In vitro biodurability of the product of thermal transformation of cement-asbestos

Our plain-English summary of In vitro biodurability of the product of thermal transformation of cement-asbestos, by A.F. Gualtieri, A. Viani, G. Sgarbi, et al (Journal of Hazardous Materials, 2012).

The study

• Authors: A.F. Gualtieri, A. Viani, G. Sgarbi, G. Lusvardi
• Journal: Journal of Hazardous Materials
• Published: 2012;205-206:63-71
• Cited by: 7 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

To safely recycle the product of the thermal transformation of cement-asbestos as secondary raw material, its toxicity potential should be assessed by in vitro biodurability tests. In this work, the acellular in vitro biodurability of the products of transformation of cement-asbestos at 1200 °C (named KRY·AS) was tested using both inorganic and organic simulated lung fluids at pH 4.5. The dissolution kinetics were followed using chemical, mineralogical and microstructural analyses. The total dissolution time estimated from the experiments with inorganic HCl diluted solution is one order of magnitude higher than that determined from the experiments with buffered Gamble solution (253 days vs. 20 days). The key parameter determining the difference in dissolution rate turns out to be the solidus/liquidus ratio which prompts a fast saturation of the solution with monosilicic acid. The calculated dissolution rate constants showed that the biodurability in vitro of KRY·AS is much lower with respect to that of standard chrysotile asbestos (total estimated dissolution time of 20 days vs. 298 days, respectively). This proves a low potential toxicity of this secondary raw material.

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In plain English

This laboratory study measured how quickly KRY·AS dissolves in fluids imitating the acid conditions inside lung cells (pH 4.5), a property called biodurability. KRY·AS is made by heating cement-asbestos slates to 1200 degrees C. No cells, animals or people were used. In the first of two set-ups, KRY·AS Sample 1 sat in hydrochloric acid solution for up to 561 days. Sample 3 had shorter tests there and behaved very similarly. In the second set-up, two KRY·AS samples and a standard chrysotile asbestos sample were placed in a buffered modified Gamble solution. Estimated total dissolution time was 253 days in the hydrochloric acid set-up. In the Gamble solution it was 20 days for both KRY·AS samples and 298 days for the chrysotile. The authors attributed the slower hydrochloric acid result mainly to a higher solid-to-liquid ratio, which saturated the solution with silica. Microscopy after 6 hours still showed rare relics of transformed asbestos fibres. The authors concluded that this further verified the very low potential toxicity of KRY·AS. They noted one limit: the chemistry and mineralogy of KRY·AS dissolution would have been better compared with the original cement-asbestos than with chrysotile.

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: acellular (no cells) in vitro dissolution tests in simulated lung fluids at pH 4.5, run as two experimental lines (hydrochloric acid solution and buffered modified Gamble solution)
• Who was studied: Three samples of KRY·AS, the product of industrial thermal treatment of sealed packs of cement-asbestos slates removed from industrial sites. Samples 1 and 2 were C2S-rich and Sample 3 was akermanite-rich. A fourth sample, standard chrysotile asbestos, was the comparison. In the hydrochloric acid line, Sample 1 was tested long-term and Sample 3 in parallel medium-short term tests.
• Exposure: Cement-asbestos slates fired for 40 hours, including 21 hours at 1200 degrees C (KRY·AS). The comparison was standard chrysotile asbestos SRM1866A from NIST. The authors chose it to assess whether KRY·AS has a lower biodurability than the dominant toxic asbestos fibres of cement-asbestos. Crocidolite, amosite and tremolite were not tested.
• Main results:
  • In the hydrochloric acid line, parallel medium-short term tests on KRY·AS Sample 3 showed dissolution behaviour very similar to Sample 1.
  • In the hydrochloric acid line (KRY·AS Sample 1), pH and weight loss levelled off after about 300 days, which the authors took as completion of the reaction.
  • In the modified Gamble solution line, KRY·AS Samples 2 and 3 reached their maximum weight loss (about 88 wt%) after about 13 days.
  • Estimated total dissolution time of KRY·AS was 253 days in experimental line 1 (hydrochloric acid) and 20 days in experimental line 2 (Gamble solution).
  • In the Gamble solution line, the estimated total dissolution time was 20 days for KRY·AS and 298 days for standard chrysotile asbestos.
  • In the hydrochloric acid line, dissolution rates at 25 degrees C and 37 degrees C were nearly identical.
  • The authors attributed the difference between the two lines mainly to early silica saturation caused by the higher solid-to-liquid ratio in line 1 (0.67 mg/ml versus 0.2 mg/ml).
  • Scanning electron microscopy of the hydrochloric acid line still showed rare relics of transformed asbestos fibres after 6 hours.
• The authors' conclusion: The authors concluded that the in vitro biodurability of KRY·AS is much lower than that of chrysotile asbestos, and that this further verifies the very low potential toxicity of this secondary raw material.
• Limitations the authors state:
  • The authors note one limit of using chrysotile as the control: from the chemical and mineralogical point of view, the dissolution process of KRY·AS would have been better compared with that of the original cement-asbestos matrix.
  • Crocidolite, amosite and tremolite were not tested as controls, because their dissolution was expected to be too slow to record within the experiment's time span.
  • The long-term hydrochloric acid experiment was carried to the end only for Sample 1. Sample 3 had only medium-short term tests in that line.
  • In the Gamble solution line, aluminium and iron concentrations were mostly below the instrument's detection limit, so no clear trends could be seen.

Topics

Medical terms in this study, explained in our Lexicon: Chrysotile.

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