Effect of Manganese on the Transformation of Ferrihydrite into Goethite and Jacobsite in Alkaline Media

Abstract--The incorporation of transition metals into hematite may limit the aqueous concentration and bioavailabity of several important nutrients and toxic heavy metals. Before predicting how hematite controls metal cation solubility, we must understand the mechanisms by which metal cations are incorporated into hematite. Thus, we have studied the mechanism for Ni 2+ and Mn 3+ uptake into hematite using extended X-ray absorption fine structures (EXAFS) spectroscopy. EXAFS measurements show that the coordination environment of Ni 2+ in hematite corresponds to that resulting from Ni z § replacing Fe 3+. No evidence for Nit or Ni(OH)z was found. The infrared spectrum of Ni-substituted hematite shows an OH-stretch band at 3168 cm 1 and Fe-OH bending modes at 892 and 796 cm i. These vibrational bands are similar to those found in goethite. The results suggest that the substitution of Ni 2+ for Fe 3+ is coupled with the protonation of one of the hematite oxygen atoms to maintain charge balance.The solubility of Mn 3+ in hematite is much less extensive than that of Ni 2~ because of the strong JahnTeller distortion of Mn 3~ in six-fold coordination. Structural evidence of Mn > substituting for Fe 3+ in hematite was found for a composition of 3.3 mole % Mn203. However a sample with nominally 6.6 mole % Mn203 was found to consist of two phases: hematite and ramsdellite (MnOz). The results indicate that for cations, such as Mn 3-showing a strong Jahn-Teller effect, there is limited substitution in hematite.

Section: Crystal Propertiesmentioning

confidence: 56%

Structural Chemistry of Fe, Mn, and Ni in Synthetic Hematites as Determined by Extended X-Ray Absorption Fine Structure Spectroscopy

Singh

Sherman

Gilkes

et al. 2000

“…Coprecipitates of Si, Cu, Co and Mn with Fe to form ferrihydrite are shown to retard the rate of ferrihydrite transformation to more crystalline phases (Anderson and Benjamin 1985;Cor-nell and Giovanoli 1987;Cornell and Giovanoli 1988;, with the retardation effect of metals following the order: Cu >> Co >> Mn. The presence of divalent cations, such as Cu, was also shown to increase the proportion of Hm relative to Gt, thus influencing the nature and quantities of the more crystalline end-products formed (Cornell and Giovanoli 1987, 1988. Metals can prevent Gt formation by interference at 2 stages: 1) by retarding dissolution of ferrihydrite, and 2) by hindering nucleation and growth in solution.…”

Section: Introductionmentioning

confidence: 99%

Coprecipitates of Cd, Cu, Pb and Zn in Iron Oxides: Solid Phase Transformation and Metal Solubility after Aging and Thermal Treatment

Martı́nez

1998

Abstract--Solid phase transformation and metal solubility were monitored after coprecipitation of Cd 2+, Cu 2+, Pb 2+ and Zn 2+ with Fe 3+ to form ferrihydrite by titration to pH 6. The (co)precipitates were aged at room temperature for up to 200 d and subsequently heated for 60 d at 70 ~ The mode of (co)precipitate formation, rapid and slow titration, was also investigated. Metal solubility was measured by anodic stripping voltammetry. Surface area, Fourier transform infrared (FTIR) and X-ray diffraction (XRD) analysis were used to follow the transformation of ferrihydrite after initial (co)precipitation. Electron microprobe analysis (EMPA) was used to show the distribution of metals within ferrihydrite aggregates. Thermal treatment produced a reduction in soluble Cd 2+ and Zn 2+, whereas Pb 2+ appeared to be expelled from the solid phase. The more stable coprecipitate (formed by slow titration) maintained a constant Cu z+ solubility after thermal treatment. Characterization of the solid phase by XRD indicated that the presence of low levels of metals did not affect the initial or final transformation products, although metals present during the slow titration seemed to stabilize a higher surface area material. The rapid titration resulted in a less ordered (I-line) ferrihydrite than the slow titration (9-line). Furthermore, FTIR analysis indicated that the presence of metals promoted the formation of mixed (microcrystalline) end-products. The initial coprecipitation products seem to determine the final thermal transformation products. These transformation products include ferrihydrite, hematite (Hm), and goethite (Gt)-and lepidocrocite-like microcrystalline structures. Although experimental conditions were favorable for the homogeneous distribution of metals throughout the coprecipitate, EMPA suggests that Cu and Zn segregation within aggregates of Fe oxides occurs.

“…The effects of soluble foreign species on the crystallization of Fe oxides from ferrihydrite (5Fe203" 9H20) has been investigated in the laboratory (Lewis and Schwertmann, 1979;Cornell and Schwertmann, 1979;Cornell 1985;Cornell et al, 1987;Cornell and Schneider, 1989). These studies have provided valuable information about the mechanisms of crystallization of two of the most widespread iron oxides in the environment, goethite (a-FeOOH) and hematite .…”

Section: Introductionmentioning

confidence: 99%

Transformation of Akaganéite Into Goethite and Hematite in the Presence of Mn

Cornell

Giovanoli

1991

Self Cite

Abstract--The interaction of Mn and akagan6ite in neutral to alkaline media has been investigated using X-ray powder diffraction and transmission electron microscopy. Akagan6ite transformed into goethite and/or hematite, whereas Mn precipitated as hausmannite and bimessite at pH > 12 and as manganite at pH 7.5-8.5. Mn influenced the kinetics of the transformation of akagan6ite: the rate-determining step, i.e., the dissolution of akagan6ite, was retarded by adsorbed Mn species. Hematite formation was not suppressed. By catalyzing the air oxidation of adsorbed Mn(II), akagan6ite promoted the formation of birnessite. Akagan6ite did not retard recrystallization of the Mn phases. The incorporation of Mn in the structure of goethite fo.rmed in this system was negligible, and jacobsite (MnFe204) did not form. The formation of mixed Mn-Fe phases appeared to require a ratio of Mn 2+ : Fe to~ > 0.02; this ratio was not achieved due to the oxidation of Mn 2 § at the akagan6ite surface.