A study of radicals in industrial raw cristobalite powders

Francesco Di Benedetto,Maurizio Romanelli,Elena Belluso,Alessandro Marinaccio,Fabio Capacci,Andrea Giaccherini,Giordano Montegrossi,Giulio Arcangeli,Alfonso Zoleo,Silvana Capella,Ottorino Gottardo

doi:10.1007/s00269-020-01127-1

Abstract

We report the results of an experimental multianalytical characterisation of industrial cristobalite powders, used as raw materials for artificial stone production. Cristobalite is considered a serious threat to human health. The study was carried out through X-ray diffraction (XRD), scanning electron microscopy with energy dispersive microanalysis (SEM/EDS), continuous-wave (cw) and pulse electron paramagnetic resonance (EPR) spectroscopy. Our results point out a sub-micrometric size of the structural coherence in cristobalite, associated with numerous stacking defects. Moreover, the material was found characterised by the presence of superoxide radicals, whose persistence appears conceivably long. Radicals in a material synthesized through a high-temperature treatment were generated during the grinding step in the industrial production of cristobalite. During this process, in fact, both superoxide generation and structural defectivity are induced. Indeed, cristobalite powders already result activated by a radical population, before any kind of process in artificial stone production.

Highlights

Among the numerous polymorphs of the crystalline silica, SiO2, cristobalite (CAS no. 14464-46-1) deserves a relevant attention in the context of occupational exposure, being rated as “carcinogenic to humans” by the IARC (2012)
The analysis reveals that cristobalite is the only species present in the X-ray diffraction (XRD) pattern
The main achievement of the present study lies in the fact that an unusually abundant and stable population of superoxide radicals has been revealed and described associated to the industrial cristobalite filler powders, and likely related to the diffused stacking disorder characterizing the investigated material

Summary

Introduction

Among the numerous polymorphs of the crystalline silica, SiO2, cristobalite (CAS no. 14464-46-1) deserves a relevant attention in the context of occupational exposure, being rated as “carcinogenic to humans” by the IARC (2012). Among the numerous polymorphs of the crystalline silica, SiO2, cristobalite The Threshold Limit Values (TLV) for crystalline silica resulted occasionally split in specific values for α-quartz and cristobalite (Andersson et al 2008). In US, OSHA reduced to 0.05 mg/m3 the TLV for the respirable fractions of all crystalline silica polymorphs. The EU in its 2398/17 adopted the carcinogenicity of crystalline silica while setting the TLV at 0.1 mg/ m3. The TLV fixed by EU directive seems only partially adequate to the carcinogenic power of crystalline silica, and to the risk associated to exposure, even at low doses, to cristobalite. Clusters of workers exhibiting silicosis have been found, in the last years, in the production of microcasted gold (Murgia et al 2007; Romanelli et al 2012) and of artificial stone

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Discussion

Conclusion