FeOOH Polymorphs Research Articles

Zinc Stable Isotope Fractionation Mechanisms during Adsorption on and Substitution in Iron (Hydr)oxides.

The Zn isotope fingerprint is widely used as a proxy of various environmental geochemical processes, so it is crucial to determine which are the mechanisms responsible for isotopic fractionation. Iron (Fe) (hydr)oxides greatly control the cycling and fate and thus isotope fractionation factors of Zn in terrestrial environments. Here, Zn isotope fractionation and related mechanisms during adsorption on and substitution in three FeOOH polymorphs are explored. Results demonstrate that heavy Zn isotopes are preferentially enriched onto solids, with almost similar isotopic offsets (Δ66/64Znsolid-solution = 0.25-0.36‰) for goethite, lepidocrocite, and feroxyhyte. This is consistent with the same average Zn-O bond lengths for adsorbed Zn on these solids as revealed by Zn K-edge X-ray absorption fine structure spectroscopy. In contrast, at an initial Zn/Fe molar ratio of 0.02, incorporation of Zn into goethite and lepidocrocite by substituting for lattice Fe preferentially sequesters light Zn isotopes with Δ66/64Znsubstituted-stocksolution of -1.52 ± 0.09‰ and -1.18 ± 0.15‰, while Zn-substituted feroxyhyte (0.06 ± 0.11‰) indicates almost no isotope fractionation. This is closely related to the different crystal nucleation and growth rates during the Zn-doped FeOOH formation processes. These results provide direct experimental evidence of incorporation of isotopically light Zn into Fe (hydr)oxides and improve our understanding of Zn isotope fractionation mechanisms during mineral-solution interface processes.

Iron oxyhydroxide polytype (γ-, δ- and β-FeOOH) structures govern Zn mobility

Iron oxyhydroxide polytype minerals, such as goethite (α-FeOOH), lepidocrocite (γ-FeOOH), feroxyhyte (δ-FeOOH) and akaganeite (β-FeOOH), widely exist in terrestrial environments, and play important roles in mediating heavy metal mobility. Previous studies mainly focus on goethite, while the other three polytypes have been rarely explored. In the present work, Zn retention contents and mechanisms in Zn-coprecipitated γ-, δ- and β-FeOOH minerals were thoroughly investigated. Additionally, Zn mobility during anoxic incubation of Zn-containing lepidocrocite and feroxyhyte samples was monitored. It demonstrates that, feroxyhyte and lepidocrocite can accommodate relatively higher amounts of Zn (7.71 ± 0.20 mol% and 8.56 ± 0.22 mol% respectively) than akaganeite that retains almost no Zn (0.054 ± 0.002 mol%). In contrast, Zn has the most significant effect on akaganeite morphology. Part of total Zn is incorporated into the lepidocrocite or feroxyhyte structure by substitution for lattice Fe while the remaining Zn is adsorbed on the mineral surfaces. The different Zn substitution levels in these FeOOH minerals are greatly related with the mineral structures, that is, the ratio of edge-sharing to corner-sharing [FeO6] and thus the spatial freedom, and the pHs of the mineral synthetic solutions. During anoxic incubation in the presence of 1 mM Fe2+ at pH 6 ± 0.05 for 60 d, both Zn-containing lepidocrocite and feroxyhyte keep stable, while the dissolved Zn concentrations rapidly increase at initial stage, then decrease after reaction for 3 d, and increase again from 10 d till to the end. The final Zn release from feroxyhyte (∼98%) is much higher than that from lepidocrocite (∼30%), probably owing to the higher Fe2+-induced reductive dissolution and desorption rates, and larger amounts of Zn adsorbed on the surfaces of the former. These results help to understand the effects of Zn on the physicochemical properties of FeOOH polymorphs and the geochemical behaviors of Zn associated with Fe oxyhydroxide minerals.

Influence of elemental sulfur on the corrosion mechanism of X80 steel in supercritical CO2-saturated aqueous phase environment

The corrosion mechanisms of X80 steel under conditions containing elemental sulfur (S) were investigated in a supercritical CO2-saturated aqueous phase environment. The uniform corrosion rate slightly increased with immersion time in the absence of S, while it decreased with immersion time in the presence of S, and pitting was induced. There were two polymorphs of FeOOH in this study: α-FeOOH, which formed in an S-free environment by conversion of Fe(NO3)3; γ-FeOOH, which formed in the presence of S by conversion of FeSO4. There were also two polymorphs of Fe2O3: α-Fe2O3 could be formed by Fe(OH)3 in both environments, while γ-Fe2O3 could be formed from α-FeOOH in an S-free environment or γ-FeOOH in the presence of S. The S could cause pitting through the acid regeneration cycle reaction and occluded corrosion defects. Moreover, FeS gradually replaced FeCO3 to protect the substrate in the S-containing environment.

Adsorption of polycyclic aromatic hydrocarbons on FeOOH polymorphs: A theoretical study

In this work we studied interactions of polycyclic aromatic hydrocarbons (PAHs) with surfaces of two iron oxyhydroxide polymorphs, goethite and lepidocrocite, by means of density functional theory (DFT) method. The first focus here was to obtain adsorption energies of selected PAH molecules with the (110) goethite surface and the (010) lepidocrocite surface by performing DFT calculations with the inclusion of dispersion interactions. To this end we have performed assessment of six different methods to show their efficiency and predictability of nonbonding interactions. The second objective was to compare adsorption capability of two FeOOH polymorphs, goethite and lepidocrocite, as common minerals in several soil types and to relate obtained adsorption energies to different surface structure and topology of the most stable surfaces of both polymorphs. The goethite (110) surface has more complicated topology and three different types of OH groups compared to the lepidocrocite (010) surface formed by a plane of OH groups of the same type. For goethite, adsorption energy did not increase monotonically with the increasing molecular size of PAHs and the strongest adsorption was observed for the linearly-shaped anthracene molecule. On the other hand, adsorption of PAHs at the lepidocrocite surface increased energetically with increasing molecular weight, and the strongest adsorption was achieved for the pyrene molecule. The results showed that the goethite (110) surface is more attractive than the (010) lepidocrocite surface, which is explained by the different surface topology, and types and surface density of the OH groups.

Phase transitions in ε-FeOOH at high pressure and ambient temperature

AbstractConstraining the accommodation, distribution, and circulation of hydrogen in the Earth's interior is vital to our broader understanding of the deep Earth due to the significant influence of hydrogen on the material and rheological properties of minerals. Recently, a great deal of attention has been paid to the high-pressure polymorphs of FeOOH (space groups P21nm and Pnnm). These structures potentially form a hydrogen-bearing solid solution with AlOOH and phase H (MgSiO4H2) that may transport water (OH–) deep into the Earth's lower mantle. Additionally, the pyrite-type polymorph (space group Pa3 of FeOOH), and its potential dehydration have been linked to phenomena as diverse as the introduction of hydrogen into the outer core (Nishi et al. 2017), the formation of ultralow-velocity zones (ULVZs) (Liu et al. 2017), and the Great Oxidation Event (Hu et al. 2016). In this study, the high-pressure evolution of FeOOH was re-evaluated up to ~75 GPa using a combination of synchrotron-based X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and optical absorption spectroscopy. Based on these measurements, we report three principal findings: (1) pressure-induced changes in hydrogen bonding (proton disordering or hydrogen bond symmetrization) occur at substantially lower pressures in ε-FeOOH than previously reported and are unlikely to be linked to the high-spin to low-spin transition; (2) ε-FeOOH undergoes a 10% volume collapse coincident with an isostructural Pnnm → Pnnm transition at approximately 45 GPa; and (3) a pressure-induced band gap reduction is observed in FeOOH at pressures consistent with the previously reported spin transition (40 to 50 GPa).

Electrocatalytic arsenite oxidation using iron oxyhydroxide polymorphs (α-, β-, and γ-FeOOH) in aqueous bicarbonate solution

α-, β-, and γ-FeOOH polymorphs have been synthesized and used for electrocatalytic As(III) oxidation into less toxic As(V) in aqueous bicarbonate solution. As-synthesized FeOOH electrodes exhibit unique crystalline structures and morphologies. All three FeOOH polymorphs have specific complexation with bicarbonate under alkaline conditions, whereas the complexation disappears at pH 6.7. α-FeOOH is active for both As(III) oxidation and competitive water oxidation, and the Faradaic efficiency (FE) of the latter is the greatest among the polymorphs. Despite the highest current density, β-FeOOH shows the lowest FE values for both As(III) and water oxidation. In contrast, γ-FeOOH exhibits a significantly higher catalytic activity for As(III) oxidation, up to two-fold and five-fold greater FE values compared to α- and β-FeOOH, respectively. The superior activity of γ-FeOOH is attributed to the combined effects of large As(III) adsorption, fast charge transfer, and facile generation of hydroxyl radicals.

Electrical conductivity of diaspore, δ-AlOOH and ε-FeOOH

AbstractElectrical conductivities of diaspore (α-AlOOH), δ-AlOOH, and ε-FeOOH were measured by impedance spectroscopy with a frequency range from 10–1 to 106 Hz at pressures from 8 to 20 GPa and temperatures from 500 to 1200 K, well below the dehydration temperatures of these phases at the relevant pressures. For diaspore, the relationship between electrical conductivity and reciprocal temperature can be well fitted by the Arrhenius formula:σ = σ 0 exp ⁡ [ − ( Δ E + P Δ V ) k T ] ,where σ0 is the pre-exponential factor, ∆E is the activation energy, and ∆V is activation volume of 56.0 ± 1.2 S/m, 0.55 ± 0.02 eV, and 1.68 ± 0.12 cm3/mol, respectively. The electrical conductivity of diaspore decreases with increasing pressure ranging from 8 to 12 GPa by a half order of magnitude, whereas the conductivity becomes almost constant in a pressure range above 12 GPa. δ-AlOOH and ε-FeOOH show one and two orders of magnitude higher electrical conductivity than diaspore. Electrical conductivities of δ-AlOOH and ε-FeOOH, which have isostructural CaCl2-type hydroxide structure, show the nearly identical activation enthalpies (0.38 ± 0.01, 0.33 ± 0.05 eV), which are relatively lower than that of diaspore. The dominant conduction mechanism of AlOOH phases can be regarded as proton conduction. The conductivity difference between diaspore and δ-AlOOH attributes to result in the different O1H bond lengths of each phase. The reduction of O1H bond length with increasing pressure could enhance the proton migration by reducing the potential barrier, thereby raising the electrical conductivity. Small polaron conduction may contribute to the conductivity of ε-FeOOH to generate higher conductivity than δ-AlOOH. Furthermore, hydrogen bond symmetrization will also play an important role in the conductivity discrepancy of these hydrous minerals with CaCl2-type hydroxide structure. For subducted sedimentary rocks, polymorphs of AlOOH and FeOOH are representative hydrous phases. Al-rich sediments show conductivity reduction with increasing depth until phase transformation occurs because diaspore represents negative pressure dependence of conductivity. After transformation to δ-AlOOH, the conductivity will jump up around 18 GPa. If ε-FeOOH is stable above 5 GPa in an iron-rich lithology, such as banded iron formation (BIF), a high conductivity zone with positive pressure dependence could be observed to the deep lower mantle.

Solubility behavior of δ-AlOOH and ε-FeOOH at high pressures

Abstract Low-pressure polymorphs of AlOOH and FeOOH are common natural oxyhydroxides at the Earth's surface, which may transport hydrogen to the deep mantle via subduction. At elevated pressures, the low-pressure polymorphs transform into δ-AlOOH and ε-FeOOH with CaCl2-type structure, which form a solid solution above 18 GPa. Nevertheless, few studies have examined the solid solution behavior of this binary system in detail. In this study, we ascertain the phase relations in the AlOOH–FeOOH binary system at 15–25 GPa and 700–1200 °C. X-ray diffraction (XRD) measurements of quenched samples show that δ-AlOOH and ε-FeOOH partly form solid solutions over wide pressure and temperature ranges. Our results demonstrate that a binary eutectic diagram is formed without dehydration or melting below 1200 °C at 20 GPa. We also observe that the maximum solubilities of Al and Fe in the solid solutions are more strongly influenced by temperature than pressure. Our results suggest that the CaCl2-type hydroxides subducted into the deep mantle form a solid solution over a wide composition range. As AlOOH and FeOOH are present in hydrous crust, these phases may be subducted into the deep interior, transporting a significant amount of hydrogen to deeper regions. Therefore, a better understanding of this binary system may help elucidate the model geodynamic processes associated with the deep water cycling in the Earth.

First principles calculations of surface dependent electronic structures: a study on β-FeOOH and γ-FeOOH.

We report a theoretical study on iron oxyhydroxide (FeOOH). The FeOOH surface is expected to act as an efficient electrochemical catalyst for the oxygen evolution reaction (OER), because it is based on iron, an element of the fourth highest Clarke number. Experimentally, the OER activity of β-FeOOH is known to be higher than that of γ-FeOOH. However, the details of the OER mechanism and the surface reactivities of the FeOOH polymorphs have not yet been fully understood. We performed first-principles calculations of bulk and surfaces of β-FeOOH and γ-FeOOH using density functional theory, to investigate their electronic structures and catalytic activities. The calculations suggest that depending on the surface indices, several surfaces may be favored for catalytic activities.

Revisiting models of Cd, Cu, Pb and Zn adsorption onto Fe(III) oxides

A generic hydrous Fe oxide surface is often used in surface complexation modeling for predicting metal behavior in aqueous and soil environments. However, this approach may lead to incorrect results because there are many different naturally-occurring Fe(III) (oxyhydr)oxide mineral phases in the environment, each exhibiting distinct adsorption characteristics. This work presents a consistent dataset of surface complexation model parameters valid at 25 °C and 1 bar that characterize the adsorption of selected divalent metals (Cd, Cu, Pb, Zn) onto environmentally relevant Fe(III) (oxyhydr)oxides, i.e., FeOOH and Fe2O3 polymorphs (goethite, hematite, lepidocrocite, maghemite), at varying metal concentrations and ionic strengths. The parameters were obtained using a simple and unified workflow and are readily implementable into widely used geochemical codes, such as Visual MINTEQ, MINEQL+, and ORCHESTRA, enabling quantification of metal adsorption onto a range of Fe(III) oxides in various aqueous environments.

Iron and nitrogen cycling, bacterioplankton community composition and mineral transformations involving phosphorus stabilisation in the ferruginous hypolimnion of a post-mining lake.

Lake Medard is an oligotrophic post-mining lake characterised by ferruginous bottom waters, with marked redox gradients resulting from iron (Fe) and nitrogen (N) speciation and accompanying depth-dependent variations in the abundance of volatile fatty acids (VFAs), pH and alkalinity. The lacustrine system is meromictic, featuring a dysoxic hypolimnion and an anoxic monimolimnion with relatively high concentrations of sulfate (SO42-, 19 ± 2 mM) and Fe(ii) (127 ± 17 μM). An increase in dissolved manganese is also observed with increasing depth, together with a general lack of sulfide, which can only be detected at the sediment-water interface at concentrations of ∼0.30 μM. In the hypolimnion, nitrate (NO3-) becomes progressively depleted and ammonium (NH4+) dominates the dissolved N inventory (up to 185 ± 13 μM). Here we describe the biogeochemical disequilibrium conditions governing critical mineralogical transformations involving Fe and phosphorus (P) co-precipitation in the dysoxic-to-anoxic bottom water column. A combination of mineral equilibrium modelling and synchrotron-based diffraction and spectroscopic techniques was applied to investigate the minerals comprising the upper anoxic sediments. The combined dataset indicates that elemental recycling on and below the hypolimnion promote the precipitation of FeOOH polymorphs that accumulate as heterogeneous mineral clusters. Changes in the relative abundance of bacterioplankton taxa with increasing water depth point to a link between the activity of certain members of Proteobacteria and the co-recycling of carbon (C), N, and Fe stocks. Such a redox recycling process seems to lead to P stabilisation into organic-rich Fe-(oxyhydr)oxides near and above the anoxic sediment-water interface (SWI).

Photocatalytic Properties of All Four Polymorphs of Nanostructured Iron Oxyhydroxides

AbstractFour different polymorphs of nanostructured iron oxyhydroxides, namely; goethite (α‐FeOOH), akaganeite (β‐FeOOH), lepidocrocite (γ‐FeOOH), and feroxyhyte (δ‐FeOOH) were synthesized and fully characterized by X‐ray diffraction, electron microscopy, UV/Visible spectrophotometry, Brunauer–Emmett–Teller (BET) measurements, and X‐ray photoemission spectroscopy. The relationship between these iron oxyhydroxide polymorphs and their photocatalytic properties was explored by examining the extent of methylene blue (MB) degradation by each polymorph under visible‐light irradiation. Feroxyhyte exhibited the best photocatalytic properties and degraded 85 % of the MB dye in five hours. In comparison, goethite, akaganeite, and lepidocrocite degraded only 40 %, 35 %, and 30 % of the MB in five hours, respectively. To understand this trend, the surface area, particle size and shape, and electronic band properties were systematically studied and discussed. It was found that the rate of MB degradation relates mainly to the surface area of the FeOOH polymorphs more than any other factor. This is the first report of a comparative study of the physical, electronic, and photocatalytic properties of all four polymorphs of nanostructured iron oxyhydroxides.

Surface and Bulk Thermal Dehydroxylation of FeOOH Polymorphs.

In this study, bulk and surface thermal decomposition of synthetic iron oxyhydroxides to iron oxides was followed using the temperature-programmed desorption (TPD) technique. Submicron-sized akaganéite (β-FeOOH), rod- and lath-shaped lepidocrocite (γ-FeOOH), and goethite (α-FeOOH) particles were heated in vacuo in the 30-400 °C range, and their OH vibrational modes were monitored by Fourier transform infrared (FTIR) spectroscopy while H2O(g) release was monitored by quadrupole mass spectrometry. Peak thermal dehydroxylation temperatures were larger in the order of lath lepidocrocite (200 °C) < akaganéite (200/260 °C) < rod lepidocrocite (268 °C) < goethite (293 °C). Pre-equilibration of these particles to aqueous solutions of HCl increased dehydroxylation temperatures of all minerals except goethite by 13-40 °C. These shifts were explained by (1) the dissolution of particles or regions of particles of lower degree of crystallinity by HCl, as well as (2) the strengthening of the hydrogen bond environment in the akaganéite bulk. The latter is a means of facilitating H2O(g) formation via interactions between two adjacent OH groups. Strongly analogous forms of interactions at the FeOOH particle surfaces were also shown to facilitate the release of singly coordinated (-OH) hydroxo groups to the gas phase at temperatures lower than 125 °C, thus creating OH vacancies that may be actively involved in the transfer of bulk to surface OH groups during thermal dehydroxylation. Doubly- (μ-OH) and triply- (μ3-OH) coordinated hydroxo groups were however resilient to exchange under those conditions, and their dehydroxylation was strongly congruent with that of bulk OH groups. By resolving the bulk and surface thermal decomposition of FeOOH polymorphs, this work provides clearer insight into the fate of these materials in natural and technological settings where important thermal gradients are commonplace.

High-pressure behavior of the polymorphs of FeOOH

The high-pressure structural and electronic behavior of α-, β-, and γ-FeOOH were studied in situ using a combination of synchrotron X-ray diffraction (XRD) and X-ray emission spectroscopy (XES). We monitored α-FeOOH by XES as a function of pressure up to 85 GPa and observed an electronic spin transition that began at approximately 50 GPa, which is consistent with previous results. In the γ-FeOOH sample, we see the initiation of a spin transition at 35 GPa that remains incomplete up to 65 GPa. β-FeOOH does not show any indication of a spin transition up to 65 GPa. Analysis of the high-pressure XRD data shows that neither β-FeOOH nor γ-FeOOH transform to new crystal structures, and both amorphize above 20 GPa. Comparing our EOS results for the β and γ phases with recently published data on the α and e phases, we found that β-FeOOH exhibits distinct behavior from the other three polymorphs, as it is significantly less compressible and does not undergo a spin transition. A systematic examination of these iron hydroxide polymorphs as a function of pressure can provide insight into the relationship between electronic spin transitions and structural transitions in these OH- and Fe 3+ -bearing phases that may have implications on our understanding of the water content and oxidation state of the mantle.

Cr(vi) removal by mesoporous FeOOH polymorphs: performance and mechanism

The mesoporous FeOOH polymorphs, i.e., goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), and feroxyhyte (δ-FeOOH) were synthesized and characterized before and after reaction with Cr(vi) using various analytical techniques.

Density functional theory study of water adsorption on FeOOH surfaces

Using density functional theory (DFT) calculations with an on-site Coulomb repulsion term, we study the composition, stability, and electronic properties of the most common FeOOH surfaces goethite(101), akaganeite(100), and lepidocrocite(010), and their interaction with water. Despite the differences in surface structure, the trends in surface stability of these FeOOH polymorphs exhibit remarkable similarities. We find that the reactivity and the binding configuration of adsorbates depend strongly on the coordination of surface iron: at the fourfold coordinated Fe2 site water is chemisorbed, whereas at the fivefold coordinated Fe1 water is only loosely bound with hydrogen pointing towards the surface. Our results show that the oxidation state of surface iron can be controlled by the surface termination where ferryl (Fe4+) species emerge for oxygen terminated surfaces and ferrous iron (Fe2+) at iron and water terminations leading to a reduced band gap. In contrast, the fully hydroxylated surfaces, identified as stable surface configurations at standard conditions from the surface phase diagram, show electronic properties and band gaps closest to bulk FeOOH with ferric surface iron (Fe3+). Only in the case of goethite(101), a termination with mixed surface hydroxyl and aquo groups is stabilized.

Structural controls on OH site availability and reactivity at iron oxyhydroxide particle surfaces

Iron oxyhydroxides (FeOOH) are highly reactive minerals of widespread occurrence in natural and industrial settings. These minerals chiefly occur as nano- to submicron-sized particles and are covered by hydroxyl functional groups coordinated to one (-OH), two (μ-OH), or three (μ(3)-OH) underlying iron atoms. These groups are reaction centers for gases, solutes as well as solvents and thereby play important roles in the fate and transformation of natural and industrial compounds. In this work we provide tools to identify hydroxyl groups on distinct crystallographic planes of two important FeOOH polymorphs, namely lepidocrocite (γ-FeOOH) and goethite (α-FeOOH). Fourier transform infrared spectroscopy was used to monitor O-H stretching vibrations of groups on particles with well-defined and distinct morphologies. Spectral responses to proton loadings and thermal gradients were used to assign bands to hydroxyl groups. These efforts were facilitated by the extraction of pure spectral components obtained by multivariate curve resolution. Molecular dynamics simulations of dominant crystallographic planes of the particles guided band assignment procedures by identifying feasible hydrogen bond networks between surface groups. Our findings provide new possibilities for molecular-scale resolution of important gas-phase processes on the surfaces of these important minerals.

Thermodynamic Stabilization of Hydrous Ferric Oxide by Adsorption of Phosphate and Arsenate

Hydrous ferric oxide (HFO) is an X-ray amorphous compound with a high affinity for anions under strongly or mildly acidic conditions. Because of the usually small particle size of HFO, the adsorption capacity is high and adsorption may significantly impact the thermodynamic properties of such materials. Here we show that adsorption of phosphate and arsenate stabilizes HFO by experimental determination of enthalpies of formation (by acid-solution calorimetry) and estimates of standard entropies for six phosphate- or arsenate-enriched HFO samples. At pH values lower than ∼5, the phosphate-doped HFO is not only less soluble than ferrihydrite (anion-free HFO) but also crystalline FeOOH polymorphs feroxyhyte and lepidocrocite. The arsenate-doped HFO is also stabilized with respect to the ferrihydrite. Phosphate availability in soils can be controlled by the phosphate-enriched HFO which is many orders of magnitude less soluble than apatite or crystalline Fe(III) phosphates, for example strengite (FePO(4)·2H(2)O). Thermodynamic dissolution models for scorodite (FeAsO(4)·2H(2)O) and As-enriched HFO show that under mildly acidic or circumneutral conditions, scorodite dissolves, As-HFO precipitates, and a substantial amount of As(V) is released into the aqueous solution (at pH 7, log m(As) ∼ -2.5). The data presented in this paper can be used to model the equilibrium concentration of Fe(III), P(V), or As(V) in soil solutions or in natural or anthropogenic sediments polluted by arsenic.

ChemInform Abstract: 2H and 7Li Solid‐State MAS NMR Study of Local Environments and Lithium Adsorption on the Iron(III) Oxyhydroxide, Akaganeite (β‐FeOOH).

2H and 7Li MAS NMR spectroscopy have been applied to characterize the surface and bulk hydroxyl groups and Li+ sorption on the iron oxyhydroxide akaganeite (β-FeOOH), a common soil mineral with a large surface area and uptake capacity for toxic cations and anions. The formation of both inner and outer-sphere complexes on the surface of akaganeite was confirmed, the former giving rise to 7Li NMR signals with large 7Li hyperfine shifts. The concentrations of these complexes was determined as a function of pH and possible Li+ binding modes and sites are proposed based on their 7Li hyperfine shifts. The binding is compared with those of the other FeOOH polymorphs, goethite and lepidocrocite. The modes of binding are similar to those of goethite, except that sites at the entrances to the tunnels become available for binding, particularly for nanosized akaganeite particles.

2H and 7Li Solid-State MAS NMR Study of Local Environments and Lithium Adsorption on the Iron(III) Oxyhydroxide, Akaganeite (β-FeOOH)

2H and 7Li MAS NMR spectroscopy have been applied to characterize the surface and bulk hydroxyl groups and Li+ sorption on the iron oxyhydroxide akaganeite (β-FeOOH), a common soil mineral with a large surface area and uptake capacity for toxic cations and anions. The formation of both inner and outer-sphere complexes on the surface of akaganeite was confirmed, the former giving rise to 7Li NMR signals with large 7Li hyperfine shifts. The concentrations of these complexes was determined as a function of pH and possible Li+ binding modes and sites are proposed based on their 7Li hyperfine shifts. The binding is compared with those of the other FeOOH polymorphs, goethite and lepidocrocite. The modes of binding are similar to those of goethite, except that sites at the entrances to the tunnels become available for binding, particularly for nanosized akaganeite particles.