Goethite substituting ions

A common method of synthesizing M-substituted oxides, particularly goethite and hematite is to add base to mixed M-Fe salt solutions to precipitate M-associated ferrihydrite. Most ions do not change their oxidation state, but incorporation of Mn and Co in goethite is preceded by oxidation of these ions to the trivalent state (Giovanoli Cornell, 1992). An indication of whether isomorphous substitution has occurred can be obtained from changes in the unit cell dimensions of the Fe oxides... 

Additives usually alter only the length-to-width or width-to-thickness ratio of the aci-cular crystals. Growth of long, thin crystals (aspect ratio >12) is induced by high levels (>0.1) of Mn or Co and is attributed to adsorption rather than substitution. These ions have the same influence on aspect ratio whether goethite is grown from Fe" or Fe " systems and over the pH range 7-13. 

Schwertmann, U. Friedl, J. Stanjek, H. (1999) From Fe(III) ions to ferrihydrite and then to hematite. J. Coll. Interface Sci. 209 215-223 Schwertmann, U. Friedl, J. Stanjek, H. Schulze, D.G. (2000) The effect of A1 on Fe oxides. XIX. Formation of Al-substituted hematite from ferrihydrite at 25°C and pH 4 to 7. Clays Clay Miner. 48 159-172 Schwertmann, U. Friedl, J. Stanjek, H. Schulze, D.G. (2000a) The effect of clay minerals on the formation of goethite and hematite from ferrihydrite after 16 years ageing at 25 °C and pH 4-7. Clay Min. 35 613-623... 

Figure 10 illustrates that Crw effectively inhibits the proton-promoted dissolution of goethite. Cr(III) adsorbs even at low pH and, as bi- or polynuclear surface complexes, blocks surface sites from being protonated. Furthermore, isomorphically substituted Cr3+ ions, characterized by an extremely low water-exchange rate, impart inertness to the surface lattice bonds. 

Ainsworth et al. (1994) observed that oxide aging did not cause hysteresis of trace element cation sorption-desorption. Aging the hydrous ferric oxide with trace elements cations resulted in hysteresis with Cd and Cu, but little hysteresis was observed with Pb. The extent of reversibility with aging for Co, Cd, and Pb was inversely proportional to the ionic radius of the ions (i.e., Co < Cd < Pb). The authors attibuted the hysteresis to Co and Cd incorporation into a recrystallizing solid (probably goethite) via isomorphic substitution, not to micropore diffusion. 

When the new compound substitutes, even partially, for the original compound, the pseudomorph is called a substitutional pseudomorph. Usually the new compound has an ion in conunon with the original, as is the case of goethite pseudomorph after pyrite (both contain iron as Fe in goethite and as Fe in pyrite) and the malachite pseudomorph after azuiite (both contain Cu, COs ", and OH ) shown in Figure 110. The malachite pseudomorph is particularly interesting, because the two minerals differ only in the stoichiometry (mole ratio) of the ions present. 

The formation of Ni-substituted goethite in the presence of Ni ions was observed by way of a significant reduction... 

The presence of Zn ions in the precipitation system [298] affected the formation of elongated Zn-substituted a-FeOOH particles as a single phase up to 9 mol% Zn. At the Zn content of I I mol% Zn-ferrite particles were formed besides elongated Zn-substituted a-FeOOH particles. At l3mol% of Zn, three phases were detected by Mossbauer spectroscopy Zn-substituted goethite, hematite, and Zn-ferrite. 

The influence of the V " " ion presence during the formation of goethite from ferrihydrite in an alkaline medium was investigated using Mossbauer spectroscopy and other techniques by Kaur et al. [249]. The presence of ions reduced HMF values in the Mossbauer spectra due to a substitution of Fe by V ions (ionic radius of 0.64 A almost equal to the radius of Fe " ") in the goethite structure. Small quantities of hematite and superparamagnetic goethite were also formed. 

Due to the similar ionic radius (0.62 A [338]) and the existence of isostructural oxyhydroxide (GaOOH), Ga ions substituted Fe " " in the structure of goethite in a broad concentration range [300]. Ga-for-Fe substitution in the structure of goethite affects the reduction in HMF and Neel temperature due to magnetic dilution by nonmagnetic Ga " ions and to the formation of small Ga-goethite particles. 

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