Identifying the reactive sites of hydrogen peroxide decomposition and hydroxyl radical formation on chrysotile asbestos surfaces

Our plain-English summary of Identifying the reactive sites of hydrogen peroxide decomposition and hydroxyl radical formation on chrysotile asbestos surfaces, by M. Walter, W.D.C. Schenkeveld, G. Geroldinger, et al (Particle and Fibre Toxicology, 2020).

The study

• Authors: M. Walter, W.D.C. Schenkeveld, G. Geroldinger, L. Gille, M. Reissner, S.M. Kraemer
• Journal: Particle and Fibre Toxicology
• Published: 2020;17(1):3
• Cited by: 8 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: Fibrous chrysotile has been the most commonly applied asbestos mineral in a range of technical applications. However, it is toxic and carcinogenic upon inhalation. The chemical reactivity of chrysotile fiber surfaces contributes to its adverse health effects by catalyzing the formation of highly reactive hydroxyl radicals (HO•) from H2O2. In this Haber-Weiss cycle, Fe on the fiber surface acts as a catalyst: Fe3+ decomposes H2O2 to reductants that reduce surface Fe3+ to Fe2+, which is back-oxidized by H2O2 (Fenton-oxidation) to yield HO•. Chrysotile contains three structural Fe species: ferrous and ferric octahedral Fe and ferric tetrahedral Fe (Fe3+tet). Also, external Fe may adsorb or precipitate onto fiber surfaces. The goal of this study was to identify the Fe species on chrysotile surfaces that catalyze H2O2 decomposition and HO• generation.

Results: We demonstrate that at the physiological pH 7.4 Fe3+tet on chrysotile surfaces substantially contributes to H2O2 decomposition and is the key structural Fe species catalyzing HO• generation. After depleting Fe from fiber surfaces, a remnant fiber-related H2O2 decomposition mode was identified, which may involve magnetite impurities, remnant Fe or substituted redox-active transition metals other than Fe. Fe (hydr)oxide precipitates on chrysotile surfaces also contributed to H2O2 decomposition, but were per mole Fe substantially less efficient than surface Fe3+tet. Fe added to chrysotile fibers increased HO• generation only when it became incorporated and tetrahedrally coordinated into vacancy sites in the Si layer.

Conclusions: Our results suggest that at the physiological pH 7.4, oxidative stress caused by chrysotile fibers largely results from radicals produced in the Haber-Weiss cycle that is catalyzed by Fe3+tet. The catalytic role of Fe3+tet in radical generation may also apply to other pathogenic silicates in which Fe3+tet is substituted, e.g. quartz, amphiboles and zeolites. However, even if these pathogenic minerals do not contain Fe, our results suggest that the mere presence of vacancy sites may pose a risk, as incorporation of external Fe into a tetrahedral coordination environment can lead to HO• generation.

In plain English

This was a laboratory study of chrysotile asbestos fibers in solution. No people or animals were studied. The authors asked which forms of iron on the fibers break down hydrogen peroxide and produce hydroxyl radicals, highly reactive molecules. The fibers were bought from a Chinese supplier, Shijiazhuang Mining IMP&EXP Trade Co. Fibers were pretreated at pH 7.4 either in buffer alone or with the iron-binding compound desferrioxamine-B (DFOB). Then 0 to 300 μmol of iron per gram was added. DFOB pretreatment halved the ferric iron in the fibers' silicon (tetrahedral) layer, from 15.0 to 7.7 μmol per gram. The fibers sped up hydrogen peroxide breakdown by a factor of 2 to 5 compared with buffer alone. In buffer-pretreated fibers, tetrahedral iron and iron oxide deposits contributed comparably. A third, remnant pathway of uncertain source also contributed. Adding iron to DFOB-pretreated fibers raised radical output from 7% to 36% of untreated fibers, still significantly below buffer-pretreated fibers. Adding iron to buffer-pretreated fibers did not consistently raise it. The authors concluded that tetrahedral ferric iron governs radical generation by chrysotile at near-neutral pH.

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: batch incubations of pretreated chrysotile fibers in solution, analyzed by chemical measurements, Mössbauer spectroscopy and EPR spin trapping
• Who was studied: Chrysotile asbestos purchased from Shijiazhuang Mining IMP&EXP Trade Co, China. The material was 86.4 ± 4.6% chrysotile fibers, with impurities including brucite, talc, chlorite, magnetite, quartz and calcite. No human or animal subjects.
• Exposure: Pristine fibers, fibers pretreated in pH 7.4 buffer (blank-altered), and fibers pretreated with 1 mmol/L DFOB (DFOB-altered), each with 0, 3, 30 or 300 μmol/g added iron. Fibers were suspended at 1 g/L and exposed to hydrogen peroxide.
• Main results:
  • Chrysotile fibers sped up hydrogen peroxide breakdown by a factor of 2 to 5 compared with the buffer-only control, depending on pretreatment.
  • Hydrogen peroxide breakdown was fastest with pristine fibers and slowest with DFOB-altered fibers.
  • DFOB pretreatment halved the fibers' tetrahedral ferric iron content, from 15.0 to 7.7 μmol/g.
  • In blank-altered fibers, tetrahedral iron and iron oxide precipitates contributed comparably to hydrogen peroxide breakdown (within a factor of 1.5).
  • A remnant pathway of hydrogen peroxide breakdown was identified. The authors said it may be related to magnetite impurities, other redox-active trace metals, or leftover iron.
  • In DFOB-altered fibers, hydroxyl radical yield (relative to pristine fibers) rose from 7% with no added iron to 36% with 300 μmol/g added iron. The rise was not proportional to the amount of iron added.
  • Even with 300 μmol/g added iron, the radical yield of DFOB-altered fibers (37 ± 14%) remained significantly lower than that of blank-altered fibers with no added iron (58 ± 6%).
  • Adding iron to blank-altered fibers did not consistently increase hydroxyl radical yield.
  • 3 mg of the iron mineral 2-line ferrihydrite gave a radical yield of 11%, relative to 11 mg of pristine chrysotile fibers.
  • In a strongly alkaline solution (0.1 mol/L NaOH, pH 12 to 13), hydrogen peroxide breakdown by pristine and preconditioned fibers was about an order of magnitude faster than at pH 7.4.
• The authors' conclusion: The authors concluded that ferric iron in the tetrahedral (silicon) layer governs hydroxyl radical generation by chrysotile at near-neutral pH. They suggested it may also contribute to the hazard of other pathogenic silicates.
• Limitations the authors state:
  • The Mössbauer measurements may have underestimated how much tetrahedral iron was restored, because isotope exchange may have occurred between iron addition and measurement.
  • The split of hydrogen peroxide breakdown into separate contributions rested on the assumption that the pathways are independent and add up.
  • The reactivity of tetrahedral iron sites may vary, so restoring these sites may not translate linearly into restored reactivity.

Topics

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

Medical terms in this study, explained in our Lexicon: Hydroxyl Radical, Chrysotile, Carcinogenic, Oxidative Stress, Silicate, Amphibole Group, Zeolite.

Related studies

• Asbestos catalyzes hydroxyl and superoxide radical generation from hydrogen peroxide — S.A. Weitzman, P. Graceffa (1984)
• Asbestos-induced decomposition of hydrogen peroxide — M.K. Eberhardt, A.A. Román-Franco, et al. (1985)
• Surface chemistry and surface reactivity of fibrous amphiboles that are not regulated as asbestos — M. Fantauzzi, A. Pacella, et al. (2012)
• The release of iron from different asbestos structures by hydrogen peroxide with concomitant O2 generation — A.J. Ghio, D.E. Taylor, et al. (1998)
• Using yeast RNA as a probe for generation of hydroxyl radicals by earth materials — C.A. Cohn, R. Laffers, et al. (2006)
• Fiber-induced hydroxyl radical formation: correlation with mesothelioma induction in rats and humans — K.R. Maples, N.F. Johnson (1992)

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