Hematite metal adsorption

Trace metal adsorption on goethite is reportedly increased by the presence of sulphate ions (Balistrieri and Murray, 1982). Arsenate and antimony oxyanions enhance uptake of cobalt on ferrihydrite and hematite, respectively (Benjamin and Bloom,... 

Honeyman, B.D., and Santschi, P.H. (1991) Coupling of trace metal adsorption and particle aggregation kinetics and equilibrium studies using 59Fe-labelled hematite. Environ. Sci. Technol. 25, 1739-1747. 

Calorimetric investigations of hydrous metal oxide suspensions are more scarce. A study of starch adsorption on hematite revealed that the adsorption process became less exothermic as surface coverage increased (25). This was attributed to a rearrangement of starch molecules to less favorable configurations. 

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. 

For iron oxides, IR spectroscopy is useful as a means of identification. Hematite crystals in films that were too thin (<70nm) to be characterized by XRD were shown by IR to be oriented with the c-axis perpendicular to the surface of the film (Yubero et al. 2000). This technique also provides information about crystal morphology, degree of crystallinity and the extent of metal (especially Al) substitution because these properties can induce shifts in some of the IR absorption bands. It is also widely used both to obtain information about the vibrational state of adsorbed molecules (particularly anions) and hence the nature of surface complexes (see Chap. 11) and to investigate the nature of surface hydroxyl groups and adsorbed water (see Chap. 10). Typical IR spectra of the various iron oxides are depicted in Figure 7.1. Impurities arising either from the method of preparation or from adsorption of atmospheric compounds can produce distinct bands in the spectra of these oxides -namely at 1700 cm (oxalate), 1400 cm (nitrate) and 1300 and 1500 cm (carbonate). 

Fig.n.6 The effect of pH on the adsorption of some heavy metal cations on goethite and hematite, showing the typical adsorption edge for heavy metals (McKenzie, 1980, with permission). 

Various well-known industrial and municipal waste products particularly those from the base metal industry, contain appreciable amounts of Fe oxides which may make them suitable for remediation purposes. Two examples from industry are the residues from the alumina and the titanium industries. The extraction of either Al or Ti from the natural ores (bauxite and ilmenite/rutile, respectively) leaves behind an alkaline and acidic (sulphuric) residue, respectively, in which Fe oxides are enriched, as indicated by their names Red Mud and Red Gypsum . A sample of Red gypsum is reported to contain ca. 35% of Fe oxide consisting of goethite and hematite, half of which was oxalate soluble (Fauziah et al., 1996). As expected, this material had an appreciable adsorption capacity for phosphate and heavy metals and, if added to soils, could confer these properties on them (Peacock Rimmer, 2000),... 

Kanai, H., Navarrete, R.C., Macisko, C.W. Scriven, L.E. (1992) Rheol. Acta 31 333 Kandori, K. Ishikawa,T. (1991) Selective adsorption of water on amorphous ferric oxide hydroxide. Langmuir 7 2213-2218 Kandori, K. Aoki,Y. Yasukawa, A. Ishikawa, T. (1998) Effects of metal ions on the morphology and structure of hematite particles produced from forced hydrolysis reaction. 

Figure 7. The effect of ligands and metal ions on surface protonation of a hydrous oxide is illustrated by two examples (1). Part a Binding of a ligand (pH 7) to hematite, which increases surface protonation. Part h Adsorption of Pb2+ to hematite (pH 4.4), which reduces surface protonation. Part c Surface protonation of hematite alone as a function of pH (for comparison). All data were calculated with the following surface complex formation equilibria (1 = 5 X 10"3 M >. Electrostatic correction was made by diffuse double layer model.

Van Riemsdijk et al. [53] were the first to show that electrostatic effects could explain non-stoichiometric exchange ratios. Predictions with the one-pKn SCG model and the two-pKn SGC model were both in a good agreement with experimentally observed proton/M ratios and metal ion isotherms at a series of pH values for rutile, hematite and amorphous iron oxide. In contrast with Benjamin and Leckie [86], Van Riemsdijk et al. [53] concluded that incorporation of surface heterogeneity is not required to describe cadmium adsorption on amorphous iron oxide. 

Cation adsorption on oxides has been analysed by several authors [72, 92-96] under the assumption that s.a. ions are not in competition with protons. Fokkink et al. [72, 94] also studied the temperature dependence of metal ion adsorption on hematite and rutile. Although they treat the metal ion sites as independent of the proton sites they state that their results form a direct indication for the occurrence of surface complex formation . 

Chang, H.C.. Healy, T.W., and Matijevic, E., Interaction of metal hydrous oxides with chelating agents. III. Adsorption on spherical colloidal hematite particles,... 

Figure 4.3. Metal cation adsorption versus pH on hematite. (Adapted from R. M. McKenzie. 1980. The adsorption of lead and other heavy metals on oxides of manganese and iron. Aust /. Soil Res. 18 61-73.)...

Fewer data, particularily calorimetric, are available for evaluating the influence of temperature on metal cation adsorption. The enthalpy of Cd(II) adsorption onto rutile was determined using isoperibol solution calorimetry and a value of +10 kJ/mole was found (6). A recent variable temperature study (25) allows enthalpies for Cd(II), Zn(II), and Ni(II) adsorption onto hematite (synthesized in the presence of 0.86% Si) to be calculated using equation (8). These data are summarized in Table IV. 

Table IV. Residual solution concentrations at 5, 20, 25 and 35 C and average isosteric adsorption enthalpies calculated using equation (8) for cadmium, zinc and nickel onto hematite at pH=6 and a total metal concentration of 10/xM. Concentration data from (25)...

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