Hematite oxide-solution

Adsorption of Pentavalent Sb Ions on Hematite. So far as we know, there are no experimental data on the adsorption equilibrium of dilute pentavalent Sb ions on metal oxides. Therefore, the pH dependence of the adsorption of pentavalent Sb ions on hematite was measured. Carrier-free pentavalent Sb-119 ions were adsorbed on 30 mg of hematite (prefired at 900°C for 2 hours) from 10 cm3 of 0.25 mol/dm3 LiCl solutions at 24 1°C. The amount of antimony employed in each run is estimated to be about 50 ng. The adsorption proceeds with a measurable rate and attains an apparent equilibrium after shaking for several hours. The reaction is second order with respect to the concentration of pentavalent Sb ions in the solution (13) The values given in Figure 4 are those obtained after 22 hours equilibration. As seen in Figure 4, strong adsorption of pentavalent Sb ions is observed below pH 7, while the percent adsorbed diminishes abruptly above that. Most of the Sb ions adsorbed on hematite from solutions of pH 2-5 are not desorbed by subsequent adjustment to alkaline conditions. Results on desorption of Sb ions pre-adsorbed at pH 4 are shown in Figure 4. 

To conclude, it is clear there is a strong dependence of the performance of synthetic hematite on the deposition technique. While methods such as spray pyrolysis and CVD consistently produce electrodes photoactive for water oxidation, solution-based methods such as sol-gel approaches have failed to produce especially photoactive hematite. This is certainly related to the quality of the prepared material in terms of crystallinity and impurity concentrations. Aqueous methods of preparing hematite typically pass through a phase containing iron hydroxide (e.g., akaganeite, lepidocrocite, or goethite) but primarily hematite is detected after at annealing at 500°C. However, it has been shown that at temperatures up to 800°C, a nonstoichiometric composition remains in hematite when prepared in this way... 

Iron Oxide Reds. From a chemical point of view, red iron oxides are based on the stmcture of hematite, a-Fe202, and can be prepared in various shades, from orange through pure red to violet. Different shades are controlled primarily by the oxide s particle si2e, shape, and surface properties. Production. Four methods are commercially used in the preparation of iron oxide reds two-stage calcination of FeS047H2 O precipitation from an aqueous solution thermal dehydration of yellow goethite, a-FeO(OH) and oxidation of synthetic black oxide, Fe O. ... 

Transparent red iron oxide is composed mainly of hematite, a-Ee202, having primary particles about 10 nm. It is prepared by a precipitation reaction from a dilute solution of an iron salt at a temperature around 30°C, foUowed by a complete oxidation in the presence of some seeding additives,... 

This hematite is not soluble in the cyanide solution. The oxidative pretreatment of gold ores thus reduces the cyanide consumption. Some impurity elements inhibit leaching reactions, examples include elements, carbon, sulfur and arsenic in gold ores are such impurities, but these can be removed by heating in air. 

The scheme E involves dead roasting of the concentrate by which Fe203 forms and the nickel content is oxidized to nickel ferrite (NiFe204). This material is selectively reduced to produce an iron-nickel alloy which is then leached in an ammoniacal ammonium carbonate solution for nickel recovery, leaving hematite as a leach residue. 

Fig. 28.1. Results (symbols) and simulations (lines) of an experiment at 25 °C by Liger et al. (1999 their Fig. 6) in which uranyl was oxidized by ferrous iron in the presence of nanoparticulate hematite, which served as a catalyst. Vertical axis is amount of NaHCCE-extractable uranyl, which includes uranyl present in solution as well as that sorbed to the nanoparticles in the experiment, nearly all the uranyl was sorbed. Broken line shows results of a simulation assuming uranyl forms a single surface complex, >Fe0U020H, which is catalytically active solid line shows simulation in which a non-catalytic site of this stoichiometry is also present. Inset is an expanded view of the first few hours of reaction.

The surface charge of metal oxides (due to surface protonation) as a function of pH can be predicted if their pHpzc are known with the help of the relationship given in Fig. 3.4. Fig. 7.6 exemplifies the effect of various solutes on the colloid stability of hematite at pH around 6.5 (pH = 10.5 for Ca2+ and Na+) (Liang and Morgan, 1990). 

The Rate of reductive Dissolution of Hematite by H2S as observed between pH 4 and 7 is given in Fig. 9.6 (dos Santos Afonso and Stumm, in preparation). The HS" is oxidized to SO. The experiments were carried out at different pH values (pH-stat) and using constant PH2s- 1.8 - 2.0 H+ ions are consumed per Fe(II) released into solution, as long as the solubility product of FeS is not exceeded, the product of the reaction is Fe2+. The reaction proceeds through the formation of inner-sphere =Fe-S. The dissolution rate, R, is given by... 

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. 

Coordinative Environment. The coordinative environment of transition metal ions affects the thermodynamic driving force and reaction rate of ligand substitution and electron transfer reactions. FeIIIoH2+(aq) and hematite (a-Fe203) surface structures are shown in Figure 3 for the sake of comparison. Within the lattice of oxide/hydroxide minerals, the inner coordination spheres of metal centers are fully occupied by a regular array of O3- and/or 0H donor groups. At the mineral surface, however, one or more coordinative positions of each metal center are vacant (15). When oxide surfaces are introduced into aqueous solution, H2O and 0H molecules... 

Adsorption Methods. Five grams of hematite were first conditioned in 0.001 M NaCl at pH 4.1. After the SDS had been added to the slurry and the pH adjusted as required, the samples were conditioned on a rotating shaker for two hours. The solutions were then centrifuged, and the supernatant liquid analyzed for its SDS content. The amount of SDS adsorbed was calculated as the difference between the initial amount added and the residual amount measured. Experimental results showed that two hours was sufficient time for equilibrium to be reached. Somasundaran ( ) observed similar equilibrium adsorption times for sulfonate adsorption on aluminum oxide. 

There are structural analogues of a number of iron oxides in the Fe-H-O system. Under certain conditions, continuous solid solutions exist between the two members of a pair. The magnetite-ulvospinel and the hematite-ilmenite pairs are well-known examples. The principle in going from the Fe oxide to the Ti-containing phase is to replace two Fe by one Fe" and one Ti , thereby increasing the unit cell size. 

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... 

The surface area of synthetic hematite depends upon whether the oxide was produced by calcination or grown in solution. The temperature of (dry) heating influences the surface area. Hematites produced at 800-900 °C have areas < 5 m g due to sintering of the particles. Hematites obtained by dehydroxylation of the various polymorphs of FeOOH or ferrihydrite at temperatures lower than 500-600 °C are mesoporous and have much higher surface areas - up to 200 m g". Commercial hematites are usually produced by calcination and hence have a low surface area. 

Litter et al. (1991) found that the dissolution of maghemite was also considerably speeded up, once a dissolved Fe "-oxalate or Fe" -EDTA complex was reduced to an Fe" complex by UV irradiation 1 = 254 nm). This system also showed an induction period which could be eliminated by addition of Fe " (see Fig. 12.28). In a study concerned with dissolution of corrosion oxide, electrons from viologen radicals produced by y-radiation ( Co) were used to dissolve hematite and goethite (Mulvaney et al., 1988) it was observed that the Fe " appearing in solution could only account for a fraction of the electrons consumed. The remainder was involved in conversion of the Fe " oxide into magnetite. 

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