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

Martin Walter,Lars Gille,Stephan M Kraemer,Walter D C Schenkeveld,Gerald Geroldinger,Michael Reissner

doi:10.1186/s12989-019-0333-1

Abstract

BackgroundFibrous 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.ResultsWe 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.ConclusionsOur 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.

Highlights

Fibrous chrysotile has been the most commonly applied asbestos mineral in a range of technical applications
Color changes related to Fe at chrysotile surfaces Complexation and mobilization of Fe from the beige pristine chrysotile fibers by DFOB resulted in the whitish color of DFOB-altered fibers (Fig. 1)
The results from this study demonstrate that both Ferric octahedral Fe (Fe3+oct) in Feoxide precipitates and Ferric tetrahedral Fe (Fe3+tet) contribute to H2O2 decomposition by chrysotile asbestos; for asbestos fibers incubated at pH 7.4 in absence of a ligand the contributions of both Fe species were comparable, despite the excess of octahedral sites

Summary

Introduction

Fibrous chrysotile has been the most commonly applied asbestos mineral in a range of technical applications. Respiratory exposure to asbestos minerals causes adverse health effects like pneumoconiosis, fibrosis of the lung, pleural plaques and effusions, carcinomas predominantly in the lung (and in the larynx and ovaries) and mesotheliomas in the pleura and peritoneum [2, 4, 6, 7]. Because of their carcinogenic potential, the WHO-IARC has classified all asbestos minerals as group 1 carcinogens [8]. In northern American countries its use has not yet been banned [10] and in some Asian countries it even increases [11, 12]

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