Journal of Hazardous Materials 179 (2010) 926–932 Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat Hydrothermal conversion of chrysotile asbestos using near supercritical conditions Kalliopi Anastasiadou, Dimosthenis Axiotis, Evangelos Gidarakos ∗ Laboratory of Toxic and Hazardous Waste Management, Department of Environmental Engineering, Technical University of Crete, Chania, P.C. 73100, Greece article info Article history: Received 11 August 2009 Received in revised form 22 March 2010 Accepted 22 March 2010 Available online 27 March 2010 Keywords: Chrysotile asbestos Mineralogical conversion Hydrothermal treatment Supercritical conditions abstract The present research investigates, develops and evaluates the transformation of chrysotile asbestos into a non-hazardous material, such as forsterite, using an economically viable and safe method. The aim of this study is to convert fibrous chrysotile asbestos into an anhydrous magnesium silicate with a non- hazardous lamellar morphology using supercritical steam. The treatment method is characterized as hydrothermal in a temperature and pressure range of 300–700 ◦ C and 1.75–5.80 MPa, respectively. Small amounts of asbestos (2.5 g) were treated in each experiment. Deionised water was used as the treatment solution. The treatment duration varied from approximately 1–5 h. Additional experiments took place using solutions of distilled water and small amounts of acetic acid, with the aim of attaining optimal treatment conditions. Crystal phases of the samples were determined by X-ray diffraction (XRD). The main phases present in the treated samples were forsterite, enstatite, and chrysotile asbestos. Lizardite and periclase were also found. The morphology of the treated chrysotile asbestos fibers was identified by scanning electron microscope (SEM). The fibrous form of chrysotile asbestos was converted into non- fibrous form of forsterite. In fact, none of the fibrous-needle-like morphology, with length equal to or greater than 5 m and diameter less than 3 m, which was responsible for the toxicity of the original material, was visible in the solid phase. The dissolution of magnesium from chrysotile asbestos was measured using volumetric determination by titration with EDTA. Leaching of magnesium into the liq- uid phase was observed. Clearly, the highest concentrations of dissolved magnesium are observed after hydrothermal treatment of chrysotile asbestos using acetic acid 1% (8.4–14.6%). Lowest concentrations of dissolved magnesium are obtained after hydrothermal treatment of chrysotile asbestos without using additives. Observing the results of the hydrothermal treatment using additives, the mineralogical con- version does not depend on the presence of a small quantity of weak organic acid (<1%). The addition of acetic acid 1% during hydrothermal treatment did not involve changes in the conditions of the chrysotile asbestos’ mineral conversion. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Asbestos has been characterized universally, and by Greek legis- lation in particular, as a harmful and hazardous material for human health (i.e. a toxic and carcinogenic substance). A key hazard is the release of fibers from products that contain asbestos [1]. The fibers penetrate respiratory systems and cause asbestosis and carcino- genesis. However, due to its resilient characteristics, asbestos has been widely utilized in industry and currently has over 3000 uses. Thus, today it can be found in a vast array of products (construc- tion materials, fire retardants, binders, etc.) in varying proportions, ranging from 5% to 100%. Chrysotile asbestos, or white asbestos, is the form of asbestos most commonly used (95%) [2]. ∗ Corresponding author. Tel.: +30 2821037789; fax: +30 2821037850. E-mail address: gidarako@mred.tuc.gr (E. Gidarakos). Chrysotile asbestos is a 1:1 trioctahedral phyllosilicate with the chemical formula Mg 6 Si 4 O 10 (OH) 8 with some Mg substituted by Fe 2+ . This 1:1 phyllosilicate comprises one Mg-octahedral sheet bonded to a Si-tetrahedral sheet. An ideal Mg-octahedral sheet has a lateral dimension of b ≈ 9.43 Å and an ideal Si-tetrahedral sheet has a lateral dimension of b ≈ 9.1 Å [3]. These dimensional differ- ences seem to cause a lateral misfit between the octahedral and tetrahedral sheets along the X and Y axes [3,4]. To compensate par- tially for this misfit, the larger octahedral Mg sheet curls over the smaller tetrahedral silica sheet, thus generating chrysotile’s tubular morphology. The outer surface of the octahedral magnesian sheet exposes hydroxyl ions (OH - ). Chrysotile’s tubular structure pro- duces four reactive sites: (1) an outer hydroxyl sheet; (2) the ends of the fiber; (3) the exposed edges of the curled sheet; and (4) the interior of the hollow central channel [5]. Fig. 1 presents chrysotile’s tubular structure, curled sheets developed from lateral misfits and cross-section showing Mg- octahedral sheet overlying Si-tetrahedral sheet. 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.03.094