Redox state and mobility of iron at the asbestos surface: a voltammetric approach

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Abstract
The mechanism by which asbestos causes the development of cancer in people exposed to airborne fibres, still unclear at the molecular level, seems to involve iron ions located at particular sites at the fibre surface. In this paper, cyclic voltammetry has been employed to investigate the oxidation state and mobility of iron ions on crocidolite and amosite, the two most common types of amphibole asbestos. Experiments have been carried out at three pH values: 0.5, at which iron is spontaneously released from the solids; neutral (close to the extracellular and cytoplasmatic value); and 4.5, representative of the lysosomial fluid, i.e. of the environment to which a phagocytized fibre is exposed. An Fe-exchanged Y zeolite and an Fe-silicalite have been used for comparison as ‘model solids’. At neutral pH iron is readily mobilised from FeY upon cycling, in contrast with Fe-silicalite. With both asbestos materials, at neutral pH iron is mobilised during the subsequent cycles and is brought into solution, amosite releasing more iron than crocidolite per unit area under the same conditions. Three couples of redox peaks are seen, centred at ca. 0.0, −0.2 and +0.6 V. The first is due to the Fe3+/Fe2+ couple in solution, the second is probably related to the Fe3+/Fe2+ at the surface of the asbestos particle, and the third is assigned to surface iron in an oxidation state higher than three. At pH 0.5, the couple of peaks due to surface Fe is absent and mobilisation immediately occurs. The release of iron at pH 0.5 from asbestos was also measured photometrically and, in the first two hours, corresponds to that expected for the outermost layer of the minerals. Voltammetric cycling markedly enhances the amount of Fe solubilised. Partial oxidation or reduction of surface iron ions was obtained by keeping the sample at a defined potential before cycling: subsequent voltammograms indicate that the smaller Fe3+ cation is the mobilised species.
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