Hematite interfaces

Above 570 °C, we observe the formation of a triple oxide layer that, from the metal-oxide interface outwards, is composed of wustite, magnetite and then hematite (Figure 9.17). The oxygen ion content of the triple layer thus increases as one moves from the interior to the outside. Different oxidation reactions take place at the interfaces. At the metal-wustite interface, iron is oxidized into Fe, whereas at the wustite-magnetite and magnetite-hematite interfaces, the Fe is transformed into Fe +. 

Quigley MS, Honeyman BD, Santschi PH (1996) Thorium sorption in the marine enviromnent equilibrium partitioning at the Hematite/water interface, sorption/desorption kinetics and particle tracing. Aquat Geochem 1 277-301... 

In situ emission Mossbauer spectroscopy provides valuable information on the chemical structure of dilute metal ions at the metal oxide/aqueous solution interface The principles of the method are described with some experimental results on divalent Co-57 and pentavalent Sb-119 adsorbed on hematite. 

We now extend the work to in situ measurements on metal ions adsorbed at the metal oxide/aqueous solution interface. In this report, our previous results are combined with new measurements to yield specific information on the chemical structure of adsorbed species at the solid/aqueous solution interface. Here, we describe the principles of emission Mossbauer spectroscopy, experimental techniques, and some results on divalent Co-57 and pentavalent Sb-119 ions adsorbed at the interface between hematite (a-Fe203) and aqueous solutions. 

In Situ Mossbauer Measurement on Hematite/Divalent Co-57. The adsorption behavior of cobaltous ions on hematite surfaces was essentially the same as that on silica reported by James and Healy (12). Appreciable adsorption begins at about pH 4 followed by an abrupt increase in adsorption between pH 6 and 8. Beyond pH 9, adsorption is practically complete. Emission Mossbauer spectra of Fe-57 arising from the divalent Co-57 ions at the interface between hematite particles and the 0.1 mol/dm3 NaCl solutions of different pH at room temperature are shown in Figure 3 The emission spectra show a marked dependence on the pH of the aqueous phase. No emission lines ascribable to paramagnetic iron species are recognized in... 

Figure 3 In situ emission Mossbauer spectra of Fe-57 arising from divalent Co-57 at the hematite/0.1 mol dm"3 NaCl solution interface for various pH values of the aqueous phase (measurement at room temperature) (A) pH 5.7, (B) pH 7.4, (C) pH 9.6, (D) pH 11.0, (E) pH 12.7, (F) pH 3.0. (F) was measured...

Figure 6. Effects of preheating of sample suspensions at 98°C for 30 min on the in situ emission Mossbauer spectra of Sn-119 arising from pentavalent Sb-119 at the hematite/0.25 mol dm 3 LiCl solution interface (measurement at room temperature) (A1) Before heating, pH 6.6 and (A2) after heating, pH 7.9 (B1)...

In situ emission Mossbauer spectroscopic measurement of the hyper-fine magnetic fields on trivalent Fe-57 and tetravalent Sn-119 arising from divalent Co-57 and pentavalent Sb—119, respectively, yields valuable information on the chemical structure of adsorbed metal ions at the interface between hematite and an aqueous solution. 

In contrast, the pentavalent Sb-119 ions at the interfaces are weakly bonded to the oxide ion layer of the hematite surfaces in neutral and slightly acidic region, while in the acidic region most of the adsorbed Sb-119 ions are in the zeroth or first metal ion layers of the substrate forming Sb-O-Fe bonds. The pentavalent Sb-119 ions having once been incorporated into the surface metal ion sites retain their chemical form, even when the pH of the aqueous phase is raised above 7. Heating of suspensions at 98°C results in chemical rearrangement of the hematite surfaces to yield pentavalent Sb-119 ions in the second or deeper metal ion layers. 

Adsorption and electrokinetic effects of amino acids, solid-aqueous interface, 311-26 Adsorption density, equilibrium PAA at various pH values, 299f PAA on hematite, 304f SDS with and without polymer, 298f,303f... 

Rust formed by atmospheric corrosion is often voluminous (Fig. 18.4) and visually appears as loose orange-brown or black masses. This type of rust is always a mixture of phases and frequently consists of two layers - magnetite at the iron/rust interface (as a result of reduced oxygen supply) with an outer layer of loose lepidocrocite and/ or goethite. Hematite is formed during high temperature aqueous corrosion and is also found in the passive layer (which forms at room temperature). 

Amal, R. Raper, J.A. Waite, T.D. (1990) Fractal structure of hematite aggregates. J. Colloid Interface Sd. 140 158-168 Amal, R. Raper, J.A. Waite, T.D. (1992) Effect of fijlvic add adsorption on the aggregation... 

Andrade, E.M. Molina, F.V. Gordillo, G.J. Posadas, D. (1994a) Adhesion of colloidal hematite onto metallic surfaces. II. Influence of electrode potential, pH, ionic strength, colloid concentration, and nature of the electrolyte on the adhesion on mercury. J. Colloid Interface Sci. 165 459-466... 

Barron,V. Torrent, J. (1996) Surface hydroxyl configuration of different crystal faces of hematite and goethite. J. Colloid Interface Sci. 177 407-411... 

Gardner, W.K. Parbery, D.G. Barber, D.A. (1982) The acquisition of phosphorus by Lu-pinus albus L. I. Some characteristics of the soil/root interface. Plant Soil 68 19-32 Garg, A. Matijevic, E. (1988) Preparation and properties of uniform coated colloidal particles. II. Chromium hydrous oxide on hematite. Langmuir 4 38-44 Garg, A. Matijevic, E. (1988) Preparation and properties of uniform coated colloidal parti-... 

Hesleitner, P. Babic, D. Kallay, N. Matijevic, E. (1987) Adsorption at solid/solution interfaces. 3. Surface charge and potential of colloidal hematite. Langmuir 3 815-820 Hesleitner, P. Kallay, N. Matijevic, E. (1991) Adsorption at solid/liquid interface. 6. The effect of methanol and ethanol on the ionic equilibrium at the hematite/water interface. Langmuir 7 178-184... 

Hug, S.J. (1997) In situ Fourier transform infrared measurements of sulfate adsorption on hematite in aqueous solutions. J. Colloid Interface Sci. 188 415-422 Hug, S.J. Canonica, L. Wegelen, M. Gechter, D. vonGrunten, U. (2001) Solar oxidation and removal of arsenic at circumneutral pH in iron containing waters. Environ. Sci. Techn. 35 2114-2121... 

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