) 2.99 6.53 2.66

(89−368) 211 (18−34) 25 69 18 0.32 3.66 90−95% (110)

52−118 (77) 7−22 (12) 122 10 0.39 5.76 94−92% (110)

10−5% (021) 3.48−3.25 3.48−3.25 8.18−8.63

6−9% (021) 3.31−3.39 3.31−3.39 8.54−8.37

a

Particle dimensions were estimated from TEM imaging. The values in parentheses are the mean values. bDetermined from 90-point N2(g) adsorption/desorption isotherms. cCrystal plane percentages were estimated assuming particle thickness equal to the particle width for all types of particles. Particle thickness of 15 nm LL was obtained by atomic force microscopy (not shown) and used in the calculations. dTotal surface site densities over all surface area. while specific surface area was determined by Brunauer−Emmett− Teller (BET) N2(g) adsorption. Powder X-ray diffraction (XRD) was carried out on a Bruker D8 Advance device working in θ−θ mode using Cu Kα radiation. TEM images were collected on air-dried samples casted onto a copper grid and covered with a Formvar film. Imaging was carried out with a JEOL-1230 instrument working at 80 kV and a point resolution of 3.4 Å. Surface area (BET) and microporosity (Barrett−Joyner−Halenda; BJH) measurements were determined from 90-point N 2 (g) adsorption/desorption isotherms (Tristar, Micrometrics). These measurements were performed on samples previously dried for 12 h at 110 °C in an atmosphere of dry N2(g). Finally, FTIR spectra (Bruker Vertex 70/V equipped with a DLaTGS detector at 298 K) were collected on dry powders pressed on an attenuated total reflectance cell (Golden Gate, single bounce diamond) under vacuum. Characteristic O−H stretching and bending modes were used to corroborate crystallographic phases identified by XRD. All XPS spectra were collected with a Kratos Axis Ultra electron spectrometer equipped with a delay line detector. A monochromatic Al Kα radiation source operated at 150 W, a hybrid lens system with a magnetic lens, and a charge neutralizer were used for all measurements. The resulting spectra were processed with the Kratos software and background-corrected with a Shirley background. All peak positions for the cryogenic spectra were calibrated against oxide component of O 1s peak (530.3 eV for GT69, GT122, RL, and LL) and fitted using 70:30 Gaussian−Lorentzian functions. Those for spectra collected at room temperature were shifted against the C 1s 285.0 peak. The spectra were determined from averaged values obtained over an analysis area of 0.3 mm × 0.7 mm2 and pertained to an analysis depth of about 6 nm. 2.2. Potentiometric Titrations. Potentiometric titrations33 were carried out to monitor proton uptake and release in 10 mM NaCl and 10 mM NaClO4 suspensions of lepidocrocite (RL and LL) and goethite (GT69 and GT122). All suspensions were kept in an atmosphere of humidified N2(g) in a sealed glass titration vessel placed in a paraffin oil bath kept to 25 ± 0.5 °C in a room that was in turn kept at 25 ± 2.0 °C. This protocol ensured constant volumetric additions of reagents throughout the course of the experiments. Mineral suspensions were first acidified to pH 3−3.5 for several hours to evacuate dissolved and adsorbed carbonate ions. Alkalimetric titrations were thereafter carried out using an automated system whereby 5−20 electromotive force (emf) measurements of the glass pH electrode were carried out at a 300 s interval. The titrant was added only when the emf drift was less than 0.6 mV/h or if the measurement exceeded 100 min. This experimental protocol ensured that (near) equilibrium with respect to proton adsorption was reached at every point.

A Metrohm pH (6.0133.100) electrode and a Ag|AgCl reference (6.0726.100) electrode were used for the experiments. The Ag|AgCl reference was filled with 10 mM NaCl to minimize variations in ionic composition during the titrations. All electrochemical cells were calibrated by separate acidimetric titrations in the ionic medium of interest to obtain both electrode constants and liquid junction potentials. 2.3. ζ-Potential Measurements. The ζ-potentials34 of mineral suspensions in 10 mM NaCl or NaClO4 were measured with a Zen3600 Zeta-sizer (Malvern Instruments Ltd.). Potentiometric titrations of mineral suspension were performed under N2(g) in an external titration vessel at constant temperature of 25 °C. Its contents were pumped to the Ζeta-sizer capillary cell through gastight tubes for each ζ-potential measurement. All measurements were performed in triplicates to test for reproducibility. Prior to each measurement, the mineral suspension was pumped through the capillary cell to avoid particle sedimentation and to ensure full mixing. 2.4. XPS Measurements. All mineral suspensions were equilibrated in 10 mM NaCl and NaClO4 between pH 4 and 10 under atmospheres of humidified N2(g). Aliquots of the suspensions were transferred to degassed polyethylene test tubes and equilibrated for a 24 h period. The samples were then centrifuged for 10 min at 5000 rpm. The centrifuged wet pastes were thereafter smeared onto a molybdenum sample holder and immediately placed onto the precooled (−170 °C) claw of a sample transfer rod into the airlock of XPS spectrometer. The samples were allowed to freeze for 45 s prior drawing a vacuum down to (4−5) × 10−5 Pa. During this cooling period a portion of free water sublimated and was deposited onto the claw of the sample transfer rod. Once frozen, the sample was transferred into the analysis chamber of the instrument and the pressure decreased to 2 × 10−7 Pa while the temperature was maintained at −155 °C. To check the reproducibility, most XPS experiments were performed in duplicates or triplicates.

3. RESULTS AND DISCUSSION 3.1. Mineral Characterization. All XRD patterns confirmed that the sole crystallographic phases in GT69 and GT122 were goethite and in RL and LL were lepidocrocite. Likewise, FTIR measurements confirmed that no other iron (oxy)hydroxide phases were associated with these materials, as was previously shown.35 TEM images showed GT69 particles of 89−368 nm in length and 18−34 nm in width, while GT122 formed 52−118 nm long and 7−22 nm wide particles (Figure 1). Both particles type are therefore approximately of the same aspect ratios. Furthermore, on the basis of previous microscopic investigations,36,37 they should be predominantly terminated by 12131


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3.2. Surface Charge Development. Our experimental data provide evidence for ion-specific effects by comparing charge uptake in 10 mM NaCl and NaClO4 (Figure 3). Both

the (110)/(100) planes and the (021) planes. The (110)/ (100) planes represent about 90−95% and (021) 10−5% of the particles’ surface area. The RL particles are 6.5−21 nm wide and 145−296 nm long, exhibiting a nearly equal distribution of (010) and (001) planes,38 while the terminal (100) plane represents about 4% of the particle surface area. LL particles are, in turn, 120−224 nm long and 29−51 nm wide platelets. The (010) plane represents about 60−73%, while the edge (001) plane represents 29% and the terminal (100) plane 6% of the particle area. The N2(g)-BET specific surface area of GT69 is 69 m2/g while that of GT122 is 122 m2/g. Those values for lepidocrocites were, on the other hand, more comparable: 71 m2/g for LL and 77 m2/g for RL (Table 1). Although values for both lepidocrocite were close to those obtained from a geometric calculation based on TEM-derived particles sizes and specific gravities, those for both goethites were considerably lower. These differences thereby suggest that lepidocrocite particle surfaces are closer to the concept of ideal terminations than those of goethite. This concept is supported further by important hysteresis of the adsorption/desorption isotherms of N2(g) of GT122 (Figure 2), possibly revealing

Figure 3. Potentiometric titrations of synthetic FeOOH particles suspended in 10 mM NaCl and NaClO4 aqueous solutions at 25 °C. The solid line corresponds to theoretical charging curve generated by fitting the GT69 10 mM NaCl data with a surface site density of 6.37 sites/nm2 with CStern = 1.16 C/m2, log KH = −9.4, log KCl = −0.58, log KNa = −0.88. The dashed line corresponds to the same model but using a site density of 50.96 sites/nm2 with the same other modeling parameters. It thus shows that a large and physically unrealistic increase in site density would be required to account for differences in the charging properties of GT69 and GT122. A small increase in capacitance would, on the other hand, be required to account for these differences, as discussed in Boily et al.22 Model calculations were carried out using the 1 pK model with the basic Stern model, the details of which can be found in Lützenkirchen et al.57

GT69 and GT122 develop more charge in NaCl than in NaClO4, an expected result due to the greater chargeneutralizing capacity of the larger charge-to-size ratio of Cl− ion (5.98 × 10−3 e pm−1) compared to ClO4− (3.47 × 10−3 e pm−1). Furthermore, GT122 consistently acquired more charge than GT69 in both ionic media considered for this work. Previous work along these lines42 suggested that goethite particles with roughened surfaces acquired more charge than smoother ones, a concept that is consistent with the larger microporosity of GT122 discussed in the previous section. We also note that equilibration times of the titration experiments were considerably longer in GT122 than in GT69 (Figure 5) in the respective electrolyte solutions. These results thereby point to possible influences of mineral porosity on the kinetics of electrolyte adsorption. Chloride-bearing systems, in particular, systematically required longer equilibration times due to additional diffusion reactions of this smaller ion into surface irregularities that cannot accommodate perchlorate. Despite these differences, point of zero charge (pzc) and isoelectric point (iep) values (Figure 4) of both goethite particle types were indistinguishable with a value of pHpzc/iep = 9.4 ± 0.2 (Figure 4), as reported in the literature for contaminant-poor synthetic particles.43 In contrast to the case of goethite, comparisons between the two types of lepidocrocite particles under study are better

Figure 2. BET N2(g) adsorption−desorption isotherms, revealing greater porosity of GT122 in comparison to other FeOOH minerals under study.

larger levels of surface microporosity than GT69. This difference is likely to have resulted from the faster rates of neutralization of the ferric nitrate solutions conditions during GT122 synthesis, a condition typically resulting in rougher surfaces. In addition to this, the presence of carbonate at early stages of GT122 precipitation produced smaller particles by hindering polymerization and aggregation rates of incipient iron (oxy)hydroxide precipitates and may be a contributing factor to microporosity. Differences in oriented aggregation39−41 mechanisms of ferrihydrite particles during goethite synthesis could also be additional contributing factors which may warrant elucidation in the future. These differences will nonetheless be highlighted more clearly in the following section devoted to surface charge development. 12132


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result also retrieved in the ζ-potential values (Figure 5). Moreover, while RL and both goethites develop more surface charge in Cl− than in ClO4−, LL acquires nearly identical charge in both electrolytes. Interactions between μ-OH groups of the (010) plane with ClO4− ions could possibly contribute to this reaction. 3.3. Cryogenic XPS. 3.3.1. Surface Fe and O Species. Room temperature XPS measurements of the dry unreacted solids reveal no surface contaminants other than traces of aliphatic carbons, as typical of these in vacuo measurements. The Fe 2p region of both goethites display, in particular, characteristic features for this phase, as can be seen in the 711.0−711.2 eV (Fe 2p3/2) and 724.9−725.2 eV (Fe 2p1/2) peaks. Those of both lepidocrocites have peaks at 711.6 eV (Fe 2p3/2) and 725.2−725.4 eV (Fe 2p1/2). The cryogenic XPS spectra of the O 1s region of all particles equilibrated in 10 mM NaCl and NaClO4 reveal coexisting O, OH, and H2O species (Figure 6a,b). As XPS preferentially retrieves surface populationsand with uncertainties of ∼10%O:OH ratios are not characteristic of the FeOOH bulk. They rather reveal an enrichment in OH due to the important populations of surface hydroxo groups. The isomorphous GT69 and GT122 particles expose similar ratios O:OH of 1:2, while those for lepidocrocite differ with values of 1:1.3 for LL and 1:1.7 for RL. Differences in particle morphologies of RL and LL are thus responsible for these different values as the predominant (010) plane of LL results in a smaller crystallographic OH density (8.4 sites/nm 2) compared to the OH density of the (001) plane (15.6 sites/ nm2).44 The total site densities calculated from TEM-based particles shapes are presented in Table 1. In an effort to effectively compare the XPS data presented in this study, all atomic abundances were normalized for Fe. This procedure shows that O and OH concentrations of all four particle types were largely unaffected by variations in pH (Figure 7), a result suggesting that these particles were stable under all experimental conditions. A possibly increasing population density of OH groups with pH in goethite (Figure 7) could potentially be explained by de(protonation) reactions of surface (hydr)oxo groups, as notably discussed by Boily and Shchukarev,45 although explanations for the absence of this trend in lepidocrocite have yet to be put forth. The impact of variations of surface charge on electrolyte ion adsorption detected by cryogenic XPS will now be discussed in the following sections. 3.3.2. Electrolyte Ions. Preliminary cryogenic XPS experiments were first carried out for various ionic strengths. Results were however better resolved in 10 mM solutions where surface-bound ions were sufficiently abundant to produce XPS data of reliable signal-to-noise ratio, and where contributions from residual solution ions were negligible. All electrolyte ion loadings exhibited a clear dependence with respect to pH, as can seen in Figure 8. The data are in line with cryogenic XPS results of other iron (oxy)hydroxide minerals studied in our laboratory37,45−47 and consistent with the concept25,48,49 that both electrolyte cations and anions coexist below and above the pzc. The largest anion surface loadings in both goethite and lepidocrocite were acquired in NaCl rather than NaClO4 at pH ∼4, where differences were more clearly manifested. Again the stronger charge-screening ability of Cl− accounts for this difference. We however note that GT122 acquired larger anion loadings than GT69 in both electrolytes at pH ∼4 (Figure 8a), a result that can be correlated with the

Figure 4. ζ-potentials of synthetic FeOOH particles suspended in 10 mM NaCl and NaClO4 aqueous solutions at 25 °C.

Figure 5. Equilibration times required for pH electrodes to achieve a drift of less than 0.6 mV/h during potentiometric titrations of goethite suspensions. GT122 requires longer equilibration times than GT69. Chloride-bearing systems also require longer equilibration times than perchlorate-bearing systems.

rationalized in terms of surface structure than roughness. RL and LL have different proportions of surface planes and thereby expose different concentrations of surface reactive groups.12 About 48% of the RL surface is represented by the (010) planeterminated by only neutrally charged and doubly coordinated (μ-OH) hydroxo groupswhile LL is terminated by over 65% of that plane (Figure 1 and Table 1). The pzc/iep values of 7.4 ± 0.2 are however unaffected by these differences (Figure 4) as the (010) plane is deemed to be charge neutral over a wide range of pH values. RL thereby acquires more surface charge than LL on a total surface area basis (Figure 3), a 12133


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Figure 6. Typical XPS spectra of goethite (a) and lepidocrocite (b) revealing characteristic O, OH, and H2O surface species. Lepidocrocite surfaces exhibit lower concentrations of OH groups than those of goethite due to the lower density of these groups at the (010) plane.

Figure 7. Fe-normalized O, OH, and H2O atomic ratios at goethite (GT69, GT122) (a) and lepidocrocite (RL, LL) (b) surfaces as a function of pH. These largely pH independent values suggest that the mineral surfaces were stable over the entire pH range considered for this work.

for differences in pdi loading. Thermodynamic adsorption calculations (Figure 3), in fact, readily show that a large (∼8fold) increase in site densities would be required to explain differences in pdi surface charge between GT122 and GT69 if all other modeling parameters (i.e., electrolyte binding constants and interfacial capacitance) were to remain unaltered. Such large increase in site density is physically unrealistic considering that GT122 surfaces would need to be severely defective, and result in considerably larger differences in BET specific surface area and TEM-observed particle morphology than those seen in the laboratory. The concept of increased and ion-specific inner-Helmholtz (or Stern) capacitance45 (C = εδ−1, where ε is the permittivity of the charge-free layer and δ its thickness) provides, on the other hand, a more effective and realistic strategy in accounting for these differences. In fact, even studies that favor enhanced site densities on roughened surfaces11 must inexorably resort to enhanced capacitances to account for differences in surface charge. Enhanced capacitance, in electrochemical parlance, translates to (1) thinner interfacial compact layers, as effectively achieved by averaging surface topographical levels, as well as (2) regions of greater dielectric constant where water can

proton adsorption data of Figure 3. A similar observation can be made in the lepidocrocite data at pH ∼4 where greater electrolyte anion loadings in RL correlate with pdi charge (Figure 8b). This correlation is more clearly expressed in Figure 9 where XPS-derived ionic loadings are shown as a function of pdi charge. Figure 9 underscores the following key aspects of the four systems under study: (1) Cl− and ClO4− loadings are not considerably different when compared on a pdi loading basis; (2) Cl− and ClO4− loadings are greater on lepidocrocite than on goethite below the pzc; (3) Na+ loadings are greater on lepidocrocite than on goethite above the pzc; (4) Na+ adsorption on both LL and RL is greater in the presence of ClO4− than of Cl− above the pzc. Implications of these findings will now be detailed for each particle type considered in this work. 3.3.2.1. Goethite. As discussed in the earlier part of section 3.2, GT122 develops more surface charge than GT69. Again, such differences in pdi loadings have previously been ascribed to important differences in surface roughness.50−52 Although differences in site density are certainly contributing to these results, realistic increases in density are insufficient to account 12134


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Figure 8. Fe-normalized Cl−, ClO4−, and Na+ atomic ratios at goethite (GT69, GT122) (a) and lepidocrocite (RL, LL) (b) surfaces as a function of pH. The data reveal large anion loadings below the pzc and large Na+ loadings above the pzc. Color-coded highlighted areas and solid lines represent arbitrarily defined trends.

solvate pdi more effectively than on a flat surface. Molecularlevel details of the latter concept, known as the edge effect, were resolved further by Rustad and Felmy53 in terms of intersecting crystallographic planes on nanosized particles. These can be readily translated to rough surfaces. Finally, we note that Na+ loadings achieved at goethite surfaces are generally independent of the type of present anion. GT69 contacted with NaClO4 however seem to develop larger Na+ loadings, as seen in two data points in Figure 9 (cf. inversed green triangles). Although the details of these interactions remain to be resolved, we suspect that ClO4− promotes the formation of thicker double layer and thereby enables the approach of additional Na+ ions to the mineral surface. This specific aspect will be discussed in a separate communication reporting results of ongoing molecular dynamics simulations of idealized FeOOH surface contacted with solutions of NaClO4.

3.3.2.2. Lepidocrocite. In contrast to GT69 and GT122, where differences in surface porosity affected surface loadings, differences in charge and electrolyte ion loadings achieved on LL and RL are primordially ascribed to morphological differences. RL particles contacted with NaCl solutions acquired more surface charge and, correspondingly, more Cl− than on LL. This can be explained by the greater density of proton active sites on RL than on LL (Table 1). It should however be stressed that the pdi-to-electrolyte responses of both minerals are highly comparable, as emphasized in Figure 9. We also note that this response is 2−3 stronger than on goethite than on lepidocrocite (Figure 9) because the neutrally charged and proton inactive (010) plane also acquires electrolyte ions, as will now be discussed. In line with the potentiometric titration data (Figure 3) showing that more pdi charge is developed on LL than on RL in NaClO4, the XPS data reveal greater perchlorate loadings on LL (Figure 8b at pH ∼4). As this was not observed on goethite, a probable cause for this result may lie in interactions between neutrally charged μ-OH groups of the (010) plane and perchlorate ions. Although μ-OH groups are generally deemed to be proton inactive under normal environmental conditions, their regular alignment (cf. Song and Boily35,38 for details on the surface structure) could facilitate such interactions.54 Ongoing molecular dynamics simulations of this plane in our group are in fact supporting this idea by showing that direct μOH···OClO3− hydrogen bonds are formed on this surface, alongside additional complexes that are completely in outersphere coordination. Cl− in these simulations remains, in contrast, predominantly separated by at least one hydration shell, yet its net charge density is closer to the surface than that of the various perchlorate species. The thicker interfacial region induced by perchlorate adsorption could thus be a viable explanation to the considerably larger sodium loadings achieved in perchlorate compared to chloride (Figure 9), especially if multiple ionic layers would be formed.55,56 It could moreover play a role in the enhanced pdi charge development in LL (Figure 3). This specific aspect would however warrant additional investigation. Interactions between the Na+ ion and the (010) plane of lepidocrocite also merit special attention in this regard. The considerably larger Na+ loadings achieved on lepidocrocite compared to goethite (Figure 9) could also be explained by

Figure 9. Variations of Fe-normalized Cl−, ClO4−, and Na+ atomic ratios at goethite (GT69, GT122) and lepidocrocite (RL, LL) surfaces as a function of pdi charge. The latter values were obtained by interpolation of the potentiometric titration data of Figure 3. Colorcoded highlighted areas and solid lines represent general trends. 12135


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direct μ-OH interactions with Na+ ion. Once more, ongoing molecular dynamics simulations of this plane are pointing to the formation bidentate sodium species at this plane, (μOH)2···Na+, as both fully and partially hydrated species. We also note that XPS-derived Na+ loadings on LL are 1.22 times greater than on RL, a value that is highly comparable to the ratio of μ-OH sites between these two planes (1.44). This final result falls in line with the role of the (010) plane of lepidocrocite in stabilizing electrolyte ions, one that is acting alongside pdi-induced effects on the amphoteric (001) plane. Consideration of the distinct reactivities of these planes is therefore essential for predicting interactions with surfaces these types of lepidocrocite particles.

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4. CONCLUSIONS This work underscored the relationships between pdi and electrolyte loadings on FeOOH minerals of different surface roughness and structure. In the case of goethite, differences in porosity explained differences in pdi and potentially ionic loadings. It was also shown that the greater surface roughness of GT122 stabilized excess ions, as can chiefly be explained by facilitated solvation of surface species.53 While anion identity did not influence cation loadings at goethite surfaces, GT69 appears to develop larger Na+ loadings in ClO4− than GT122 (Figure 8a), a finding warranting further investigations. In the case of lepidocrocite, results showed how the larger charge-tosize ratio of Cl− is responsible for the effective chargeneutralizing capacity of this ion, hence the larger pH-dependent pdi loadings. At the same time ClO4− could be a strong adsorbent at the (010) plane of lepidocrocite. We also provided clues for important interactions of Na+ at this plane. Ionic loadings could moreover be enhanced by perchlorate due to a thickening of the interfacial region. In the same token, the larger pdi adsorption achieved in chloride-bearing media would be associated with a thinner inner-Helmholtz layer and thereby a greater capacitance than in perchlorate-bearing solutions. We nonetheless stress that the actual response of the counteranion to pdi adsorption is generally unaffected by its charge-to-size ratio, as underscored in Figure 9.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.A.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Swedish research council (VR 2012-2976) as well as by the Kempe and Wallenberg Foundations. The authors declare no competing financial interest.

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REFERENCES

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Electrolyte Ion Binding at Iron Oxyhydroxide Mineral Surfaces

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