Surface charge, hematite, effect

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). 

Effects of Pentavalent Sb on the Adsorption of Divalent Co-57. The emission Mossbauer spectra of divalent Co-57 adsorbed on hematite with pentavalent Sb ions (Figure 8) are complex and we have not yet succeeded in their analysis. It is certain, however, from the spectra that trivalent Fe-57 ions produced by the EC decay of Co-57 are interacting magnetically with the ferric ions of the substrate. This means that the divalent Co-57 are not adsorbed on the pentavalent Sb ions, but on hematite directly. The [Sb(OH)g]- anions are considered to facilitate direct adsorption of divalent Co-57 ions on the positively charged surfaces of hematite in the acidic region. 

The presence of pre-adsorbed polyacrylic acid significantly reduces the adsorption of sodium dodecylsulfonate on hematite from dilute acidic solutions. Nonionic polyacrylamide was found to have a much lesser effect on the adsorption of sulfonate. The isotherm for sulfonate adsorption in absence of polymer on positively charged hematite exhibits the typical three regions characteristic of physical adsorption in aqueous surfactant systems. Adsorption behavior of the sulfonate and polymer is related to electrokinetic potentials in this system. Contact angle measurements on a hematite disk in sulfonate solutions revealed that pre-adsorption of polymer resulted in reduced surface hydrophobicity. 

The surfactant sodium dodecyl sulphonate (Ci2H2sS03Na) and its sulphate adsorb electrostatically on hematite at low solute concentrations (Han et al., 1973). Hydro-phobic effects operate at high concentrations due to the incompatibility of the hydrocarbon part of the molecule with water. This involves condensation of the alkyl chains at the surface (hemi-micelle interactions), which lowers the free energy of the system and reverses the surface charge. 

Hesleitner, P. Babic, D. Kallay, N. Matijevic, E. (1987) Adsorption at solid/solution interfaces. 3. Surface charge and potential of colloidal hematite. Langmuir 3 815-820 Hesleitner, P. Kallay, N. Matijevic, E. (1991) Adsorption at solid/liquid interface. 6. The effect of methanol and ethanol on the ionic equilibrium at the hematite/water interface. Langmuir 7 178-184... 

Colic, M. et al., Lyotropic effect in surface charge, electrokinetics, and coagulation of a hematite dispersion. Colloids Surf., 59, 169, 1991. 

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... 

Fig. 6.9. Log-log plot of W, for hematite (a-Fe203) colloids at pH 3.44, versus o-phosphate concentration. The arrow indicates the point of zero charge, defined operationally as the concentration of o-phosphate at which W p = 1.0 (data from L. Liang, Effects of surface chemistry on kinetics of coagulation of submicron iron oxide particles (ar-F Oj) in water, Ph.D. dissertation, California Institute of Technology, Pasadena, CA, 1988. Environmental Quality Laboratory Report No. AC-5-88).

PZC and lEP are important parameters for surface characterization of oxide minerals. The flotation of these minerals is best understood in terms of the electrical double layer theories. Simple oxide minerals such as hematite, goethite, magnetite and corundum float well with cationic collectors above their PZC. Fig. 3.14 shows the flotation of goethite using both anionic and cationic collectors. The PZC of this mineral is pH 6.7. Anionic collectors are effective for goethite below pH 6.7 since the mineral is then positively charged. 

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