Reactive sites at the surface of crocidolite asbestos

The reactive sites at the surface of crocidolite (amphibole asbestos) which may play a role in the toxicity of the fibrous mineral have been investigated by measuring the adsorption of nitric oxide (infrared spectroscopy and adsorption calorimetry) and the release of free radicals in aqueous suspensions. The fibers have been thermally modified-progressively heated in vacuo up to 800°C, calcined in air at 400°C, and kept under hydrothermal conditions at 200°C - in order to mimic modifications likely to occur upon weathering. While the crystal structure, as revealed by XRD, was stable up to 600°C, IR spectroscopy indicated that from 400 to 600°C OH groups are eliminated and changes in the lattice vibrations occur. Calcination at 400°C, which oxidizes part of Fe²⁺ to Fe³⁺, as evidenced by diffuse reflectance UV-vis spectroscopy, eliminates OH groups and modifies lattice vibrations. The hydrothermal treatment causes oxidation of Fe²⁺ only to a limited extent. Upon contact with deuterated water, irreversibly adsorbed H₂O is displaced by D₂O and exposed OH groups are exchanged into OD. Nitric oxide adsorbs onto poorly coordinated surface cations from which water and hydroxyl groups have been removed, the maximum adsorptive capacity being attained on samples heated at 400°C. Mononitrosylic species irreversibly held on different types of surface iron ions are formed, and a part of them is converted into dinitrosyls when increasing the NO pressure. The heat of adsorption decreases from 130 kJ mol^-1 (strongest mononitrosyls) to 20 kJ mol^-1 (addition of NO to a mononitrosyl). NO is also adsorbed on hydrothermally treated crocidolite, but the IR spectra reveal a different site distribution. No nitric oxide is adsorbed on calcined crocidolite. Original crocidolite generates free radicals in aqueous suspensions, revealed by a carboxylate radical originating from the formate ion, but this property is lost both upon calcination and hydrothermal treatment.