Identifying the reactive sites of hydrogen peroxide decomposition and hydroxyl radical formation on chrysotile...

Fibrous chrysotile has been the most commonly applied asbestos mineral in a range of technical applications. However, it is toxic and carcinogenic upon inhalation. The chemical reactivity of chrysotile fiber surfaces contributes to its adverse health effects by catalyzing the formation of highly reactive hydroxyl radicals (HO^•) from H₂O₂. In this Haber-Weiss cycle, Fe on the fiber surface acts as a catalyst: Fe³⁺ decomposes H₂O₂ to reductants that reduce surface Fe³⁺ to Fe²⁺, which is back-oxidized by H₂O₂ (Fenton-oxidation) to yield HO^•. Chrysotile contains three structural Fe species: ferrous and ferric octahedral Fe and ferric tetrahedral Fe (Fe³⁺_tet). Also, external Fe may adsorb or precipitate onto fiber surfaces. The goal of this study was to identify the Fe species on chrysotile surfaces that catalyze H₂O₂ decomposition and HO^• generation.

We demonstrate that at the physiological pH 7.4 Fe³⁺_tet on chrysotile surfaces substantially contributes to H₂O₂ decomposition and is the key structural Fe species catalyzing HO^• generation. After depleting Fe from fiber surfaces, a remnant fiber-related H₂O₂ decomposition mode was identified, which may involve magnetite impurities, remnant Fe or substituted redox-active transition metals other than Fe. Fe (hydr)oxide precipitates on chrysotile surfaces also contributed to H₂O₂ decomposition, but were per mole Fe substantially less efficient than surface Fe³⁺_tet. Fe added to chrysotile fibers increased HO^• generation only when it became incorporated and tetrahedrally coordinated into vacancy sites in the Si layer.

Our results suggest that at the physiological pH 7.4, oxidative stress caused by chrysotile fibers largely results from radicals produced in the Haber-Weiss cycle that is catalyzed by Fe³⁺_tet. The catalytic role of Fe³⁺_tet in radical generation may also apply to other pathogenic silicates in which Fe³⁺_tet is substituted, e.g. quartz, amphiboles and zeolites. However, even if these pathogenic minerals do not contain Fe, our results suggest that the mere presence of vacancy sites may pose a risk, as incorporation of external Fe into a tetrahedral coordination environment can lead to HO^• generation.