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Thermochemical destruction of asbestos-containing roofing slate and the feasibility of using recycled waste sulfuric acid
Journal of Hazardous Materials 265 (2014) 151–157
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Thermochemical destruction of asbestos-containing roofing slate and the feasibility of using recycled waste sulfuric acid Seong-Nam Nam a,∗ , Seongkyeong Jeong b , Hojoo Lim c a
Engineering Research Institute, Seoul National University, Daehak-dong, Gwanak-gu 151-744, South Korea Environmental Resource Recirculation Division, National Institute of Environmental Research, Environmental Research Complex, Kyeongseo-dong, Seo-gu, Incheon 404-708, South Korea c Indoor Environment and Noise Division, National Institute of Environmental Research, Environmental Research Complex, Kyeongseo-dong, Seo-gu, Incheon 404-708, South Korea b
h i g h l i g h t s • • • • •
Asbestos-containing roofing slates (ACS) were thermochemically treated. 5 N H2 SO4 with 100 ◦ C heating for 10–24 h showed complete disappearance. Asbestiform of ACS was changed to non-asbestiform after treatment. Favorable destruction was occurred at the Mg(OH)2 layer rather than SiO2 sheet. Equivalent treatability of waste acid brightened the feasibility of this approach.
a r t i c l e
i n f o
Article history: Received 30 May 2013 Received in revised form 22 October 2013 Accepted 5 November 2013 Available online 13 November 2013 Keywords: Asbestos containing slate Chrysotile Thermochemical destruction X-ray diffraction Polarized light microscopy Scanning electron microscopy
a b s t r a c t In this study, we have investigated the feasibility of using a thermochemical technique on ∼17% chrysotile-containing roofing sheet or slate (ACS), in which 5 N sulfuric acid-digestive destruction was incorporated with 10–24-h heating at 100 ◦ C. The X-ray diffraction (XRD) and the polarized light microscopy (PLM) results have clearly shown that raw chrysotile asbestos was converted to nonasbestiform material with no crystallinity by the low temperature thermochemical treatment. As an alternative to the use of pricey sulfuric acid, waste sulfuric acid discharged from a semiconductor manufacturing process was reused for the asbestos-fracturing purpose, and it was found that similar removals could be obtained under the same experimental conditions, promising the practical applicability of thermochemical treatment of ACWs. A thermodynamic understanding based on the extraction rates of magnesium and silica from a chrysotile structure has revealed that the destruction of chrysotile by acid-digestion is greatly influenced by the reaction temperatures, showing a 80.3-fold increase in the reaction rate by raising the temperature by 30–100 ◦ C. The overall destruction is dependent upon the breaking-up of the silicon-oxide layer – a rate-limiting step. This study is meaningful in showing that the low temperature thermochemical treatment is feasible as an ACW-treatment method. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Asbestos and asbestos-containing materials have been widely used in many applications, such as insulators, asbestos cement and fireproof construction materials, because of their low thermal conductivity and high mechanical strength. However, asbestos is known to be extremely carcinogenic, especially in causing a severe asbestosis, lung cancer and pleural mesothelioma when the respiratory system is exposed to it. As a result, nowadays, in most
∗ Corresponding author. Tel.: +82 2 880 4321; fax: +82 2 877 7376. E-mail address: [email protected] (S.-N. Nam). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.11.004
countries the mining, refinement and use of asbestos have been banned, apart from some exceptional applications. From a toxicological point of view on asbestos, although there have been many studies by toxicologists and clinical research scientists to elucidate the clear mechanisms that cause severe toxicities, little is known about the crucial processes at the cellular/molecular levels, and still such studies leave unanswered which chemical or physical properties of asbestos are key factors in disease causation. Chemical reasons are explained with mineralogical compositions of asbestos. For instance, magnesium from chrysotile or iron from iron-containing asbestos (e.g., amosite or crocidolite) may be leached intracellularly, thereby inducing toxicity of the fibers [1,2], or causing cytotoxicity through generating a highly reactive species (e.g., a hydroxyl radical or reactive oxygen) [3].
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Pathogenic mechanisms by the physical properties of asbestos are related to the size and length of the fibers, morphology (i.e., asbestiform or non-asbestiform), etc. [4]. Thus, if any treatment technique can achieve the “detoxification” of asbestos through both chemical (i.e., decomposition) and physical destruction, it should be the target goal of treatment, regardless of which property would play an important role in inducing toxicity to life. In South Korea, the most common use of asbestos occurred in the 1960–70s by the government-led project “New Village Movement”. As a part of the project, straw roofs in rural areas had been replaced with asbestos-cement roofing slates, and most of them are now obsolete and are being removing according to the Waste Asbestos Management Plan (2012–2021). According to the Korea Waste Statistics 2009, it was reported that 16 million tonnes of ACWs would be cumulatively generated by 2016. Currently, the Waste Management Law of South Korea classifies ACWs containing greater than 1 wt.% of asbestos as “Specific Hazardous Waste”, which must be disposed of only by controlled landfill in accordance with safety regulations. Regarding interim treatments before landfill, few thermal or mechanochemical techniques such as hightemperature incineration, or solidification with cement are legally permitted [5]. However, the confining and displacement of ACWs in landfill do not essentially destroy asbestos, and yet provide a load to landfill sites for several decades or for a permanent period, especially in a small country like South Korea. Thermal treatment involves the conversion of asbestos into non-asbestiform materials by melting the compositional elements at temperatures ranging from ca. 800 to 1400 ◦ C or above [6,7]. Similarly, microwave heating applies high power to inert asbestos materials [8]. Despite the efforts to lower operating costs, thermal treatments or microwave application are still energy-intensive and cost-demanding. Therefore, new or different treatment techniques need to be considered for significant amounts of ACWs. One approach, which was studied, is a chemical method using caustic acids [9,10] or alkalis to transform the crystalline structures to noncrystalline forms or to decompose the constituents of asbestos. In fact, such chemical treatment is not a discovery, but a novel technology which has been applied a great deal in various media [10–16]. Of the acidic agents, sulfuric acid is known to be the most effective attacking chemical and this was also experimentally observed in our previous tests [17]. Therefore, in this research, sulfuric acid (H2 SO4 )-based chemical dissolution was used and in order to enhance or complete the breakage of chemical bonds thermal process (i.e., heating) was incorporated to the process of chemical digestion. Furthermore, sulfuric acid, one of the most widely applied chemicals in many industrial processes, accounts for about 20% (89,000 tonnes discharged in 2009) of the entire waste acids discharged annually in South Korea, so the reuse or recycling of waste sulfuric acid is strongly desirable [9]. Therefore, the main objective of this research is to evaluate the treatability of asbestos-containing roofing slate waste using lowtemperature thermochemical treatment and to demonstrate the feasibility of reusing waste sulfuric acid as a replacement for a commercial acid. Concentration of acid and heating temperature applied in this study were chosen mainly for practical reasons such as estimated costs per tonne of ACWs, comparable to other treatment methods [18]. The authors expect that the results shown in this research would give an insight to future guidelines with regard to the disposal of asbestos-containing wastes. 2. Experimental methods 2.1. Asbestos materials Asbestos-containing (roofing) slate wastes (hereafter, ACS) were taken from a government-registered hazardous waste
treating company. Waste materials were shipped polyethylene double-packed to a HEPA filtration equipped laboratory, and were briefly rinsed with tap water in order to remove the attached dirt such as soil and moss. After drying them at room temperature, the slate samples were ground with a blade mill for 3 min and sieved to get a particle size below 0.5 mm. The presence of chrysotile asbestos was identified and confirmed by XRD, SEM and PLM analytical techniques. The raw fiber materials of chrysotile asbestos obtained from a mine located in Jecheon city, South Korea were used without any further purification process. 2.2. Thermochemical treatment Based on preliminary tests, the sulfuric acid was determined to be effective at concentrations above 5 N [17]. Thermochemical experiments were performed as follows: 5 g of the ACS sample was placed in 10 mL of 5 N H2 SO4 in a porcelain crucible with a small lip (i.e., solid/liquid = 500 g/L) and heated at 100 ◦ C for 10 and 24 h. During the treatment, a porcelain lid was covered to prevent gas emissions, and the gases were gently emitted through a lip and were trapped in a potable air cleaning system. The treated residual product was 0.45 m pore-sized filtered in a vacuum filtration unit while rinsing with deionized water ( ≥ 18.2) until the pH of the filtrate was neutralized, and the filtrate residues were dried overnight at 100 ◦ C. The dried samples were then analyzed in powder form for further characterization (XRD, PLM, SEM/EDS). The waste sulfuric acid was obtained from an electronic company in South Korea, and the concentration of the acid was determined by the acid-base titration method using NaOH. Detailed descriptions on the waste sulfuric acid were included in Section 3.4. 2.3. Analytical techniques Regarding solid materials, the chemical compositions of the untreated chrysotile and slate powder below 500 m were measured with X-ray fluorescence (XRF-1700, Shimazdu Co. Ltd, Japan). In order to examine the changes in the crystalline phases of asbestos, the powdered samples were analyzed by X-ray diffractometer (XRD: RIGAKU Ultima III, Japan) with CuK␣ radiation ( = 0.1584 nm) generated at 40 kV and 25 mA. The XRD spectrum was obtained in the 5–75 degree range of 2 and the obtained diffraction patterns were compared with the JCPDS archives in the PDXL software program. Polarized light microscopy (PLM: DP72, JP/BX51 Olympus, Japan) using dispersion staining, extinction, etc., was also used in order to confirm if chrysotile remained after acidic digestion, or if asbestiform remained or was not detectable. The microstructures and/or compositional changes of the treated asbestos waste were observed using scanning electron microscopy with an electron dispersion spectrum (SEM/EDS: Shimadzu Co., Ltd., Japan). In addition, the concentration of magnesium, silicon, calcium, and iron extracted into solution were measured using inductively coupled plasma-optical emission spectrometry (ICPOES, Horiba Jobin Yvon Ultima, Japan). 3. Results and discussion 3.1. Characterization of untreated asbestos samples Table 1 shows the chemical compositions of natural chrysotile asbestos and an ACS sample measured by X-ray fluorescence. Raw chrysotile is mostly composed of about 42 wt.% MgO and 36 wt.% SiO2 together with 4 wt.% Fe2 O3 and 1 wt.% Al2 O3 which may be contained due to replacements of Mg and Si, respectively as reported in the literature [19]. Since ACS materials are mixtures of asbestos with cement, their chemical matrices are similar to
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Fig. 1. SEM-EDS images of raw chrysotile (upper) and ACS (lower); a cross mark (+) indicates the point that the EDS was taken.
ordinary Portland cement (OPC), showing about 40 wt.% CaO and 19 wt.% SiO2 (59 wt.% and 23 wt.% for OPC, respectively). The SEM images of raw chrysotile and ACS samples (Fig. 1) show the chrysotile asbestos bundles with a long and serpentine-like fibrous morphology. The EDS spectrum of raw chrysotile showed the composition to be MgO 37.9%, SiO2 57.3%, FeO 3.38% and Al2 O3 1.35% which generally agreed with the reported values [19,20]. Chrysotile fibers in ACS were present and were entangled with co-existing materials in slates such as cement constituents, most probably CaO and SiO2 [21]. The presence of chrysotile asbestos in ACS was also confirmed by EDS, revealing that the major elements of the fiber were MgO (40.6%), SiO2 (59.4%) and Al2 O3 (∼0.8%). CaO was also detected at ∼5%, and it is thought to be mostly present in the fine particles attached onto the chrysotile fiber since the surrounding white-colored amorphous materials mostly consisted of Ca and O. Table 2 summarizes the X-ray diffraction angles and lattice spaces of the three major peak positions (0 0 2, 0 0 4 and 0 2 0 in order of intensity) obtained from the XRD spectra of untreated raw chrysotile and ACS samples (shown in Figs. 2 and 3,
Fig. 2. XRD patterns for untreated raw and treated chrysotile asbestos (10- and 24-h heating at 100 ◦ C in 5 N H2 SO4 ): C: chrysotile (Mg3 Si2 O5 (OH)4 .
Table 1 Chemical compositions (wt.%) of raw chrysotile and untreated ACS material by XRF. Material
SiO2
Al2 O3
TiO2
Fe2 O3
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
LOI
Raw chrysotile Raw ACS SY OPCa NIST cementb
36.01 19.43 23.25 20.66
1.09 3.23 4.12 3.89
0.04 0.12 0.14 0.227
3.99 1.91 2.12 1.937
0.07 0.06 0.09 0.2588
42.20 5.63 2.89 0.814
0.36 39.62 58.83 65.34
0.00 0.00 0.18 0.195
0.00 0.31 1.14 0.605
0.00 0.08 0.17 0.110
15.65 28.82 6.99 3.28
a b
SY OPC: an ordinary Portland cement that is being producing by the SY cement company (Korea). NIST cement: product number #1889a, NIST-Standard Reference Material of Portland cement (blended with limestone).
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Table 2 Comparisons of X-ray diffraction angles and lattice spaces of raw chrysotile and ACS material with reference asbestos (i.e., clinochrysotile-2Mc1). hkl (0 0 2)
(0 2 0)
(0 0 4)
Clinochrysotile-2Mc1 (00–052–1562) ˚ d (A) 7.27 12.10 2 (◦ )
4.55 19.62
3.65 24.37
Raw chrysotile this study ˚ d (A) 7.29 12.12 2 (◦ )
4.52 19.6
3.65 24.39
4.52 19.62
3.65 24.39
Raw ACS this study ˚ d (A) 2 (◦ )
7.28 12.15
respectively), and it was found that based on the major three characteristic peaks, the type of chrysotile asbestos was mostly well matched to the clinochrysotile-2Mc1 (PDF No.: 00-052-1562) data in the ICDD database. 3.2. Thermochemical decomposition of chrysotile asbestos It is believed that the chemical digestion of chrysotile by sulfuric acid takes place through diffusion of hydrogen ions released from sulfuric acid, and the subsequent attacking of the reactive site or the surface of the chrysotile [22,23]. As resultants of the reaction, magnesium or silica is extracted from the asbestos structure [23,24]. Based on the analytical results of the final products, the chemical reactions at room temperature can be understood as follows (R1)–(R4). Chrysotile will react with water, resulting in a raised pH due to the released hydroxide ions (OH− ) (R1). However, the reaction is not important since the increase in the pH and the extracted concentration of magnesium ions were negligible for the 5-h experiment duration. Hence, reaction (R2) is likely to be dominant in chrysotile destruction through sulfuric acid digestion. Mg3 Si2 O5 (OH)4 (s) + 5H2 O → 3Mg2+ + 6OH− + 2H4 SiO4 (aq) (R1) Mg3 Si2 O5 (OH)4 (s) + 6H+ (i.e., 3H2 SO4 ) → 3Mg2+ + (3SO4 2− ) + 2SiO2 (s) + 5H2 O (or 2H4 SiO4 (aq) + H2 O) (R2)
On the other hand, regarding the ACS sample, chemical digestions will take place in two ways; one is a reaction with chrysotile and the other is a reaction with the constituents of ACS. However, a significant portion of the sulfuric acid will be used to react with calcite (CaCO3 ) since it exists as a major constituent of the ACS, resulting in gypsum or calcium sulfate (CaSO4 ) as the final product (R4). Although the reaction of calcite with water raises the pH of a solution by releasing hydroxide ions (as described in (R3)), it is also negligible when the solution is a strong acid like sulfuric acid. Thus, (R2) competes with (R4) and unknown reactions between the sulfuric acid and the other constituents of the ACS. CaCO3 + H2 O → Ca2+ + 2OH− + CO2 (g)
(R3)
CaCO3 (s) + H2 SO4 → CaSO4 (s) (or Ca2+ + SO4 2− ) + H2 O + CO2 (g)
(R4)
Therefore, the chemical destruction of asbestos in the ACS takes place competitively by (R2) and (R4), and can be accelerated by heating (i.e., thermochemical destruction). Figs. 2 and 3 show the XRD patterns of the samples before and after thermochemical treatment with 100 ◦ C heating in 5 N sulfuric acid for 10 and 24 h. For the untreated chrysotile asbestos (Fig. 2), characteristic 2-theta peaks corresponding to 0 0 2, 0 2 0 and 0 0 4 positions can be distinctively detected. However, after the thermochemical treatment, a broad and hump-like peak was observed without showing any characteristic diffraction angles, and this implies that the product does not have the original crystallinity of raw chrysotile. For the ACS sample (Fig. 3), a characteristic peak of calcite (CaCO3 ) was observed with crystalline clinochrysotile2Mcl in the untreated sample. After treatment, the peak patterns showed crystalline calcium sulfate (CaSO4 ) that was formed as a final product of the reaction between sulfuric acid and the ACS, as seen in R4. With an increased treatment time up to 24 h, reduced peak strengths of calcium sulfate have been observed. PLM images presented in Fig. 4 were obtained by the dispersion staining method, in which 1–1.5 drops of a liquid with nD (refractive index) = 1.550 at 25 ◦ C were added to a small amount of sample on a slide glass. The chrysotile fiber was observed as having a blue color in a 2–8 o’clock position, as shown in Fig. 4a, while the treated or the transformed products revealed their non-asbestiform structure with a pale blue color, which means that their structure and compositions must have been changed. As seen in Fig. 4b and c, fibrous chrysotile asbestos was not observed and its morphology has been transformed to a fine particulate form. 3.3. Temperature effects
Fig. 3. XRD patterns for untreated raw chrysotile, and treated chrysotile asbestos (10- and 24-h heating at 100 ◦ C in 5 N H2 SO4 ), C: chrysotile (Mg3 Si2 O5 (OH)4 , S: calcium sulfate (CaSO4 ), L: calcite (CaCO3 ).
In order to understand the temperature effects on chrysotile destruction, concentrations of extracted magnesium were compared at 30 ◦ C, 50 ◦ C, 70 ◦ C and 100 ◦ C (Fig. 5) as the reaction time increased. Due to a limited amount of available raw chrysotile material, the reaction dosage was set to 1 g:50 mL (i.e., 500 g/L in chrysotile:5 N H2 SO4 solution) and triplicate experiments were performed by stirring at 200 rpm. As shown, at 30 ◦ C, 50 ◦ C and 70 ◦ C, extraction rates were slow, and they started to gradually increase from 30 to 40 min of reaction time. In contrast at 100 ◦ C, a significant amount of magnesium (ca. 1500 mg/L) was extracted within just a 10-min reaction and afterward, a pseudo steadystate resulted, meaning that the magnesium extraction had taken place very quickly at the initial stage. When calculating reaction rates based on the amount of magnesium extracted in 10 min, the apparent reaction constants under temperatures of 30 ◦ C,
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Fig. 4. PLM images of chrysotile before and after low-temperature thermochemical treatments: untreated raw chrysotile (left), treated raw chrysotile (middle), 5 N H2 SO4 treated ACS.
Fig. 5. Concentration of extracted Mg depending on temperature variations (left), and comparisons of extracted Mg and Si concentrations at 100 ◦ C (right), Error bars mean the 95% confidence level; (reaction dosage: 1 g chrysotile in 50 mL of 5 N H2 SO4 ).
50 ◦ C, 70 ◦ C and 100 ◦ C, have been determined to be 2.51 × 10−5 , 9.78 × 10−5 , 3.17 × 10−4 , and 2.02 × 10−3 mole/L-min, respectively, and the rates at 50 ◦ C, 70 ◦ C and 100 ◦ C increased by 3.9-fold, 12.6fold and 80.3-fold compared to the one at 30 ◦ C. In particular, the reaction constant at 100 ◦ C implies that theoretically it takes only 5.4 min for the magnesium to be completely extracted from the 1 g of chrysotile used in the test, and approximately 75% of the entire magnesium was released in 10 min. The temperature dependency can be expressed by Arrhenius’s law (Eq. (1)).
k = Ae−Ea /RT
(1)
where k is the reaction rate, A is the frequency factor or attempt frequency factor, Ea is the activation energy (kJ/mol), T is the absolute temperature (K), and R is the gas constant (8.314 J/kmol-K). By plotting the apparent rate constants determined for the experiments, the activation energy and the frequency factor have been obtained (Fig. 6) and compared with the results in previous studies (Table 3). Our results seem to agree with Tier et al. [23] and Jonckbloedt [25]. In addition, the activation energy and frequency factor of the apparent rate constants based on the silica concentrations released within 10 minutes were calculated to be 107.82 kJ/mol and 1.25 × 1014 s−1 , respectively, and this activation energy value means that under these experimental conditions it is 1.77 times more difficult to break down the Si O Si structure than the brucite layers, Mg(OH)2 . Therefore, the destruction of chrysotile takes place favorably at the brucite layers, and the breakdown of silicon-oxide tetrahedron is a rate-limiting process of the overall reaction. The later disappearance of a (0 0 2) peak rather than a (0 0 4) peak supports this statement.
3.4. Treatability by waste sulfuric acid From a practical perspective, the reuse of waste sulfuric acid as an alternative chemical reagent will reduce industrial waste as well as lower the operating cost, resulting in an increased feasibility of this technique (low-temperature thermochemical treatment) to treat ACWs. The concentration of waste sulfuric acid was found to be ∼30 N using the acid-base titration method (NaOH ranged over 0.5–5 N), and the concentrated waste sulfuric acid was diluted with deionized water to prepare it to a normality of 5 N. Heasman and Baldwin [9] reported that the presence of extraneous material in the waste sulfuric acid hampered detailed examination of asbestos samples, and also showed negative impact on the morphology of the materials remaining after treatment, i.e., still fibrous form even
Fig. 6. An Arrhenius plot showing the effect of temperature on the extraction rate of Mg from chrysotile.
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Table 3 Arrhenius parameters. Study
Activation energy (E, kJ/mol)
Frequency factor (A, s−1 )
Experimental conditions
Ref.
This study Tier et al. (2007) Jonckbloedt (1998) Fouda et al. (1996)
58.588 68.092 ± 7.32 66.5 ± 2 35.6
2.983 × 105 8.89 × 106 ± 14.9 – –
5 N H2 SO4 , T = 50–100 ◦ C with as-received chrysotile 4 N H2 SO4 , T = 30–70 ◦ C with ground serpentine 6 N H2 SO4 , T = 60–90 ◦ C with ground olivine 3 N H2 SO4 , T = 30–75 ◦ C with serpentine ore roasted at 800 ◦ C for 2–3 h
[23] [25] [26]
when magnesium leaching was more than 90%. Meanwhile, authors also postulated an increase of the leaching potential when using three different industrial waste acids including sulfuric acid, probably owing to impurities such as metal cations, even though they did not present the levels and types of residual contaminants. Thus, in this study, residual impurity concentrations in waste sulfuric acid have been measured by ICP-OES in order to check out their possible influences on removals. From the results shown in Table 4, the Sn (tin) concentration was the highest (38.3 mg/L) followed by the Fe (iron), Se (selenium) and Cu (copper) concentrations, and the other impurities were not detected or were present only in small quantities. Fig. 7 compares the XRD results for thermochemically treated ACS using 5 N waste sulfuric acid, and 5 N commercial sulfuric acid incorporated with 100 ◦ C-low temperature heating for 24 h. The peak chrysotile asbestos was not detected in either the sample treated with waste sulfuric acid or the one treated with commercial sulfuric acid, and the two XRD patterns were overall very much similar to each other even though the peak intensities of the former seem to be less than those of the latter, showing the dominant presence of calcium sulfate (CaSO4 ) in the final treated product, while no other distinguishing peaks were found. This implies the residual impurities in waste sulfuric acid did not effectively affect (either increase or hinder) the strength of the acid. Thus, the reuse of waste sulfuric acid is a good way to reduce industrial wastes via recycling. Additionally, since CaSO4 , a treated by-product, has a wide variety of applications in many industries, it can be open to recycling after sufficient assessment about potential risks. However, in cases that waste acids contain high amounts of impurity depending on its discharged industries or processes, also they may be reused after impurities in acids are thoroughly identified and treated in terms of their toxic characteristics or reactivity with asbestos. Table 4 Concentration of metal species in waste sulfuric acid. Element
Wavelength (nm)
Concentration (mg/L)
Ag As Ba Be Ca Cd Co Cr Cu Fe Hg Mg Mn Ni Pb Sb Se Si Sn Ti V Zn
338.289 193.695 455.403 313.107 317.933 228.802 238.892 267.716 223.008 259.940 194.163 280.270 254.610 216.556 261.418 206.833 196.090 252.412 189.989 323.904 311.838 213.856
ND 0.80 ND ND ND ND 0.43 ND 1.36 5.66 ND ND ND ND ND ND 3.01 ND 38.3 ND ND 0.262
ND: not detected.
Fig. 7. Comparisons of XRD patterns for thermochemically treated ACS using 5 N waste sulfuric acid, and 5 N commercial sulfuric acid with 100 ◦ C-low temperature heating for 24 h, C: chrysotile (Mg3 Si2 O5 (OH)4 , S: calcium sulfate (CaSO4 ), L: calcite (CaCO3 ).
4. Conclusions This study has demonstrated that chrysotile asbestos existing in both a pure form and a mixed form with cement components can be destroyed by chemical treatment incorporated with a lowtemperature heating process (i.e., thermochemical treatment). By applying a 10–24-h thermochemical treatment using 5 N H2 SO4 heated at 100 ◦ C, raw chrysotile fibers lost their chemical compositions and asbestiform structure. Determination of an activation energy using a rate constant indicates that chemical destruction occurs favorably in the brucite layer through magnesium extraction rather than in the silicon oxide layer. The results also show that waste sulfuric acid used had almost same strength as the commercially produced, and can be used as a replacement for the other without any noticeable hindering in removal efficiency. However, it should be taken into consideration that treatability may be affected by high amounts of impurities in waste acids. The findings of the study have implications in providing experimental evidence that the thermochemical destruction of asbestos is a viable technique which makes it possible for ACWs to be disposed in more sustainable ways considering the treatability and recyclability of wastes, and the results of the study would give insight into the waste management guidelines/regulations related to ACWs in South Korea as well as in many other countries.
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Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Thermochemical destruction of asbestos-containing roofing slate and the feasibility of using recycled waste sulfuric acid Seong-Nam Nam a,∗ , Seongkyeong Jeong b , Hojoo Lim c a
Engineering Research Institute, Seoul National University, Daehak-dong, Gwanak-gu 151-744, South Korea Environmental Resource Recirculation Division, National Institute of Environmental Research, Environmental Research Complex, Kyeongseo-dong, Seo-gu, Incheon 404-708, South Korea c Indoor Environment and Noise Division, National Institute of Environmental Research, Environmental Research Complex, Kyeongseo-dong, Seo-gu, Incheon 404-708, South Korea b
h i g h l i g h t s • • • • •
Asbestos-containing roofing slates (ACS) were thermochemically treated. 5 N H2 SO4 with 100 ◦ C heating for 10–24 h showed complete disappearance. Asbestiform of ACS was changed to non-asbestiform after treatment. Favorable destruction was occurred at the Mg(OH)2 layer rather than SiO2 sheet. Equivalent treatability of waste acid brightened the feasibility of this approach.
a r t i c l e
i n f o
Article history: Received 30 May 2013 Received in revised form 22 October 2013 Accepted 5 November 2013 Available online 13 November 2013 Keywords: Asbestos containing slate Chrysotile Thermochemical destruction X-ray diffraction Polarized light microscopy Scanning electron microscopy
a b s t r a c t In this study, we have investigated the feasibility of using a thermochemical technique on ∼17% chrysotile-containing roofing sheet or slate (ACS), in which 5 N sulfuric acid-digestive destruction was incorporated with 10–24-h heating at 100 ◦ C. The X-ray diffraction (XRD) and the polarized light microscopy (PLM) results have clearly shown that raw chrysotile asbestos was converted to nonasbestiform material with no crystallinity by the low temperature thermochemical treatment. As an alternative to the use of pricey sulfuric acid, waste sulfuric acid discharged from a semiconductor manufacturing process was reused for the asbestos-fracturing purpose, and it was found that similar removals could be obtained under the same experimental conditions, promising the practical applicability of thermochemical treatment of ACWs. A thermodynamic understanding based on the extraction rates of magnesium and silica from a chrysotile structure has revealed that the destruction of chrysotile by acid-digestion is greatly influenced by the reaction temperatures, showing a 80.3-fold increase in the reaction rate by raising the temperature by 30–100 ◦ C. The overall destruction is dependent upon the breaking-up of the silicon-oxide layer – a rate-limiting step. This study is meaningful in showing that the low temperature thermochemical treatment is feasible as an ACW-treatment method. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Asbestos and asbestos-containing materials have been widely used in many applications, such as insulators, asbestos cement and fireproof construction materials, because of their low thermal conductivity and high mechanical strength. However, asbestos is known to be extremely carcinogenic, especially in causing a severe asbestosis, lung cancer and pleural mesothelioma when the respiratory system is exposed to it. As a result, nowadays, in most
∗ Corresponding author. Tel.: +82 2 880 4321; fax: +82 2 877 7376. E-mail address: [email protected] (S.-N. Nam). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.11.004
countries the mining, refinement and use of asbestos have been banned, apart from some exceptional applications. From a toxicological point of view on asbestos, although there have been many studies by toxicologists and clinical research scientists to elucidate the clear mechanisms that cause severe toxicities, little is known about the crucial processes at the cellular/molecular levels, and still such studies leave unanswered which chemical or physical properties of asbestos are key factors in disease causation. Chemical reasons are explained with mineralogical compositions of asbestos. For instance, magnesium from chrysotile or iron from iron-containing asbestos (e.g., amosite or crocidolite) may be leached intracellularly, thereby inducing toxicity of the fibers [1,2], or causing cytotoxicity through generating a highly reactive species (e.g., a hydroxyl radical or reactive oxygen) [3].
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Pathogenic mechanisms by the physical properties of asbestos are related to the size and length of the fibers, morphology (i.e., asbestiform or non-asbestiform), etc. [4]. Thus, if any treatment technique can achieve the “detoxification” of asbestos through both chemical (i.e., decomposition) and physical destruction, it should be the target goal of treatment, regardless of which property would play an important role in inducing toxicity to life. In South Korea, the most common use of asbestos occurred in the 1960–70s by the government-led project “New Village Movement”. As a part of the project, straw roofs in rural areas had been replaced with asbestos-cement roofing slates, and most of them are now obsolete and are being removing according to the Waste Asbestos Management Plan (2012–2021). According to the Korea Waste Statistics 2009, it was reported that 16 million tonnes of ACWs would be cumulatively generated by 2016. Currently, the Waste Management Law of South Korea classifies ACWs containing greater than 1 wt.% of asbestos as “Specific Hazardous Waste”, which must be disposed of only by controlled landfill in accordance with safety regulations. Regarding interim treatments before landfill, few thermal or mechanochemical techniques such as hightemperature incineration, or solidification with cement are legally permitted [5]. However, the confining and displacement of ACWs in landfill do not essentially destroy asbestos, and yet provide a load to landfill sites for several decades or for a permanent period, especially in a small country like South Korea. Thermal treatment involves the conversion of asbestos into non-asbestiform materials by melting the compositional elements at temperatures ranging from ca. 800 to 1400 ◦ C or above [6,7]. Similarly, microwave heating applies high power to inert asbestos materials [8]. Despite the efforts to lower operating costs, thermal treatments or microwave application are still energy-intensive and cost-demanding. Therefore, new or different treatment techniques need to be considered for significant amounts of ACWs. One approach, which was studied, is a chemical method using caustic acids [9,10] or alkalis to transform the crystalline structures to noncrystalline forms or to decompose the constituents of asbestos. In fact, such chemical treatment is not a discovery, but a novel technology which has been applied a great deal in various media [10–16]. Of the acidic agents, sulfuric acid is known to be the most effective attacking chemical and this was also experimentally observed in our previous tests [17]. Therefore, in this research, sulfuric acid (H2 SO4 )-based chemical dissolution was used and in order to enhance or complete the breakage of chemical bonds thermal process (i.e., heating) was incorporated to the process of chemical digestion. Furthermore, sulfuric acid, one of the most widely applied chemicals in many industrial processes, accounts for about 20% (89,000 tonnes discharged in 2009) of the entire waste acids discharged annually in South Korea, so the reuse or recycling of waste sulfuric acid is strongly desirable [9]. Therefore, the main objective of this research is to evaluate the treatability of asbestos-containing roofing slate waste using lowtemperature thermochemical treatment and to demonstrate the feasibility of reusing waste sulfuric acid as a replacement for a commercial acid. Concentration of acid and heating temperature applied in this study were chosen mainly for practical reasons such as estimated costs per tonne of ACWs, comparable to other treatment methods [18]. The authors expect that the results shown in this research would give an insight to future guidelines with regard to the disposal of asbestos-containing wastes. 2. Experimental methods 2.1. Asbestos materials Asbestos-containing (roofing) slate wastes (hereafter, ACS) were taken from a government-registered hazardous waste
treating company. Waste materials were shipped polyethylene double-packed to a HEPA filtration equipped laboratory, and were briefly rinsed with tap water in order to remove the attached dirt such as soil and moss. After drying them at room temperature, the slate samples were ground with a blade mill for 3 min and sieved to get a particle size below 0.5 mm. The presence of chrysotile asbestos was identified and confirmed by XRD, SEM and PLM analytical techniques. The raw fiber materials of chrysotile asbestos obtained from a mine located in Jecheon city, South Korea were used without any further purification process. 2.2. Thermochemical treatment Based on preliminary tests, the sulfuric acid was determined to be effective at concentrations above 5 N [17]. Thermochemical experiments were performed as follows: 5 g of the ACS sample was placed in 10 mL of 5 N H2 SO4 in a porcelain crucible with a small lip (i.e., solid/liquid = 500 g/L) and heated at 100 ◦ C for 10 and 24 h. During the treatment, a porcelain lid was covered to prevent gas emissions, and the gases were gently emitted through a lip and were trapped in a potable air cleaning system. The treated residual product was 0.45 m pore-sized filtered in a vacuum filtration unit while rinsing with deionized water ( ≥ 18.2) until the pH of the filtrate was neutralized, and the filtrate residues were dried overnight at 100 ◦ C. The dried samples were then analyzed in powder form for further characterization (XRD, PLM, SEM/EDS). The waste sulfuric acid was obtained from an electronic company in South Korea, and the concentration of the acid was determined by the acid-base titration method using NaOH. Detailed descriptions on the waste sulfuric acid were included in Section 3.4. 2.3. Analytical techniques Regarding solid materials, the chemical compositions of the untreated chrysotile and slate powder below 500 m were measured with X-ray fluorescence (XRF-1700, Shimazdu Co. Ltd, Japan). In order to examine the changes in the crystalline phases of asbestos, the powdered samples were analyzed by X-ray diffractometer (XRD: RIGAKU Ultima III, Japan) with CuK␣ radiation ( = 0.1584 nm) generated at 40 kV and 25 mA. The XRD spectrum was obtained in the 5–75 degree range of 2 and the obtained diffraction patterns were compared with the JCPDS archives in the PDXL software program. Polarized light microscopy (PLM: DP72, JP/BX51 Olympus, Japan) using dispersion staining, extinction, etc., was also used in order to confirm if chrysotile remained after acidic digestion, or if asbestiform remained or was not detectable. The microstructures and/or compositional changes of the treated asbestos waste were observed using scanning electron microscopy with an electron dispersion spectrum (SEM/EDS: Shimadzu Co., Ltd., Japan). In addition, the concentration of magnesium, silicon, calcium, and iron extracted into solution were measured using inductively coupled plasma-optical emission spectrometry (ICPOES, Horiba Jobin Yvon Ultima, Japan). 3. Results and discussion 3.1. Characterization of untreated asbestos samples Table 1 shows the chemical compositions of natural chrysotile asbestos and an ACS sample measured by X-ray fluorescence. Raw chrysotile is mostly composed of about 42 wt.% MgO and 36 wt.% SiO2 together with 4 wt.% Fe2 O3 and 1 wt.% Al2 O3 which may be contained due to replacements of Mg and Si, respectively as reported in the literature [19]. Since ACS materials are mixtures of asbestos with cement, their chemical matrices are similar to
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Fig. 1. SEM-EDS images of raw chrysotile (upper) and ACS (lower); a cross mark (+) indicates the point that the EDS was taken.
ordinary Portland cement (OPC), showing about 40 wt.% CaO and 19 wt.% SiO2 (59 wt.% and 23 wt.% for OPC, respectively). The SEM images of raw chrysotile and ACS samples (Fig. 1) show the chrysotile asbestos bundles with a long and serpentine-like fibrous morphology. The EDS spectrum of raw chrysotile showed the composition to be MgO 37.9%, SiO2 57.3%, FeO 3.38% and Al2 O3 1.35% which generally agreed with the reported values [19,20]. Chrysotile fibers in ACS were present and were entangled with co-existing materials in slates such as cement constituents, most probably CaO and SiO2 [21]. The presence of chrysotile asbestos in ACS was also confirmed by EDS, revealing that the major elements of the fiber were MgO (40.6%), SiO2 (59.4%) and Al2 O3 (∼0.8%). CaO was also detected at ∼5%, and it is thought to be mostly present in the fine particles attached onto the chrysotile fiber since the surrounding white-colored amorphous materials mostly consisted of Ca and O. Table 2 summarizes the X-ray diffraction angles and lattice spaces of the three major peak positions (0 0 2, 0 0 4 and 0 2 0 in order of intensity) obtained from the XRD spectra of untreated raw chrysotile and ACS samples (shown in Figs. 2 and 3,
Fig. 2. XRD patterns for untreated raw and treated chrysotile asbestos (10- and 24-h heating at 100 ◦ C in 5 N H2 SO4 ): C: chrysotile (Mg3 Si2 O5 (OH)4 .
Table 1 Chemical compositions (wt.%) of raw chrysotile and untreated ACS material by XRF. Material
SiO2
Al2 O3
TiO2
Fe2 O3
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
LOI
Raw chrysotile Raw ACS SY OPCa NIST cementb
36.01 19.43 23.25 20.66
1.09 3.23 4.12 3.89
0.04 0.12 0.14 0.227
3.99 1.91 2.12 1.937
0.07 0.06 0.09 0.2588
42.20 5.63 2.89 0.814
0.36 39.62 58.83 65.34
0.00 0.00 0.18 0.195
0.00 0.31 1.14 0.605
0.00 0.08 0.17 0.110
15.65 28.82 6.99 3.28
a b
SY OPC: an ordinary Portland cement that is being producing by the SY cement company (Korea). NIST cement: product number #1889a, NIST-Standard Reference Material of Portland cement (blended with limestone).
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Table 2 Comparisons of X-ray diffraction angles and lattice spaces of raw chrysotile and ACS material with reference asbestos (i.e., clinochrysotile-2Mc1). hkl (0 0 2)
(0 2 0)
(0 0 4)
Clinochrysotile-2Mc1 (00–052–1562) ˚ d (A) 7.27 12.10 2 (◦ )
4.55 19.62
3.65 24.37
Raw chrysotile this study ˚ d (A) 7.29 12.12 2 (◦ )
4.52 19.6
3.65 24.39
4.52 19.62
3.65 24.39
Raw ACS this study ˚ d (A) 2 (◦ )
7.28 12.15
respectively), and it was found that based on the major three characteristic peaks, the type of chrysotile asbestos was mostly well matched to the clinochrysotile-2Mc1 (PDF No.: 00-052-1562) data in the ICDD database. 3.2. Thermochemical decomposition of chrysotile asbestos It is believed that the chemical digestion of chrysotile by sulfuric acid takes place through diffusion of hydrogen ions released from sulfuric acid, and the subsequent attacking of the reactive site or the surface of the chrysotile [22,23]. As resultants of the reaction, magnesium or silica is extracted from the asbestos structure [23,24]. Based on the analytical results of the final products, the chemical reactions at room temperature can be understood as follows (R1)–(R4). Chrysotile will react with water, resulting in a raised pH due to the released hydroxide ions (OH− ) (R1). However, the reaction is not important since the increase in the pH and the extracted concentration of magnesium ions were negligible for the 5-h experiment duration. Hence, reaction (R2) is likely to be dominant in chrysotile destruction through sulfuric acid digestion. Mg3 Si2 O5 (OH)4 (s) + 5H2 O → 3Mg2+ + 6OH− + 2H4 SiO4 (aq) (R1) Mg3 Si2 O5 (OH)4 (s) + 6H+ (i.e., 3H2 SO4 ) → 3Mg2+ + (3SO4 2− ) + 2SiO2 (s) + 5H2 O (or 2H4 SiO4 (aq) + H2 O) (R2)
On the other hand, regarding the ACS sample, chemical digestions will take place in two ways; one is a reaction with chrysotile and the other is a reaction with the constituents of ACS. However, a significant portion of the sulfuric acid will be used to react with calcite (CaCO3 ) since it exists as a major constituent of the ACS, resulting in gypsum or calcium sulfate (CaSO4 ) as the final product (R4). Although the reaction of calcite with water raises the pH of a solution by releasing hydroxide ions (as described in (R3)), it is also negligible when the solution is a strong acid like sulfuric acid. Thus, (R2) competes with (R4) and unknown reactions between the sulfuric acid and the other constituents of the ACS. CaCO3 + H2 O → Ca2+ + 2OH− + CO2 (g)
(R3)
CaCO3 (s) + H2 SO4 → CaSO4 (s) (or Ca2+ + SO4 2− ) + H2 O + CO2 (g)
(R4)
Therefore, the chemical destruction of asbestos in the ACS takes place competitively by (R2) and (R4), and can be accelerated by heating (i.e., thermochemical destruction). Figs. 2 and 3 show the XRD patterns of the samples before and after thermochemical treatment with 100 ◦ C heating in 5 N sulfuric acid for 10 and 24 h. For the untreated chrysotile asbestos (Fig. 2), characteristic 2-theta peaks corresponding to 0 0 2, 0 2 0 and 0 0 4 positions can be distinctively detected. However, after the thermochemical treatment, a broad and hump-like peak was observed without showing any characteristic diffraction angles, and this implies that the product does not have the original crystallinity of raw chrysotile. For the ACS sample (Fig. 3), a characteristic peak of calcite (CaCO3 ) was observed with crystalline clinochrysotile2Mcl in the untreated sample. After treatment, the peak patterns showed crystalline calcium sulfate (CaSO4 ) that was formed as a final product of the reaction between sulfuric acid and the ACS, as seen in R4. With an increased treatment time up to 24 h, reduced peak strengths of calcium sulfate have been observed. PLM images presented in Fig. 4 were obtained by the dispersion staining method, in which 1–1.5 drops of a liquid with nD (refractive index) = 1.550 at 25 ◦ C were added to a small amount of sample on a slide glass. The chrysotile fiber was observed as having a blue color in a 2–8 o’clock position, as shown in Fig. 4a, while the treated or the transformed products revealed their non-asbestiform structure with a pale blue color, which means that their structure and compositions must have been changed. As seen in Fig. 4b and c, fibrous chrysotile asbestos was not observed and its morphology has been transformed to a fine particulate form. 3.3. Temperature effects
Fig. 3. XRD patterns for untreated raw chrysotile, and treated chrysotile asbestos (10- and 24-h heating at 100 ◦ C in 5 N H2 SO4 ), C: chrysotile (Mg3 Si2 O5 (OH)4 , S: calcium sulfate (CaSO4 ), L: calcite (CaCO3 ).
In order to understand the temperature effects on chrysotile destruction, concentrations of extracted magnesium were compared at 30 ◦ C, 50 ◦ C, 70 ◦ C and 100 ◦ C (Fig. 5) as the reaction time increased. Due to a limited amount of available raw chrysotile material, the reaction dosage was set to 1 g:50 mL (i.e., 500 g/L in chrysotile:5 N H2 SO4 solution) and triplicate experiments were performed by stirring at 200 rpm. As shown, at 30 ◦ C, 50 ◦ C and 70 ◦ C, extraction rates were slow, and they started to gradually increase from 30 to 40 min of reaction time. In contrast at 100 ◦ C, a significant amount of magnesium (ca. 1500 mg/L) was extracted within just a 10-min reaction and afterward, a pseudo steadystate resulted, meaning that the magnesium extraction had taken place very quickly at the initial stage. When calculating reaction rates based on the amount of magnesium extracted in 10 min, the apparent reaction constants under temperatures of 30 ◦ C,
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Fig. 4. PLM images of chrysotile before and after low-temperature thermochemical treatments: untreated raw chrysotile (left), treated raw chrysotile (middle), 5 N H2 SO4 treated ACS.
Fig. 5. Concentration of extracted Mg depending on temperature variations (left), and comparisons of extracted Mg and Si concentrations at 100 ◦ C (right), Error bars mean the 95% confidence level; (reaction dosage: 1 g chrysotile in 50 mL of 5 N H2 SO4 ).
50 ◦ C, 70 ◦ C and 100 ◦ C, have been determined to be 2.51 × 10−5 , 9.78 × 10−5 , 3.17 × 10−4 , and 2.02 × 10−3 mole/L-min, respectively, and the rates at 50 ◦ C, 70 ◦ C and 100 ◦ C increased by 3.9-fold, 12.6fold and 80.3-fold compared to the one at 30 ◦ C. In particular, the reaction constant at 100 ◦ C implies that theoretically it takes only 5.4 min for the magnesium to be completely extracted from the 1 g of chrysotile used in the test, and approximately 75% of the entire magnesium was released in 10 min. The temperature dependency can be expressed by Arrhenius’s law (Eq. (1)).
k = Ae−Ea /RT
(1)
where k is the reaction rate, A is the frequency factor or attempt frequency factor, Ea is the activation energy (kJ/mol), T is the absolute temperature (K), and R is the gas constant (8.314 J/kmol-K). By plotting the apparent rate constants determined for the experiments, the activation energy and the frequency factor have been obtained (Fig. 6) and compared with the results in previous studies (Table 3). Our results seem to agree with Tier et al. [23] and Jonckbloedt [25]. In addition, the activation energy and frequency factor of the apparent rate constants based on the silica concentrations released within 10 minutes were calculated to be 107.82 kJ/mol and 1.25 × 1014 s−1 , respectively, and this activation energy value means that under these experimental conditions it is 1.77 times more difficult to break down the Si O Si structure than the brucite layers, Mg(OH)2 . Therefore, the destruction of chrysotile takes place favorably at the brucite layers, and the breakdown of silicon-oxide tetrahedron is a rate-limiting process of the overall reaction. The later disappearance of a (0 0 2) peak rather than a (0 0 4) peak supports this statement.
3.4. Treatability by waste sulfuric acid From a practical perspective, the reuse of waste sulfuric acid as an alternative chemical reagent will reduce industrial waste as well as lower the operating cost, resulting in an increased feasibility of this technique (low-temperature thermochemical treatment) to treat ACWs. The concentration of waste sulfuric acid was found to be ∼30 N using the acid-base titration method (NaOH ranged over 0.5–5 N), and the concentrated waste sulfuric acid was diluted with deionized water to prepare it to a normality of 5 N. Heasman and Baldwin [9] reported that the presence of extraneous material in the waste sulfuric acid hampered detailed examination of asbestos samples, and also showed negative impact on the morphology of the materials remaining after treatment, i.e., still fibrous form even
Fig. 6. An Arrhenius plot showing the effect of temperature on the extraction rate of Mg from chrysotile.
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Table 3 Arrhenius parameters. Study
Activation energy (E, kJ/mol)
Frequency factor (A, s−1 )
Experimental conditions
Ref.
This study Tier et al. (2007) Jonckbloedt (1998) Fouda et al. (1996)
58.588 68.092 ± 7.32 66.5 ± 2 35.6
2.983 × 105 8.89 × 106 ± 14.9 – –
5 N H2 SO4 , T = 50–100 ◦ C with as-received chrysotile 4 N H2 SO4 , T = 30–70 ◦ C with ground serpentine 6 N H2 SO4 , T = 60–90 ◦ C with ground olivine 3 N H2 SO4 , T = 30–75 ◦ C with serpentine ore roasted at 800 ◦ C for 2–3 h
[23] [25] [26]
when magnesium leaching was more than 90%. Meanwhile, authors also postulated an increase of the leaching potential when using three different industrial waste acids including sulfuric acid, probably owing to impurities such as metal cations, even though they did not present the levels and types of residual contaminants. Thus, in this study, residual impurity concentrations in waste sulfuric acid have been measured by ICP-OES in order to check out their possible influences on removals. From the results shown in Table 4, the Sn (tin) concentration was the highest (38.3 mg/L) followed by the Fe (iron), Se (selenium) and Cu (copper) concentrations, and the other impurities were not detected or were present only in small quantities. Fig. 7 compares the XRD results for thermochemically treated ACS using 5 N waste sulfuric acid, and 5 N commercial sulfuric acid incorporated with 100 ◦ C-low temperature heating for 24 h. The peak chrysotile asbestos was not detected in either the sample treated with waste sulfuric acid or the one treated with commercial sulfuric acid, and the two XRD patterns were overall very much similar to each other even though the peak intensities of the former seem to be less than those of the latter, showing the dominant presence of calcium sulfate (CaSO4 ) in the final treated product, while no other distinguishing peaks were found. This implies the residual impurities in waste sulfuric acid did not effectively affect (either increase or hinder) the strength of the acid. Thus, the reuse of waste sulfuric acid is a good way to reduce industrial wastes via recycling. Additionally, since CaSO4 , a treated by-product, has a wide variety of applications in many industries, it can be open to recycling after sufficient assessment about potential risks. However, in cases that waste acids contain high amounts of impurity depending on its discharged industries or processes, also they may be reused after impurities in acids are thoroughly identified and treated in terms of their toxic characteristics or reactivity with asbestos. Table 4 Concentration of metal species in waste sulfuric acid. Element
Wavelength (nm)
Concentration (mg/L)
Ag As Ba Be Ca Cd Co Cr Cu Fe Hg Mg Mn Ni Pb Sb Se Si Sn Ti V Zn
338.289 193.695 455.403 313.107 317.933 228.802 238.892 267.716 223.008 259.940 194.163 280.270 254.610 216.556 261.418 206.833 196.090 252.412 189.989 323.904 311.838 213.856
ND 0.80 ND ND ND ND 0.43 ND 1.36 5.66 ND ND ND ND ND ND 3.01 ND 38.3 ND ND 0.262
ND: not detected.
Fig. 7. Comparisons of XRD patterns for thermochemically treated ACS using 5 N waste sulfuric acid, and 5 N commercial sulfuric acid with 100 ◦ C-low temperature heating for 24 h, C: chrysotile (Mg3 Si2 O5 (OH)4 , S: calcium sulfate (CaSO4 ), L: calcite (CaCO3 ).
4. Conclusions This study has demonstrated that chrysotile asbestos existing in both a pure form and a mixed form with cement components can be destroyed by chemical treatment incorporated with a lowtemperature heating process (i.e., thermochemical treatment). By applying a 10–24-h thermochemical treatment using 5 N H2 SO4 heated at 100 ◦ C, raw chrysotile fibers lost their chemical compositions and asbestiform structure. Determination of an activation energy using a rate constant indicates that chemical destruction occurs favorably in the brucite layer through magnesium extraction rather than in the silicon oxide layer. The results also show that waste sulfuric acid used had almost same strength as the commercially produced, and can be used as a replacement for the other without any noticeable hindering in removal efficiency. However, it should be taken into consideration that treatability may be affected by high amounts of impurities in waste acids. The findings of the study have implications in providing experimental evidence that the thermochemical destruction of asbestos is a viable technique which makes it possible for ACWs to be disposed in more sustainable ways considering the treatability and recyclability of wastes, and the results of the study would give insight into the waste management guidelines/regulations related to ACWs in South Korea as well as in many other countries.
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