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Wpływ metody syntezy na właściwości fizykochemiczne ferrihydrytu i Si-ferrihydrytu

Pieczara G., Mendsaikhan N., Manecki M., Rzepa G.

Przemysł Chemiczny

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2015

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T. 94, nr 10

1828-1831

PL

Przeprowadzono syntezę ferrihydrytów (Fe5HO8·4H2O) czystych (bez domieszki Si) i zawierających domieszki Si (stosunek molowy Si:Fe = 0,1) przez wytrącanie z roztworów wodnych. Porównano produkty otrzymane w reakcji zobojętniania roztworów siarczanu żelaza(III) (metoda siarczanowa) lub azotanu żelaza(III) (metoda azotanowa). Wykazano, że czyste ferrihydryty otrzymane metodą azotanową charakteryzują się mniejszymi rozmiarami cząstek, większą powierzchnią właściwą, mniejszym stopniem uporządkowania struktury oraz mniejszą trwałością termiczną. Sposób syntezy ma mniejszy wpływ na różnice we właściwościach ferrihydrytów z domieszką Si.

EN

Pure Fe5HO8·4H2O and Si-doped ferrihydrites (Si/Fe = 0.1 molar ratio) were pptd. from aq. solns. of Fe2 (SO4)3 and Fe(NO3)3 with NaOH optionally in presence of Na2SiO3 under stirring. Si-contg. ferrihydrites had smaller particles and showed higher thermal stability and sp. surface area than the pure ones.

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“Four-letters” (γ → ε → β → α) thermal transformation of Si-rich ferrihydrite

Pieczara G., Rzepa G., Gaweł A., Tomczyk A.

Geology, Geophysics and Environment

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2015

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Vol. 41, no. 1

121--122

EN

Ferrihydrite is a poorly ordered iron (oxyhydr)oxide, ubiquitous in near-surface environments, where it is an important scavenger of numerous toxic metals and metalloids (Cornell & Schwertmann 2003). Being metastable, ferrihydrite transforms with time into stable phases, usually goethite and/or hematite. The latter is also a final product of ferrihydrite thermal conversion via both, hydrothermal and dry-heating pathways. The transformation course is affected by many factors and for this reason its details are still under debate. Pure ferrihydrite practically does not exist in nature. The admixtures present in its composition affect many properties of the oxyhydroxide, including surface chemistry, sorption effectiveness, crystallinity, magnetic ordering and solubility. Silicate, probably the most important natural impurity, was shown to hamper thermal transformation of ferrihydrite. The process was studied in detail only for relatively low-Si samples (Campbell et al. 2002). In some environments, such as modern seafloor hydrothermal vents, higher Si/Fe proportion have been found (Sun et al. 2013). The objective of this work was to determine if (and if so, how) the high silicate content in ferrihydrite modifies its thermal transformation. Ferrihydrite samples of high Si/Fe molar ratios (0.50, 0.75, 1.00, and 1.50) were obtained by reaction of Fe2(SO4)3 with NaOH in the presence of Na2SiO3 at pH 8.2 (Vempati & Loeppert 1989). The products were characterized using X-ray powder diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and scanning electron microscopy (SEM). Simultaneous thermal analyses, including differential thermal analysis (DTA), thermogravimetry (TG) and quadrupole mass spectrometry of evolved gases (QMS) were also performed. All of the ferrihydrites were heated from 30°C • min−1to 1000°C • min−1, at 10°C • min−1in flowing air, using preheated sample as an inert material. The samples were also heated to various temperatures, chosen on the basis of the analysis of their thermal patterns. The heating was stopped immediately when reaching the desired temperature. The sample was removed, rapidly cooled in air and characterized again by XRD, FTIR and SEM. The thermal patterns revealed that the presence of Si in ferrihydrite hampers its conversion to hematite, which is reflected by shifting of the hematite crystallization exotherm to temperature as high as ca. 920°C, in comparison to 350–460°C when Si-free ferrihydrite is annealed (Pieczara et al. 2014). XRD patterns and FTIR spectra of the samples heated to 1000°C show the presence of hematite and cristobalite. It was observed that the higher molar Si-to-Fe ratio in initial mineral results in the higher cristobalite concentration in the product. SEM observation showed that hematite crystallites, embedded in cryptocrystalline silica, are distinctly smaller than those produced from pure ferrihydrite. They also exhibit wider range of crystal habits – isometric grains, plates, rods and even needles were encountered. Elongated crystallites (rods and needles) appear to be more common in the highest-Si products. Moreover, whilst during transformation of pure ferrihydrite into hematite no distinct intermediate phase is formed, the conversion of high-Si ferrihydrite proceeds by much more complex pathway. After low-temperature dehydration, the material still exhibits two-peak ferrihydrite-like XRD pattern, but a gradual amorphisation is observed up to ca. 600°C. Then, at 650°C amorphous silica emerges which is followed by the formation of nanocrystalline maghemite (γ-Fe2O3) between 700°C and 800°C. Subsequently, maghemite is transformed to orthorhombic ε-Fe2O3phase at 800–850°C. In the samples of 0.50, 0.75 and 1.0 Si/Fe ratios, further increase of temperature results in ε-Fe2O3conversion to hematite and crystallization of cristobalite-like phase. However, XRD patterns of annealed the highest-Si sample (Si/Fe = 1.5) show also the presence of β-Fe2O3between 907°C and 930°C. At 907°C hematite appears and ε-Fe2O3 vanishes by 930°C, so at the latter temperature two Fe2O3 polymorphs (α and β) are present. At 1000°C hematite is the sole iron oxide in all the products. Thus siliceous ferrihydrites transform into hematite via γ–ε or γ–ε–β pathway, depending on the Si/Fe ratio. Formation of rare epsilon and beta iron oxide polymorphs from high-Si ferrihydrites can be explained by emerging of amorphous SiO2during annealing, which subsequently acts as antisintering agent stabilizing maghemite precursor against its direct thermal conversion to hematite. Our results show that high silicate content causes not only the retarding of ferrihydrite conversion to hematite but also affects crystallinity of the product and complicates the transformation pathway. In the authors’ opinion the formation of rare epsilon and beta Fe2O3 is noteworthy for at least two reasons. Firstly, β- and, especially, ε-Fe2O3have been recently found to exhibit many interesting physical properties, which make them attractive nanomaterials for use in a wide range of applications (Machala et al. 2011). Annealing of Si-ferrihydrites might offer an alternative method of obtaining these oxides. Secondly, both these oxides have not been found in nature yet. This work suggests that their formation is possible in places where siliceous ferrihydrites or other Si-Fe amorphous sediments have been heated to high temperature, as in e.g. hydrothermal systems of mid-ocean ridges.

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The influence of silicate content on thermal stability of 2-line ferrihydrite and properties of its transformation products

Pieczara G., Rzepa G., Gaweł A.

Geology, Geophysics and Environment

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2014

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Vol. 40, no. 1

160--161

EN

Naturally occurring ferrihydrite (Fe5HO8 • 4H2O) is a poorly ordered iron (oxyhydr)oxide mineral, with non-stoichiometric composition and not fully understood structure. Because of its unique chemical and physical properties, such as low crystallinity, high surface area and surface reactivity, ferrihydrite plays significant role in e.g. inorganic weathering processes, biochemical cycling of iron and as a sorbent in various near-surface environments. Ferrihydrite is a metastable phase and transforms with time into stable oxides: goethite and/or hematite, through dissolution-reprecipitation and dehydration-rearrangement mechanisms, respectively. Ferrihydrite structure provides numerous sorption sites and for this reason substantial amounts of admixtures are present in its chemical composition. The most common and well documented impurities include silicate, phosphate, arsenate, sulphate, calcium, aluminum and organic compounds. These ions affect ferrihydrite composition, surface molecular structure and sorption properties. Silicate, probably the most important impurity, causes decreasing crystallinity of this nanomineral, modifies magnetic ordering and solubility. Thus, natural ferrihydrite distinctly differ from synthetic pure analogue. As it was previously shown, the association of Si with ferrihydrite surface hindered thermal transformation to hematite. The implications of this observation for the understanding of Si-ferrihydrite stability in geochemical systems are obvious. The aim of this work was determining of the influence of the Si/Fe ratio in ferrihydrites on its thermal transformation processes and the properties of the products. Ferrihydrite samples having different Si/Fe molar ratios: 0.00, 0.05, 0.10, 0.20, 0.25, 0.50, 0.75, 1.00, and 1.50, were obtained by reaction of Fe2(SO4)3 with NaOH in the presence of Na2Si03 at pH 8.2. After four-day incubation, dialysis and freeze-drying, the precipitates were characterized using X-ray powder diffraction (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), Raman spectroscopy, and scanning electron microscopy (SEM). Then thermal analyses (DTA/DTG/TG) were performed. All of the ferrihydrites were heated from 30°C to 1000°C, at 10°C min-1 in flowing air, using hematite as inert material. After heating all the samples were again characterized by XRD and SEM-EDS methods. DTA curve of pure ferrihydrite shows typical dehydration endotherm at 160°C and. sharp exotherm at 350°C, attributed to hematite formation. Additional weak peaks at 550°C and 710°C were probably originated form decomposition of relic sulphate. The presence of Si in ferrihydrite appears to stabilize its structure and prevents conversion to hematite: the ferrihydrite-hematite transformation peak weakens and broadens and is shifted towards higher temperatures, up to ca. 900°C for high-Si materials. However, no simple linear relationship between silica content in ferrihydrite and the position of this peak has been found. X-ray diffraction patterns indicate that the main product of thermal transformation of all ferrihydrites is hematite (α-Fe2O3). For low-Si samples (Si/Fe < 0.20), a gradual broadening of 104, 214, 300, 110 hematite reflexes has been noticed, indicating the decrease of its crystallinity. On the other hand, for high-Si materials the broadening appears to be less distinct. Increasing Si/Fe molar ratio (≥ 0.10) in the initial material took an effect also in the appearance of a cristobalite-type oxide, the content of which increases drastically for the highest-Si samples. Additionally, the XRD pattern of the Si/Fe 0.10 sample reveals the presence of some spinel phase. Hematite originating from the heating of Si-free ferrihydrite forms quite large (up to 1 mm in size) isometric and prismatic crystals, often exhibiting pseudohexagonal shape. In contrast, the oxide particles become tenfold smaller even in the lowest-silicate material (Si/Fe = 0.05) and reveal pseudospheric morphology. In higher-silicate products (Si/Fe > 0.10) the crystallites are getting elongated and the elongation increases with increasing Si/Fe ratio. This preliminary study demonstrates that silicate content causes the retarding of ferrihydrite (thermal) transformation to hematite and affecting crystallinity of the latter. Even small Si admixture in the precursor reduces crystal size of the product. During roasting of low-Si ferrihydrites some Si probably enter the hematite structure, but higher-Si hematites cannot form and for this reason heating of high-Si ferrihydrites produces two-phase composition.

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Synthesis and preliminary characterization of 2-line ferrihydrite containing different amounts of Si

Pieczara G., Rzepa G.

Geology, Geophysics and Environment

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2012

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Vol. 38, no. 4

517--518

EN

Ferrihydrite (Fe5HO8 • 4H2O) is the reddish-brown, nearly amorphous hydrous ferric oxyhydroxide mineral with variable composition, widespread in various near-surface environments. Being thermodynamically unstable, it transforms with time into goethite (a-FeOOH) or hematite (a-Fe2O3). Due to its low crystallinity and high surface area, ferrihydrite is highly reactive and plays, through coprecipitation and adsorption reactions, an essential role in e.g. geochemical cycling of various trace elements and capturing of contaminants from streams and groundwater in such environments as ironladen springs, mine wastes and acid mine drainage. The environmental importance is one of the main reasons for numerous studies on ferrihydrite properties which have been carried out recently. These studies have been dealing with, among others, solubility, thermodynamic features, surface chemistry, sorption and catalytic properties etc. However, in the majority of experimental works synthetic ferrihydrite analogues with chemical composition close to ideal have been applied. Such approach might cause oversimplification, because ferrihydrite always contain substantial amounts of admixtures, with Si, C, P, As, Ca, Al being probably most common. One of the most important and the most common impurity is Si, which in the form of silicate ion has strong affinity for a hydrous ferric oxyhydroxide surface. An association of ferrihydrite with Si not only retards the rate of its transformation to the stable phases (goethite or hematite), but also seriously affects e.g. surface chemistry. Although Si-ferrihydrite was successfully synthesized in several studies, relatively little is known about its properties. The aim of this work was to fill that gap. Ferrihydrite samples having different Si/Fe molar ratios: 0.00, 0.05, 0.10, 0.20, 0.25, 0.50, 0.75, 1.00, and 1.50, were obtained by reaction of Fe2(SO4)3 with NaOH in the presence of Na2Si03 at pH 8.2. The precipitates were incubated for four days at room temperature, then the suspensions were dialyzed to remove an excess of salt, and finally freezedried. The products were characterized using a variety of analytical techniques, including X-ray powder diffraction (XRD), inductively coupled plasma atomic emission spectrometry (ICP-AES), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and Raman spectroscopy. The X-ray pattern of pure ferrihydrite reveals two asymmetric broad bands with maxima at 2.55 A and 1.50 A, characteristic for 2-line ferrihydrite. With increasing Si/Fe molar ratio, shifting in position of the first (ca. 35°20) peak towards lower angles (up to ca. 29°20) was observed. Gradual broadening of the peak and declining its asymmetry were noticed as well. Both the position and the shape of the second band did not shift at the same time. These features indicate reducing crystallinity and lowering grain size of Si-ferrihydrite in comparison to those for the pure ferrihydrite. Infrared spectrum of the pure (Si-free) ferrihydrite shows a broad band at ca. 400 cm"1, with a shoulder at 600 cm"1, attributable to Fe-0 stretching vibrations. Distinct bands at 1635 cm"1 and 3400 cm"1, related to OH stretching, are apparent as well. The presence of small peaks at 975 cm" , 1055 cm" and 1125 cm" is probably an effect of sulfate complex formation on the ferrihydrite surface. Increasing Si concentration strongly affects infrared spectra of ferrihydrite: additional intensive band at ca. 990 cm" (Si-0 stretching) appears and is getting stronger with increasing Si/Fe ratio. The position of this band is shifted slightly towards higher wavenumbers (up to 1003 cm"1) at higher-Si-ferrihydrite spectra. At the same time, ~ 600 cm"1 shoulder and sulfate peaks disappear. Results of Raman spectroscopy are in general consistent with those of FTIR and gave more specific information about the band at ca. 400 cm"1, which is quite indistinct on infrared spectra and attributed to Fe-OH unsymmetrical-stretching vibrations. The band is getting broader and is slightly shifted to higher wavenumbers with increasing Si/Fe ratio but its intensity decreases drastically for the highest-Si samples (Si/Fe > 0.75). At the same time, characteristic 720 cm"1 peak and ca. 500 cm"1 shoulder become hardly visible and the spectra are getting dominated by broad but intensive band of ca. 1500-1700 cm"1, typical for amorphous silica. Additionally, sharp peak at 980 cm"1 present on lower-Si spectra is probably an effect of relic sulfate ion adsorption onto ferrihydrite surface. Preliminary results indicate that silicate ions not only cause decreasing crystallinity and retard ferrihydrite transformation but also strongly affect its surface properties. To verify this hypothesis and to enhance characteristics of Si-ferrihydrite, additional analyses are planned, including solubility, surface area and pHPZC determinations, thermal analyses and electron microscopy. Sorption/desorption studies involving cations and anions binding are planned as well.