Thermal inertization of amphibole asbestos modulates Fe topochemistry and surface reactivity

Our plain-English summary of Thermal inertization of amphibole asbestos modulates Fe topochemistry and surface reactivity, by A. Pacella, M. Tomatis, C. Viti, et al (Journal of Hazardous Materials, 2020).

The study

• Authors: A. Pacella, M. Tomatis, C. Viti, A. Bloise, L. Arrizza, P. Ballirano, F. Turci
• Journal: Journal of Hazardous Materials
• Published: 2020;398:123119
• Cited by: 8 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

This study discloses the morphological and chemical-structural modifications that occur during thermal degradation of amphibole asbestos. Low-iron tremolite and iron-rich crocidolite were heated at temperatures ranging from r.t. to 1200 °C. Heating promoted a complex sequence of iron oxidation, migration and/or clustering and, finally, the formation of brittle fibrous pseudomorphs consisting of newly formed minerals and amorphous nanophases. The effects of the thermal modifications on toxicologically relevant asbestos reactivity were evaluated by quantifying carbon- and oxygen-centred, namely hydroxyl (OH), radicals. Heating did not alter carbon radicals, but largely affected oxygen-centred radical yields. At low temperature, reactivity of both amphiboles decreased. At 1200 °C, tremolite structural breakdown was achieved and the reactivity was further reduced by migration of reactive iron ions into the more stable TO4 tetrahedra of the newly formed pyroxene(s). Differently, crocidolite breakdown at 1000 °C induced the formation of hematite, Fe-rich pyroxene, cristobalite, and abundant amorphous material and restored radical reactivity. Our finding suggests that thermally treated asbestos and its breakdown products still share some toxicologically relevant properties with pristine fibre. Asbestos inertization studies should consider morphology and surface reactivity, beyond crystallinity, when proving that a thermally inactivated asbestos-containing material is safe.

Copyright © 2020 Elsevier B.V. All rights reserved.

In plain English

This laboratory study examined how two amphibole asbestos minerals change when heated, as in treatments to make asbestos waste inert. The authors heated a low-iron tremolite from Maryland (USA) to 930°C and 1200°C, and the iron-rich UICC standard crocidolite to 650, 800 and 1000°C. They examined the fibres with microscopes and X-ray methods. In cell-free tests, they measured hydroxyl and carbon-centred free radicals from the fibres. Tremolite heated to 1200°C kept its fibrous shape but had turned mainly into two pyroxene minerals. Crocidolite heated to 1000°C broke down mostly into hematite, cristobalite, aegirine and abundant amorphous material. Carbon-centred radicals were tested with ascorbic acid, a reducing agent, added; without it, unheated fibres produced virtually none. Under that condition, heating did not significantly change their yields. Hydroxyl radical yields dropped for tremolite (about 40% lower at 930°C, lower still at 1200°C) and for crocidolite at 650°C. Crocidolite's reactivity was partly restored at 800°C, and its 1000°C breakdown products were still reactive. The authors concluded that thermally inactivated asbestos may still share some toxicologically relevant properties with the original fibre.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory experiment on mineral samples (heating, structural analysis, and cell-free free-radical tests); no humans or animals
• Who was studied: Two asbestos samples: a low-iron tremolite asbestos from Maryland (USA) and the iron-rich UICC standard crocidolite (fibrous riebeckite)
• Exposure: Heating in an oxidizing atmosphere: tremolite up to 930°C and 1200°C; crocidolite up to 650°C, 800°C and 1000°C
• Main results:
  • Tremolite heated to 1200°C kept its fibrous morphology
  • Heated tremolite broke down into two pyroxenes (a subcalcic diopside making up about 70 wt.% of the mixture, plus a pigeonite-like pyroxene), minor cristobalite and traces of hematite
  • Crocidolite heated to 1000°C broke down into hematite (54.0%), cristobalite (24.0%) and aegirine (21.1%), with abundant amorphous material
  • Under scanning electron microscopy, crocidolite heated to 1000°C kept a fibrous morphology with very irregular edges. Under transmission electron microscopy, it showed mostly a nanogranular texture, with fibrous pseudomorphs only in some cases
  • In crocidolite, the ratio of ferrous to ferric iron fell from 1.07 at room temperature to 0.39 at 650°C, indicating extensive iron oxidation
  • Before heating, tremolite produced more hydroxyl radicals than crocidolite, despite its very low iron content
  • Without a reducing agent, the unheated fibres showed virtually no reactivity toward formate (the carbon-centred radical test). Reactivity was restored when surface iron was reduced with ascorbic acid
  • In the presence of ascorbic acid, heating did not significantly change carbon-centred radical yields
  • The hydroxyl radical signal of tremolite heated at 930°C was about 40% lower than that of unheated tremolite, and the 1200°C breakdown products were lower still
  • Crocidolite's hydroxyl radical reactivity was significantly reduced at 650°C and partially restored at 800°C
  • Crocidolite breakdown at 1000°C restored radical reactivity. Of the new minerals tested separately, aegirine and hematite produced both radical types, but cristobalite did not
• The authors' conclusion: The authors concluded that thermally inactivated asbestos may still share some toxicologically relevant properties with the original fibre. They recommended that inertization studies consider aspect ratio and surface reactivity, not only crystallinity, when proving that an asbestos-containing material is safe.
• Limitations the authors state:
  • Amorphous material was hard to detect by X-ray diffraction because the glass capillary partly masked its signal
  • Gentle grinding before electron microscopy may have destroyed much of the fibrous shape of the heated crocidolite seen under that microscope
  • Cristobalite and hematite were not detected by electron microscopy in heated tremolite, probably because of their low amounts and the very small sample volume examined

Topics

Medical terms in this study, explained in our Lexicon: Amphibole Asbestos, Surface Reactivity, Tremolite, Crocidolite, Amphibole Group, Pyroxene, Hematite, Asbestos-Containing Material (ACM).

Related studies

• Iron topochemistry and surface reactivity of amphibole asbestos: relations with in vitro toxicity — A. Pacella, G.B. Andreozzi, et al. (2012)
• Surface reactivity of amphibole asbestos: a comparison between crocidolite and tremolite — G.B. Andreozzi, A. Pacella, et al. (2017)
• Surface chemistry and surface reactivity of fibrous amphiboles that are not regulated as asbestos — M. Fantauzzi, A. Pacella, et al. (2012)
• Recycling of the product of thermal inertization of cement-asbestos for various industrial applications — A.F. Gualtieri, C. Giacobbe, et al. (2011)
• Surface reactivity and cell responses to chrysotile asbestos nanofibers — F. Turci, M. Colonna, et al. (2012)
• Surface and bulk modifications of amphibole asbestos in mimicked gamble's solution at acidic PH — A. Pacella, P. Ballirano, et al. (2021)

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