Structures, Formation And Transformation, And Surface Physicochemical Properties Of Several Meta-stable Iron Oxides

Iron oxides, including oxides, hydroxides and oxyhydroxides, are ubiquitous in environments with small particle size, large surface area, high surface reactivity, and commonly associate with elements of phosphorus（P）, sulfur（S）, silicon（Si）, manganese（Mn）, etc. Thus, iron oxides influence the bioavailability and toxic of the related elements by adsorption and co-precipitation. Meanwhile, during the processes of Fe2+ oxidation or Fe3+ hydrolysis, the co-exist ions regulate the crystallization and growth of iron oxides, changing the mineralogical properties and surface reactivity of the products. Due to the limitation of methods and techniques, the previous studies more focus on the crystalline iron oxides, such as goethite, hematite and lepidocrocite, while the environmental geochemical processes and mechanisms of poor crystalline and high reactivity metastable iron oxides have been less addressed. Therefore, in this study, ferrihydrite, green rust, and schwertmannite were chosen as the research subject. The structure, formation and transformation, and surface physicochemical properties of ferrihydrite, and formation and transformation of green rust, and sulfate local atomic environment in schwertmannite structure were investigated by combining various solution chemical experiments, X-ray diffraction（XRD）, horizontal attenuated total reflection-Fourier transform infrared（HATR-FTIR）, X-ray absorption spectroscopy（XAS） structure, X-ray pair distribution function（PDF）, and so on. The main results are listed as following:1. The effects of Fe（III） hydrolysis rates and Si concentrations on the formation of ferrihydrite, and the role of aqueous Mn（II） on transformation of two-line ferrihydrite（2LFh） were examined. Results indicate that Fe（III） hydrolysis products evolve with decreasing hydrolysis rates from 2LFh to six-line ferrihydrite（6LFh） and goethite, and to 6LFh, lepidocrocite and goethite. This increase in structural order coupled with decreasing hydrolysis rates can be attributed to the decreased supply of hydrolyzed Fe（III） species. Fe（II） oxidation products also vary with increasing Si concentration from an assemblage of lepidocrocite and goethite to a mixture of 6LFh with small amount of poorly crystalline lepidocrocite and goethite, and to 2LFh. The strong adsorption and polymerization of Si on the surface of iron（oxyhydr）oxides limits nucleation of hydroxychloride green rust（GR1（Cl-）） and is likely responsible for hindering the formation of lepidocrocite and goethite in the presence of Si. During aging in suspension at 60 oC, 2LFh converts into goethite and hematite. The conversion rate increases as p H increases from 4 to 12 and at alkaline p H values（7, 9 and 12） a portion of the pre-formed goethite converts into hematite. In the presence of low concentrations of Mn（II）（8 m M） at acidic or neutral p H（4, 5.5 and 7） aging overwhelmingly favors the formation of goethite over hematite. This is likely caused by the catalytic effect of trace Fe（II） produced from Mn（II） oxidation by ferrihydrite. However, when 24 m M Mn（II） is added instead, the transformation of ferrihydrite is largely inhibited due to the limited accessibility of surface sites caused by increasing Mn（II） adsorption.2. The impacts of ferrihydrite surfaces on the oxidation of dissolved Mn（II） and concurrent formation of Mn（III/IV）（oxyhydr）oxides under various conditions were explored. In the absence of ferrihydtite, the products of 24 m M Mn（II） oxidation by atmospheric O2 are manganite（γ-Mn OOH） at p H 7.5 and 8.5, feitknechtite（β-Mn OOH）, groutite（α-Mn OOH） and manganite at p H 8, and hausmannite（Mn3O4） at p H 9. In contrast, in the presence of ferrihydrite, manganite is formed at p H 6.5- 8, manganite and hausmannite at p H 8.5, hausmannite and birnessite（δ-Mn O2） at p H 9. When 24 m M Mn（II） is oxidized by pure O2 at p H 9 and room temperature, the products are mainly hausmannite and birnessite with a small amount of feitknechtite in the absence of ferrihydrite, while only birnessite is obtained in the presence of ferrihydrite. These results suggest that the presence of ferrihydrite promotes the formation of manganite at lower p H（p H ≤ 7） and the oxidation of Mn（II） to form higher valence of Mn（oxyhydr）oxides, such as birnessite.3. Compared with goethite and hematite, the adsorption-desorption behavior of phosphate on ferrihydrite and underlying mechanisms were investigated. The phosphate adsorption kinetics for these three minerals are described by a fast initial adsorption followed by a slow reaction, both of which comply with pseudo first-order kinetics. Ferrihydrite shows a higher amount of phosphate adsorption and OH- release, a faster slow reaction and exhibits an extra diffusion reaction. Analysis of OH- released kinetics reveal that the adsorption process includes two steps: 1) phosphate preferentially exchanges with two groups of Fe-OH21/2+: it occurs in an extremely short time for ferrihydrite and goethite, yet more time for hematite; 2) phosphate exchange with Fe-OH21/2+ and Fe-OH1/2-: the amount of OH- released and phosphate adsorbed shows a linear correlation. Isotherms of the three minerals show better conformity to the Freundlich model than the Langmuir model due to their surface heterogeneity. The desorption percentage of adsorbed phosphate using KCl in order are hematite（12.5 %） > goethite（10 %） > ferrihydrite（8.5 %）. The lower desorption for ferrihydrite indicates a higher percentage of specific adsorption. On the other hand, citric acid promotes desorption on ferrihydrite more than on goethite and hematite via a dissolution mechanism.4. The structure and magnetic properties of the nano-sized ferrihydrite samples were systematically investigated. Difference size ferrihydrite samples were synthesized, and they are 1.6, 2.6, 3.4 and 4.4 nm, respectively. The XRD data of all samples are fitted well with the Michel model parameters and their PDF are similar, indicating that the long- and medium- range ordered structure does not vary with crystallite size. XAS analysis shows that the ferrihydrite samples also have similar local atomic structure with similar Fe-O and Fe-Fe interatomic distances but increasing numbers of neighboring Fe atoms as crystallite size increases. Magnetic data indicate that ferrihydrite samples are antiferromagnetic with a ferromagnetic-like moment at the lower temperature（100 K and 10 K）, but paramagnetic at room temperature. In addition, magnetization decreases with increasing crystallite size, and the smallest sample, two-line ferrihydrite（2LFh₁）, has much higher coercivities（Bc） than the other samples, implying that the surface structure may predominate the changes in magnetism with size for the disordered that is very small ferrihydrites. Smaller-sized ferrihydrite has less magnetic hyperfine splitting and a lower unblocking temperature（TB）.5. The influence of crystallite size on the adsorption reactivity of phosphate on ferrihydrite samples was investigated. With increasing crystallite size, the specific surface area（SSABET） decreases from 427 to 234 m2 g-1 and micropore volume（Vmicro） from 0.137 to 0.079 cm3 g-1. Proton adsorption at p H 4.5 and 0.01 M KCl ranges from 0.73 to 0.55 mmol g-1. Phosphate adsorption capacity at p H 4.5 and 0.01 M KCl for the ferrihydrites decreases from 1690 to 980 μmol g-1 as crystallite size increased, while the adsorption density normalized to SSABET was similar. Phosphate adsorption on the ferrihydrites exhibited similar behavior with respect to both kinetics and the adsorption mechanism. The kinetics could be divided into three successive first-order stages: relatively fast adsorption, slow adsorption and a very slow stage. With decreasing crystallite size ferrihydrites exhibited increasing rate constants per mass for all stages. Analysis of OH- release and attenuated total reflectance infrared spectroscopy（ATR-IR） and differential pair distribution function（d-PDF） results indicated that initially phosphate preferentially bound to two Fe-OH21/2+ groups to form a binuclear bidentate surface complex without OHrelease, with smaller size ferrihydrites exchanging more Fe-OH21/2+ per mass. Subsequently, phosphate exchanged with both Fe-OH21/2+ and Fe-OH1/2- with a constant amount of OHreleased per phosphate adsorbed. Also in this stage binuclear bidentate surface complexes were formed with a P-Fe atomic pair distance of ³.25 ?.6. The transformation of hydroxycarbonate green rust GR1（CO32-） by air oxidation at different conditions and the underlying mechanisms were studied. At the synthesis stage, the formation of GR1（CO32-） was completed when the suspension p H attained a minimum and the ratio n（FeII）/n（Fetotal） in the mineral attained a maximum. At the subsequent transformation stage, a dissolution- oxidation- reprecipitation process was observed, in which GR1（CO32-） firstly dissolves to less crystalline iron hydroxide and Fe2+, and is then oxidized to crystalline iron（oxyhydr）oxides. When the p H increases from 6.5 to 10 at 25 °C, the transformation rate and oxidation rate of GR1（CO32-） gradually decrease, the final products change from lepidocrocite to goethite and magnetite, and the crystallite size of goethite gradually increases with p H. When temperature increases from 15 °C to 45 °C at about p H 7.7 8.8, the dissolution- reprecipitation rate of GR1（CO32-） gradually increases while the oxidation rate decreases, and the final products also change from lepidocrocite to goethite and magnetite. When the airflow rate increases from 0 to 0.1 m3·h-1 at 25 °C, the oxidation rate gradually increases, the final products vary from goethite to lepidocrocite and their crystallite sizes and crystallinity gradually decrease. In general, the Fe2+ oxidation rate is governed by p H, temperature and O2 concentration. When the oxidation rate increases, the products follow the order from magnetite to goethite to lepidocrocite, and the crystallite size and crystallinity decrease.7. The transformation of GR1（CO32-） in the presence of different orthophosphate（P） or silicate（Si） concentration were conducted. Results show that both P and Si significantly affect the transformation of GR1（CO32-） through adsorption on the minerals. The time of transformation and the type of transformation products and their crystallinity and morphology all depend on the Fe/anion molar ratio. The transformation can be promoted（Si） or retarded（P） as compared to the control without P or Si. When decreasing Fe/P ratio, the products change from acicular goethite（absence of P） to tabular lepidocrocite（Fe/P: 120- 48） to mixed phases of platelets of ferric GR1（CO32-）（EX-GR1）, lepidocrocite and ferrihydrite（Fe/P: 24- 3）. In the case of Si, the products are goethite for Fe/Si ratios down to 12, but the crystallinity and particle size decrease and the morphology changes from acicular（absence of Si） to plate-like or isodimensional particles（Fe/Si: 48- 12）, comparable to those of natural goethite samples commonly found in soils. At Fe/Si = 3, the products are platelets of EX-GR1 with some goethite and ferrihydrite. The possible pathway of oxidative GR1（CO32-） transformation in the presence low concentrations of Si（Fe/Si ≥ 12） and in the control system could be denoted as GR1（CO32-） â†' amorphous γ-Fe OOH-like phase â†' α-Fe OOH via a dissolution-oxidation-precipitation mechanism. In addition, the transformation of γ-Fe OOH-like phase to goethite is rapidly catalyzed by Fe2+ released during dissolution of GR1（CO32-）. For the P system, most released Fe2+ may complex with P, which inhibits the catalytic role of Fe2+ and goethite formation.8. The sulfate atomic environment was firstly explored using S K-edge X-ray absorption near edge structure（XANES） and extended X-ray absorption fine structure（EXAFS） spectroscopy. Results indicate that sulfate exists as both inner- and outer-sphere complexes in schwertmannite. Regardless of the sample preparation conditions, the EXAFS-determined S-Fe inter-atomic distances are 3.22 – 3.26 ?, indicative of bidentate-binuclear sulfate inner-sphere complexes. A XANES linear combination fitting analysis shows that the proportion of the inner-sphere complexes decreases with increasing p H for both wet and dried samples and that the dried samples contained much more innersphere complexes than the wet ones at any given p H. These results are consistent with the qualitative analysis of the XANES pre-edge feature. Assuming that schwertmannite is an akaganéite-like structure, the sulfate inner-sphere complexation suggests that, the double chains of the edge-sharing Fe octahedra, enclosing the tunnel, must contain defects, on which reactive singly-Fe coordinated hydroxyl functional groups form for ligand exchange with sulfate. The drying effect suggests that the tunnel contains H2 O molecules in addition to sulfate ions.