Decomposition of Chrysotile Asbestos by Fluorosulfonic Acid


Dec 1, 1997 - The effect of a fluorosulfonic acid (FSO3H) aqueous solution on decomposing the chrysotile asbestos fibers was investigated by using FT-...
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Ind. Eng. Chem. Res. 1998, 37, 79-88

79

Decomposition of Chrysotile Asbestos by Fluorosulfonic Acid T. Sugama, R. Sabatini, and L. Petrakis* Department of Applied Science, Brookhaven National Laboratory, Upton, New York 11973

The effect of a fluorosulfonic acid (FSO3H) aqueous solution on decomposing the chrysotile asbestos fibers was investigated by using FT-IR, XRD, and XPS. From the equilibrium of FSO3H in an aqueous medium (FSO3H + H2O ) HF + H2SO4), the resulting H2SO4 had a strong affinity for the external Mg(OH)2 layers in the tubular, scroll-like chrysotile structure. This acid-base reaction led to the precipitation and lixiviation of MgSO4‚H2O, MgO, and Mg2+ ion. Once the breakage of the outer Mg(OH)2 layers occurred, HF readily diffused into the inner silicious layers and then reacted with silicates, converting them into SiO2 hydrate and H2SiF6, while the ionic reaction between lixiviated Mg2+ and F- resulted in precipitating MgF2, thereby destroying the fibrous nature of the asbestos. An optimum combination of HF and H2SO4 contributed significantly to enhancing the rate of conversion of asbestos into nonfibrous materials in a short treatment time without any physical agitation. Introduction Asbestos is a commercial term applied to a group of silicate minerals which occur in fibrous form. There are six principal asbestos minerals: crocidolite [Na2Fe23+(Fe2+Mg)3Si8O22(OH)2]; tremolite [Ca2Mg5Si8O22(OH)2]; actinolite [Ca2(MgFe2+)5Si8O22(OH)2]; anthophyllite [Mg7Si8O22(OH)2]; amosite [(Fe2+Mg)7Si8O22(OH)2]; and chrysotile [Mg3(Si2O5)(OH)4]. Only one among them, chrysotile asbestos, belongs to the group classified as serpentine, having a layered silicate structure. The other five asbestos forms are of the amphibole type, having an SiO4 chain structure [Speil and Leineweber (1969), Harington et al. (1975)]. Due to their desirable thermal, physical, and chemical properties, asbestos minerals were widely used for fire protection, moisture control, and thermal insulation in numerous building products. Most of these products contain chrysotile asbestos, which supplied over 90% in the worldwide asbestos consumption [Heasman and Baldwin (1986)]. The distinct morphological feature of chrysotile asbestos is a long-serpentine shape that reflects the tubular scroll-like structure of the fibrils which are 20-50 nm in diameter [Michaels and Chissick (1979)]. This crystalline structure is characterized by spiralling of both the silicate (SiO4)2- tetrahedral sheet as an inner layer and the magnesium hydroxide [Mg(OH)2] octahedral sheet as an outer layer. Thus, the fibers can also be represented as hollow cylindrical tubes with their external surfaces covered with hydroxyl groups [Mg(OH)2 layer] and with internal hollow centers of bridging oxygen atoms (silicate layer) [Yada (1967)]. Early studies implicated the fibrous nature of these materials for their health effects [Gilson (1973), Lewisohn (1974)]. Several authors [Goni et al. (1979), Jaurand et al. (1981), Cozak et al. (1983)] have reported that when chrysotile asbestos is exposed to organic and mineral acids (pH < 6), leaching and lixiviation of magnesium from the chrysotile readily occur because of the interaction between the hydroxyl groups in the Mg(OH)2 layer and the hydrogen ions liberated from the acids, thereby * To whom all correspondence should be addressed. Telephone: (516) 344-3037. Fax: (516) 344-6288. E-mail: [email protected]

eliminating the magnesium hydroxide layers. This also causes the morphological alteration of a roll-shaped fiber structure into the open or unrolled one. Wagner et al. (1973) reported that, after the magnesium hydroxide layers were removed, the silicious residues retained a glass fiber-like morphology. Baldwin and Heasman (1986) investigated the rate of magnesium leaching from chrysotile by two experimental methods: one static, where asbestos is left standing in the acidic medium, and the other involving physical agitation. When the proportion of fiber to acid fluid was high, the physical agitation was found to speed up the leaching process and the maximum leaching rate was achieved sooner. In an attempt to convert the asbestos fibers into nonfibrous materials, several investigators [Mirick (1991), Mirick and Forrister (1993)] demonstrated that incorporating fluoride ions into the acidic media significantly promotes the rate of chemical decomposition of asbestos. Presumably the attack by the acids on the asbestos causes hydrolysis of the outer Mg(OH)2 layers in the chrysotile structure, while fluoride ions favorably react with Si in the inner silicious layers, thereby destroying the fibrous nature of the chrysotile. The source of fluoride ions was water-soluble fluoride compounds, such as ammonium fluoride, ammonium bifluoride, and sodium fluoride. Considering that the decomposition of asbestos is significantly promoted by fluoridated organic and mineral acids without requiring any physical agitation, our particular interest was in using water-soluble inorganic fluoro acid compounds, such as fluorosulfonic acid (FSO3H) and fluorophosphoric acid [FP(O)(OH)2], which form both the acid and fluoride in an aqueous medium. These compounds have the potential for replacing the mixed solution of acid and fluoride systems. Our main effort in this work was to identify the reaction products and byproducts of the interaction between the FSO3H solution and chrysotile asbestos and also to explore the resulting morphological alterations of the asbestos fibers. Mechanisms and pathways of degradation for the FSO3H-treated asbestos under static treatment conditions were investigated also. The equilibrium of FSO3H in an aqueous medium can be considered as FSO3H + H2O ) HF + H2SO4. Thus, an

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80 Ind. Eng. Chem. Res., Vol. 37, No. 1, 1998

actual HF and H2SO4 solution was also examined as the F-- and H+-liberating reference reagents, respectively. Experimental Section Quebec Class 9 chrysotile asbestos fibers with the formula Mg3(Si2O5)(OH)4 were obtained from LAB Chrysotile, Inc. The inorganic fluoro acid used was fluorosulfonic acid (FSO3H), supplied by Aldrich Chemical Co., Inc. Two additional chemical reagents, 48% hydrofluoric acid (HF) and 96.1% sulfuric acid (H2SO4), were employed as the F-- and H+-liberating reference reagents at concentrations of 5, 10, and 15 wt %. Fourier transform infrared spectroscopy (FT-IR; Model 1600, Perkin Elmer) and X-ray powder diffraction (XRD; Philips Electronic Instruments) were used to determine the amorphous and crystalline reaction products and byproducts of the reacted chrysotile asbestos and also to estimate the extent of decomposition of asbestos fibers by FSO3H, HF, and H2SO4 solutions. The samples for FT-IR and XRD studies were prepared in the following manner: 0.7 g of asbestos was placed in a plastic test tube (# 3.0 cm × 8.0 cm), and then 10 g of the chemical reagent was poured in the tube and left to react for 2 h at room temperature, without any physical agitation. Then, the mixture of asbestos and reagent was filtered through a 0.45 µm Millipore filter and washed with 30 mL of deionized water to remove any extra chemical reagent adsorbed on the conversion products and nonreacted asbestos. Following this, the solid residue was dried for ≈16 h in an oven at 110 °C. It was very important to carefully wash the sample with water in order to prevent any further reaction between excess reagent and asbestos in the oven. For FT-IR analysis, disks were made by mixing 200 mg of KBr and 3-4 mg of the sample that had been ground to a powder in a pestle. This powder sample was also used for XRD examination. X-ray photoelectron spectroscopy (XPS; CLAM 100, VG Scientific Ltd., Sussex, England) was employed to identify the chemical states and atomic fractions of the conversion products at the interfaces between the asbestos fibers and the chemical reagents. The excitation radiation was provided by an Al KR (1486.6 eV) X-ray source operated at a constant power of 200 W. The vacuum in the analyzer chamber of the instrument was maintained at 10-9 Torr (≈1.33 × 10-7 Pa) throughout. XPS samples were prepared by pressing the chrysotile fibers into a disk, 12 mm in diameter and 0.5-1.0 mm thick, with 100 MPa pressure. To precipitate the conversion products on the chrysotile disk surfaces, the samples were dipped for few seconds into 5, 10, or 15 wt % FSO3H, HF, or H2SO4 solutions, then left in air at room temperature for 2 h, and finally dried for 24 h in a vacuum oven at 60 °C. The thickness of the layers which were explored by XPS was ≈5 nm, corresponding to the escape depth of an aluminum photoelectron at an electron take-off angle of 38°. The same disk samples as those used in the XPS were examined in a SEM to investigate the microstructure and the chemical components of the surfaces using scanning electron microscopy (SEM; Model JSM-6400, JEOL, Peabody, MA) coupled with energy-dispersion X-ray spectrometry (EDX; TN-5502, Tracor Northern, Madison, WI). Results and Discussion FT-IR and XRD. Figure 1 shows FT-IR spectra for

Figure 1. FT-IR absorption spectra for the 0, 5, 10, and 15 wt % FSO3H-treated chrysotile asbestos samples and the asbestos sample treated with a 3 wt % HF-14.7 wt % H2SO4 solution, corresponding to the equilibrium relation of 15 wt % FSO3H in an aqueous medium.

the untreated and 5, 10, and 15 wt % FSO3H-treated chrysotile asbestos samples, over the two frequency ranges, from 4000 to 2500 cm-1 and from 1900 to 550 cm-1. The quantities of HF and H2SO4 components computed from the equilibrium relation of 15 wt % FSO3H aqueous solution correspond to 3 and 14.7 wt %, respectively. The asbestos samples treated with this mix formulation were also examined by FT-IR, as were fibers exposed to the 3.0 wt % HF-14.7 wt % H2SO482.3 wt % water system, to reveal the effect of the cotreatment with fluoride and acid on destroying the asbestos. A typical spectrum from the untreated chrysotile samples, denoted as 0 wt % FSO3H, showed the following absorption bands: at 3680 cm-1, concomitant with a shoulder at 3640 cm-1, ascribed to the stretching vibration of hydroxyl in the magnesium hydroxide [Mg(OH)2] and underlayers; at 3410 and 1640 cm-1, corresponding to the stretching and bending modes in H2O; at 1080, 1050, and 970 cm-1, originating from the Si-O-Si, Si-O-Mg, and Si-O- stretching frequencies, respectively; and at 610 cm-1, reflecting the stretching mode of the Mg-O linkage [Farmer and Russell (1964), Khorami and Leimieux (1987)]. When the chrysotile asbestos was treated with 5 wt % FSO3H, striking differences in spectral features are noted: (1) a considerable reduction of peak intensity at 3680 and 3640 cm-1; (2) the growth of peaks at 3410 and 1640 cm-1; (3) the disappearance of 1080 and 1050 cm-1 frequencies; and (4) the emergence of new bands at 1220, 1090, and 810 cm-1, while the Si-O- and Mg-O linkagerelated bands at 970 and 610 cm-1 remain. According to the literature [Nyquist and Kagel (1971)], these new bands in relating to item 2 seem to be associated with the formation of hydrated silica (SiO2). Increasing the concentration of FSO3H to 10 wt % caused further changes in the spectral features; namely, the SiO2 hydrate-related bands became the dominant ones, whereas the Mg-OH and Si-O- bands at 3680 and 970 cm-1 were converted into a shoulder. With 15 wt % FSO3H, it was very difficult to identify the presence of Mg-OH- and Si-O--related bands. This

Ind. Eng. Chem. Res., Vol. 37, No. 1, 1998 81

Figure 3. Changes in absorbance at 3680 cm-1 for the FSO3H-, HF-, and H2SO4-treated asbestos fibers as a function of the concentrations of chemical reagents. Figure 2. FT-IR spectra for 5, 10, and 15 wt % HF-treated and 10 and 15 wt % H2SO4- treated fibers.

spectrum also exhibited the development of additional bands at 880 and 690 cm-1, which may be due to the formation of MgSO4 hydrates as the reaction products, derived from the reaction between FSO3H and chrysotile [Nyquist and Kagle (1971)]. Accordingly, the peaks at 3410, 1640, 1220, and 1090 cm-1 not only correspond to the SiO2 hydrates but also are attributable to MgSO4 hydrates. The presence of the band at 610 cm-1 strongly suggests that it may be dependent upon the newly formed MgSO4 but independent of the Mg-O linkages in chrysotile. Of particular interest was the comparison between the spectral features of the 15 wt % FSO3H and 3 wt % HF-14.7 wt % H2SO4 samples. As seen in Figure 1, the spectra from the samples closely resemble each other, thereby suggesting that the destruction of the chrysotile fibers by the FSO3H solution is due mainly to the cooperative functions of HF and H2SO4. From this information, chrysotile fibers treated with FSO3H appear to be converted into two precipitates, SiO2 hydrates as byproducts and MgSO4 hydrates as reaction products. To obtain more detailed information on the decomposition mechanisms and conversion products of chrysotile fibers by FSO3H, we investigated the FT-IR spectra of the fibers treated with either HF or H2SO4 solutions. Figure 2 depicts the changes in the IR spectra of HFand H2SO4-treated fibers as a function of the concentration of these reagents. With HF, a 5 wt % treated fiber shows a spectrum containing SiO2 hydrate-related bands at 3410, 1220, 1090, and 810 cm-1 and also the residual chrysotile, represented by the remaining bands at 3680, 970, and 610 cm-1. This finding strongly supports the contention that HF is responsible for converting chrysotile into SiO2 hydrates. Although a certain amount of unreacted chrysotile fibers remained in the samples, treatment with 10 wt % treated fiber introduced two new bands at 1450 and 740 cm-1. These new bands are assigned to the Mg-F stretching in the MgF2 compounds formed by the interaction of HF with Mg(OH)2 in chrysotile [Nyquist and Kagel (1971)]. This would imply that the MgF2 reaction products co-exist with SiO2 hydrates in the HF-treated samples. However, for >5 wt % HF solutions, these MgF2-related bands were hardly visible. A conspicuous decrease in

the intensity of chrysotile-related bands can be seen in the 15 wt % HF-induced spectrum. In contrast, the MgF2 and SiO2 hydrate-related bands became the major features of the spectra. Hence, it seems that if only the HF reagent is used, a highly concentrated HF solution is essential to obtain a high degree of decomposition of fibers. On the other hand, the H2SO4 solution by itself is not particularly effective in destroying the fibers. Despite using a high concentration of 15 wt %, the resultant spectrum clearly showed the presence of chrysotile-related bands. However, the spectra also strongly suggested that the external Mg(OH)2 layers in the chrysotile structure are very vulnerable to the attack of H2SO4, so that they are transformed into MgSO4 hydrates as determined by the five bands at 3410, 1640, 1220, 1090, and 610 cm-1. Since the first four bands reflect the formation of SiO2 hydrates, it is reasonable to assume that the conversion of the outer Mg(OH)2 units (superposed on the inner silicious units) into MgSO4 hydrates by H2SO4 would promote the rate of transition of the silicious unit to SiO2 hydrate. We were also interested in determining the rate of decomposition of fibers by FSO3H, HF, and H2SO4. This determination was made by measuring the absorbance values of the IR absorption band at 3680 cm-1, due to the Mg-OH linkage, as a function of the concentrations of FSO3H, HF, and H2SO4. The reason for selecting the Mg-OH band at 3680 cm-1 was primarily due to the fact that, although the particle size of chrysotile varies, there is no shift in the frequencies of the peak position [Langer et al. (1978)]. The absorbance-concentration relation is given in Figure 3. The value of absorbance for all the chemical reagents tends to decrease with an increase in their concentration. However, the change of absorbance as a function of increasing concentration depended mainly upon the nature of the reagents, the greatest change being with the FSO3H solution. The absorbance of 1.1 × 10-2 for a 15 wt % FSO3H was lower by 2 orders of magnitude, compared with that of the untreated chrysotile. Almost as dramatic is the change in absorbance with HF; however, the absorbance change with H2SO4 is small, even at the highest concentration. Lower absorbance values mean lesser amounts of residual asbestos. Thus, the most efficient reagent of those investigated in this work for destroying asbestos is a 15 wt % solution of FSO3H. The amount of HF, computed from the equilibrium relation of 15 wt %

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Figure 4. Comparison between XRD patterns for 0, 5, 10, and 15 wt % FSO3H-treated chrysotile asbestos.

amorphous SiO2 hydrate and crystalline kieserite as the major conversion phases and sellaite crystals as the minor one. The XRD tracing (Figure 5) for the HFtreated fibers showed the precipitation only of the sellaite phase as the reaction product. The rate of the chrysotile to sellaite phase conversion increased with an increase in HF concentration, as shown by the enhanced sellaite line intensity with increasing HF. Correspondingly, a remarkable diminution in chrysotile’s spacings can be seen in the pattern from 15 wt % HF-treated fibers, implying that the fibers are also vulnerable to the attack of a highly concentrated fluoride solution. Nevertheless, the two precipitates, amorphous SiO2 hydrate and sellaite crystals, were the major conversion products of chrysotile by HF. The treatment of asbestos by H2SO4 promoted the chrysotile to MgSO4‚H2O phase conversion. XPS and SEM-EDX. Attention was also focused on understanding the interaction occurring at interfaces between the liquid chemical reagents, and the solid chrysotile fibers, and also on identifying the reaction products and byproducts. The critical interfacial zone was explored using XPS. In this study, the internally generated Mg2p, Si2p, C1s, O1s, F1s, and S2p peak areas were used to obtain the atomic fractions, which can be determined according to the equation [Wagner et al. (1979)]:

Cx ) (Ix/Sx)/

Figure 5. XRD patterns for 5, 10, and 15 wt % HF-treated asbestos.

FSO3H, is only 3 wt %, suggesting that the co-existence of HF and H2SO4 resulted in a requirement for a lesser amount of fluoride. As pointed out, using a solution of H2SO4 alone has drawbacks because the decomposition of asbestos occurs slowly under static conditions. Accordingly, we believe that a proper proportion of H2SO4 to HF plays an important role in enhancing the rate of conversion of chrysotile fibers into nonfibrous, nonchrysotile materials. The IR results are supported by XRD studies using the same samples (Figures 4 and 5). Using the 5 wt % FSO3H solutions (Figure 4), the diffraction pattern of the treated chrysotile showed that, although the XRD line intensity is relatively weak, a crystalline magnesium fluoride, MgF2 (sellaite), phase is formed as the minor reaction product in conjunction with the predominant lines of nonreacted chrysotile. A considerable attenuation of the intensities of the chrysotile-related lines can be seen from the 10 wt % solution, while an additional crystalline phase, MgSO4‚H2O, forms together with the sellaite. This finding directly suggests that the MgSO4 hydrate (identified in the FT-IR study) corresponds to the formation of kieserite. With 15 wt % FSO3H, the pattern was characterized by the dominant kieserite-related d-spacings as the principal crystal phase present in the samples and a weak line of sellaite; there were no clear d-spacings of chrysotile. In addition, no SiO2-related crystal compounds were found in the XRD tracings. Relating this finding to the FT-IR results, it appears that the reaction between chrysotile and FSO3H precipitated hybrid components containing

∑(Ii/Si)

where Cx is the concentration of respective atoms at the fiber’s surface; Ix is the peak area, defined by using the differential cross-sections for core-level excitation; and Sx is a sensitivity factor (0.07, 0.17, 0.205, 0.63, 1.00, and 0.35 for Mg, Si, C, O, F, and S, respectively). The atomic composition (in percent) of “as-received” chrysotile fiber surfaces also was used for comparison. The surface atomic compositions from these samples are given in Table 1. The surface of “as-received” fibers had 31.58% Mg, 17.31% Si, 7.18% C, and 43.98% O. Although some C contaminants were present at the outermost surface side, the atomic ratio of Mg/Si was 1.82. When the surface of fibers was treated with the HF solution, F atoms were incorporated into the fiber’s surfaces. Of particular interest is the variation in the percentages of Mg, Si, O, and F as a function of the concentration of HF: the amounts of Mg and F atoms tend to increase with increasing HF concentration, whereas the amounts of Si and O decreased, reflecting the increase in the value of Mg/Si ratio with an increase in HF concentration. From the aspect of the surface domination of Mg and F atoms, a possible interpretation for the increased Mg/ Si ratios is that using a highly concentrated HF solution promotes the precipitation of a large amount of MgF2 and F-enriched reaction products. The identification of F-enriched products was later carried out by inspecting the high-resolution F1s core-level spectrum. However, there is no evidence whether or not the interleaved silicious layers were removed from the chrysotile structure. After dipping the fiber disks in H2SO4 solution, their surfaces exhibited a considerably higher Mg/Si ratio, compared with that of the HF-treated samples. Also, this value was enhanced as the concentration of H2SO4 was increased. In connection with the migration of a certain amount of S and O atoms from the solution to the fiber’s surfaces, the reason for such a high ratio

Ind. Eng. Chem. Res., Vol. 37, No. 1, 1998 83 Table 1. Atomic Composition of Untreated and HF-, H2SO4-, and FSO3H-Treated Chrysotile Fiber Surfaces chemical reagents

Mg

Si

5% HF 10% HF 15% HF 5% H2SO4 10% H2SO4 15% H2SO4 5% FSO3H 10% FSO3H 15% FSO3H 3% HF-14.7% H2SO4

31.58 22.81 23.61 27.59 19.76 20.53 21.36 12.84 7.84 4.11 4.79

17.31 11.82 11.67 7.57 2.03 1.06 0.80 24.13 27.43 28.79 27.76

atomic composition, % C O 7.18 10.48 14.51 17.27 11.81 12.27 13.24 9.43 9.37 7.02 7.27

perhaps is due to the coverage of the fiber’s surfaces by thick MgSO4‚H2O layers as reaction products. In contrast, the surfaces of FSO3H-treated fibers exhibited a change in atomic composition as a function of the FSO3H concentration; namely, there was a conspicuous decrease in the amount of Mg and a marked increase in Si, while the additional F and S atoms migrated from the FSO3H solution to the fiber’s surfaces. This finding is related directly to the fact that the value of the Mg/Si ratio was considerably reduced with increasing FSO3H concentration. By comparison with that of H2SO4-treated fibers, the amount of S incorporated into the top surface layer was much less. Thus, the FSO3H reagent, which can liberate fluoride and acid in aqueous media, had the ability to extensively remove the Mg(OH)2 layers from the chrysotile structures, thereby exposing the inner silicious layers and promoting the precipitation of Si-related conversion products of chrysotile. As expected, the atomic fraction and Mg/ Si ratio similar to that of the 15 wt % FSO3H sample were obtained when fibers were treated with a mixed solution of the two components, i.e., 3 wt % HF and 14.7 wt % FSO3H (computed from the equilibrium relation of a 15 wt % FSO3H solution). This suggests that such a fluoride-acid solution has the same ability as FSO3H to eliminate the Mg(OH)2 layers. To identify the conversion products formed at interfaces between the fibers and HF, H2SO4, or FSO3H solutions, we inspected the high-resolution Mg2p, F1s, Si2p, and S2p core-level spectra for chrysotile disks treated with 5, 10, and 15 wt % HF, H2SO4, or FSO3H solutions. For all the XPS core-level spectra, the scale of the binding energy (BE) was calibrated with the C1s of the principal hydrocarbon-type carbon peak fixed at 285.0 eV as an internal reference standard. A curvedeconvolution technique, using a Du Pont curve resolver, was employed in determining the respective chemical states from the high-resolution spectrum of each element. In the HF-treated sample surfaces, the Mg2p, Si2p, and F1s core-level spectra are shown in Figure 6. The Mg2p spectrum of the chrysotile reveals only a single symmetric peak at 49.8 eV, originating from Mg in the chrysotile, Mg3(Si2O5)(OH)4. However, noticeable changes in the features of the Mg2p signal were observed from the 5, 10, and 15 wt % HF-treated samples. A new signal at a BE of 51.7 eV may have originated from the Mg in the MgF2 compounds [Wagner (1980)]. With concentrations higher than 10 wt % HF, this new signal became the principal peak reflecting the major chemical state, whereas the peak at 49.8 eV, corresponding to the Mg in chrysotile, had decayed significantly. The Si2p spectra gave additional information on the interfacial reaction products. The surfaces of 5 wt % HF-treated

43.98 33.80 20.46 18.92 51.59 50.75 48.52 49.11 49.21 51.64 51.66

F

S

atomic ratio, Mg/Si

14.82 15.40 16.08 2.10 3.14 4.11 4.43

1.82 1.93 2.02 3.65 9.73 19.37 26.70 0.53 0.29 0.14 0.17

21.09 29.75 28.65

2.38 3.02 4.32 4.09

fibers had three resolvable Gaussian components, at 102.8 eV as the most intense peak, at 103.5 eV as the secondary strong peak, and at 104.3 eV as a weak signal. By comparison with untreated chrysotile, this major peak reveals the Si in chrysotile. Our particular interest was to determine the contributions to the other two minor peaks. The literature [Klasson et al. (1974), Morgan and Van Wazer (1973), Nefedov et al. (1977)] suggested that the shoulder peaks at 103.5 and 104.3 eV may be attributable to the Si in hydrated SiO2 and in fluorosilicic acid (H2SiF6), respectively. This would indicate that both the hydrated SiO2 and H2SiF6 increasingly precipitated on the fiber’s surfaces with an increase in HF concentration. With a 20 wt % HF, these shoulder peaks at 103.5 and 104.3 eV were changed into the principal peak and secondary intense one, respectively, while the chrysotile’s Si signal at 102.8 eV was markedly attenuated. The information on the conversion products identified from the Mg2p and Si2p core-level excitations was further supported by inspecting the F1s region of these samples. As seen in the F1s region, the spectra for all samples had two Gaussian components at 686.3 eV as the major peak and at 684.8 eV as the shoulder one. The former peak is attributable to the F in H2SiF6 [Nefedov et al. (1977)], and the latter corresponds to the F in MgF2 [Wagner (1980)]. The spectra in the F1s region also demonstrated that the area of the overall F1s signal increases with an increased HF concentration, implying that this had enhanced the extent of coverage by H2SiF6 and MgF2 precipitates over the fiber surfaces. As with the FT-IR and XRD results, XPS supports the assertion that the interaction between HF and the chrysotile fibers leads to the precipitation of amorphous SiO2 hydrates as the major conversion phase, H2SiF6 as the secondary phase, and MgF2 crystal as the minor one. These results also afford the possibility of elucidating the reaction route for the overall decomposition mechanism of chrysotile by HF. It is well-known [Moody (1965)] that when HF reacts with the silica of silicate, silicon tetrafluoride (SiF4) is formed:

SiO2 + 4HF f SiF4v + 2H2O SiF4 gas is rapidly hydrolyzed by water to form gelatinous hydrated silica and fluorosilicic acid:

SiF4 + 2H2O f hydrated SiO2V + 4HF SiF4 + 2HF f H2SiF6 It is reasonable to assume that a similar reaction might precipitate H2SiF6 when HF comes into contact with the inner silicious layers of the chrysotile structure. Be-

84 Ind. Eng. Chem. Res., Vol. 37, No. 1, 1998

Figure 6. XPS high-resolution spectra in Mg2p, Si2p, and F1s regions for HF-treated chrysotile surfaces as a function of the concentration of HF.

cause H2SiF6 is soluble in water, this could be the reason why we did not identify H2SiF6 in our FT-IR and XRD studies which were carried out on samples washed with deionized water. On the other hand, hydrogen ions dissociated from HF favorably react with the hydroxyl groups in the external Mg(OH)2 layers to lixiviate or leach out magnesium ions from chrysotile:

Mg(OH)2 + 2H+ f Mg2+ + 2H2O Next, the ionic interaction between the leached Mg2+ and the F- from HF leads to the precipitation of MgF2 as the reaction product. From this information, the reaction of HF with chrysotile may take place as follows:

Mg3(Si2O5)(OH)4 + 14HF f 2SiF4v + 3MgF2V + 9H2O Furthermore, the rapid hydrolysis of SiF4 can cause the precipitation of the two conversion products, SiO2 hydrate and H2SiF6. Figure 7 shows the changes in the spectral features of the Mg2p and S2p regions of the H2SO4-treated samples as a function of the concentration of acid. In Mg2p, attention was focused on the possible contributions to the two signals at 51.2 and 50.1 eV. These signals are assigned as follows: the main Mg-related component is sulfate, reflecting the major line at 51.2 eV, while the Mg in the magnesium oxides at 50.1 eV [Inove and Yasumori (1981)] is present as a secondary component. Relating this information to the XRD data, the magnesium sulfate compound corresponds to the formation of MgSO4‚H2O. Because the signal at 50.1 eV markedly grows with an increased H2SO4 concentra-

Figure 7. Mg2p and S2p regions for H2SO4-treated chrysotile fibers.

tion, using a highly concentrated H2SO4 solution seems to promote the precipitation of amorphous magnesium oxides as byproducts. No signal from the Mg of chrysotile was found in these spectra, thereby showing that the surface of fibers was extensively covered by thick layers of precipitates of MgSO4‚H2O and magnesium oxides. Because the escape depth of a photoelectron is ≈5 nm, the thickness of such layers appears to be more than 5 nm. Inspection of the S2p region directly supported the information on the MgSO4‚H2O formed as reaction product. The S2p region for all the samples

Ind. Eng. Chem. Res., Vol. 37, No. 1, 1998 85

Figure 8. Changes in Mg2p, Si2p, and F1s core-level spectral features as a function of the concentration of FSO3H for FSO3H-treated chrysotile surfaces. Table 2. Binding Energy for Reference Conversion Products Obtained from XPS Core-Level Inspection of HF- and H2SO4-Treated Chrysotile Fiber Surfaces reference conversion product chrysotile MgO MgF2 SiO2 hydrate MgSO4‚H2O H2SiF6

Mg2p 49.8 50.1 51.7

binding energy, eV Si2p Fls

S2p

102.8 684.8 103.5

51.2

169.7 104.3

686.3

showed one strong signal at 169.7 eV. The XPS study on the sulfur-oxygen bonds [Lindberg et al. (1970), Mixan and Lawbert (1975)] suggested that an increase in the rate of oxidation of S shifts the peak position to a high BE site, for instance, sulfoxide (>SdO) around 165.9 eV, sulfone (>SO2-) at ≈167.5 eV, and sulfate (-SO4) at ≈169.5 eV. From this information, we are convinced that the contribution at 169.7 eV is due to the SO4 in the MgSO4‚H2O. In the Si2p region, it was very difficult to gain detailed information on the Sirelated reaction products and byproducts because of weak signals for all samples. Taken together the data obtained from FT-IR, XRD, and XPS for H2SO4-treated chrysotile fibers indicate crystalline MgSO4‚H2O as the reaction product and amorphous magnesium oxide and SiO2 hydrates as the byproducts. These were precipitated (MgSO4‚H2O > MgO >> SiO2‚H2O) on the fiber’s surfaces through the following reaction:

Mg3(Si2O5)(OH)4 + 2H2SO4 f 2(MgSO4‚H2O) + 2(SiO2‚H2O) + MgO On the basis of the binding energy positions (Table 2) of the reference conversion products, we can determine the contributions to the peaks in the Mg2p, Si2p, F1s, and S2p regions for the FSO3H-treated samples (Figure 8). The spectral feature of the Mg2p region was characterized by a broad signal which includes at least three resolvable peaks at 51.7, 51.2, and 50.1 eV. By comparison with the BE positions of the reference conversion products, these peaks are attributed to the formation of MgF2, MgSO4‚H2O, and magnesium oxides,

Figure 9. Hypothetical decomposition model of chrysotile fiber by a FSO3H solution.

respectively. The Si2p and F1s spectra suggested that three major compounds, SiO2 hydrate, H2SiF6, and MgF2, were precipitated by the interactions between the FSO3H solution and chrysotile on the fiber’s surfaces. The S2p spectra indicated the presence of a single peak at 169.7 eV, reflecting the formation of MgSO4‚H2O. Hence, the FSO3H-chrysotile interaction provided the same conversion products as those derived from the combined HF and H2SO4 individual solutions. Thus, the decomposition of chrysotile fibers by FSO3H can be accounted for through the model shown in Figure 9. The morphological features of the chrysotile fibers are represented by scroll-tubular structures assembled by rolling up double sheets consisting of a Mg(OH)2 octahedron layer as the outer sheet and silicate tetrahedron as the inner sheet. When the FSO3H solution comes into contact with the fiber’s surfaces, a highly concentrated solution of H2SO4 acid, generated from

86 Ind. Eng. Chem. Res., Vol. 37, No. 1, 1998

Figure 10. SEM-EDX examinations of the untreated (top) and 15 wt % FSO3H-treated chrysotile fiber (bottom) disk surfaces.

FSO3H in the aqueous medium, preferentially reacts with the external Mg(OH)2 layers to precipitate MgSO4‚H2O and MgO and to lixivate Mg2+ ions from the Mg(OH)2 layers, rather than the interleaved silicious sheets. Such a breakdown of the Mg(OH)2 sheets could accelerate the rate of diffusion and penetration of HF derived from FSO3H into the silicious layers. The chemical affinity of HF for the silicious layers causes the formation of SiF4, and, concurrently, the ionic reaction between the F- from HF and the lixiviated Mg2+ results in precipitating MgF2. The rapid hydrolysis of SiF4 forms the SiO2 hydrate as a byproduct and H2SiF6 as the reaction product, thereby destroying the fibrous nature of chrysotile. An important question remains to be answered, namely, why the Mg/Si atomic ratio of FSO3H-treated fiber surfaces was so low. The solubility of Mg-related conversion products in 100 cm-3 water at 20 °C is 35.8 g for MgSO4‚H2O, 0.00062 g for MgO, and 0.66 g for MgF2 [Weast (1968)]. The silicate to SiO2 hydrate and H2SiF6 conversion may promote the rate of leaching of MgSO4‚H2O, which is very susceptible to dissolution in water from the product layers. Thus, we believe that a proper proportion of fluoride to acid can contribute significantly to enhancing the rate of conversion of chrysotile fibers into nonfibrous materials. Morphology by SEM and EDX. To further support this finding, we explored the morphological features and

elemental compositions of the surfaces of the disks used in XPS examination by SEM-EDX. By comparison with XPS, which identifies the chemical states and compositions of the top layers (5 nm), EDX is extremely useful for quantitative analysis of individual elements on a solid surface layer, up to ≈1.5 µm thick. The abscissa of the EDX spectrum is the X-ray energy characteristic of the element, and the intensity of the gross peak count is related to the amount of each element present. Therefore, bulk analyses with EDX can greatly facilitate the interpretation of SEM images. Compared with the untreated fiber surfaces (Figure 10, top), a characteristic feature of surface morphology of FSO3H-treated fibers (Figure 10, bottom) was their complete coverage by precipitated conversion products. No fibrous image was observed in this micrograph. The EDX spectrum concomitant with the SEM image indicated that the predominant elements were O, Mg, and S, with lesser amounts of F, Si, Au, and Fe. The detected Au is the coating material deposited on the SEM sample’s surfaces. Because O, Mg, and S are associated with MgSO4‚H2O and MgO, this observation strongly suggested that a large amount of these Mgrelated compounds as conversion products is present in the subsurface layers (≈1.5 µm). The amounts of Siand F-related conversion products seem to be less. From XPS, the dominant conversion product in the superficial 5 nm layer is SiO2 hydrate, not Mg-related

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Figure 11. SEM-EDX data for the 15 wt % HF-treated (top) and 15 wt % H2SO4-treated (bottom) chrysotile fiber surfaces.

compounds. Although no signal was found in the XPS spectrum, some Fe atoms are present in the subsurface layers. The interesting morphology in the HF-treated fiber surfaces is shown in Figure 11, top. This image discloses the interlocking structure of the fibrils. There was no reticulate-like fiber texture, which was observed on the untreated fiber surfaces (Figure 10, top). Also, there was no heavy coverage of the fiber surfaces by conversion products. The EDX spectrum exhibited strong signals for Mg and Si elements, a moderate intensity of O, and weak F and Fe lines. Such strong Mg and Si signals reveal chrysotile fibers, with the attack of HF on the fiber’s surfaces having contributed to their alteration. In contrast, the SEM image of the H2SO4-treated fiber surfaces (Figure 11, bottom) shows that the conversion products in this case seem to blend together partially reacted and unreacted fibers. In fact, the fibers present in this blend are identical with the images in which the fibrous materials are coated with the conversion products. From the EDX spectrum which have S, Mg, and O as the predominant elements, the major conversion product appears to be MgSO4‚H2O. Such binding of MgSO4‚H2O might suppress the further diffusion and penetration of H2SO4 into the asbestos layers. Thus, this would explain why H2SO4 by itself does such a poor job in decomposing asbestos.

Conclusions The decomposition of chrysotile asbestos by fluorosulfonic acid (FSO3H) is determined by the establishment of the equilibrium in aqueous media, FSO3H + H2O ) HF + H2SO4. Sulfuric acid, derived from FSO3H aqueous solution, preferentially reacts with the outer Mg(OH)2 layers of the tubular scroll-like fibers of chrysotile, rather than with its inner silicious layers. This reaction precipitates crystalline MgSO4‚H2O (kieserite) and amorphous MgO and leads to the lixiviation of Mg2+ ions. Once the breakdown of Mg(OH)2 layers is initiated, the diffusion of FSO3H-derived HF into the silicious layers promotes the breakage of SiO-Si bonds in the silicious structure, while ionic reaction between the lixiviated Mg2+ and the F- from HF causes the formation of crystalline MgF2 (sellaite). Such Si-O bond breakage, caused by the attack of HF on the Si-O-Si linkages, results in the production of silicon tetrafluoride, which is rapidly hydrolyzed by water to precipitate SiO2 hydrate and fluorosilicic acid (H2SiF6). Based on this mechanism of decomposition of asbestos by FSO3H, chrysotile can also be treated effectively with an optimum mixture of H2SO4 and HF. Such a mixture can enhance the rate of conversion of asbestos fibers into nonfibrous materials in a short treatment time without any physical agitation and also reduce significantly the concentration of HF needed for disintegrating the asbestos.

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Acknowledgment The work at Brookhaven National Laboratory was performed under Contract No. DE-AC02-76CH00016, U.S. Department of Energy, Office of Energy Research and Office of Environmental Management. The NSLS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Division of Chemical Sciences. Literature Cited Baldwin, G.; Heasman, L. A. In Chemicals in the Environment Proceedings of the International Conference; Lester, J. N., Perry, R., Sterritt, R. M., Eds.; Selper: London, 1986; pp 36-46. Cozak, D.; Barbeau, C.; Gauvin, F.; Barry, J. P.; DeBlois, C.; DeWolf, R. Can. J. Chem. 1983, 61, 2753. Farmer, V. C.; Russell, J. D. Spectrochim. Acta 1964, 20, 1149. Gilson, J. C. Proc. R. Soc. Med. 1973, 66, 395. Goni, J.; Thomasssin, J. H.; Jaurand, M. C.; Touray, J. C. Phys. Chem. Earth 1979, 11, 807. Harington, J. S.; Allison, A. C.; Badami, D. V. Adv. Pharmacol. Chemother. 1975, 12, 291. Heasman, L. A.; Baldwin G. Waste Manage. Res. 1986, 4, 215. Inove, Y.; Yasumori, I. Bull. Chem. Soc. Jpn. 1981, 54, 1505. Jaurand, M. C.; Magne, L.; Boulner, J. L.; Bignon, J. Toxicology 1981, 21, 323. Khorami, J.; Lemieux, A. Can. J. Chem. 1987, 65, 2268. Klasson, M.; Berndtsson, A.; Hedman, J.; Nilsson, R.; Nyholm, R.; Nordling, C. J. Electron Spectrosc. Relat. Phenom. 1974, 3, 427. Langer, A. M.; Wolff, M. S.; Rohl, A. N.; Selikoff, I. J. J. Toxicol. Environ. Health 1978, 4, 173. Lewisohn, H. C. J. Soc. Occup. Med. 1974, 24, 2.

Lindberg, B. J.; Hamrin, K.; Johansson, G.; Gelius, U.; Fahlman, A.; Nordling, C.; Siegbahn, K. Phys. Scr. 1970, 1, 286. Michaels, L.; Chissick, S. S. AsbestossProperties, Application, and Hazards; John Wiley & Sons Ltd.: New York, 1979; Vol. 2. Mirick, W. U.S. Patent 5,041,277, 1991. Mirick, W.; Forrister, W. B. U.S. Patent 5,264,655, 1993. Mixan, C. E.; Lawbert, J. B. J. Org. Chem. 1975, 38, 1350. Moody, B. J. Comparative Inorganic Chemistry; Edward Arnold Publishers Ltd.: London, 1965; p 275. Morgan, W. E.; Van Wazer, J. R. J. Phys. Chem. 1973, 77, 96. Nefedov, V. I.; Salyn, Y. V.; Leonhardt, G.; Scheibe, R. J. Electron Spectrosc. Relat. Phenom. 1977, 10, 121. Nyquist, R. A.; Kagel, R. O. Infrared Spectra of Inorganic Compounds; Academic Press: New York, 1971. Speil, S.; Leineweber J. P. Environ. 1969, 2, 166. Wagner, C. D. J. Electron Spectrosc. Relat. Phenom. 1980, 18, 345. Wagner, J. C.; Berry, G.; Timbrell, V. Br. J. Cancer 1973, 23, 173. Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. Handbook of X-Ray Photoelectron Spectroscopy; Perkin-Elmer Corp.: Eden Prairie, MN, 1979. Weast, R. C. Handbook of Chemistry and Physics, 49th ed.; The Chemical Rubber Co.: Cleveland, OH, 1968. Yada, K. Acta Crystallogr. 1967, 23, 704.

Received for review April 18, 1997 Revised manuscript received October 8, 1997 Accepted October 13, 1997X IE9702744

X Abstract published in Advance ACS Abstracts, December 1, 1997.