How are the physical and chemical properties of chrysotile asbestos altered by a 10-year residence in water and up to 5 days in simulated stomach acid?

Although there have been a number of studies on the ingestion of asbestos, few studies exist on how the chrysotile asbestos itself is altered by the exposure to the acid stomach environment. This study has found that there are changes in the physical, chemical and surface properties of chrysotile asbestos as a result of exposure to water, strong acids, and simulated gastric juices. It was observed that the charge on the surface (the zeta potential) is changed from positive to negative; the surface becomes silicalike; and the magnesium is lost from the fibers of asbestos upon exposure to water and acid. It was also noted that the smaller the fiber diameter, the faster the loss of the magnesium. Notable among the changes in physical properties is a change in the refractive index. This means that asbestos exposed to acids or water may not be detectable using the dispersion staining techniques that identify asbestos based on the refractive index. Other physical property changes include the destruction of the gross crystallinity of the fibers. The x-ray diffraction signal disappears when fibers are exposed to acid. However, this study shows that the fibers may still be detected by electron diffraction. It appears that upon acid exposure, the magnesium ions are leached out, leaving a magnesium-free silica network. A positive ion, possibly the proton (H+) or the hydronium ion (H3O+), replaces the lost magnesium ion.


Introduction
The prevalence of asbestos in drinking water in the United States has been cataloged (1,2), and the effects of magnesium leaching on the biological effects of inhaled asbestos have been studied (3)(4)(5). However, no studies have been found that have considered the effects of magnesium leaching in ingested asbestos.
Several mineralogists (6,7) have studied how asbestos changes under heat and presssure, while others (8,9) have studied the surface properties of asbestos fibers. The physical and chemical properties of asbestos have been cataloged (10,11). The optical properties of chrysotile have been studied (12), and dye absorption on chrysotile has been investigated (13,14). The study summarized here attempted to use these various techniques and tests on asbestos fibers altered by exposure to simulated gastric juices and fibers stored in water for long periods of time. Specific details of the studies can be found in the more comprehensive final report, which will be available from the U.S. Environmental Protection Agency (U.S. EPA) at a future date.
This work impacts two areas. The first is the area of chrysotile identification in the environmental matrix. Acid-treated fibers may not be easily detectable by conventional techniques used for asbestos identification. Second, there is the biomedical implication: What changes in fiber properties caused by exposure to water or gastric juice will alter the biological effects of asbestos? Light and Wei have pointed out the possibility of a connection between surface charge and toxicity (15).

Materials and Methods
This project studied the changes in the physical, chemical, and surface properties of chrysotile asbestos after exposure to doubly distilled (DD) water for 10 years, 1 N hydrochloric acid for 1/2hr to 5-day intervals, and to simulated gastric juices for up to 5 days. The simulated gastric juices were produced by adding 2 g of NaCl, 3.2 g of pepsin (hog extract), and 7.0 mL of HCI to a liter of distilled water (16). The pH of the juice was 1.2.
Chrysotile asbestos samples were obtained from three sources: International Union Against Cancer (UICC), National Institute of Environmental Health Sciences (NIEHS), and Globe, AZ.
A variety of tests, including X-ray and electron diffraction, energy-dispersive X-ray analysis, and various surface tests, were performed. The tests may be divided into three main categories: physical, chemical, and surface charge investigations.
Changes in surface charge as a result of acid exposure were determined by the measurement of zeta potential (ZP) (17,18) versus pH. Untreated fibers were compared with those treated in 1 N HCl and those treated in simulated gastric juices. A nitrogen absorption (19) experiment was performed to determine changes in surface area. Changes in surface were also studied by a dye adsorption method (20).
Physical changes to the fiber after acid exposure were found by measuring the refractive index of the fibers. X-ray and electron diffraction were used to study the differences between acidtreated, water-treated, and untreated fibers. Chemical changes to the fiber after acid treatment were studied by performing atomic absorption (AA) analyses of the liquid in which the fibers were treated. The total amount of magnesium lost from the fibers was determined. Energy-dispersive X-ray analysis on individual fibers was performed using a scanning transmission electron microscope (STEM). In this case the Mg/Si ratio, as a function of residence time in acid, was determined.

Results
Upon exposure to simulated gastric juice, the ZP of NIEHS, UICC, and Globe chrysotile asbestos goes from positive to negative in less than 1 hr ( Fig.1). Figure 2 shows the results of asbestos exposure to 1 N HCl. In 8 hr the ZP still remained positive. The different results suggest that the NaCl and pepsin have an important part to play in changing surface charge.
ZP-pH measurements can be used to understand surface changes. Figures 3 and 4 compare ZP-pH curves for untreated and 0.1 N HCltreated chrysotile. The point at which the ZP curve cuts the x-axis, called the zero point charge (ZPC), has moved from a pH of 6.5 for untreated fibers to a pH of 4 for the treated chrysotile. Since chrysotile has a ZPC of 6 and silica has a ZPC of 4, Figures 3 and 4 show that acid exposure has turned the chrysotile surface to a silicalike surface.   Table 1 shows that the surface area of the treated NIEHS chrysotile is about double that of the untreated chrysotile as measured by the nitrogen adsorption experiment. The chrysotile from Globe was not affected by the acid treatment. The difference cannot be attributed to larger fibers in the NIEHS material breaking up into smaller fibers with more surface area because there was no difference in nitrogen adsorption after sonication of both materials.
The results of the dye adsorption measurement of surface area are shown in Figure 5. The HCl is shown to have the greatest effect on the asbestos fibers. One of the important physical properties that changes upon acid treatment of chrysotile is its refractive index. This change for 1 N HCl and simulated gastric juice treatments is shown in Figure 6. The refractive index decreases from 1.54 for the untreated fiber to 1.44 for the treated fibers. A refractive index of 1.4 is approaching that of the zeolite minerals. This is consistent with the removal of magnesium from the chrysotile, leaving an open framework type of silicate. The result suggests that methods used to identify chrysotile based on its refractive index alone will not be effective in identifying acid-treated fibers.
Acid treatment also destroys the X-ray diffraction pattern of chrysotile. The effect on the X-ray pattern of chrysotile after 3 and 5 days in 1 N HCl is shown in Figure 7. Crocidolite, an amphibole asbestos form, remains unchanged (Fig. 8). The results of an electron diffraction study of acidtreated chrysotile are shown in Table 2. It was observed that after 5 days in 1 N HCl, the electron diffraction patterns lost clarity; this is assumed to be related to the loss of magnesium ions from the fiber.   Chrysotile asbestos has the formula Mg3ISi2O5](OH)4. The results of the AA analysis of the liquid in which the asbestos materials had been placed (Fig.9) leads to the clear conclusion ___O that acid leaches magnesium from chrysotile fibers. The excellent agreement with electron mi-'°°'°°°croprobe results of Monchaux et al. (4) may also be seen in Figure 9.

Discussion
Some of the changes caused by HCl and gastric acid would make it difficult to identify acidtreated fibers. It will be necessary to develop new techniques for the optimum identification of fibers after they have been placed in the gstrointestinal tract through ingestion. The simulated gastric juices used in this study did not contain all components of human stomach acid. The complex organic compounds such as muco-and glycoproteins may play a large part in coating the fibers after they are in the stomach and have a great effect on such parameters as surface charge and magnesium leaching.

Conclusions
Chrysotile asbestos from three sources (UICC, NIEHS, and Globe, Arizona) has been shown to change its physical, chemical and surface properties after exposure to HCI and simulated gastric juice.
The research described in this report was supported by Research Resources, International Institutes of Health. The assistance of J. Millette and that of my various students is gratefully acknowledged.
The research described in this paper has been peer and administratively reviewed by the U.S. Environmental Protection Agency and approved for presentation and publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.