Chemical differences between long and short amosite asbestos: differences in oxidation state and coordination sites of iron, detected by infrared spectroscopy

Our plain-English summary of Chemical differences between long and short amosite asbestos: differences in oxidation state and coordination sites of iron, detected by infrared spectroscopy, by A. Graham, J. Higinbotham, D. Allan, et al (Occupational and Environmental Medicine, 1999).

The study

• Authors: A. Graham, J. Higinbotham, D. Allan, K. Donaldson, P.H. Beswick
• Journal: Occupational and Environmental Medicine
• Published: 1999;56(9):606-11
• Cited by: 11 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Objectives: Short fibres of amosite asbestos (SFA), obtained by ball milling of long fibres (LFA), have been shown to be less pathogenic than long fibres. Accumulating evidence suggests an important role for differences in surface chemistry between fibres. Iron has been implicated in the pathogenesis of asbestos fibres. In this study infrared (IR) spectroscopy was used to compare LFA and SFA in terms of the coordination and oxidation state of iron at the three cation sites (M1, M3, M1).

Methods: Infrared was used to examine LFA ad SFA, when dry and when hydrated in the presence and absence of the chelators desferroxamine and ferrozine. With appropriate software the proportions of iron and its oxidation states in the overlapping peaks were resolved and assigned, and the three coordination sites were identified. Data were obtained from 10 samples of both lengths of fibre for each of the four treatments. Iron release was also monitored.

Results: Iron was significantly more oxidised in LFA than SFA. Further oxidation of the dry fibres with water, ferrozine, or desferroxamine tended to abolish these differences. There were also significant differences between the proportions of iron held in the different coordination sites of the fibres. For LFA, a higher proportion of its iron was held in the cation sites coordinating less with iron and more with Mg. Interestingly, the sites coordinating single irons were significantly more oxidised than multiple sites. The single iron sites were more oxidised in LFA than SFA and were more readily oxidised by the treatments.

Conclusions: Important chemical differences between LFA and SFA were found. There seemed to be some mobility of iron near the surface. Based on these data it is speculated that the 1 iron surface site may be important in pathogenesis.

In plain English

This laboratory study compared long-fibre amosite asbestos with a short-fibre sample made from it by ball milling, which earlier studies had found less harmful. The authors used infrared spectroscopy to examine the iron in the fibres: how much was in reduced (ferrous) or oxidised (ferric) form, and how it was arranged among neighbouring iron and magnesium atoms. They tested 80 one-milligram samples, 10 of each fibre type under each of four conditions: untreated, or soaked in water or in one of two iron-binding chemicals. In both untreated samples most iron was reduced, but more was oxidised in the long fibres (16.1%) than in the short fibres (10.2%). Iron held next to two magnesium atoms, called the one-iron site, was the most oxidised and the most easily oxidised by the treatments. After treatment, oxidation in the short fibres was similar to that of untreated long fibres. The authors speculated that the one-iron site, which they concluded tends to lie at the fibre surface, may be important in amosite's harmful effects.

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 study of asbestos fibre samples using infrared spectroscopy, with iron release measured in the treatment solutions; no people or animals were studied
• Who was studied: Samples of long-fibre amosite (LFA) and short-fibre amosite (SFA) supplied by the Canadian Asbestos Institute. There were 80 samples of 1 mg each: 10 samples of each fibre type for each of four treatments.
• Exposure: Amosite asbestos only. The short sample was produced from the long sample by milling; earlier animal studies cited by the authors had found the short sample not pathogenic. Samples were either left untreated or treated with 2 ml distilled water, or with 2 ml 4 mM ferrozine (which binds Fe2+) or desferrioxamine (which binds Fe3+) in citrate/phosphate buffer (pH 4.6), for 24 hours.
• Main results:
  • In untreated (dry) fibres, most iron was in the reduced (Fe2+) form in both samples. Long-fibre amosite had a greater proportion of oxidised iron than short-fibre amosite at all coordination sites: 16.1% of total iron was in the Fe3+ form in LFA versus 10.2% in SFA (Table 4). The Fe2+/Fe3+ ratio for total iron was 8.77 for SFA and 5.21 for LFA (95% CI for the difference 1.51 to 5.61, p less than 0.05).
  • Iron at the one-iron site (one iron with two magnesium atoms) was far more oxidised than iron at the two-iron and three-iron sites. In dry fibres, 38.2% (SFA) and 39.5% (LFA) of one-iron-site iron was in the Fe3+ form, compared with 7.0% (SFA) and 10.8% (LFA) at the two-iron site.
  • In untreated fibres, LFA held relatively more of its iron in the lower-occupancy (one-iron) sites than SFA. The ratio of total iron in one-iron to three-iron sites was 0.490 for LFA and 0.273 for SFA.
  • Water alone significantly oxidised iron at all coordination sites in SFA but only at the one-iron site in LFA. At the one-iron site, the Fe3+ share rose from 38.2% to 55.1% in SFA and from 39.5% to 59.5% in LFA after water treatment.
  • After all treatments, the degree of iron oxidation in SFA was similar to that of untreated LFA.
  • In SFA, all treatments shifted iron (both Fe2+ and Fe3+) from the three-iron site to the one-iron site, most markedly for Fe3+. In LFA this shift was seen only for Fe3+ with water treatment.
  • Negligible iron was released into distilled water. With ferrozine, Fe2+ released was 34.2 (3.1) nmol/mg for SFA and 47.0 (4.8) nmol/mg for LFA. With desferrioxamine, Fe3+ released was 753 (57) nmol/mg for SFA and 261 (34) nmol/mg for LFA.
  • From their calculations and oxidation data, the authors concluded that one-iron sites tend to lie on the surface of the fibre, unlike the two-iron and three-iron sites.
• The authors' conclusion: The authors speculated that the one-iron coordination site, which is the most oxidised site, the most able to change its oxidation state, and more prevalent in LFA than SFA, may be important in the damaging processes caused by amosite. They called for further work.
• Limitations the authors state:
  • Because of variations in sample preparation, absolute iron content could not be measured; only ratios of peak areas were compared.
  • Single measurements of LFA and SFA could not be compared directly, because exact sample replicates could not be made.
  • The method measured total iron in the M1, M3, M1 sites rather than surface iron alone, so the greater oxidation of LFA at the surface is probably understated.
  • Results for ferric (Fe3+) iron were less statistically significant because Fe3+ concentrations were lower.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Amosite.

Medical terms in this study, explained in our Lexicon: Amosite, Infrared Spectroscopy (IR), Cation Site.

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