Dark fermentation process as pretreatment for a sustainable denaturation of asbestos containing wastes

Our plain-English summary of Dark fermentation process as pretreatment for a sustainable denaturation of asbestos containing wastes, by D. Spasiano (Journal of Hazardous Materials, 2018).

The study

• Author: D. Spasiano
• Journal: Journal of Hazardous Materials
• Published: 2018;349:45-50
• Cited by: 12 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

A cement asbestos compound (CAC) sample was detoxified by a treatment train based on a dark fermentation (DF) process followed by a hydrothermal phase, which led to the complete degradation of the chrysotile fibers. During the biological pretreatment, the glucose was converted in biogas rich in H2 and volatile fatty acids (VFA). The latter caused the dissolution of all the Ca-based compounds and the solubilisation of 50% brucite-like layers of chrysotile fibers contained in the CAC suspended in the bioreactor (5 g/L). XRD analysis of the solids contained in the effluents of the DF process highlighted the disappearance of the chrysotile fiber peaks. However, a complete destruction of all the asbestos fibers is hard to prove and a hydrothermal treatment was carried out to dissolve the "brucite" layers still present in solution. Due to the presence of the VFA produced during the DF, a complete destruction of chrysotile fibers was achieved by a 24 h hydrothermal process performed with a [H2SO4]/[CAC] ratio 50% lower than that adopted in a previous finding. Consequently, the DF pre-treatment can contribute to lower the H2SO4 and the energy consumption of a CAC hydrothermal treatment, due to the production of VFA and H2.

Copyright © 2018 Elsevier B.V. All rights reserved.

In plain English

This laboratory experiment in Italy tested a two-step method for breaking down chrysotile asbestos fibers in a sample of Eternit cement-asbestos slate supplied by a hazardous-waste company. In the first step, bacteria from dairy-farm digester sludge fermented glucose without oxygen (dark fermentation), producing hydrogen gas and organic acids, and 5 g/L of the milled slate was added. The acids dissolved all of the cement matrix and half of the magnesium-containing layer of the chrysotile fibers. X-ray diffraction no longer showed chrysotile peaks. The author noted that this could not prove complete destruction, because a few fibers might remain below the test's detection limit. In the second step, the mixture was heated to 100 °C. Without added sulfuric acid, 24.1% of the magnesium stayed solid after 24 hours. With 1.25 g/L or 2.5 g/L of sulfuric acid, the magnesium fully dissolved within 24 hours and 8 hours respectively. The author treated full magnesium dissolution as full destruction of the fibers. The author concluded that this needed half the acid-to-waste ratio reported in an earlier study.

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 on treating asbestos-containing waste: dark fermentation pretreatment followed by hydrothermal treatment
• Who was studied: a sample of Eternit slate (cement asbestos compound) provided by a hazardous waste handling company; milled, sieved below 2.0 mm, and found to contain 30% w/w calcium and 3.1% w/w magnesium by EDX
• Exposure: chrysotile fibers in cement asbestos compound (5.0 g/L) exposed to dark fermentation of glucose at 35 °C, then hydrothermal treatment at 100 °C with 0, 1.25 or 2.5 g/L sulfuric acid
• Main results:
  • During the start-up phase without asbestos cement (65 h), fermentation lowered the pH to 5.45 and produced 233.8 mmol of hydrogen
  • After the cement asbestos sample was added, hydrogen production stopped after 62 h (127 h in total), reaching 465.4 mmol
  • By the end of dark fermentation, all the cement matrix of the sample had dissolved
  • By the end of dark fermentation, 50% of the brucite-like layer of the chrysotile fibers had dissolved
  • X-ray diffraction of the suspended solids at the end of dark fermentation (127 h) showed complete disappearance of chrysotile and calcite peaks
  • Hydrothermal treatment without sulfuric acid left 24.1% of magnesium in solid form (75.9% dissolution within 24 h)
  • With 1.25 g/L and 2.5 g/L sulfuric acid, complete dissolution of the brucite layer was reached within 24 h and 8 h respectively
  • The author reported that with 1.25 g/L sulfuric acid, an acid-to-waste ratio of 0.25 was enough to destroy all the chrysotile fibers in the sample
  • COD of the filtered solution fell only from 55.6 g/L after fermentation to 51.2 g/L after hydrothermal treatment with 1.25 g/L sulfuric acid
• The authors' conclusion: The author concluded that complete magnesium dissolution can be reached with a sulfuric acid to cement asbestos ratio of 0.25, half of what was previously reported
• Limitations the authors state:
  • Complete denaturation after dark fermentation could not be shown, because XRD may have reached its detection limit while a few asbestos fibers remained
  • Without added sulfuric acid, complete denaturation after hydrothermal treatment was not proven

Topics

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

Related studies

• Sustainable bio-hydrothermal sequencing treatment for asbestos-cement wastes — D. Spasiano, V. Luongo, et al. (2019)
• From hazardous asbestos containing wastes (ACW) to new secondary raw material through a new sustainable inertization process: A multimethodological mineralogical study — N.M. Marian, G. Giorgetti, et al. (2021)
• Thermochemical destruction of asbestos-containing roofing slate and the feasibility of using recycled waste sulfuric acid — S.N. Nam, S. Jeong, et al. (2014)
• Treatment and recycling of asbestos-cement containing waste — F. Colangelo, R. Cioffi, et al. (2011)
• Hydrothermal process development for the treatment of crocidolite asbestos waste — C.T. Nzogo Metoule, S. Delaby, et al. (2019)
• Treatments of asbestos containing wastes — D. Spasiano, F. Pirozzi (2017)

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