Iron/manganese phase

A portion of the isothermal section of an aluminum-iron-manganese phase diagram at 600 °C is shown in Figure 6.10. Assuming equilibrium, list the phases present, give their compositions, and calculate the relative amounts of them for... 

Yoshimura et al. [193] carried out microdeterminations of phosphate by gel-phase colorimetry with molybdenum blue. In this method phosphate reacted with molybdate in acidic conditions to produce 12-phosphomolybdate. The blue species of phosphomolybdate were reduced by ascorbic acid in the presence of antimonyl ions and adsorbed on to Sephadex G-25 gel beads. Attenuation at 836 and 416 nm (adsorption maximum and minimum wavelengths) was measured, and the difference was used to determine trace levels of phosphate. The effect of nitrate, sulfate, silicic acid, arsenate, aluminium, titanium, iron, manganese, copper, and humic acid on the determination were examined. 

It was assumed that there were no limitations on the rates of oxidation due to mass transport as discussed in detail by Schwartz and Freiberg (1981), this assumption is justified except for very large droplets (> 10 yarn) and high pollutant concentrations (e.g., 03 at 0.5 ppm) where the aqueous-phase reactions are very fast. It was also assumed that the aqueous phase present in the atmosphere was a cloud with a liquid water content (V) of 1 g m-3 of air. As seen earlier, the latter factor is important in the aqueous-phase rates of conversion of S(IV) thus the actual concentrations of iron, manganese, and so on in the liquid phase and hence the kinetics of the reactions depend on the liquid water content. 

Mill-). Barnes (1967) examined the depth dependence of the mineralogy in nodules taken Iron) the Pacific. His data indicate that above 3.500 ni in depth, the only important manganese phase is <5 MnO , but. below the 3.51 Hi m depth, both 10-A manganitc and 7-A manganitc coexist with the /i MnO-. The observed phase changes may be pressure induced. 

Analytical Chemical Data for Natural Waters. While elemental compositions of various natural waters usually can be determined with good reliability, analytical methods to distinguish between free and complex-bound species, oxidized and reduced forms, simple and polynuclear metal ion forms, and even between dissolved and colloidal or suspended phases are often lacking. Data on the nature and amounts of the individual substances which make up the total concentrations of organic material found in different natural waters are not yet extensive. These analytical deficiencies relate almost solely to the highly reactive, non-conservative elements—e.g., iron, manganese, phosphorus, carbon, nitrogen, aluminum, and other metal ions. 

Shan and Chen [32] reported that various proportions of metals released from exchangeable, carbonate-bound iron, manganese oxide-bound and organic-bound fractions were readsorbed onto the other solid geochemical phases during sequential extractions. 

In operationally defined speciation the physical or chemical fractionation procedure applied to the sample defines the fraction isolated for measurement. For example, selective sequential extraction procedures are used to isolate metals associated with the water/acid soluble , exchangeable , reducible , oxidisable and residual fractions in a sediment. The reducible, oxidisable and residual fractions, for example, are often equated with the metals associated, bound or adsorbed in the iron/manganese oxyhydroxide, organic matter/sulfide and silicate phases, respectively. While this is often a convenient concept it must be emphasised that these associations are nominal and can be misleading. It is, therefore, sounder to regard the isolated fractions as defined by the operational procedure. Physical procedures such as the division of a solid sample into particle-size fractions or the isolation of a soil solution by filtration, centrifugation or dialysis are also examples of operational speciation. Indeed even the distinction between soluble and insoluble species in aquatic systems can be considered as operational speciation as it is based on the somewhat arbitrary definition of soluble as the ability to pass a 0.45/Am filter. 

In view of literature data and our (in vitro) results on iron-manganese interaction, we considered it worthwhile to study the effect of iron on manganese metabolism in very young rats when absorptive, homeostatic and competitive mechanisms in the intestinal tract are not yet functioning properly or are in a developing phase. We performed experiments to see what role iron dose, duration of treatment and animals age have in iron-manganese competition at an early period of rat s life. 

Cobalt is mostly incorporated into the iron phase of manganese nodules. However, theoretically, it would suit much better in the manganese phase which has experimentally been confirmed by Giovanoli and Brutsch There is a theory, not yet proved by experiments, staling that Co is oxidized to Co in sea-water. This process should be catalyzed by Fe(OH)3, The final product of oxidation, Co(OH)3, forms a solid solution with Fe(OH)3 The theory does not include a discussion about the possibilities of diadochic incorporation of cobalt after cristalliza-tion of iron hydroxide. 

Ferromanganese nodules and seamount crusts are characterized by distinct alternating layers of iron oxides and manganese oxides in addition to other components such as aluminosilicates (shale), CFA, and sometimes biogenic barite and carbonates in seamount crusts. Iron-oxide phases are enriched in elements that exist in normal seawater mainly as hydroxyl and carbonate complexes of tri- to pentavalent cations (e.g.. As, B, Bi, In, Ir, Rh, REEs, Y, Ti, Th, U, Zr, Hf, Nb, and Ta), oxyanions (e.g., P, Re, Ru, Os, S, Se, Te, V, and W), and divalent cations... 

Many of the important chemical reactions controlling arsenic partitioning between solid and liquid phases in aquifers occur at particle-water interfaces. Several spectroscopic methods exist to monitor the electronic, vibrational, and other properties of atoms or molecules localized in the interfacial region. These methods provide information on valence, local coordination, protonation, and other properties that is difficult to obtain by other means. This chapter synthesizes recent infrared, x-ray photoelectron, and x-ray absorption spectroscopic studies of arsenic speciation in natural and synthetic solid phases. The local coordination of arsenic in sulfide minerals, in arsenate and arsenite precipitates, in secondary sulfates and carbonates, adsorbed on iron, manganese, and aluminium hydrous oxides, and adsorbed on aluminosilicate clay minerals is summarized. The chapter concludes with a discussion of the implications of these studies (conducted primarily in model systems) for arsenic speciation in aquifer sediments. 

It is evident from Table 8.7 that, independent of the depth (or age) of the sediment, negligible quanties of Cs were dissolved by reagents selected to remove the available, exchangeable, organic bound and secondary iron/manganese bound components. In each case, at least 80% of the Cs was tightly bound in the residual phase and between 11 and 15% was associated with the fraction extracted by 1 MHNOj. This is consistent with the observation by Allan et al. (1991) of a Kj of about 10 applicable to desorption of Cs from this sediment. 

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