Cited by 7Occupational and environmental lung diseasesIron oxide chemistry and applicationsMineralogy and Gemology Studies

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Redox Cycling of Tetrahedral Iron Drives the Fenton Reactivity of Chrysotile Asbestos

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Abstract

High Resolution Image Download MS PowerPoint Slide Chrysotile asbestos is a carcinogenic fibrous mineral. Its pathogenicity is partly governed by the ability of Fe on the fiber surface to catalyze the Fenton reduction of H 2 O 2 (which is produced during inflammatory processes) to form the highly toxic hydroxyl radical (HO • ). Recently, tetrahedrally coordinated Fe (Fe tet ) in the fibers’ Si sheets was identified as the principal Fe species to catalyze this process. However, as only ferric Fe tet (Fe 3+ tet ) substitutes Si tetrahedra in chrysotile, Fe tet needs to redox cycle to ferrous Fe tet (Fe 2+ tet ) to facilitate fiber-mediated reductions of H 2 O 2 to HO • . This redox cycling has never been experimentally investigated. Here we demonstrate, by consecutive ascorbate and O 2 treatments, that structural Fe tet in exposed Si sheets of chrysotile fibers can redox cycle between Fe 3+ tet and Fe 2+ tet . Reduction, back-oxidation, and rereduction of Fe 3+ tet did not labilize the exposed Si sheet and, consequently, did not promote fiber dissolution. However, in the presence of H 2 O 2, prolonged redox cycling of Fe tet increased fiber dissolution, presumably by accelerating Fe tet dissolution and subsequent labilization of the exposed Si sheet. Chrysotile fibers for which the concentration of Fe tet surface sites undergoing redox cycling was lowered through selective Fe removal showed a rebound in Fe 3+ tet surface site concentration and associated Fenton reactivity once Fe 3+ tet -depleted Si sheets were dissolved off from the fiber surfaces. To conclude, our results demonstrate that redox cycling of Fe tet on chrysotile surfaces produces Fe 2+ tet surface sites, which, as the ultimate Fenton reactive iron species on chrysotile, contribute to the fibers’ adverse chemical reactivity.

Overview

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TL;DR

Chrysotile asbestos surface tetrahedral Fe (Fe_tet) can redox cycle between Fe³⁺ and Fe²⁺ in exposed Si sheets, and this cycling supplies the Fe²⁺ responsible for Fenton/Haber-Weiss generation of HO•. Prolonged H₂O₂-driven cycling also enhances fiber dissolution, while Fe-depleted surfaces can recover Fenton reactivity as deeper Si sheets dissolve.

Key findings

  • Anoxic ascorbate + Ferrozine showed Fe³⁺_tet reduction to Fe²⁺_tet and ligand-enhanced Fe²⁺ dissolution: 29.5, 23.5, and 3.5 μmol L⁻¹ Fe mobilized after 336 h from pristine, blank-altered, and DFOB-altered fibers, respectively.
  • One full redox cycle (ascorbate reduction then air oxidation) and 1.5 cycles did not labilize the exposed Si sheet or promote Si dissolution; reoxidized ascorbate-altered fibers with ascorbate + Ferrozine mobilized up to 21.8 μmol L⁻¹ Fe after 336 h.
  • In oxic H₂O₂ incubations, prolonged Fe_tet redox cycling increased dissolution: after 336 h, H₂O₂ + DFOB mobilized 983 μmol L⁻¹ Mg from pristine fibers vs ~600 μmol L⁻¹ in other treatments, and Si exceeded 200 μmol L⁻¹ for all three fiber types.
  • DFOB-altered fibers had HO• yield 9% relative to pristine; after 336 h at pH 7.4 it rose to 13%, and after pH 3.0 to ~20% (2-fold), indicating newly exposed Fenton-active Fe³⁺_tet.

Methods

  • Batch dissolution/preconditioning with Shijiazhuang chrysotile at 1 g L⁻¹, pH 7.4 (or pH 3.0 for recovery), 20 ± 2 °C, dark, anoxic or oxic; fiber types: pristine, blank-altered, DFOB-altered, ascorbate-altered, reoxidized ascorbate-altered.
  • Treatments included 1 mmol L⁻¹ DFOB, Ferrozine, ascorbate, combinations, and 3.3 g L⁻¹ (~100 mmol L⁻¹) H₂O₂ starting; solution Mg, Fe, Si by ICP-OES; HO• by EPR spin trapping with DMPO; one-way ANOVA α = 0.05.
  • Sampling was destructive at 0.5–336 h; dissolution rates calculated from 24–336 h; duplicates n = 2, EPR n = 4.

Limitations

  • Stated: the H₂O₂ starting concentration (3.3 g L⁻¹) is at least 2 orders of magnitude above (patho)physiological H₂O₂ in inflamed tissues, so H₂O₂-driven Fe_tet dissolution is likely much slower in vivo; the cause of increased fiber dissolution under high H₂O₂ could not be established, and the number of H₂O₂-induced redox cycles is unknown.
  • Stated: ascorbate was applied at supra-physiological concentrations.
  • Evident: findings come from one chrysotile specimen (Shijiazhuang) in abiotic batch experiments up to 336 h, with no cellular or in vivo validation; extrapolation to other asbestos/mineral dusts and physiological conditions is therefore limited.

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Integrity signals

Registry

Crossref · 10.1021/acsearthspacechem.3c00189

Cites

65 of 65 references verified0 citation errors · 0 not checkable

Statistics

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Figures

Image 1
CC-BY · From the PDF
Image 2
CC-BY · From the PDF
Figure 1 Mobilized Fe concentrations in anoxic incubations of pristine, blank-altered and DFOB-altered fibers at pH 7.4 in blank (a), 1 mmol L -1 Ferrozine (b), 1 mmol L -1 ascorbate (c) or 1 mmol L -1 ascorbate + Ferrozine (Asc + Ferr, d) suspensions. Error bars indicate standard deviations ( n = 2). The Fe 3+ tet content of the chrysotile fibers in this experiment underwent half a redox cycle (reduction only) in suspensions to which ascorbate was added (c, d). The data for this figure are presented in Table S2.
Figure 1 Mobilized Fe concentrations in anoxic incubations of pristine, blank-altered and DFOB-altered fibers at pH 7.4 in blank (a), 1 mmol L -1 Ferrozine (b), 1 mmol L -1 ascorbate (c) or 1 mmol L -1 ascorbate + Ferrozine (Asc + Ferr, d) suspensions. Error bars indicate standard deviations ( n = 2). The Fe 3+ tet content of the chrysotile fibers in this experiment underwent half a redox cycle (reduction only) in suspensions to which ascorbate was added (c, d). The data for this figure are presented in Table S2.CC-BY · From the PDF
Figure 2 Mobilized Mg and Si concentrations in anoxic incubations of pristine, blank-altered, and DFOB-altered fibers at pH 7.4 in 1 mmol L -1 ascorbate or 1 mmol L -1 ascorbate + Ferrozine (Asc + Ferr) solutions. Mobilized Mg (a) and Si (b) concentrations by the ascorbate treatment are presented in subpanels 1 and 2 and by the ascorbate + Ferrozine treatment in subpanels 2. Error bars indicate standard deviations ( n = 2). Results for the blank and 1 mmol L -1 Ferrozine treatments are presented in Figure S1. The data for this figure are presented in Table S3.
Figure 2 Mobilized Mg and Si concentrations in anoxic incubations of pristine, blank-altered, and DFOB-altered fibers at pH 7.4 in 1 mmol L -1 ascorbate or 1 mmol L -1 ascorbate + Ferrozine (Asc + Ferr) solutions. Mobilized Mg (a) and Si (b) concentrations by the ascorbate treatment are presented in subpanels 1 and 2 and by the ascorbate + Ferrozine treatment in subpanels 2. Error bars indicate standard deviations ( n = 2). Results for the blank and 1 mmol L -1 Ferrozine treatments are presented in Figure S1. The data for this figure are presented in Table S3.CC-BY · From the PDF
Figure 3 Observed fiber colors after different experimental incubations. From left to right: pristine fibers, DFOB-altered fibers, and ascorbate-altered fibers. The pale pinkish color of ascorbate-altered fibers was observed by incubating pristine, blank-altered, and DFOBaltered fibers at pH 7.4 in 1 mmol L -1 ascorbate solutions under anoxic conditions for 336 h. The color change of DFOB-altered fiber treatments from whitish to pale pinkish, however, only occurred in later stages of the incubation. Blank-altered fibers had the same color as pristine fibers (Figure S2a).
Figure 3 Observed fiber colors after different experimental incubations. From left to right: pristine fibers, DFOB-altered fibers, and ascorbate-altered fibers. The pale pinkish color of ascorbate-altered fibers was observed by incubating pristine, blank-altered, and DFOBaltered fibers at pH 7.4 in 1 mmol L -1 ascorbate solutions under anoxic conditions for 336 h. The color change of DFOB-altered fiber treatments from whitish to pale pinkish, however, only occurred in later stages of the incubation. Blank-altered fibers had the same color as pristine fibers (Figure S2a).CC-BY · From the PDF
Figure 4 Fe (a), Mg (b), and Si (c) concentrations mobilized from reoxidized ascorbate-altered fibers. Fibers were incubated under anoxic conditions at pH 7.4. Blank, 1 mmol L -1 Ferrozine (Ferr), 1 mmol L -1 ascorbate (Asc), and 1 mmol L -1 ascorbate + Ferrozine (Asc + Ferr) treatments were applied. Error bars indicate standard deviations ( n = 2). The number of redox cycles of Fe 3+ tet is noted for each treatment. The data for this figure are presented in Table S4.
Figure 4 Fe (a), Mg (b), and Si (c) concentrations mobilized from reoxidized ascorbate-altered fibers. Fibers were incubated under anoxic conditions at pH 7.4. Blank, 1 mmol L -1 Ferrozine (Ferr), 1 mmol L -1 ascorbate (Asc), and 1 mmol L -1 ascorbate + Ferrozine (Asc + Ferr) treatments were applied. Error bars indicate standard deviations ( n = 2). The number of redox cycles of Fe 3+ tet is noted for each treatment. The data for this figure are presented in Table S4.CC-BY · From the PDF
Figure 5 Fe (a), Mg (b), and Si (c) concentrations mobilized from pristine, blank-altered, and DFOB-altered fibers after 96 h (subpanels 1) and 336 h (subpanels 2) of incubation at pH 7.4 under oxic conditions in blank, 3.34 g L -1 H2O2, 3.34 g L -1 H2O2 + 1 mmol L -1 Ferrozine (Ferr), 1 mmol L -1 DFOB, and 3.34 g L -1 H2O2 + 1 mmol L -1 DFOB solutions. 3.34 g L -1 was the starting concentration of all H 2 O2 incubations. Dashed lines indicate the concentrations in the blank treatment. Error bars indicate standard deviations ( n = 2). Blank and DFOB treatment data for pristine fibers were taken from Walter et al., 18 and data for H 2 O2 treatments were taken from Walter et al. 38 The data for this figure are presented in Table S5. Mobilization of Fe, Mg, and Si by 1 mmol L -1 Ferrozine alone (without 3.34 g L -1 H2O2) under oxic conditions at pH 7.4 was exclusively examined for pristine fiber suspensions (1 g L -1 ); no notable differences compared to the mobilized concentrations in the 3.34 g L -1 H2O2 + 1 mmol L -1 Ferrozine treatment were found (Table S8).
Figure 5 Fe (a), Mg (b), and Si (c) concentrations mobilized from pristine, blank-altered, and DFOB-altered fibers after 96 h (subpanels 1) and 336 h (subpanels 2) of incubation at pH 7.4 under oxic conditions in blank, 3.34 g L -1 H2O2, 3.34 g L -1 H2O2 + 1 mmol L -1 Ferrozine (Ferr), 1 mmol L -1 DFOB, and 3.34 g L -1 H2O2 + 1 mmol L -1 DFOB solutions. 3.34 g L -1 was the starting concentration of all H 2 O2 incubations. Dashed lines indicate the concentrations in the blank treatment. Error bars indicate standard deviations ( n = 2). Blank and DFOB treatment data for pristine fibers were taken from Walter et al., 18 and data for H 2 O2 treatments were taken from Walter et al. 38 The data for this figure are presented in Table S5. Mobilization of Fe, Mg, and Si by 1 mmol L -1 Ferrozine alone (without 3.34 g L -1 H2O2) under oxic conditions at pH 7.4 was exclusively examined for pristine fiber suspensions (1 g L -1 ); no notable differences compared to the mobilized concentrations in the 3.34 g L -1 H2O2 + 1 mmol L -1 Ferrozine treatment were found (Table S8).CC-BY · From the PDF
Figure 6 HO · yield in % relative to pristine fibers ( ≙ 100%). DFOBaltered fibers ('2w pH 7.4 DFOB') were incubated for 336 h at pH 7.4 ('2w pH 7.4 DFOB → 2w pH 7.4') and at pH 3.0 ('2w pH 7.4 DFOB → 2w pH 3.0') in blank treatments under oxic conditions. * indicates a statistically significant increase determined in an ANOVA with p = 0.05 relative to DFOB-altered fibers. Error bars indicate standard deviations ( n = 4). The Mg, Si, and Fe concentrations mobilized during these incubations are presented in Table S1. The data for this figure are presented in Table S6.
Figure 6 HO · yield in % relative to pristine fibers ( ≙ 100%). DFOBaltered fibers ('2w pH 7.4 DFOB') were incubated for 336 h at pH 7.4 ('2w pH 7.4 DFOB → 2w pH 7.4') and at pH 3.0 ('2w pH 7.4 DFOB → 2w pH 3.0') in blank treatments under oxic conditions. * indicates a statistically significant increase determined in an ANOVA with p = 0.05 relative to DFOB-altered fibers. Error bars indicate standard deviations ( n = 4). The Mg, Si, and Fe concentrations mobilized during these incubations are presented in Table S1. The data for this figure are presented in Table S6.CC-BY · From the PDF
Image 9
CC-BY · From the PDF

Verification

Claims the paper makes

55 Own result · 35 Own interpretation · 18 Not checked yet · 11 Check pending · 11 No full text of the sources · 6 No basis given · 2 Sources not found · 1 Supported

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Show all 96 claims

  • It may state two separate claims as one. 34
  • Probably not a substantial claim. 11
43 of 43
  • The authors state this as an assumption.

    Under the experimental H2O2 conditions, Fe_tet undergoes a large number of full Haber-Weiss cycles.

    “Under these conditions, Fe tet is presumed to undergo a large number of full Haber-Weiss cycles.”
  • The paper relies on this without support.

    pH 7.4 is the physiological lung pH.

    “These hypotheses were tested in batch dissolution experiments at the physiological lung pH of 7.4 (unless mentioned otherwise);”
  • The paper relies on this without support.

    Blank-altered fibers host non-Fenton-active secondary Fe phases, such as iron (hydr)oxides on their fiber surface.

    “Blank-altered fibers also host non-Fenton-active secondary Fe phases, such as iron (hydr)oxides on their fiber surface.”
  • The paper relies on this without support.

    Redox cycling of Fe_tet on chrysotile surfaces has never been experimentally investigated.

    “This redox cycling has never been experimentally investigated.”
  • The paper relies on this without support.

    It remains unexplored whether redox cycling of Fe_tet on chrysotile surfaces accelerates Fe dissolution kinetics and subsequently promotes the labilization and dissolution of the exposed Si sheets.

    “Additionally, it remains unexplored whether redox cycling of Fe tet on chrysotile surfaces accelerates Fe dissolution kinetics and subsequently promotes the labilization and dissolution of the exposed Si sheets, as observed when Fe 3+ tet undergoes proton-promoted or ligand-enhanced dissolution.18”
  • The paper relies on this without support.

    It is not known whether surface depletion permanently rids the surface of Fe_tet or whether Fenton reactivity is recovered to some extent as fiber dissolution proceeds.

    “However, it is not known whether surface depletion permanently rids the surface of Fe tet or whether Fenton reactivity is recovered to some extent as fiber dissolution proceeds.”
  • The paper relies on this without support.

    The preconditioned fiber types (blank-altered, DFOB-altered, ascorbate-altered) are representative of chrysotile fibers in environmental or physiological settings.

    “For chrysotile fibers depleted in Fe tet surface sites (e.g., due to selective Fe removal), our results demonstrate that new reactive surface Fe 3+ tet sites can become freshly exposed as fiber dissolution proceeds.”
  • The paper relies on this without support.

    The HO• yield measured by EPR spin trapping is a valid measure of Fenton reactivity.

    “we measured the HO • yield of blankaltered fibers that were incubated for 336 h at pH 7.4 in blank or ascorbate solutions under anoxic conditions (Figure S4).”
8 of 8

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1 of 1

Citations

Cites

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Research works printed but not linked: 3

  1. WHO-IARC, 2012, IARC monographs on the evaluation of carcinogenic risks to humans 100 C, 219
  2. WHO-IPCS., 2014, International programme on chemical safety: Chrysotile asbestos
  3. Stroink G., 1980, Canadian Mineralogist, 18, 285

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Metadata

Authors
Walter, M., Schenkeveld, W. D. C., Geroldinger, G., Gille, L., & Kraemer, S. M.
Affiliations
  • University of Vienna (AT)
  • University of Veterinary Medicine Vienna (AT)
Corresponding author
Schenkeveld, Walter D. C.
Venue
ACS Earth and Space Chemistry
Published
2023-12-21
Volume
8
Issue
1
Pages
1-13
ISSN
2472-3452 (ISSN-L)
OpenAlex
W4390060026
Cited by
7
References
65
Citations by year
2026: 6 · 2024: 1
FWCI
1.19
Type
journal article
Language
en
Open access
Locations
  • ACS Earth and Space Chemistry (publishedVersion, cc-by)
  • Data Archiving and Networked Services (DANS) (submittedVersion)
  • Mendeley Data (cc-by)
Funders
Universität Wien
Topics
  • Health Sciences › Medicine › Pulmonary and Respiratory Medicine › Occupational and environmental lung diseases
  • Physical Sciences › Materials Science › Materials Chemistry › Iron oxide chemistry and applications
  • Physical Sciences › Earth and Planetary Sciences › Geochemistry and Petrology › Mineralogy and Gemology Studies
Keywords
chrysotile, tetrahedral iron, redox cycling, Fenton reaction, hydroxyl radicals, hydrogen peroxide reduction, fiber dissolution, vitamin C, oxygen exposure, iron removal, oxidative stress

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