CC BY
15Journal of Siberian Federal University. Engineering & Technologies 7 (2013 6) 779-785
УДК 622.341
Can the Toxicity of Asbestos be Reduced?
Fathi Habashi*
Department of Mining, Metallurgical and Materials Engineering Laval University, Quebec City, Canada G1V 0A6
Received 17.08.2013, received in revised form 22.09.2013, accepted 03.10.2013
Asbestos, an extremely useful natural resource, has been studied extensively mineralogically and for industrial application. Its extraction from ores has been greatly improved with respect to safety in the work place. In spite of that, there is a decisive effort in many countries to ban its use on the grounds that it is a toxic substance. This action resulted in drastic decrease in production and the possibility of destroying the industry. The chelation of asbestos with organic dyes seems to be a promising way to abate its toxicity. Dyeing can be done when the fibers are slurred in water at ambient conditions in the same way as textile fibers.
Keywords: asbestos, toxicity.
Introduction
Dyestuffs have been used for many decades to stain cells and microorganisms for identification purposes [1]. They have also been used to identify clay minerals through a color reaction, e.g., montomorillonite gives a blue color with benzidine but illite does not [2]. Reactions like these were exploited by engineers to separate minerals by flotation [3]. For example, Alizarin Red S is more adsorbed on apatite than calcite, hence separation by flotation can be made possible [4]. Fly ash, which is mainly a silicate, is suggested as a cheap material for removal of certain dyestuffs from textile waste water [5]. Activated charcoal is also used for the same purpose. Dyestuffs are also adsorbed on metallic surfaces to inhibit their corrosion [6] and used to color certain metals on which an anodic oxide film has formed [7]. This shows the diversified reactions of dyestuffs with materials other than textile fibers - their most important application. In the present communication the reaction of dyestuffs with chrysotile asbestos is reviewed with respect to the toxicity issue [8-11].
Structure of absestos
Asbestos is an industrial mineral of great economic importance. It occurs in nature in a variety of forms but the most important is chrysotile which represents more than 90 % of the asbestos used; it is a hydrated magnesium silicate, 3MgO.2SiO2.2H2O. The given formula, however, represents only the ratio of the components and a more exact formula would be Mg3(Si2O5)(OH) 4 [12].
© Siberian Federal University. All rights reserved Corresponding author E-mail address: [email protected]
*
Fig. 1. Asbestos fibers under the microscope. The fibers that are visible to the naked eye are composed of hundreds of thousands mono-fibers called fibrils in close; packed parafiel arrangement
Fig. 2. Cross section oii^ fiLTbrrils at high resolution electron microscope showing the scroll-like structure. A ty pical fiber consists 01s 12 yo 20 layers rolled up like a scroll
The asbestos fibers that are visible to the naked eye are composed of hundreds of thousands mono-fibers called fibrils in close packed parallel arrangement (Fig. 1). The mono-fibers have a tubular shape formed by the rolling up of a sheet composed of hydrattd magnesium silicate. This sheet is composed of silica groups in tetrahedral pyramid form linked to each other to form a first layer and conjugated to octahedral groups of hydrated magnesia. The latter groups are bulkier than their conjugate silica groups and the sheet that they form has to curve to accommodate the excess encumberment of the magnesia. Lateral crystal growth of the sheet eventually leads to the formation of a tube. A typical fiber consists of 12 to 20 layers rolled up like a scroll (Fig. 2). The inner and outer diameters are respectively in the order of 50 to 250 Â. The hydration represents about 14 % of the mass and is in the form of hydroxyl groups linked to the magnesium and covering the external surface (Fig. 3).
A sliver of chrysotile asbestos with cross section 0.1 mm square contains about 20 x 106 fibrils all in parallel orientation. It is possible therefore to strip from asbestos fiber bundle very fine threads,
Fig. 3. Schematic illustration of the hydeated magnesium silicate sheets rolled in form of scrolls to forman asbestos fiber. Hydroxyl groups linked to the magnesium and covering; the external surface
each of which still contains many thousand fibrils. Aqueous suspensions of chrysotile asbestos show a marked alkaline, e. g., 10 g suspended in 1 L of distilled water shows a pH of 9. 5 at room temperature. This is due to the reaction:
Mg3Si2O5(OH)4 + 5 H2O -— 3 Mg^ + 6 OH-^) + 2 ^SiO^ The toxicity issue
A sbestos is claimed by so me as a workplace problem that can be adequately addie ssed with cutrent dusn control technology and appropeiate ptactices and not an environmental issue, and bp others as a toxic and carcinogenic material that must be banned immediately. It was demonstrated, for example, that heavy smokers in particular are more susceptible to have cancee of dhe lung; it seems that any fiber lodging in the lung because o!" ims large surface area is csppble oa adsorbing carcinogdnic fubstances from tobacwo smoke. The accumulation of siliceous dust in the lungs may result in causing a dieease c alled silicosis, and this may b e cured by sungem In case ob lung cavaer due to asbestos, howevep this is not possibte because the d^ease spreads eapidly from the lunge to other organs.
Ae a result of this controveisy, the co nsumption of asbestos in many countries has been decreasing gradually over tin pasn few years, which forced production decline, and in some cases plant shut downs. The industry is looking for substitutes for asbestos, but the long term environmental effects of other minerals or synthetic substitutes that have been proposed are unknown. There are two widely accepted screening toxicity tests for asbestos: hemolytic and cyto-toxicity.
□ Hemolytic: In these tests red blood cells are left in contact with certain amount of fiber and after various periods of time, the cells are centrifuged; and the hemoglobin content of the supernatant solution is measured using spectrophotometric methods. Usually, 72 % of the cells undergo hemolysis when contacted with dry fibers, and 65 % when contacted with wet fibers.
□ Cytotoxicity. In these tests P 388Dj cells are incubated with a certain amount of fibers for various periods of time. The number of healthy and dead cells is then measured using Trypan Blue dye. Usually, 81 % of the cells die when contacted with dry fibers and 50 % when contacted with wet fibers.
While hemolysis is a short-term test in which the membrane stability is involved, cytotoxicity as outlined above gives a global picture of the toxicity of the fibers. Hence, the two tests do not necessarily correlate together since the factors involved are not the same.
The chelation of asbestos
Chrysotile asbestos has the advantage of a large surface area and, unlike activated charcoal, is white in color so that the action of dyes can be followed easily. When a sample of asbestos fibers is agitated at room temperature with an aqueous solution of an organic dye, the fibers become immediately colored. When the colored fibers are separated by filtration, dried and examined by the scanning electron microscope they are homogeneous and contain no precipitates. When boiled with distilled water under reflux condenser, they do not lose their color thus indicating that the dye is chemically bound to the fiber. Table 1 gives the color of the dye solution and the color of the asbestos treated by these dyes.
The appearance of new peaks in X-ray diffraction patterns and in infrared spectra together with the absence of precipitates supports tTev iew that chelates are formed with asbestos, apparently with its Mg(OH)2 component (Fig. 4). This resembles the pro cess of mordant dyeing of textile fibers where Mg(OH)2, Al(OH) 3, or Cr(OH)3 are first precipitated on textile fibers then known as "lakes" [13]. Some odgabic compounds, e.d- , bromofresol purple and pXanolphdhale in, althoughthey are not dyesoucfs, they color asbestos. They have nearly similar structure and behdve neadly the same way towards asbestos. The toxic^y of the colored fibers was measured by the aests mentione d earliee and compared with tho uncolored sample. It was found that some dyes, e.g., Thiazol Yellow G and Trypan Blue decreased the toxictty of the filoers (Table 2).
Coxclusions
Chrygotile a^ctes can be colored by a variety oe dyes ah ambient conditions like textile fibers forming stable magnesium chelates. Certain dyes render it less toxic as measured by the homolytic
Fig. 4. Formation of a chelaFe between Mg(OH )2 and alizarin
Table 1. Color of dye solutions and color of the asbestos treated by these dyes [Habashi, 1992, Awadalla and Habashi, 1990]
pH of application Color of dye solution Color of asbestos
Alizarin 9.5-10 Violet Blue-violet
5-6 Brown Brown
2-3 Yellow Yellow
Basic Fuchsin 9.5-10 Purple Colorless
5-6 Red rose Pale red
2-3 Pale red Pale red
Brilliant Blue R 9.5-10 Colorless Blue
6-7 Blue Blue
2-3 Blue Blue
Brilliant Yellow 9.5-10 Red Red
6-7 Orange Orange
3-5 Yellow Yellow
1-1.5 Green
Precipitate
Bromocresol Purple* 9.5-10 Violet Blue
6-7 Yellow Yellow
2-3 Yellow Yellow
Crystal Violet 9.5-10 Violet Violet
5-6 Violet Violet
2-3 Greenish yellow Greenish yellow
Hydroxyquinoline 1-2 Yellow Gray
>2 Insoluble --
Malachite Green 9.5-10 Greenish blue Pale blue
6-7 Greenish blue Pale blue
2-3 Green Greenish blue
Methyl Blue 9.5-10 Colorless Blue
6-7 Blue Blue
3-5 Blue Blue
Methylene Blue 2-10 Pale blue Pale blue
Methyl Violet 9.5-10 Violet Violet
6-7 Violet Violet
1-3 Greenish yellow Greenish yellow
Phenolphthalein* 9.5-10 Pink Pink
6-7 Colorless Pale pink
2-3 Colorless Colorless
Thiazol Yellow G 9.5-10 Red Yellow
6-7 Yellow Yellow
2-3 Orange Orange
Trypan Blue 2-9.5 Blue Blue
* Not a dye
Table 2. Toxicity of chelated asbestos [Habashi et al., 1988, 1991]
Chelating Concentration of chelating Toxicity
agent on asbestos grade 7 RF66, % Hemolysis Cytotoxicity
None (dry asbestos) 0 72 81
None (asbestos suspended in water) 0 65 50
Thiazol Yellow G 0.2 54 59
2.0 1 13
2.1 6 0
2.5* 6 12
Trypan Blue 4.0 1 4
Brilliant Yellow 0.2 57 85
0.9 40 74
1.0 73 14
Methyl Blue 0.5 69 61
2.0 23 51
2.4 12 46
Alizarin 0.4 54 40
2.7 46 57
3.9* 90 13
5.3 64 24
* Chelating agent contacted with asbestos at the boiling point.
and cyto toxicity tests. This is a promising area of research that may lead to the preparation of colored asbestos that is nontoxic. Since the major application of asbestos, about 70 %, is in the fabrication of asbestos cement whereby the fibers are slurried in water with Portland cement, the addition of a dye in this step is a simple matter. Colored asbestos is stable with respect to boiling water that suggests a chemical reaction with magnesium hydroxide in the silicate structure.
References
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[3] Somasundaran P. and Nagaraj D.R. in Reagents in the Mineral Industry, edited by M.J. Jones and R. Oblatt, Institution of Mining and Metallurgy, London, 1984, p. 209-219.
[4] Fu E. and Somasundaran P. // Int. J. Min. Processing 18 (1986) 287.
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[7] Giles C.H. // Chem. Ind. (London) (1959) 1400.
[8] Habashi F. et al. Can. Patent, 1, 220, 492 (1988).
[9] Habashi F. et al. // Bull. Can. Inst. Min. & Met. 84 (945), 67-79 (1991).
[10] Awadalla FT, Habashi F. // J. Materials Sci. 25, 87-92 (1990).
[11] Habashi F. // Textile Chemist & Colorist 24 (4), 23-25 (1992).
[12] Michaels L. and Chissick S.S. (editors) «Asbestos», vol. 1 and 2 (Wiley, New York 1979).
[13] Trotman S.R. and Trotman E.R. The Bleaching, Dyeing, and Chemical Technology of Textile Fibers, Griffin, London 1948.
Токсичность асбеста может быть уменьшена?
Фатхи Хабаши
Кафедра горной металлургии и материаловедения Университет Лаваль Квебек, Канада G1V 0A6
Асбест - чрезвычайно полезный природный ресурс - был широко изучен для минералогического и промышленного использования. Извлечение его из руд было значительно улучшено в целях безопасности на рабочем месте. Несмотря на это, во многих странах предпринимают усилия для запрета его использования на том основании, что он является токсичным веществом. Это приводит к резкому снижению производства и возможности разрушения промышленности. Хелатирование асбеста с органическими красителями кажется перспективным способом уменьшить его токсичность.
Ключевые слова: асбест, токсичность.
