title: Hydrothermal conversion of chrysotile asbestos using near 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 degrees 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 microm and diameter less than 3 microm, 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 liquid 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 conversion 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.
authors.affiliation: Laboratory of Toxic and Hazardous Waste Management, Department of Environmental Engineering, Technical University of Crete, Chania, PC 73100, Greece.
authors.forename: Kalliopi
authors.initials: K
authors.lastname: Anastasiadou
authors.forename: Dimosthenis
authors.initials: D
authors.lastname: Axiotis
authors.forename: Evangelos
authors.initials: E
authors.lastname: Gidarakos
mesh_terms: D017632:Asbestos, Serpentine / Q000737:chemistry*; D002273:Carcinogens / Q000737:chemistry*; D002614:Chelating Agents / Q000737:chemistry; D025924:Chromatography, Supercritical Fluid; D004492:Edetic Acid; D006358:Hot Temperature; D008855:Microscopy, Electron, Scanning; D019016:Mineral Fibers; D013696:Temperature; D014961:X-Ray Diffraction
chemical_list: D017632:Asbestos, Serpentine; D002273:Carcinogens; D002614:Chelating Agents; D019016:Mineral Fibers; D004492:Edetic Acid