Hematite surfaces

This discrepancy might be explained if after about an hour the reaction approached equilibrium and slowed due to a diminishing thermodynamic drive. If the Fe+++ produced did not precipitate on the hematite surface, and did not form either hematite or goethite (FeOOH), it would accumulate in solution and weaken the drive for uranyl reduction. As the saturation index for hematite reached about 1.7, or about 1.25 for goethite, reaction would cease. 

Comparison of hematite surface charge, [coul g 1], electrophoretic mobility, and stability ratio, Wexp, as a function of pH. Note that at pHpzc the net surface charge and mobility are both zero, and the stability is a minimum. 

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... 

Sb carrier ions. The Sb-119 ions were adsorbed on 30 mg of hematite from 10 cm3 of a 0.25 mol/dm3 KC1 solution containing about 1 mg of pentavalent Sb ions. About 0.3 mg of Sb was adsorbed at pH 2.5 and 4.0. The amounts of Sb adsorbed are less than that required to cover all the hematite surfaces as a monolayer. The emission Mossbauer spectra obtained are shown in Figure 7. It is seen from Figure 7 that the width of the emission Mossbauer spectrum at pH 2.5 is much smaller than that of the carrier-free one, while essentially no effect of carrier Sb ions is observed at pH 4.0. 

In the slightly acidic region, adsorbed Co-57 ions are distributed between at least two chemical forms one attributable to the Co-57 ions coordinatively bound to surface sites and the other to Co-57 weakly bound to the hematite surfaces. In the alkaline region, most of the adsorbed Co-57 ions are in the zeroth or first metal-ion layers of the substrate forming Co-O-Fe bonds. Desorption of divalent Co-57 from strongly coordinated surface complexes occurs when the pH is lowered from alkaline to acidic values. 

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. 

F. Herrera, A. Lopez, G. Mascolo, P. Albers and J. Kiwi, Catalytic combustion of Orange II on hematite. Surface species responsible for the dye degradation. Appl. Catal. B Environ., 29 (2001) 147-162. 

At equilibrium surfactant concentrations of less than 0.0003 M SDS where the hematite surface is still positively charged, adsorption of surfactant follows its normal pattern due to the electrostatic forces which provide the driving force for adsorption. Sufficient effective surface area must be available for this level of SDS adsorption density. As surfactant adsorption... 

Calculation of the electronic structure of freshly cleaved hematite surfaces suggest that the local electronic structure of the surface may be very different from the bulk... 

The results of the XSW study by Templeton et al. [180] show that Pb2+ binds initially to reactive sites on the a-Al203 (1-102) and a-Fe203 (0001) surfaces even with a biofilm coating that covers essentially the entire mineral surface, as shown by SEM and confocal microscopy studies. The order of reactivity of these biofilm-coated surfaces for Pb(II) [a-Fe203 (0001) > a-Al203 (1-102) >> a-Al203 (0001)] is the same as that observed in uptake and EXAFS studies of Pb(II) sorption on biofilm-free alumina and hematite surfaces (Figure 7.22) (see [155-157]). 

By following the reaction scheme proposed by dos Santos Afonso and Stumm (22) for the reductive dissolution of hematite surface sites (Scheme 1), we were able to explain perfectly the observed pH pattern of the oxidation rate of H2S. The rate is proportional to the concentration of inner-sphere surface complexes of HS" formed with either the neutral (>FeOH) or the protonated (>FeOH2+) ferric oxide surface sites. 

Neal A. L., TechkarnjanarukS., Dohnalkova A., McCready D., Peyton B. M., and Geesey G. G. (2001) Iron sulfides and sulfur species produced at hematite surfaces in the presence of sulfate-reducing bacteria. Geochim. Cosmochim. Acta... 

Maurice, P. A., Hochella, M. F. jr., Parks, G. A., Sposito, G. and Schwertmami, U. Evolution of hematite surface microtopography upon dissolution by simple organic acids. Clays Clay Min. 43 29-38. 

Wavelet analysis has great potential in image processing applications of interest in mineralogy (see Moktadir and Sato (2000) for an illustrative example for silicon). As an illustration, in Figure 4 we show a version of Equation (3) over a one-dimensional trace across a two-dimensional AFM image of a hematite surface where there are some traces of bacterially mediated reduction reactions. One-dimensional wavelets with the second-derivative of the Gaussian function, also known as Mexican-hat wavelets because of their... 

Note All atoms within 2 A of the hematite surface are assumed to be on the surface. 

In the interpretation 2-pK model was employed and binding of Co to the hematite surface was considered by three possible reaction mechanisms... 

Chibowski, S., Study on adsorption properties of polyvinyl alcohol on hematite surface, Pol. J. Chem., 59, 1193, 1985. 

We have chosen hematite oxalate as a model system, since the photochemical properties of colloidal hematite (Stramel and Thomas, 1986) and the photochemistry of iron(III) oxalato complexes in solution (Parker and Hatchard, 1959) have been studied extensively. The experiments presented in this section were carried out as batch experiments with monodispersed suspensions of hematite (diameter of the particles 50 and 100 nm), synthesized according to Penners and Koopal (1986) and checked by electron microscopy and X-ray diffraction. An experimental technique developed for the study of photoredox reactions with colloidal systems (Sulzberger, 1983) has been used. A pH of 3 was chosen to maximize the adsorption of oxalate at the hematite surface. This case study is described in detail by Siffert (1989) and Siffert et al. (manuscript in preparation). 

Part of the photochemically formed iron(II) is readsorbed on the hematite surface, with oxalate acting as a bridging ligand. Two possible structures of such a ternary surface complex are shown in reactions 8 and 9. The adsorbed Fe11 may... 

The estimation of equilibrium constants with a SCF/DLM model can be facilitated by using computer programs, such as SURFEQL and FITEQL (23-24). Hematite surface equilibrium acidity constants are listed in Table I, together with other surface physical parameters. Here we summarize the effects of organic solutes on kinetics of hematite coagulation and discuss the results on the basis of surface chemical concepts. 

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