The chemical environment of iron in mineral fibres. A combined X-ray absorption and Mössbauer spectroscopic study

Our plain-English summary of The chemical environment of iron in mineral fibres. A combined X-ray absorption and Mössbauer spectroscopic study, by S. Pollastri, F. D'Acapito, A. Trapananti, et al (Journal of Hazardous Materials, 2015).

The study

• Authors: S. Pollastri, F. D'Acapito, A. Trapananti, I. Colantoni, G.B. Andreozzi, A.F. Gualtieri
• Journal: Journal of Hazardous Materials
• Published: 2015;298:282-93
• Cited by: 35 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Although asbestos represents today one of the most harmful contaminant on Earth, in 72% of the countries worldwide only amphiboles are banned while controlled use of chrysotile is allowed. Uncertainty on the potential toxicity of chrysotile is due to the fact that the mechanisms by which mineral fibres induces cyto- and geno-toxic damage are still unclear. We have recently started a long term project aimed at the systematic investigation of the crystal-chemistry, bio-interaction and toxicity of the mineral fibres. This work presents a systematic structural investigation of iron in asbestos and erionite (considered the most relevant mineral fibres of social and/or economic-industrial importance) using synchrotron X-ray absorption and Mössbauer spectroscopy. In all investigated mineral fibres, iron in the bulk structure is found in octahedral sites and can be made available at the surface via fibre dissolution. We postulate that the amount of hydroxyl radicals released by the fibers depends, among other factors, upon their dissolution rate; in relation to this, a ranking of ability of asbestos fibres to generate hydroxyl radicals, resulting from available surface iron, is advanced: amosite > crocidolite ≈ chrysotile > anthophyllite > tremolite. Erionite, with a fairly high toxicity potential, contains only octahedrally coordinated Fe(3+). Although it needs further experimental evidence, such available surface iron may be present as oxide nanoparticles coating and can be a direct cause of generation of hydroxyl radicals when such coating dissolves.

Copyright © 2015 Elsevier B.V. All rights reserved.

In plain English

This laboratory spectroscopy study examined the chemical form of iron in eight mineral fibre samples: amphibole asbestos (amosite, anthophyllite, crocidolite, tremolite), three chrysotile asbestos samples and fibrous erionite from Jersey, Nevada. In the asbestos minerals, iron sat in octahedral sites. Tetrahedral iron in the chrysotile samples was attributed to the impurity magnetite, partly oxidized to maghemite. Amosite and anthophyllite held mainly ferrous iron (Fe2+), while crocidolite held comparable amounts of ferrous and ferric iron (Fe3+). Erionite held only octahedral Fe3+. The authors judged this iron likely to sit in iron-enriched surface nanoparticles under 10 nm, findings they called preliminary. The authors made several assumptions: a 0.1 micrometre fibre diameter, pH 4, dissolution data from earlier studies, and that anthophyllite dissolves like crocidolite. On that basis they estimated the iron each asbestos type would release in 239 days, chrysotile's estimated dissolution time. They proposed, as a conjecture, a ranking of ability to generate hydroxyl radicals from surface iron: amosite, then crocidolite and chrysotile about equal, then anthophyllite, then tremolite. Erionite was excluded. The authors concluded that, regarding surface iron, chrysotile's toxicity potential may not be lower than that of amphiboles.

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What the study did and found

• Design: Laboratory spectroscopy study of mineral fibre samples (synchrotron X-ray absorption and Mössbauer spectroscopy, with electron microscopy); no human or animal subjects
• Who was studied: Eight mineral fibre samples: UICC standard amosite, anthophyllite and crocidolite; tremolite asbestos from Val d'Ala (Italy); UICC standard chrysotile; chrysotile from Balangero and from Val Malenco (Italy); fibrous erionite from Jersey, Nevada (USA)
• Exposure: Not an exposure study. The materials examined were asbestos fibres (chrysotile and the amphiboles amosite, anthophyllite, crocidolite, tremolite) and the zeolite fibre erionite.
• Main results:
  • In all investigated fibres, iron was found in octahedral (six-fold) coordination.
  • In UICC chrysotile, the authors assigned all tetrahedral iron to the impurity magnetite/maghemite rather than to chrysotile itself.
  • In Balangero chrysotile, the tetrahedral iron seen by EXAFS was attributed to ferric iron from magnetite. Magnetite in this sample showed 10% oxidation to maghemite.
  • UICC amosite showed a predominance of ferrous iron (Fe2+) in octahedral position.
  • UICC anthophyllite was mainly ferrous iron (Fe2+) with a very low amount of ferric iron (Fe3+).
  • UICC crocidolite contained comparable amounts of ferrous and ferric iron.
  • In UICC chrysotile, the XANES data showed ferric iron (Fe3+) in larger fractions than ferrous iron (Fe2+).
  • In the Balangero chrysotile sample, Mössbauer data attributed about 30% of the total iron to the impurity magnetite and 70% to chrysotile.
  • Within the chrysotile of the Balangero sample, ferrous iron (Fe2+) made up 40% and ferric iron (Fe3+) 30% of total iron.
  • In the Val Malenco chrysotile sample, Mössbauer data assigned 27% of the total iron to magnetite and 73% to chrysotile.
  • The erionite from Jersey, Nevada contained only ferric iron (Fe3+).
  • In erionite, the authors considered the octahedral Fe3+ likely to be in oxide nanoparticles on the fibre surface.
  • Electron microscopy (TEM) of the erionite showed spherical particles smaller than 10 nm on the fibre surface.
  • Clusters of these particles on the erionite were enriched in iron compared with the bulk of the fibre.
  • The authors assumed a 0.1 micrometre fibre diameter, pH 4 and dissolution times from earlier studies (chrysotile ca. 239 days). On that basis they estimated the iron released within 239 days: amosite 3.0 wt%, chrysotile 1.1-2.3 wt%, crocidolite 1.3 wt%, tremolite 0.02 wt%. Anthophyllite was 0.37 wt% if assumed to dissolve at crocidolite's rate.
  • From these estimates the authors proposed a ranking of ability to generate hydroxyl radicals from surface iron: amosite, then crocidolite and chrysotile about equal, then anthophyllite, then tremolite. Erionite was deliberately left out of the ranking.
• The authors' conclusion: The authors concluded that chrysotile's much shorter dissolution time gives an iron release comparable to amphiboles. This may indicate that chrysotile's overall toxicity potential is not lower than that of amphiboles, as far as surface iron availability is concerned. For erionite, they proposed that iron sits in a surface coating of oxide nanoparticles that dissolve in acid conditions.
• Limitations the authors state:
  • The quality of the EXAFS data for anthophyllite was poor, and a better fit could not be achieved.
  • The erionite results were preliminary and need further evidence to rule out iron sitting inside the zeolite cages instead.
  • The link between dissolution time and iron release (the ranking) was a semi-quantitative conjecture; a specific study is planned.
  • The estimates assumed a single mean fibre diameter, chosen for convenience.
  • Anthophyllite's dissolution rate was assumed to equal crocidolite's.

Mines named here

Asbestos mines this study is about. Each mine's page says who ran it, when it worked, and what it produced.

• Balangero mine, Balangero, about 20 km north of Turin, Piedmont

All asbestos mines

Topics

Medical terms in this study, explained in our Lexicon: Amphibole Group, Controlled Use, Chrysotile, erionite, Octahedral Site, Hydroxyl Radical, Amosite, Crocidolite.

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