Chemical characterization and reactivity of iron chelator-treated amphibole asbestos

Our plain-English summary of Chemical characterization and reactivity of iron chelator-treated amphibole asbestos, by J. Gold, H. Amandusson, A. Krozer, et al (Environmental Health Perspectives, 1997).

The study

• Authors: J. Gold, H. Amandusson, A. Krozer, B. Kasemo, T. Ericsson, G. Zanetti, B. Fubini
• Journal: Environmental Health Perspectives
• Published: 1997;105 Suppl 5(Suppl 5):1021-30
• Cited by: 26 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Iron in amphibole asbestos is implicated in the pathogenicity of inhaled fibers. Evidence includes the observation that iron chelators can suppress fiber-induced tissue damage. This is believed to occur via the diminished production of fiber-associated reactive oxygen species. The purpose of this study was to explore possible mechanisms for the reduction of fiber toxicity by iron chelator treatments. We studied changes in the amount and the oxidation states of bulk and surface iron in crocidolite and amosite asbestos that were treated with iron-chelating desferrioxamine, ferrozine, sodium ascorbate, and phosphate buffer solutions. The results have been compared with the ability of the fibers to produce free radicals and decompose hydrogen peroxide in a cell-free system in vitro. We found that chelators can affect the amount of iron at the surface of the asbestos fibers and its valence, and that they can modify the chemical reactivity of these surfaces. However, we found no obvious or direct correlations between fiber reactivity and the amount of iron removed, the amount of iron at the fiber surface, or the oxidation state of surface iron. Our results suggest that surface Fe3+ ions may play a role in fiber-related carboxylate radical formation, and that desferrioxamine and phosphate groups detected at treated fiber surfaces may play a role in diminishing and enhancing, respectively, fiber redox activity. It is proposed that iron mobility in the silicate structure may play a larger role in the chemical reactivity of asbestos than previously assumed.

In plain English

This laboratory study used no cells, animals or people. It examined standard UICC samples of crocidolite and amosite asbestos after soaking them for 3 days in one of four iron-binding (chelating) solutions: desferrioxamine, sodium ascorbate, ferrozine or phosphate buffer. The authors measured the iron released into the solutions, the iron inside the fibers and at their surfaces, and the fibers' ability to generate free radicals and to break down hydrogen peroxide. The treatments removed an estimated 1 to 5 atomic percent of the fibers' total iron, with the least removed by phosphate buffer. They did not significantly change the balance of iron types inside the fibers. Most treatments reduced fiber reactivity. However, phosphate buffer increased hydroxyl radical production by both fiber types, and ferrozine did so for crocidolite. Traces of desferrioxamine and phosphate appeared to remain on treated fiber surfaces. The authors found no correlation between the iron released and the iron left at the surface. They proposed that iron mobility within the mineral structure may matter more than previously assumed. They noted that the free radical signals were at or near detection levels.

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 (cell-free, in vitro) chemical study of asbestos fibers treated with iron chelators, using Mossbauer spectroscopy, X-ray photoelectron spectroscopy, and EPR spin-trapping tests of free radical release and hydrogen peroxide decomposition
• Who was studied: No people or animals; standard UICC (Union Internationale Contre le Cancer) reference samples of crocidolite and amosite asbestos, tested untreated and after chelator treatment
• Exposure: Fibers incubated in 10-mM solutions of desferrioxamine, sodium ascorbate, ferrozine or phosphate buffer, at 10 mg fibers per ml, for 3 days at 37 degrees C; pH was not controlled
• Main results:
  • The chelator treatments removed an estimated 1 to 5 atomic percent of the total iron in the fibers; the phosphate buffer solution mobilized the least iron
  • Inside the fibers, amosite contained only ferrous iron (Fe2+) and crocidolite about 59 at% Fe2+ and 41 at% Fe3+; chelator treatments did not significantly change bulk ferrous to ferric ratios
  • The surface iron-to-silicon ratio averaged about 0.8 on untreated fibers; it was significantly lower on desferrioxamine-treated crocidolite (0.53) and ascorbate-treated crocidolite (0.64). On amosite it was lower after desferrioxamine, ferrozine and phosphate buffer treatment, but not after ascorbate treatment
  • Most surface iron on all fibers was Fe3+; chelator-treated amosite surfaces showed an average increase of 6 to 10 at% in Fe2+ content
  • Nitrogen was elevated (around 2 at%) on desferrioxamine-treated fibers, and 0.8 to 1.2 at% phosphorus was found on phosphate buffer-treated fibers; the authors took this as evidence that these substances stayed on the surfaces
  • In the carboxylate radical test, all chelator treatments eliminated or reduced reactivity. All treatments eliminated the signal for amosite, but for crocidolite only ascorbate eliminated it
  • In the hydroxyl radical test, phosphate buffer treatment made hydroxyl radicals detectable from amosite and increased the signal 3-fold for crocidolite; ferrozine also increased hydroxyl radical activity for crocidolite, desferrioxamine eliminated it, and ascorbate caused no change
  • Hydrogen peroxide breakdown by both fiber types decreased after chelator treatments, except for ascorbate-treated amosite; ferrozine gave the greatest reduction, and desferrioxamine and phosphate buffer reduced it by about one-half
  • No correlation was found between the iron released into the chelator solutions and the amount and valence of iron remaining at the fiber surface
• The authors' conclusion: The authors concluded that chelators can change the amount and valence of surface iron and the fibers' chemical reactivity, but found few correlations between reactivity and iron measures. Their results indicated that surface Fe3+ is important for carboxylate radical formation. They proposed that iron mobility in the silicate structure may play a larger role in asbestos reactivity than previously assumed
• Limitations the authors state:
  • EPR free radical signals had low intensities at or near detection levels, which may complicate interpretation
  • Curve fitting of iron valence states in the surface (XPS) spectra is difficult and questionable because the signals overlap
  • pH of the chelator solutions was not controlled; ferrozine likely became acidic
  • Observations could be complicated by iron diffusing from the bulk to the surface, precipitation of iron complexes from solution, and simultaneous dissolution of the silicate structure

Topics

Medical terms in this study, explained in our Lexicon: Amphibole Asbestos, Reactive Oxygen Species (ROS), Crocidolite, Amosite, Silicate.

Related studies

• Phytic acid, an iron chelator, attenuates pulmonary inflammation and fibrosis in rats after intratracheal instillation of asbestos — D.W. Kamp, V.A. Israbian, et al. (1995)
• Iron mobilization from asbestos by chelators and ascorbic acid — L.G. Lund, A.E. Aust (1990)
• Surface chemistry and surface reactivity of fibrous amphiboles that are not regulated as asbestos — M. Fantauzzi, A. Pacella, et al. (2012)
• Iron topochemistry and surface reactivity of amphibole asbestos: relations with in vitro toxicity — A. Pacella, G.B. Andreozzi, et al. (2012)
• Free radical activity of synthetic vitreous fibers: iron chelation inhibits hydroxyl radical generation by refractory ceramic fiber — D.M. Brown, C. Fisher, et al. (1998)
• Free radical activity of natural and heat treated amphibole asbestos — C. Otero Areán, F. Barceló, et al. (2001)

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