Environ. Sci. Technol. 2008, 42, 1638–1642
Novel Selective Dyeing Method for Chrysotile Asbestos Detection in Concrete Materials Y O S H I H I K O O K E , * ,† N A K A M I C H I Y A M A S A K I , ‡ G O Y A M A M O T O , †,§,| K A Z U H I R O S A S A K I , ⊥ NAOMI MAETA,| HIROKAZU FUJIMAKI,⊥ AND T O S H I Y U K I H A S H I D A * ,†,| Fracture and Reliability Research Institute, Graduate School of Engineering; Institute of Fluid Science; Graduate School of Environmental Studies; and Department of Earth and Planetary Sciences, Graduate School of Science, Tohoku University, Aramaki, Aoba-ward, Sendai, Miyagi, 980-8579, Japan and Center for Advanced Science and Innovation, Osaka University, 2-1, Yamadaoka, Suita, Osaka, 565-0871, Japan
Received July 20, 2007. Revised manuscript received November 19, 2007. Accepted December 10, 2007.
There are a tremendous number of asbestos-containing buildings without any surveys on the presence of asbestos because of the difficulty to detect asbestos in building materials simply and quickly, although a great deal of worldwide effort was put into removing asbestos of which inhalation causes serious diseases. In this study, we newly developed a simple dyeing method to detect chrysotile asbestos, the most commonly used type of asbestos, in asbestos-cement composite materials using magnesium-chelating organic dyes. As an essential process for selective dyeing of chrysotile asbestos, special pretreatment with a calcium-chelating agent was developed to prevent the dyes from reacting with calcium, which is the major component of concrete materials. Our developed selective dyeing method was shown to possess sufficient sensitivity for detecting chrysotile asbestos in an amount greater than 0.1 mass% in concrete specimens, and there was an approximately linear relationship between the area fraction of dyed spots and the mass fraction of chrysotile asbestos. Our results may provide a basis for further development of a simple on-site detection method for chrysotile asbestos in building materials and may facilitate the progress of control and removal of asbestos in the environment.
Introduction Large amounts of asbestos were installed in many buildings because of its strength, chemical resistance, thermal insulating, and sound-proofing properties. Asbestos is a general term for two groups of fibrous silicate minerals known as serpentines (chrysotile) and amphiboles (amosite, crocidolite, anthophyllite, tremolite, and actinolite). Asbestos is present * Address correxpondence to either author. Phone: +81-22-7957524(Y.O.); +81-22-795-7523(T.H.). Fax: +81-22-795-4311 (Y.O.); +8122-795-4311 (T.H.). E-mail:
[email protected] (Y.O.);
[email protected] (T.H.). † Fracture and Reliability Research Institute, Tohoku University. ‡ Osaka University. § Institute of Fluid Science, Tohoku University. | Graduate School of Environmental Studies, Tohoku University. ⊥ Department of Earth and Planetary Sciences, Tohoku University. 1638
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as bundles of fibers that may become airborne when asbestoscontaining buildings are damaged or broken. Excessive inhalation of asbestos fibers causes serious diseases, such as silicosis, mesothelioma, and pulmonary carcinoma (1, 2). Although a worldwide effort has been made to remove asbestos from the biosphere of human, there still remains large amounts of asbestos-containing buildings without appropriate survey of the presence or absence of asbestos. Especially, chrysotile asbestos is currently mined in any quantity today and accounts for approximately 95% of the asbestos used commercially (3). Furthermore, more than 90% of the world production of chrysotile asbestos has been used in the manufacture of chrysotile-cement. Therefore, a simple and quick detection method for chrysotile asbestos needs to be developed for surveying a large number of existing buildings. Conventional techniques for the analysis of asbestos fibers in building materials includes polarized light microscopy (PLM) (4), X-ray diffraction (XRD) (5), scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis (6), transmission electron microscopy with EDX, selected area electron diffraction (SAED) (7), and Raman spectroscopy (8, 9). These laboratory-based techniques may be inappropriate for assessing a large number of building materials for asbestos. First of all, all the methods require expensive equipments. PLM requires a complicated process as well as the competence and experience of the analyst. The analyses of XRD, SEM with EDX, TEM with EDX, and SAED require sample preparation, which can be time-consuming. Furthermore, the biggest disadvantage is the difficulty to determine asbestos content, though most government regulations require identification of asbestos in an amount greater than 1% or 0.1% by weight. The dyeing method may provide a simple and safe route for detection of asbestos contained in building materials without generating airborne asbestos. Indeed, it has been reported that pure chrysotile asbestos was dyed with some magnesium-chelating organic dyes, because the dyes formed the chelates with magnesium in chrysotile asbestos (10). However, further study is needed to investigate whether the dyeing method could be useful for detecting chrysotile asbestos in building materials. In general, magnesiumchelating organic dyes can also easily form chelates with calcium, and almost all the matrices of chrysotile asbestoscontaining building materials are cementitious materials in which calcium is contained as a principal component. Therefore, it may be difficult to distinguish chrysotile asbestos from building materials by dyeing, because the matrices of calcium-containing building materials as well as magnesiumcontaining chrysotile asbestos may be dyed with magnesiumchelating organic dyes. To develop a simple detection method for chrysotile asbestos in building materials by dyeing, only magnesium in chrysotile asbestos must be chelated with the dyes. Thus, an additional treatment needs to be developed that prevents magnesium-chelating organic dyes from forming the chelates with calcium in building materials. In this study, we present a novel detection method for chrysotile asbestos in concrete materials by selective dyeing with magnesium-chelating organic dyes. In the developed selective dyeing method, pretreatment with a calciumchelating agent was performed to inhibit the dyeing of cement matrices. The selective dyeing method has the sufficient sensitivity to determine the quantity of chrysotile asbestos in concrete materials. Our results demonstrate that the masking of calcium can be useful process for developing a 10.1021/es071805a CCC: $40.75
2008 American Chemical Society
Published on Web 01/26/2008
simple on-site detection method by selective dyeing for chrysotile asbestos in building materials.
Experimental Section Sample Preparation. An asbestos cement slate was used to develop a simple detection method for chrysotile asbestos by selective dyeing because the cement slate was widely used for building wall materials. To examine the sensitivity of the dyeing method, concrete specimens having various mass fractions of chrysotile asbestos were also prepared. The cement slate was shown to contain chrysotile asbestos, even though no detailed information was available about the chrysotile asbestos in the cement slate. The thickness of the cement slate was approximately 5 mm. To conduct dyeing experiments, the cement slate was cut into rectangular plates of 20 × 20 × 5 mm using a diamond wheel saw. To prepare concrete specimens having various mass fractions of chrysotile asbestos, predetermined quantities of chrysotile asbestos (Wako Pure Chemical Industries, Ltd., Japan) and water were first placed into a Hobart type mixer, and the contents were mixed for approximately 1 min to disperse fiber bundles. Then, portland cement was added to the mixture and further mixed for approximately 10 min. The water-cement ratios were varied in the range of 30-120%, depending on the mass fraction of chrysotile asbestos, to maintain the appropriate workability. The mass fractions of chrysotile asbestos used were zero, 0.01, 0.05, 0.1, 0.5, 1.0, 2.5, 5.0, 7.5, 10, and 15 mass%. The mixtures were cast under high frequency vibration for 3 min using a steel mold 40 × 40 × 160 mm in dimensions. After the casting, the mixtures were kept at a laboratory (approximately 60% relative humidity, room temperature) for 12 h prior to demolding. The hardened concrete plates were cut into rectangular specimens of about 20 × 40 × 10 mm in dimensions using a diamond wheel saw. No further surface finish was made to all specimens for dyeing experiments. Instrumental Analysis. To identify the type of fibers in the cement slate, PLM and EDX were used. Thin sections were prepared from the cement slate for PLM. The thickness of the thin sections was approximately 30 µm. The thin sections of the cement slate were observed with a polarizing microscope (OPTIPHOT-POL; Nikon, Japan), followed by photomicroscopy with a digital camera (COOLPIX 4500; Nikon, Japan). The thin section was placed on the rotating stage of the polarizing microscope, and a fiber bundle in the thin section was aligned along the polarization direction. The angular reading of the stage was recorded. The stage was next rotated until the fiber bundle appeared maximally dark in the polarized light. Then, the reading of the stage was recorded again. The extinction angle of the fiber bundle was revealed by the difference in the first reading and the second. The measurements of the extinction angle were performed on 80 bundles in the thin sections. To analyze the detailed morphology and the elemental composition of the fibers in the cement slate, the fibers in the rectangular plate specimens were examined by using an SEM (S-4700, Hitachi, Japan) with EDX (EDX; Phoenix, EDXA, USA). The specimens were coated with carbon for SEM and EDX analysis. As a comparison test, pure chrysotile asbestos fibrils were analyzed. The pure chrysotile asbestos fibrils were prepared from natural chrysotile mineral mined at Furano in Japan (NICHIKA Corp. Japan). Dyeing Experiments. All specimens were washed using a cleaning solution (pH 9.0) containing Tween 20 of 10 g/l in purified water before the dyeing experiment. The specimen was immersed in 40 mL of masking solution containing the calcium-chelating agent for 20 min at 80 °C as a pretreatment. The calcium-chelating agent used in this study was O,O′Bis(2-aminoethyl)ethyleneglycol-N,N,N′,N′-tetraacetic acid
FIGURE 1. Polarised light microscopy images of a chrysotile asbestos fiber bundle in a thin section of the asbestos cement slate. (a) open nicol. (b) crossed nicol. Bar, 1 mm.
FIGURE 2. SEM images of the pure chrysotile asbestos fibril (a) and the fiber bundle in the asbestos cement slate (b). (EGTA). The pH of the masking solution was set to be at 9.0, and the concentration of EGTA was adjusted to 10 g/L in cleaning solution. Specimens of the cement slate without pretreatment were also prepared as a comparison. Trypan Blue or Ponceau S was solved in cleaning solution at 10 g/L (pH 9.0) to prepare dye solutions. The specimen was agitated in 40 mL of each dye solution for 20 min at 80 °C. Then, the specimen was washed in cleaning solution for 20 min at 80 °C to remove the dye residue. The specimen was finally immersed in 1 M NaHCO3-Na2CO3 buffer (pH 9.0) to stabilize the color appearance of the dye.
Results and Discussion Development of Selective Dyeing Method. Optical observation indicated that the asbestos cement slate used in this study contained a large number of fiber bundles; the diameter of the fiber bundles was approximately 0.1 mm, and their length was in the range of 0.5–2 mm (Figure 1a). The analyses of PLM, SEM, and EDX were performed to assess the type of fibers in the cement slate. The result of the PLM analysis is presented first. All fiber bundles were shown to have the extinction angle between 0 and 7 degrees. The angle values were consistent with the extinction angles of chrysotile fibers. No other types of fibers were observed in the thin sections of the cement slate with PLM. A typical SEM image obtained from the pure chrysotile fibrils is presented in Figure 2a, and that from the fiber bundles in the cement slate is shown in Figure 2b, respectively. It is noted that the fiber bundles observed in the cement slate exhibit quite similar morphology to the pure chrysotile fibrils. Furthermore, SEM observation with higher magnifications revealed that each fiber in the bundles was approximately 50 nm or less in diameter, which is also consistent with the reported diameter of natural pure chrysotile asbestos fibers (11). Typical EDX spectrum of the pure chrysotile asbestos fibrils is shown in Figure 3a, and that of the fiber bundles in the cement slate in Figure 3b. Both spectra possess identical wave peaks. Three peaks appear at about 50, 125, and 170 keV, which correspond to oxygen, magnesium, and silicon, respectively. The above-mentioned results strongly suggest that the fiber bundles in the cement slate consist of chrysotile asbestos whose chemical formula is Mg3Si2O5(OH)4. Therefore, all fiber bundles in the cement slate are judged to consist of chrysotile asbestos. VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. EDX spectra of the pure chrysotile asbestos fibrils (a) and the fiber bundles in the asbestos cement slate (b). The above-mentioned cement slate containing chrysotile asbestos fiber bundles was used as a test specimen to develop a selective dyeing method for chrysotile asbestos in building materials. It is well accepted that magnesium-chelating organic dyes can easily form chelates with calcium as well as magnesium. Further, calcium is one of the major chemical components in matrices of asbestos-cement composite materials such as asbestos cement slates. Therefore, the addition of magnesium-chelating organic dyes onto cementbased materials may simultaneously dye the cement matrices
as well as chrysotile asbestos. To distinguish chrysotile asbestos from the cement slate, the dyes should react only with magnesium in chrysotile asbestos and not with calcium in the matrices of the cement slate. A typical calciumchelating agent, EGTA, can form the chelate with calcium and not with magnesium. Therefore, EGTA is expected to form the chelate with calcium in the cement slate when the specimen is treated with EGTA before dyeing. After this reaction, calcium in the cement slate should be masked and should no longer react with the dyes. The masking effect for calcium in the cement slate enables the dyes to form the chelates only with magnesium in chrysotile asbestos. Thus, the above-mentioned masking effect of a calcium-chelating agent is expected to offer the dyeing selectivity of chrysotile asbestos in concrete materials. In this study, the cement slate specimens pretreated with EGTA were prepared in addition to the specimens without the EGTA-pretreatment. In the dyeing experiment, the specimens were immersed in either Ponceau S or Trypan Blue. The results of the dyeing experiment are demonstrated in Figure 4 for two types of magnesium-chelating organic dyes, Ponceau S and Trypan Blue. It is noted that the dyed spots appeared on the crosssection surfaces of the cement slate specimens with the EGTApretreatment (Figure 4, panels c and d). The dyed spots were identified as chrysotile fiber bundles by EDX analysis. Indeed, it has been shown that the pure chrysotile fibrils exhibited the same colors as shown in the dyed spots of Figure 4, panels c and d, when dyed with either Ponceau S or Trypan Blue. On the other hand, uniform dyeing was observed on the cross-section surfaces of the cement slate specimens without the EGTA-pretreatment, and no distinction could be made
FIGURE 4. Photographs of the dyed section of the asbestos cement slate. The specimens of the asbestos cement slate were dyed using either Ponceau S (c and e) or Trypan Blue (d and f) with (middle row) or without (bottom row) the EGTA-pretreatment. Upper row indicates the sections of the cement slate specimens before dyeing. Bar, 1 mm. 1640
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FIGURE 5. Photographs of the dyed section of chrysotile asbestos-containing concrete specimen. Each specimen was dyed with Ponceau S after the EGTA-pretreatment. The figure given in each image indicates the mass fraction of chrysotile asbestos used. Bar, 5 mm. between cement matrices and chrysotile asbestos (Figure 4, panels e and f). These results indicate that the combination of dyeing with magnesium-chelating dyes and with EGTApretreatment may provide a simple detection method of chrysotile asbestos in cementitious composite materials as well as pure chrysotile asbestos. Sensitivity of the Selective Dyeing Method. To assess the sensitivity of the developed selective dyeing method for chrysotile asbestos in concrete materials, we conducted the dyeing experiment on the concrete specimens having various mass fractions of chrysotile asbestos. Photographs of the dyed concrete specimens are shown in Figure 5 for the chrysotile asbestos mass fractions of zero, 0.1, 1.0, and 5.0 mass%. It is seen that the chrysotile asbestos can be detected as pinky dyed spots on the sections of the concrete specimens. The number and area of dyed spots increased with the increasing mass fraction of chrysotile asbestos. In the case of 5.0 mass%, the almost entire section of the concrete specimen appears to be dyed. It should be mentioned that no dyed spots were observed for the specimen without chrysotile asbestos (0 mass%). The observation supports the validity of the calcium-masking procedure. Building materials including the asbestos content greater than 0.1 mass% are defined to be asbestos-containing products in some countries, such as Japan and Germany. The threshold value (0.1 mass%) may be the lowest in the world. It is particularly noticed that the dyed spots can be readily observed on the section of the concrete specimen containing 0.1 mass% of chrysotile asbestos. The result demonstrates that the selective dyeing method may have the sufficient sensitivity for detecting the regulated quantity of chrysotile asbestos in concrete materials. We measured the area fractions of dyed spots on the sections of the concrete specimens. The area fractions were quantified with Scion Image software (Scion Co.). The relationship between the area fraction and the mass fraction of chrysotile asbestos is shown in Figure 6. A magnified version of the relationship is provided in the inset. The figures in the parenthesis indicate the number of the specimens used for the observation. It is seen that there is an approximately linear relationship between the two parameters, although a significant scatter is observed in the measured area fractions. In principle, the area fraction of chrysotile asbestos should be equal to its volume fraction in the concrete. The argument may support the relationship between the area and mass fractions observed in Figure 6. The magnified diagram also demonstrates the relatively high sensitivity of the dyeing method developed in this study.
FIGURE 6. Relationship of the area fraction of dyed chrysotile asbestos with the mass fraction of chrysotile asbestos used. The inset gives the range of mass fraction less than 1 mass %. Thus, our developed selective dyeing method with the calcium-masking pretreatment can be a fundamental approach to estimate quantities of chrysotile asbestos in concrete materials. In summary, we newly developed a simple and quick method for detecting chrysotile asbestos in concrete materials by means of a selective dyeing technique. Cement slate specimens and concrete specimens including chrysotile asbestos were used to examine the applicability of the detection method in this study. In our developed detection method, the selective dyeing of chrysotile asbestos in the cementitious materials was accomplished by a pretreatment with calcium-chelating agent, EGTA, and chrysotile asbestos was delineated as dyed spots on the section of the cementitious materials. The pretreatment with EGTA is an essential process to mask solid calcium in the matrices of concrete materials. EGTA might form the chelates with solid calcium in the matrices of concrete materials, and consequently, magnesium-chelating organic dyes could selectively form chelates with magnesium in chrysotile asbestos. Our developed method was shown to have the sufficient sensitivity for detecting the regulated quantity of chrysotile asbestos. Furthermore, there was an approximately liner relationship between the area fraction of dyed spots and the mass fraction of chrysotile asbestos used. Therefore, our developed selective dyeing method utilizing a calcium-masking process is expect to provide a fundamental basis for a simple and quick onsite detection method for chrysotile asbestos in building materials. VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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This work was supported by the Grant-in-aid for Urgent Development of Fundamental Technologies for the Practical Reduction of Asbestos of The New Energy and Industrial Technology Development Organization (NEDO) in 2006.
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(8) Bard, D.; Tylee, B.; Williams, K.; Yarwood, J. Use of fibre-optic probe for the identification of asbestos fibres in bulk materials by Raman spectroscopy. J. Raman Spectrosc. 2004, 35, 541–548. (9) Rinaudo, C.; Gastaldi, D.; Belluso, E.; Capella, S. Application of Raman Spectroscopy on asbestos fibre identification. N. Jb. Miner. Abh. 2005, 182, 31–36. (10) Awadalla, F. T.; Habashi, F. Reaction of chrysotile asbestos with triphenylmethane dyes. J. Mater. Sci. 1990, 25, 87–92. (11) Yoda, K. Study of chrysotile asbestos by a high resolution electron microscope. Acta Crystallogr. 1967, 23, 704–707.
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