Pentavalent hematite

In situ emission Mossbauer spectroscopy provides valuable information on the chemical structure of dilute metal ions at the metal oxide/aqueous solution interface The principles of the method are described with some experimental results on divalent Co-57 and pentavalent Sb-119 adsorbed on hematite. 

We now extend the work to in situ measurements on metal ions adsorbed at the metal oxide/aqueous solution interface. In this report, our previous results are combined with new measurements to yield specific information on the chemical structure of adsorbed species at the solid/aqueous solution interface. Here, we describe the principles of emission Mossbauer spectroscopy, experimental techniques, and some results on divalent Co-57 and pentavalent Sb-119 ions adsorbed at the interface between hematite (a-Fe203) and aqueous solutions. 

The procedure for separating Sb-119 from an alpha-irradiated tin target has been described elsewhere (10,11). The amounts of cobalt and antimony coexisting with the nuclides are estimated to have been about 400 ng/mCi and 300 ng/mCi, respectively, i.e., to have been much smaller than that required for monolayer coverage of 30 mg of the hematite sample. About 10 cm3 of an aqueous solution containing 1 - 2 mCi of divalent Co-57 or 0.1 - 1 mCi of pentavalent Sb-119 was adjusted to an appropriate pH value in a Teflon vessel with a 0.5 mm-thick Teflon window at the bottom, and about 30 mg of hematite powder was added to the solution. The suspension was shaken for 30 min at room temperature. After settling of the powder at the bottom of the vessel, the pH was remeasured. 

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. 

In Situ Mossbauer Measurement on Hematite/Pentavalent Sb-119. The Mossbauer measurement on Sb-119 was continued for 1 - 3 days after shaking the suspension for 30 min and waiting for settling of the... 

Figure 4. pH dependence of the adsorption and desorption of carrier-free pentavalent Sb—119 on hematite at room temperature (30 mg of hematite prefired at 900°C in 10 cm3 of 0.25 mol/dm3 LiCl solutions). Desorption was measured on pentavalent Sb-119 pre-adsorbed at pH 4. Shaking time was 22 hours for the adsorption and was 5 days for the desorption. 

In order to study the effect of the amount of pentavalent Sb ions on adsorption state, in situ emission Mossbauer measurement was made on Sb-119 adsorbed on hematite with non-radioactive pentavalent... 

Figure 5. In situ emission Mossbauer spectra of Sn-119 arising from pentavalent Sb-119 at the hematite/0.25 mol dm-3 LiCl solution interface for various pH values of the aqueous phase (measurement at room temperature) (A) pH 8.5, (B) pH 6.6, (C) pH 4.6, (D) pH 3.4, (E) pH 2.5, (F) pH 8.6. (F) was measured...

Sb carrier ions. The Sb-119 ions were adsorbed on 30 mg of hematite from 10 cm3 of a 0.25 mol/dm3 KC1 solution containing about 1 mg of pentavalent Sb ions. About 0.3 mg of Sb was adsorbed at pH 2.5 and 4.0. The amounts of Sb adsorbed are less than that required to cover all the hematite surfaces as a monolayer. The emission Mossbauer spectra obtained are shown in Figure 7. It is seen from Figure 7 that the width of the emission Mossbauer spectrum at pH 2.5 is much smaller than that of the carrier-free one, while essentially no effect of carrier Sb ions is observed at pH 4.0. 

Effects of Pentavalent Sb Ions on the Adsorption of Divalent Co-57 on Hematite. Benjamin and Bloom reported that arsenate ions enhance the adsorption of cobaltous ions on amorphous iron oxyhydroxide (J 6). Similarly, when divalent Co-57 ions were adsorbed on hematite together with pentavalent Sb ions, an increase of adsorption in the weakly acidic region was observed. For example, when 30 mg of hematite was shaken with 10 cm3 of 0.1 mol/dm3 KC1 solution at pH 5.5 containing carrier-free Co-57 and about 1 mg of pentavalent Sb ions, 95 % of Co-57 and about 30 % of Sb ions were adsorbed. The emission spectra of the divalent Co-57.ions adsorbed under these conditions are shown in Figure 8 together with the results obtained under different conditions. As seen in Figure 8, the spectra of divalent Co-57 co-adsorbed with pentavalent Sb ions are much different from those of Co-57 adsorbed alone (Figure 3). These observations show a marked effect of the.co-adsorbed pentavalent Sb ions on the chemical structure of adsorbed Co-57. 

Figure 7. In situ emission Mossbauer spectra of pentavalent Sb-119 adsorbed on hematite with Sb carrier from 0.25 mol/dm3 KC1 solution (A) pH 4.0 and (B) 2.5.

Figure 8. In situ emission Mossbauer spectra of divalent Co-57 adsorbed on hematite with pentavalent Sb ions from 0.1 mol/dm3 KCI solution (A) pH 5.5, (B) pH 9.2. (C) From 0.3 M KOH.

The results of Weiss field calculation on ferric ions at the surface metal ion sites are given in Figure 6 of ref 4, and the values for room temperature are shown in Figure 10. Since both ferric and pentavalent Sb ions can occupy octahedral or distorted octahedral sites with six ligand oxide ions and bulk hematite is considered to accommodate pentavalent Sb—119 ions in the metal ion sites (3 ), we can estimate STHF interactions on tetravalent Sn-119 ions at the surface metal ion sites of hematite. Using the magnetization of surface ferric ions at room temperature, the STHF magnetic fields on tetravalent Sn-119 ions at the surface sites are calculated to be... 

Figure 9. Simplified model of the (111) surface of the corundum-type structure, (a) A view of the surface from a direction slightly shifted from <111>. Only metal ions of the zeroth, first, and second layers are shown, (b) A section of the surface along the arrows depicted in part a. Hexagonally close-packed oxide ion layers are shown with lines. Surface protons are not shown, (c) A divalent Co-57 or pentavalent Sb-119 ion on the zeroth metal ion layer, (d) Aquo or hydroxyl complex of divalent Co-57 or pentavalent Sb-119 hydrogen-bonded to the surface oxide ion layers of hematite.

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. 

In situ emission Mossbauer spectroscopic measurement of the hyper-fine magnetic fields on trivalent Fe-57 and tetravalent Sn-119 arising from divalent Co-57 and pentavalent Sb—119, respectively, yields valuable information on the chemical structure of adsorbed metal ions at the interface between hematite and an aqueous solution. 

In contrast, the pentavalent Sb-119 ions at the interfaces are weakly bonded to the oxide ion layer of the hematite surfaces in neutral and slightly acidic region, while in the acidic region most of the adsorbed Sb-119 ions are in the zeroth or first metal ion layers of the substrate forming Sb-O-Fe bonds. The pentavalent Sb-119 ions having once been incorporated into the surface metal ion sites retain their chemical form, even when the pH of the aqueous phase is raised above 7. Heating of suspensions at 98°C results in chemical rearrangement of the hematite surfaces to yield pentavalent Sb-119 ions in the second or deeper metal ion layers. 

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