The Effect of pH and Biogenic Ligands on the Weathering of Chrysotile Asbestos: The Pivotal Role of Tetrahedral Fe in Dissolution Kinetics and Radical Formation

Our plain-English summary of The Effect of pH and Biogenic Ligands on the Weathering of Chrysotile Asbestos: The Pivotal Role of Tetrahedral Fe in Dissolution Kinetics and Radical Formation, by M. Walter, W.D.C. Schenkeveld, M. Reissner, et al (Chemistry, 2019).

The study

• Authors: M. Walter, W.D.C. Schenkeveld, M. Reissner, L. Gille, S.M. Kraemer
• Journal: Chemistry
• Published: 2019;25(13):3286-3300
• Cited by: 7 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Chrysotile asbestos is a soil pollutant in many countries. It is a carcinogenic mineral, partly due to its surface chemistry. In chrysotile, FeII and FeIII substitute Mg octahedra (Fe[6]), and FeIII substitutes Si tetrahedra (Fe[4]). Fe on fiber surfaces can generate hydroxyl radicals (HO. ) in Fenton reactions, which damage biomolecules. To better understand chrysotile weathering in soils, net Mg and Si dissolution rates over the pH range 3.0-11.5 were determined in the presence and absence of biogenic ligands. Also, HO. generation and Fe bulk speciation of pristine and weathered fibers were examined by EPR and Mössbauer spectroscopy. Dissolution rates were increased by ligands and inversely related to pH with complete inhibition at cement pH (11.5). Surface-exposed Mg layers readily dissolved at low pH, but only after days at neutral pH. On longer timescales, the slow dissolution of Si layers became rate-determining. In the absence of ligands, Fe[6] precipitated as Fenton-inactive Fe phases, whereas Fe[4] (7 % of bulk Fe) remained redox-active throughout two-week experiments and at pH 7.5 generated 50±10 % of the HO. yield of Fe[6] at pristine fiber surfaces. Ligand-promoted dissolution of Fe[4] (and potentially Al[4]) labilized exposed Si layers. This increased Si and Mg dissolution rates and lowered HO. generation to near-background level. It is concluded that Fe[4] surface species control long-term HO. generation and dissolution rates of chrysotile at natural soil pH.

© 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.

In plain English

This was a laboratory experiment on chrysotile asbestos fibers bought from a Chinese mining company. It was not a study of people or animals. The authors kept the fibers for up to 336 hours (two weeks) at six pH levels. pH 3.0 to 8.5 stood for soil and pH 11.5 for cement. Some solutions also contained metal-binding substances (citrate, oxalate, DFOB or HEDTA). A fifth, HBED, was tested only at pH 11.5. Dissolution of the fibers generally rose as pH fell and was generally faster with these substances, but there were exceptions: oxalate inhibited magnesium dissolution at pH 6.0 and above. At pH 11.5, magnesium dissolved only with HEDTA, citrate or HBED, and silicon dissolved only with HBED. About 7% of the fibers' iron sat in the silicon layers. Without binding substances at pH 7.5, the fibers' ability to generate hydroxyl radicals (reactive molecules) settled at about 50% (plus or minus 10%) of untreated fibers. With DFOB it fell to 9% after 336 hours. The authors concluded that, in the long term, this silicon-layer iron was the only radical-generating surface iron in chrysotile at natural soil 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 dissolution tests on chrysotile fibers, with chemical analysis of the solutions, measurement of hydroxyl radical generation (EPR spectroscopy) and analysis of iron forms in the fibers (Mössbauer spectroscopy)
• Who was studied: Chrysotile asbestos fibers from Shijiazhuang Mining IMP&EXP Trade Co. LTD, China. By X-ray analysis the material was 86.4 plus or minus 4.6% chrysotile, with brucite, talc, chlorite, magnetite, quartz and calcite as impurities. No humans or animals were studied.
• Exposure: Fibers were incubated at pH 3.0, 4.5, 6.0, 7.5, 8.5 and 11.5. pH 3.0 to 8.5 represented soil and pH 11.5 represented cement. Solutions contained either no ligand (blank) or one of citrate, oxalate, DFOB, HEDTA or HBED. HBED was used only at pH 11.5. Samples were taken at times from 0.5 to 336 hours.
• Main results:
  • In general, the ligands increased dissolution rates of the chrysotile fibers, and rates were inversely related to pH. There were treatment-specific exceptions.
  • With oxalate, magnesium dissolution rates increased at low pH but were inhibited at pH 6.0 and above.
  • At pH 11.5, no magnesium was released from the outermost layer except in the HEDTA, citrate and HBED treatments.
  • At pH 11.5, silicon was released only in the HBED treatment.
  • Chrysotile dissolved congruently at pH 4.5 and 6.0, but incongruently at pH 3.0 and at pH 7.5 and above.
  • Iron in the tetrahedral (silicon-layer) position, FeIII[4], made up 7.0% of the bulk iron in the chrysotile.
  • In blank treatments at pH 7.5, the fibers' hydroxyl radical yield fell to 77% of untreated fibers within 0.5 hours. The authors attributed the remaining yield of about 50 plus or minus 10% to FeIII[4], which stayed active for the whole incubation.
  • With DFOB at pH 7.5, the fibers' hydroxyl radical yield decreased gradually to 9% of untreated fibers after 336 hours.
  • At pH 11.5, fibers in blank, oxalate, citrate and DFOB treatments had hydroxyl radical yields within 15% of untreated fibers.
  • The authors assumed that the mix of iron forms at the surface is the same as in the bulk fiber. On that assumption, they estimated that a single Fe[4] center is about an order of magnitude more potent than a single Fe[6] center in generating hydroxyl radicals.
• The authors' conclusion: The authors concluded that, in the long term, FeIII[4] was the only radical-generating (Fenton-active) surface iron in chrysotile in the natural soil pH range (3 to 9). They also concluded that silicon dissolution controls the rate of chrysotile weathering at pH 3.0 to 8.5.
• Limitations the authors state:
  • Magnetite contamination affects the measurements of total bulk iron.
  • Iron in the chlorite impurity may interfere with the dissolution experiments and the Mössbauer analysis.
  • The pH dependence of silicon dissolution rates cannot be attributed unambiguously to iron being released from the silicon sheet.
  • The authors could not interpret why the different ligands affected the formation of amorphous silica-rich material differently.

Topics

Medical terms in this study, explained in our Lexicon: Chrysotile, Carcinogenic, Hydroxyl Radical, Fenton Reaction.

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