Hematite particles reducibility

As a result, nanometer-sized metallic Au particles were selectively deposited onto monodispersed polycrystalline ellipsoidal hematite particles without addition of any specific reducing agent, as shown in Figure 5. 

It seems that Au ions of Au(OH) Cl4 complex, formed by the first aging at room temperature, are reduced to Au particles by electron transfer from the coordinated OH ions on the surface of hematite as a catalyst of the electron transfer. As a consequence, the essential reducing agent is water. The optimum pH to yield the maximum quantity of Au particles was ca. pH 5.9, as measured at room temperature, corresponding to the pH of the above standard system. Au ions are reduced to metallic Au by electron transfer from coordinated OH ions on the surfaces of hematite particles through their catalytic action. 

Preliminary adsorption experiments revealed that the adsorption of FA on hematite surfaces is of the high affinity type. Electrophoretic mobility data (Figure 5(b)) indicate that adsorption of fulvic acid on hematite is not only a consequence of electrostatic interaction between polyanions and positive surface sites, but depends on specific chemical interactions between surface iron (=Fe) and carboxyl (-COO ) groups as well. If electrostatic interaction were the sole driving force for adsorption, the mobility would be expected to be reduced to zero as the FA concentration increases, and would remain at zero as the concentration of FA is further increased. The strong reversal in mobility observed arises from the chemical interaction in addition to the electrostatic force. Consequently, the mobility data are in accord with the observed stability ratio, where at (FA)< 1 x 10 g/l the stability results from net repulsions of positive surfaces, and at (FA)> 1 x 10 g/l the stability arises from repulsion between negative surfaces. Similar conclusions can be drawn with respect to the driving forces for HA adsorption on hematite particles. 

When the DMAEMA content of NVP - DMAEMA copolymers was reduced from 20% to 8%, the silica fines stabilization effectiveness appeared to improve slightly. When the 80/20 NVP - DMAEMA copolymer was converted to a terpolymer containing 8% DMAEMA (CH SO, silica fines stabilization was substantially unaffected. However, stabilization of silica/kaolinite fines was greatly improved. This suggested that the interaction of polymer quaternary nitrogen atoms with anionic sites on mineral surfaces was important for the stabilization of migrating clays but a different interaction was important for the stabilization of silica fines. Calcite fines stabilization improved while hematite fines stabilization effectiveness decreased. This also indicated the nature of the adsorbed polymer - fine particle complex varied for different minerals. 

Figure 8 (10) shows the interrelationship of some of the forms of iron oxide. The important pigments are a-Fe203 (hematite) and a-FeOOH (goethite). While these are generally red and yellow, respectively, there are many variations which we believe arise from particle size distribution and the presence of other minerals. Reduced forms of... 

Suspensions of hematite have also been used and studied for other aims than photooxidation of water, e.g. catalytic oxidation of sulphur dioxide in aqueous solutions [52]. Aqueous dispersion of semiconductor particles could be an easy and attractive way to photooxidise water, but they have the drawback that dihydrogen and dioxygen are produced simultaneously in the same suspension. Apart from the separation problem the two produced gases may create a pathway for back reactions that reduces the yield of the overall photo-oxidation process. The latter obstacle can partly be avoided by addition of Na2C03, which was successfully shown by Arakawa et al [115]. 

Figure 21 Relationship of Fe(III)-reducing bacteria activity and growth to oxide surface area, (a) Percent Fe(III) reduced as a function of oxide surface area. Surface area corresponded to different mineral types and included hematite, goethite, and ferrihydrite. (b) The density of Shewanella oneidensis cells as a function of the amount of structural Fe(III) reduction to Fe(It) in smectite clay, a strongly crystalline, high-surface-area Fe mineral. Differences in Fe(II) content reflect different amounts of clay particles inoculated into a minimal basal media (after Roden and Zachara, 1996 and Kostka et al, 2002a, respectively).

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