Y-FeOOH

For steel, passivation is achieved by the surface formation of a tough, adherent mixture of oxides. The passive film is primarily gamma-magnetite (y-Fe203) but also contains gamma-hydrated ferric oxide (y-FeOOH). The film thickness is perhaps 15 A to 30 A (angstrom units). 

First attempts to incorporate pre-formed magnetite colloids within alginate/silica nanocomposites via a spray-drying process have been described, but formation of lepidocrocite y-FeOOH and fayalite Fe2Si04 was observed, attributed to Fe2+ release during the aerosol thermal treatment [53],... 

The verification of the presence of hydrogen in the film has proved more controversial, primarily because many of the structural investigations have been carried out after the film has been dried in vacuo. An example of the problems here is the fact that electron diffraction, which has to be carried out in vacuo, reveals a relatively well-crystallised spinel lattice whose origin may be the comparatively high sample heating encountered in the electron beam. Moreover, the use of in situ techniques, such as Mossbauer and X-ray absorption spectroscopy, clearly reveals marked differences between the spectra of the films in situ and the spectra of the same films ex situ as well as the spectra of y-Fe203 and y-FeOOH standards. These differences are most naturally ascribed to hydration of the spinel forms. 

Sung, W. and J. J. Morgan, 1981, Oxidative removal of Mn(II) from solution catalysed by the y-FeOOH (lepidocrocite) surface. Geochimica et Cosmochimica Acta 45, 2377-2383. 

The amorphous iron oxide is observed to be considerably more photoactive than the crystalline oxide - presumably as a result of the greater number of surface-located ferric hydroxy chromophores (the BET surface area of the synthesized Y-FeOOH is only 34 m2/g... 

Table I. Initial Rates of Photodissolution of y-FeOOH and am-FeOOH Suspended in 0.01M NaCl Using a Simulated Solar Spectrum of Total Radiation Output 300 pEinsteins cm-2 min-1.

Figure 2. Dissolution of 5uM Y-FeOOH under dark and light conditions in a) the absence, and b) the presence of 10-1M citrate. Light source simulated solar spectrum of total intensity 300 uEinsteins cm- min . (Reproduced with permission from Ref. 41. Copyright 1984, Academic Press, Inc.)...

Figure 5. Dependence of rate of dissolution of 5pM Y-FeOOH in pH 4.0, 0.01M NaCl on concentration of a) tartaric acid, and b) salicylic acid. Fitted parameters obtained for rectangular hyperbolic model are given. Light source mercury arc lamp with 365nm band-pass filtering.

Figure 6. Dissolution of 5pM Y-FeOOH on photolysis of a) pH 4.0, and b) pH 6.5 solutions containing either 10-1 M citrate, 10 mg/L aquatic fulvic acids, or no added organic agent. Light source simulated solar spectrum of total intensity 300 pEinsteins cm- min-1. (Reproduced from Ref. 32. Copyright 1985, American Chemical Society.)...

Figure 8. Rate of dissolution of 5iiM Y-FeOOH suspended in pH 3.0, 0.01M NaCl containing various concentrations of mercaptoacetic acid under dark and light conditions. Light source unfiltered 150W Xenon arc lamp. (Reproduced from Ref. 49.)...

Oxide composition and lattice structure influences the coordin-ative environment of surface sites, and should have an impact on rates of ligand substitution. Hematite (Fe203), goethite (a-FeOOH), and lepidocrocite (y-FeOOH), for example, are all Fe(III) oxide/ hydroxides, but may exhibit different rates of surface chemical... 

Mn(II) oxidation is enhanced in the presence of lepidocrocite (y-FeOOH). The oxidation of Mn(II) on y-FeOOH can be understood in terms of the coupling of surface coordination processes and redox reactions on the surface. Ca2+, Mg2+, Cl, S042-, phosphate, silicate, salicylate, and phthalate affect Mn(II) oxidation in the presence of y-FeOOH. These effects can be explained in terms of the influence these ions have on the binding of Mn(II) species to the surface. Extrapolation of the laboratory results to the conditions prevailing in natural waters predicts that the factors which most influence Mn(II) oxidation rates are pH, temperature, the amount of surface, ionic strength, and Mg2+ and Cl" concentrations. 

This paper discusses the oxidation of Mn(II) in the presence of lepidocrocite, y-FeOOH. This solid was chosen because earlier work (18, 26) had shown that it significantly enhanced the rate of Mn(II) oxidation. The influence of Ca2+, Mg2+, Cl", SO,2-, phosphate, silicate, salicylate, and phthalate on the kinetics of this reaction is also considered. These ions are either important constituents in natural waters or simple models for naturally occurring organics. To try to identify the factors that influence the rate of Mn(II) oxidation in natural waters the surface equilibrium and kinetic models developed using the laboratory results have been used to predict the... 

Preparation and Characterization of y-FeOOH. The preparation and characterization of the bulk and surface properties of the y-FeOOH studied is described in Davies (26). The surface properties of the oxide are given in Table I. 

Oxidation Studies. These studies were performed at 25+0.2°C in a carbonate buffered 0.1 M NaClO solution. The buffer was equilibrated with air or an 02/C02 gas mixture for at least 24 hours. The pH was adjusted to 8.3 using HCIO, NaOH, or Na2C03. Where y-FeOOH was present the suspension was deaerated by bubbling N2/C02 for 1 hour, the Mn(II) was added and was allowed to equilibrate with the solid for 30 minutes and then the oxidation was commenced by switching to 02/C02 or air bubbling. Where no solid was present the solution was not deaerated. The rate of oxidation was monitored by following the loss of filterable Mn. 

Mn(II) adsorption on metal oxide surfaces. The binding of Mn(II) on y-FeOOH can be understood in a surface coordination chemical framework. The surface groups on a metal oxide are amphoteric and the hydrolysis reactions can be written ... 

As shown in Figure 1, the adsorption of Mn(II) on y-FeOOH can be successfully described using a constant capacitance model. In these calculations the hydrolysed surface complex =FeO-Mn-OH was not considered. The reason for not considering both the bidentate (sS0)2Mn and hydrolysed surface species is that both have virtually the same pH dependence, so it is impossible using the available data to make anything other than an arbitrary choice about the relative proportions of these two species. Based on the model calculations, in the pH range 8-9, the predominant Mn(II) species on the y-FeOOH surface is the bidentate surface complex or the hydrolysed surface complex. 

Mn(II) oxidation on metal oxides. The rate of oxidation of Mn(II) in the presence of y-FeOOH can be described by the following equation (26) ... 

Table III. Interactions of anions, Ca2+, and Mg2+ with the y-FeOOH surface.

Phosphate and silicate experiments in 0.98 mM y-FeOOH. Sulphate experiments in 2.5 mM y-FeOOH. All other experiments in l.OmM y-FeOOH. 

It has been shown elsewhere (26) that in natural waters the degree of enhancement of Mn(II) oxidation predicted on the basis of model calculations is as follows y-FeOOH > a-Fe00H > silica > alumina. It has also been shown that the rate of Mn(II) oxidation is strongly influenced by pH, y-FeOOH concentration, temperature and ionic strength. Depending on the conditions, the predicted half-life 1/2 = ln 2/ki ) f°r Mn(II) oxidation may vary from a few days to thousands of years. By way of example, at pH 8, p02 0.21 atm, 25°C in waters containing 4(iM y-FeOOH and 0.2uM Mn(II), the half-life for oxidation is about 30 days. 

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