Hematite experimental data

Fig. 3.4a gives plots of charge resulting from surface protonation vs pH for various oxides. Dots represent experimental data from different authors (Table 3.1a) from titration curves at ionic strength I = 0.1 M (hematite = 0.2 M). It is interesting to note that the data "of different oxides" can be "normalised" i.e., made congruent, if we chose the master variable... 

Surface protonation isotherms. Dots represent experimental data from titration curves at ionic strength I = 0.1 (Hematite, I = 0.2). References are indicated in Table 3.1. The concentration of protonated sites MOH is given in moles nr2. BET surface data were used to calculate the surface concentration. 

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. 

The surface complexation models quantify adsorption with experimentally determined equilibrium constants. Another, less widely used approach considers the relationship between the equilibrium constant for the adsorption reaction and the associated free energy change (James and Healy, 1972). Attempts have been made to determine the chemical contribution to the overall adsorption free energy by fitting adsorption isotherms to the experimental data values of -50, -33 and —45 kj mol were found for the change in chemical free energy associated with adsorption of Cr, Ni and Zn, respectively, on ferrihydrite (Crawford et al., 1993). Values ranging from -21 to 241 kJ mol were found for Ni on hematite the actual value depended upon the hydrolysis species that were assumed to exist (Fuerstenau and Osseo-Assare, 1987). 

The model solutions discussed above were compared with experimental data collected using a laser diffraction particle size analyzer. Hematite was dispersed in solutions at various pH values and moderate ionic strength (0.02 M) such that aggregation rates varied from slow to fast, and aggregation was allowed to occur in quiescent solution. The aggregating dispersion was sampled over time, and data was represented in three dimensions as the change in particle size distributions over time. Figure 10a shows hematite... 

In addition, new experimental data were given on the conditions of formation of goethite, hematite, grunerite, and iron-magnesian amphiboles and pyroxenes. 

The conditions of metamorphism of BIF of different original composition are examined on the basis of thermodynamic calculations and some experimental data. In oxide iron-formations the conversion of goethite to hematite was accomplished even before the beginning of regional metamorphism the P-T curve of the dehydration reaction falls within 110° and... 

From a critical review of the experimental data (Mel nik, 1972b) it was concluded that the temperature of equilibrium conversion of goethite to hematite at the pressure of a vapor-saturated aqueous solution varies from 70 to 125°C. Experimental investigations at high pressures have been limited to up to 3kbar (Smith and Kidd, 1949 Schmalz, 1959 Wefers, 1966), and their results are contradictory the discrepancies in determinations of the goethite hematite equilibrium temperature reached 50-60°C, and the slope of the P-T curve was not established. 

In another example, five test cases were computed by PHREEQE and EQ3/6 and the same thermodynamic database was run for each program (INTERA, 1983) to test for any code differences. The five examples were speciation of seawater with major ions, speciation of seawater with complete analysis, dissolution of microchne in dilute HCl, reduction of hematite and calcite by titration with methane, and dedolomitization with gypsum dissolution and increasing temperature. The results were nearly identical for each test case. Test cases need to become standard practice when using geochemical codes so that the results will have better credibility. A comparison of code computations with experimental data on activity coefficients and mineral solubilities over a range of conditions also will improve credibility (Nordstrom, 1994). 

Christl and Kretzschmar (2001) studied the interaction of copper with fulvic acid and hematite, and the results were compared with model calculations based on the linear additivity assumption. The sorption data for the ion-single sorbent systems were modeled with a basic Stem model (BSM) (Section 12.2.2) for hematite and the NICA-Donnan model for fulvic acid. In the second step, pH-dependent sorption of Cu and fulvic acid in systems containing Cu, hematite, and fulvic acid was investigated in batch sorption experiments. Comparison of the experimental data with model calculations shows that Cu sorption in binary hematite-fulvic acid systems is systematically underestimated by up to 30% by using the linear additivity assumption, as shown in Figure 14.4. 

FIGURE 14.4 Cu sorption to the solid phase in binary systems hematite-fulvic acid at three different ionic strengths. Experimental data ( ) 0.1 M (O) 0.03 M (A) 0.01 M, and corresponding model calculations (lines) based on the additivity assumption. (Reprinted from Geochimica et CosmochimicaActa, 65, Christl, I. and Kretzschmar, R., Interaction of copper and fulvic acid at the hematite-water interface, 3435-3442. Copyright 2001, with permission from Elsevier.)... 

Figure 13. Lower Species distribution in 2-10 mol dm citric acid solution as calculated by using K = 1.2-10, Ki = 4.9-10, and Ki, = 2.5-10 in the presence of I-IO mol dm NaN03. Upper Surface concentration (or coverage, 0) of citric acid on hematite as a function of the pH, calculated for the adsorption of its HX + X anions (solid line). The circles represent the experimental data taken from ref [25]. Dashed lines give the calculated contributions of individual ions to the total adsorption density [26].

FIG. 5.1. Experimental data on the direct reduction of hematite by graphite in the presence of a lithium oxide catalyst [6]. 

Figure 14.1 Comparison of predicted (solid lines) and experimental (symbols) time evolution of mean diameter of hematite aggregates at different temperatures mean primary particle diameter 100 nm solids concentration 2.25 x 10 m [KCI] = 50mM (18). (Experimental data from Ref. [14].)...

Some more recent attempts were made to construct deformation mechanism maps for the three oxides, namely wustite, magnetite and hematite [118-120]. Deformation mechanism maps were introduced by Ashby and Frost [121] as a tool to identify the predominant deformation mechanisms and the rates of deformation (strain rates) imder certain stress and temperature conditions for solid metals and ceramics. However, the maps constructed for iron oxides were primarily based on theoretical calculation because experimental data available were very limited and estimation was based on data from other oxide systems. 

Fig. 4.2. Experimental test of Eq. 4.47a for the reaction in liq. 4.45a as applied to hematite (ff-Fe203) suspended in 2 mol m perchlorate solution at pi I 2 5 (data from It. I). Astumian ct al.M). A linear plot is consistent with liq. 4.47a.

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