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New model for the sulfation of marble by dry deposition Sheltered marble—the indicator of air pollution by sulfur dioxide
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Atmospheric Environment 39 (2005) 913–920 www.elsevier.com/locate/atmosenv
New model for the sulfation of marble by dry deposition Sheltered marble—the indicator of air pollution by sulfur dioxide Tran Thi Ngoc Lana,, Nguyen Thi Phuong Thoaa, R. Nishimurab, Y. Tsujinoc, M. Yokoid, Y. Maedab a
Department of Physical Chemistry, College of Natural Sciences, Vietnam National University, 227 Nguyen Van Cu, HoChiMinh, Vietnam b Department of Material Science, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai 599-8531, Osaka, Japan c Osaka Prefectural Environmental Pollution Control Center, 1-3-62 Nakamichi, Higashinari-ku, Osaka 537-0025, Japan d Technology Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi, Osaka 594-1157, Japan Received 8 January 2004; received in revised form 7 September 2004; accepted 27 September 2004
Abstract This paper was concerned with evaluating the effect of dry deposition on deterioration of marble. Two types of marble were exposed to atmospheric environment with a rain shelter at four exposure sites in the south of Vietnam for 3-month, 1-year and 2-year periods from July 2001 to September 2003. X-ray diffraction (XRD) and X-ray fluorescent (XRF) methods were applied to study the products of marble deterioration. Ion chromatography was used to analyze dry depositions on marble. The main product of marble deterioration was gypsum (CaSO4 2H2O). The amount of sulfate ions deposited on marble was found to be proportional to SO2 concentration in the air, relative humidity and duration of the exposure. In addition, sulfation of marble caused by SO2 at a relative humidity lower than 70% is almost half of that at relative humidity higher than 70%. Moreover, marble consisting of calcite (CaCO3) was more sensitive to SO2 than marble consisting of dolomite (CaCO3 and MgCO3). A good relation between the amount of sulfate ions deposited on marble and SO2 concentration in the air suggested that marble could serve as an indicator for atmospheric pollution by SO2. r 2004 Elsevier Ltd. All rights reserved. Keywords: Acidification; Passive sampler; Sulfation; Dolomite; Calcite
1. Introduction Corresponding author. Department of Material Science,
Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuencho, Sakai 599-8531, Osaka, Japan. Tel.: +84 8 839 7720. E-mail address: [email protected] (Tran Thi Ngoc Lan).
Air pollution problem caused by industrial development is the reality which the modern world is facing. A huge amount of sulfur dioxide and other pollutants like nitrogen oxides, carbon monoxide, etc. are discharged into the atmosphere. Consequently, the atmospheric acidification, with high SO2 concentrations in the air and
1352-2310/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.09.074
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acid rains, becomes more and more serious, especially in developing countries (Leygraf and Graedel, 2000; Maeda et al., 2001). Damage to architectural and cultural constructions caused by acidifying atmospheric pollution imposes huge socio-economic costs (Soon-Bok et al., 1996). Therefore, considerable attention has been paid to the effect of air pollutants on architectural material, and especially on the cultural assets made of carbonate stones. Many reports concerning deterioration of carbonate stones were published (Alonso and Martinez, 2003; Boke and Lal Gauri, 2003; Graedel, 2000; Franzini, 1996; Hadler et al., 1996; Johnson et al., 1996; Katsanos et al., 1999; Komeiji, 1992; Maeda et al., 2001; Metallo et al., 1995; Tecer, 1999). Most of them were laboratory-simulated experiments and were concerned with European countries where the climate is moderate and the temperature is low. Vietnam is a tropical country. Temperature and relative humidity are rather high. The behavior of the deterioration of carbonate stones was unknown. It was found that SO2 caused deterioration (corrosion) of carbonate stones by sulfation, i.e. the reaction of SO2 with carbonate calcium and carbonate calcium was consequently changing to more soluble gypsum (CaSO4 2H2O). However, the kinetics of deterioration and sulfation of marble was not well studied. The purpose of our research is to clarify the effect of air pollutants due to dry deposition on marble and the kinetics of its deterioration. Since products of marble corrosion tends to be lost due to the washing effect of rains (Leygraf and Graedel, 2000), to investigate the sulfation process, we focused on marble that was exposed under a rain shelter. Understanding the effects of environmental factors on marble enables us to predict the material damage and to propose protective measures.
2. Experiment 2.1. Climate conditions at four exposure sites in the south of Vietnam Four exposure sites at HoChiMinh (HCMC), Bien Hoa, Vung Tau and My Tho representing urbanindustrial, urban, urban-marine and rural climate, respectively, in the south of Vietnam were chosen. HoChiMinh is the biggest industrial center in Vietnam. The other sites are located about 30 to 140 km around HoChiMinh. The climatic parameters like temperature and relative humidity were obtained from the meteorological stations where the test specimens were set up. There are only two seasons in the south of Vietnam: dry (from the end of November to the end of May) and rainy (from the end of May to the end of November) seasons. The period from January to March had no rain at all and was termed as the absolute dry period.
Table 1 Climatic conditions at the four exposure sites: HoChiMinh (HCMC), My Tho, Vung Tau and Bien Hoa Period
Jul/01–Sep/01 Oct/01–Dec/01 Jan/02–Mar/02 Apr/02–Jun/02 Jul/02–Sep/02 Jul/01–Jun/02
Temperature (1C) HCMC
My Tho
Vung Tau
Bien Hoa
28.0 27.3 27.0 29.8 28.2 28.0
27.1 26.2 25.8 28.3 27.0 26.8
27.8 26.9 26.0 28.9 27.8 27.4
27.6 26.5 26.4 28.9 27.9 27.4
Fig. 1. Three-month and 1-year (solid line) average relative humidities at the four exposure sites.
The 3-month average temperatures at the four exposure sites in the period from July 2001 to September 2002 are summarized in Table 1. In the south of Vietnam, the temperature is relatively stable all the year round and from year to year. During the period from July 2001 to September 2003, the 1-year average temperature at the four exposure sites was in the range of 26.8–28.0 1C (Table 1), while the 3-month average was in the range of 25.8–29.8 1C. Fig. 1 represents the seasonal dependence of relative humidity at the exposure sites. In principle, the 3-month average relative humidity at all exposure sites was the lowest in the absolute dry period, and highest in the period from July to September when there was a lot of rain. The lowest relative humidity of 67.3% was observed against the highest one of 84.8%. However, the 1-year average relative humidities varied a little from one site to another and were in the range of 73.8%–80.3%. Moreover, the 1-year average relative humidities at each individual site in periods from July 2001 to June 2002 and from July 2002 to June 2003 were almost the same, with the difference between them being less than 2%. In addition, at any exposure period, the
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average relative humidity was the lowest at HoChiMinh. Especially, a very low relative humidity was observed at HoChiMinh and Bien Hoa during the absolute dry period. The big difference in relative humidity depending on the season allowed elucidating the effect of relative humidity on degradation of marble based on short-term exposure. 2.2. Sampling method of SO2 A passive sampling method was used for the determination of the gaseous SO2 concentration in the air (SO2 concentration) at the four exposure sites (Tran Thi Ngoc Lan et al., 2004). A SO2 concentration was estimated from an amount of SO2 collected by the passive samplers. The passive samplers were sampled once a month from July 2001 to September 2003. 2.3. Material and exposure methods Before exposure to the atmosphere, the test specimens with a size of 20 mm 20 mm 5 mm of Italian white marble consisting of CaCO3 were washed in Millipore water, dried in a desiccator for 24 h and weighed with a precision of 0.01 mg. The duplicated test specimens were exposed to the outdoor atmospheric environments at the four exposure sites under the rain shelter with an exposure angle of 901. The exposure was carried out for each 3-month (July 2001–September 2003), 1-year (July 2001–June 2003) and 2-year (July 2001–June 2003) period. The test specimens were denoted as the 3-month, the 1-year and the 2-year specimens, respectively, to the 3-month, 1-year and 2-year exposure. To compare the effects of air pollutants on different types of marble, test specimens of Italian white marble consisting of dolomite (CaMg(CO3)2) of the same size were exposed for 3-month, 1-year and 2-year periods together with the test specimens consisting of calcite. The test specimens are designated as calcite and dolomite specimens, representing the two types of marble.
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Normally, one immersion for 24 h is sufficient to achieve this. The washing filtrates of 20 ml were analyzed with 761 Metrohm ion chromatograph (IC) systems to measure the amounts of anions and cations deposited on the test specimens. However, the most interesting was the amount of SO2 ions on the test specimens that 4 correlated to the amount of gypsum. The amount of SO2 ions (MS—mg cm2) on the test specimens was 4 evaluated as follows: MS ¼ V C=S;
(1)
where V is the volume of washing filtrates (20 ml), C is the concentration of SO2 ions in particular washing 4 filtrates (mg ml1) and S is the surface area of the test specimens (12 cm2). 2.5. Regression analysis Regression analysis with a stepwise method was performed by using the computer software SPSS 10.0 to identify factors influencing the sulfation of marble. Regression analysis was conducted independently for the amount of SO2 4 ions on the test specimens at the four exposure sites after the 3-month, 1-year and 2-year exposure separately. The independent variables were the SO2 concentration (C SO2 ), the relative humidity (RH), and the time of exposure, temperature, and their combinations.
3. Results 3.1. SO2 concentration in the air at the exposure sites Fig. 2 shows the 3-month, 1-year (solid line) and 2year (dashed line) average SO2 concentrations at the four exposure sites. The 3-month and 1-year average SO2 concentrations increased in the order of My
2.4. Analysis of the test specimens The corrosion products formed on the test specimens were identified by X-ray diffraction method (XRD) using X-ray diffractometer (40 kV and 150 mA, Rigaku Co.) with Cu target. The scan was performed at a speed of 4.0001 min1, a scan step of 0.0201 and a scan range from 51 to 851. The Rigaku RIX 3000 fluorescence analyzer was used to determine the amount of sulfur on the test specimens (XRF method). After being analyzed by XRD and XRF methods, the test specimens were immersed and slightly shaken in 20 ml of the Millipore water for sufficient time to completely dissolve soluble corrosion products (e.g. gypsum, as described later).
Fig. 2. Average 3-month, 1-year (solid line) and 2-year (dashed line) SO2 concentrations at the four exposure sites.
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ThooVung TauoBien Hoa andoHoChiMinh with comparable ones at My Tho and Vung Tau. Nevertheless, the variation in the 3-month average SO2 concentrations at each exposure site was small in comparison with that among the sites (except between My Tho and Vung Tau). HoChiMinh was the most polluted site, since it is the biggest industrial and commercial center in Vietnam with many enterprises inside and outside the city. Bien Hoa is the urban site located about 30 km in north-east of HoChiMinh City and the atmospheric environment is affected by industrial parks outside the city. My Tho and Vung Tau have no heavy industries, hence they are clean areas. The 1year average SO2 concentrations during the period from July 2002 to June 2003 at HoChiMinh, My Tho and Vung Tau reduced slightly, but were almost the same as those of the first year of exposure. However, the 1-year average SO2 concentration at Bien Hoa during the second year of exposure increased by about 20% compared with that of the first year of exposure. The increasing SO2 concentrations at Bien Hoa was related to the fast expansion of industrial parks outside the city and moving industrial enterprises from HoChiMinh into industrial parks.
3.3. The amount of sulfate ions on the calcite specimens The washing extracts of the test specimens were 2 analyzed by IC. Na+, K+, Cl, NO ions 3 and SO4 were found on all test specimens, but not NO2 , SO2 3 and PO3 4 . However, the main anions deposited on + + marble were SO2 4 ions. Na , K , Cl and NO3 were in 2 small quantities compared with SO4 ions. Oxalate and NH+ 4 ions were also detected on many test specimens, but in negligible quantities in comparison with the other ions. This indicates that corrosion of marble was mainly caused by sulfation. The amount of SO2 ions eluted 4 from the test specimens was measured for all of the specimens and was estimated as an average value of the duplicated ones. Fig. 4 shows the amount of SO2 4 ions on the 3-month, 1-year (solid line, right axis) and 2-year (dashed line, right axis) calcite specimens at the four exposure sites. The site dependence of the amount of SO2 ions on the 3-month calcite specimens of the 4 second year of the exposure was similar to that of the first year of the exposure. The amount of SO2 ions 4 increased in the order of My ThooVung TauoBien HoaoHoChiMinh for all of the 3-month, 1-year and 2year exposures. In addition, the amount of SO2 4 ions on
3.2. Corrosion products on the test specimens Fig. 3 shows a representative result of the XRD analysis for the 3-month specimens at HoChiMinh, where the diffractions of gypsum (CaSO4 2H2O) at d ¼ 7:635; 4.287 and 2.880 A˚ were indicated by the symbols G and the other strong diffractions corresponding to those of calcite. At the four exposure sites, gypsum was found on all of the calcite and dolomite specimens. It was found that the corrosion products were only gypsum. All of the test specimens increased in weight after the exposure due to the transformation of CaCO3 into gypsum by the following reaction (Graedel, 2000): oxidation
SO2 g 3SO2 aq 3
3CaSO4 2H2 O:
SO2 4 aq 3CaSO4 wH2 O ð2Þ
Fig. 4. Amounts of SO2 ions on the 3-month, 1-year (solid 4 line) and 2-year (dashed line) calcite specimens at the four exposure sites.
Fig. 3. XRD pattern of the 3-month sheltered specimen at HoChiMinh.
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5000
XRD intensity (cps)
SO4-2(µg cm -2)
200 150 y = 3.7292x R2 = 0.9593
100 50 0 0
20
40
60
XRF Intensity (kcps)
80
917
d:7.622
4000 3000 2000
y = 69.407x R2 = 0.9115
1000 0
0
20
40
60
80
XRF Intensity (kcps)
( Fig. 5. Relationships among the amount of SO2 4 ions, intensity of the highest XRD diffraction of gypsum at d ¼ 7:622 A; and XRF intensity of sulfur on the test specimens at the four exposure sites.
the 3-month calcite specimens at individual exposure sites was the lowest in the absolute dry period lasting from January to March. An exception was the rural site of My Tho; nevertheless, the amount of SO2 4 ions on the 3-month calcite specimens at My Tho observed in the absolute dry period was almost the same as the lowest one at this site. From Figs. 2 and 4, it was found that the site dependence of the amount of SO2 4 ions on the 1-year and 2-year calcite specimens was associated with that of the 1-year and 2-year average SO2 concentration. In addition, for each identical 3-month exposure period, the variation in the amount of SO2 4 ions on the calcite specimens at the four exposure sites reflected the variations in the 3-month average SO2 concentrations. Furthermore, it was seen from Figs. 1 and 4 that at each exposure site, the change in the 3month average relative humidities corresponded to that in the amounts of SO2 ions on the 3-month calcite 4 specimens. Thus, the amount of SO2 ions on the 34 month calcite specimens was related to the 3-month average SO2 concentration, and the 3-month average relative humidity, while that on the 1-year calcite specimens seemed to relate only to the 1-year average SO2 concentration. From Fig. 4, at HoChiMinh, My Tho and Vung Tau, the amount of SO2 4 ions on the 1-year calcite specimens of the first and second year of exposure were almost the same, with the difference between them being less than 10%. However, at Bien Hoa, the amount of SO2 4 ions on the 1-year calcite specimens of the second year was about 20% higher than that of the first year of exposure. This should relate to the increase of SO2 concentration at Bien Hoa, as mentioned before. The amount of SO2 ions on the 2-year calcite 4 specimens at each exposure sites was almost twice that on the 1-year calcite ones. In addition, the amount of SO2 4 ions on the 1-year calcite specimens was comparable to the cumulative amount of SO2 4 ions on the 3month calcite specimens of the year. This indicates that the amount of SO2 4 ions deposited on calcite specimens correlated with the duration of exposure.
3.4. Relationship among the amounts of sulfate ions, XRF intensities of sulfur and XRD intensities of gypsum on the calcite specimens X-ray analysis was performed and the relationships among the amounts of SO2 4 ions, the intensities of the ( and highest XRD diffraction of gypsum at d ¼ 7:622 A the XRF intensities of sulfur on the calcite specimens are shown in Fig. 5. Linear functions fit well. Therefore, the XRF intensity was proportional to both the amount of SO2 ions and the XRD intensities of gypsum, which 4 suggests that the amount of SO2 4 ions was proportional to the XRD intensity of gypsum, that is, the amount of gypsum.
4. Discussions 4.1. Effects of environmental factors on the sulfation of marble In this section, we consider the sulfation of marble from the amount of SO2 4 ions, which was proportional to the amount of gypsum, as described in Sections 3.3 and 3.4. As pointed out in Section 3.3, the amount of SO2 ions of the calcite specimens is related to the 4 average SO2 concentration, the average relative humidity and the duration of the exposure period. From Figs. 2 and 4, for the 1-year and 2-year exposure period, the increase of the 1-year average SO2 concentrations resulted in that of the amounts of SO2 4 ions of the 1year and 2-year calcite specimens, respectively. This consideration can be applied to the 3-month exposure, from the behavior of the amounts of SO2 4 ions and the 3-month average SO2 concentrations at the four exposure sites for the same 3-month exposure period. This means that the amounts of SO2 ions were 4 proportional to the average SO2 concentrations. Moreover, it was found from the comparison of the amounts of SO2 4 ions on the 3-month calcite specimens (Fig. 4) and the 3-month average relative humidities at each
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Table 2 Amount of sulfate ions on calcite specimens exposed in absolute dry periods (January to March) and in the other periods with the comparable SO2 concentration at HoChiMinh and Bien Hoa HoChiMinh
SO2 4
2
(mg cm ) Amount of SO2 concentration (ppb) Relative humidity (%)
Bien Hoa
Jan/02–Mar/02
Jan/03–Mar/03
Other periods
Jan/02–Mar/02
Jan/03–Mar/03
Other periods
38.6 18.3 67.0
40.4 16.8 67.5
72–81 16.4–19.1 72.2–78.4
19.9 10.22 68.9
17.8 10.32 70.2
42–51 10.0–11.5 74.8–84.4
exposure site (Fig. 1), that the higher 3-month average relative humidity was correlated to the higher amounts of SO2 ions. Thus, the amount of SO2 ions was 4 4 proportional to the average relative humidity at the four exposure sites. Consequently, SO2 concentrations and relative humidity were the factors strongly affecting the corrosion of marble under the sheltered exposure conditions, although the correlation of each factor to the amounts of SO2 ions on the sheltered specimens 4 was more or less evident depending on the 3-month or 1year exposure periods. For the same exposure period (including 1-year and 3-month), the difference among average relative humidities at the four exposure sites was not large. Therefore, the correlation of the average relative humidities was not seen, although the average SO2 concentrations showed a clear correlation, as they were largely different among the four exposure sites. On the other hand, at each exposure site such as at HoChiMinh, the 3-month average SO2 concentration was rather stable, but the 3-month average relative humidity largely depended on the 3-month period. Thus, the correlation of the 3-month average relative humidity was shown more clearly. It has been pointed out above that the amounts of SO2 ions deposited on calcite 4 specimens were almost proportional to the duration of the exposure. As mentioned in Section 3.3, the amounts of SO2 4 ions deposited on calcite specimens at HoChiMinh and Bien Hoa exposed to the absolute dry periods from January to March were very low in comparison to those on the ones exposed to the other periods at these sites, although the average SO2 concentrations of the absolute dry periods were comparable with those of the other 3month periods. As shown in Fig. 1, the relative humidity at HoChiMinh and Bien Hoa during the absolute dry period was very low compared with that during the other 3-month periods at these sites and with the 3month relative humidity at the other sites in total. For comparison, the amounts of SO2 4 ions deposited on the calcite specimens exposed in absolute dry period and other periods with comparable SO2 concentrations, the relative humidity, and the SO2 concentration at HoChiMinh and Bien Hoa are given in Table 2. From Table 2, in absolute dry periods at HoChiMinh and Bien Hoa,
the relative humidity was lower than 70.2%, and the amounts of SO2 4 ions deposited on the calcite specimens was about half of those on the ones exposed in the other periods with comparable SO2 concentration. This pointed out, that at a relative humidity lower than 70.2%, sulfation occurred much more slowly than at the relative humidity above 72.2%, and there was some critical value of the relative humidity, below which the kinetics of sulfation changed to a lower rate.
4.2. Regression equations for the sulfation of calcite specimens Regression analysis was carried out as described in Section 2.4. Based on the observed results, the regression analysis was conducted separately for the amounts of SO2 4 ions deposited on the calcite specimens exposed at Bien Hoa and HoChiMinh in the absolute dry periods, and for those deposited on other calcite specimens, i.e. regression analysis was done separately for two ranges of relative humidity: (1) relative humidity above 72.2%, and (2) relative humidity below 70.2%. Regression equations obtained for the range of relative humidity above 72.2% showed that the most substantial environmental factors influencing the sulfation, i.e. corrosion of marble were C SO2 and RH. This was in agreement with the observed results. It was difficult to elucidate the effect of temperature on the sulfation of marble, since the average temperature during the exposure periods at the exposure sites was almost unchanged. Moreover, regression analysis showed that the relative humidity and SO2 concentration affected the sulfation of marble as interacting factors. We obtained the following functions for the relation between the amount of sulfate ions on calcite specimens and the environmental factors for the range of relative humidity above 72.2%: MS ¼ 0:0188 t RH C SO2 ; ð3-month exposureÞ
R2 ¼ 0:917;
n ¼ 32; ð3Þ
MS ¼ 0:0153 t RH C SO2 ; ð1-year exposureÞ
R2 ¼ 0:958;
n ¼ 8; ð4Þ
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R2 ¼ 0:946;
n ¼ 4;
120
ð5Þ
SO2 4
where MS is the amount of ions deposited on the calcite specimens (mg cm2), t is the duration of the exposure (month), C SO2 is the average SO2 concentration in the air during the exposure period (ppb), RH is the average relative humidity during the exposure period (%), and n is the number of data processed. The empirical kinetic equation of calcite sulfation derived from Eqs. (3)–(5) is as follows: MS ¼ 0:0219 t0:857 RH C SO2 ;
(6)
where the duration of the exposure is expressed in months, or MS ¼ 0:185 t0:857 RH C SO2 ;
(7)
where the duration of the exposure is expressed in years. RH and SO2 are expressed in % and ppb as in Eqs. (3)–(5). The amount of sulfate ions on unit marble surface is considered as a level of sulfation or corrosion of marble by sulfur dioxide. The fact that the time dependence of sulfation level of marble was stated in terms of time, in the power of 0.857, pointed out that the corrosion rate of marble caused by dry deposition decreases slightly with the duration of exposure. This means, that unlike some metals, the corrosion products in this case have a somewhat protective ability against further corrosion, while corrosion products of marble do not have this feature. Regression analysis was also performed for the amounts of SO2 4 ions deposited on the calcite specimens exposed for 3 months at the relative humidity below 70.2%. The following equations were obtained: MS ¼ 0:0106 t RH C SO2 ;
R2 ¼ 0:820;
n ¼ 4; (8)
MS ¼ 0:0721 t C SO2 ;
R2 ¼ 0:855;
n ¼ 4:
(9)
Regression analysis gave higher correlation coefficient if independent variable was t C SO2 instead of t RH C SO2 : It seemed that at low relative humidity, the effect of relative humidity on the sulfation of marble was minor and the amount of SO2 ions deposited on the 4 calcite specimens was correlated with SO2 concentration, but not the relative humidity. All the above obtained regression equations were in agreement with the observed data. Considering that the squares of the regression coefficient R2 were large, the proposed model for the sulfation of marble is credible. Therefore, the empirical kinetic Eqs. (6) and (7) can be applied to predict deterioration of calcite marble by dry deposition at the four exposure sites. Assuming that the amount of SO2 4 ions deposited on the marble specimens is proportional to the SO2 concentration in the air,
100 SO4-2 on dolomite (µg cm-2)
MS ¼ 0:0140 t RH C SO2 ; ð2-year exposureÞ
919
80 60 40
y = 0.6738x 2
R = 0.9357 20 0 0
30
60 90 SO4-2 on calcite (µg cm-2)
120
150
Fig. 6. Amount of SO2 ions on dolomite specimens versus 4 amount of SO2 4 ions on calcite specimens exposed under the same conditions.
marble can serve as an indicator for the pollution level at a site exposed to SO2. The sulfation rate of marble decreases remarkably at relative humidity below 70%; hence the first step to protect marble artifacts in indoor conditions from attack by sulfur dioxide is to reduce the relative humidity below the above critical value. We suggest to reduce relative humidity to lower than 65%. This has a significant social and economical meaning, especially for the preservation of cultural artifacts in museums. 4.3. Sulfation of dolomite To elucidate the effect of environmental factors on different types of marbles, 20 dolomite specimens were exposed to the environment under rain shelter together with calcite specimens at the exposure sites for 3-month, 1-year and 2-year periods. Fig. 6 shows the amount of SO2 4 ions on dolomite specimens versus the amount of SO2 4 ions on calcite specimens exposed under the same conditions. The sulfation level of dolomite was about 67% that of calcite. Thus, dolomite was less sensitive to sulfur dioxide than calcite. It was pointed out in the literature (Leygraf and Graedel, 2000) that in principle, dolomite is the most stable among the carbonate stones.
5. Conclusions (1) Deterioration of marble by dry deposition is mainly caused due to sulfation by sulfur dioxide in the air. The product of sulfation was gypsum (CaSO4 2H2O). (2) Marble consisting of dolomite is less sensitive to sulfur dioxide than marble consisting of calcite. (3) The sulfation of marble at relative humidity above 72% was related to both the SO2 concentration in
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the air and relative humidity; while at relative humidity below 70%, it was controlled by SO2 concentration in the air only. (4) We obtained the following dose-response functions for the sulfation of marble consisting of calcite at relative humidity above 72%: MS ¼ 0:0185 t0:857 RH C SO2 ; (10) where MS is the amount of SO2 4 deposited on marble (mg cm2), C SO2 is average SO2 concentration (ppb) and RH is average relative humidity (%).
Acknowledgments This research is supported by The Japan Society for Promotion of Science. We greatly appreciate the help from Osaka City Institute of Public Health and Environmental Science, Osaka Prefectural Environmental Pollution Control Center, Technology Research Institute of Osaka Prefecture, College of Engineering, Osaka Prefecture University, Vietnam National University and Meteorological Center of south Vietnam.
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Hadler, N.J.C., Iunes, P.J., Mello, T.C.W.P., Navia, L.M.S., Paulo, S.R., Kirkitsos, P., Sikiotis, D., 1996. Deterioration of pentelic marble, portland limestone and Baumberger sandstone in laboratory exposures to NO2: a comparison with exposures to gaseous HNO3. Atmospheric Environment 30 (6), 941–950. Johnson, J.B., Montgomery, M., Thompson, G.E., Wood, G.C., Sage, P.W., Cooke, M.J., 1996. The influence of combustion-derived pollutants on limestone deterioration: 2. The wet deposition of pollutant species. Corrosion Science 38 (2), 267–278. Katsanos, N.A., De Santis, F., Cordoba, A., RoubaniKalantzopoulou, F., Pasella, D., 1999. Corrosive effects from the deposition of gaseous pollutants on surfaces of cultural and artistic value inside museums. Journal of Hazardous Materials 64 (1), 21–36. Komeiji, T., 1992. The effect of air pollution on cultural resources. Corrosion Engineering 41, 161–174. Leygraf, C., Graedel, T.E., 2000. Atmospheric Corrosion. Wiley-InterScience. A John Wiley & Sons, New York, pp. 261–268. Maeda, Y., Morioka, J., Tsujino, Y., Satoh, Y., Zhang, X., Mizoguchi, T., Hatakeyama, S., 2001. Material damage caused by acidic air pollution in east Asia. Water, Air, and Soil Pollution 130, 141–150. Metallo, M.C., Poli, A.A., Diana, M., Persia, F., Cirillo, M.C., 1995. Air pollution loads on historical monuments: an air quality model application to the marble Arch of Titus in Rome. The Science of the Total Environment 171 (1), 163–172. Soon-Bok, L., Cowell, D., Apsimon, H., 1996. Estimating the cost of damage to buildings by acidifying atmospheric pollution in Europe. Atmospheric Environment 30 (17), 2959–2968. Tecer, L., 1999. Laboratory experiments on the investigation of the effects of sulfuric acid on the deterioration of carbonate stones and surface corrosion. Water, Air, and Soil Pollution 114 (1/2), 1–12. Tran Thi Ngoc Lan, Nishimura, R., Tsujino Y., Imamura, K., Warashina, M., Nguyen Thai Hoang, Maeda, Y., 2004. Atmospheric concentrations of sulfur dioxide, nitrogen oxides, ammonia, hydrogen chloride, nitric acid, formic and acetic acids in the south of Vietnam measured by the passive sampling method. Analytical Science, January in press.
Atmospheric Environment 39 (2005) 913–920 www.elsevier.com/locate/atmosenv
New model for the sulfation of marble by dry deposition Sheltered marble—the indicator of air pollution by sulfur dioxide Tran Thi Ngoc Lana,, Nguyen Thi Phuong Thoaa, R. Nishimurab, Y. Tsujinoc, M. Yokoid, Y. Maedab a
Department of Physical Chemistry, College of Natural Sciences, Vietnam National University, 227 Nguyen Van Cu, HoChiMinh, Vietnam b Department of Material Science, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai 599-8531, Osaka, Japan c Osaka Prefectural Environmental Pollution Control Center, 1-3-62 Nakamichi, Higashinari-ku, Osaka 537-0025, Japan d Technology Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi, Osaka 594-1157, Japan Received 8 January 2004; received in revised form 7 September 2004; accepted 27 September 2004
Abstract This paper was concerned with evaluating the effect of dry deposition on deterioration of marble. Two types of marble were exposed to atmospheric environment with a rain shelter at four exposure sites in the south of Vietnam for 3-month, 1-year and 2-year periods from July 2001 to September 2003. X-ray diffraction (XRD) and X-ray fluorescent (XRF) methods were applied to study the products of marble deterioration. Ion chromatography was used to analyze dry depositions on marble. The main product of marble deterioration was gypsum (CaSO4 2H2O). The amount of sulfate ions deposited on marble was found to be proportional to SO2 concentration in the air, relative humidity and duration of the exposure. In addition, sulfation of marble caused by SO2 at a relative humidity lower than 70% is almost half of that at relative humidity higher than 70%. Moreover, marble consisting of calcite (CaCO3) was more sensitive to SO2 than marble consisting of dolomite (CaCO3 and MgCO3). A good relation between the amount of sulfate ions deposited on marble and SO2 concentration in the air suggested that marble could serve as an indicator for atmospheric pollution by SO2. r 2004 Elsevier Ltd. All rights reserved. Keywords: Acidification; Passive sampler; Sulfation; Dolomite; Calcite
1. Introduction Corresponding author. Department of Material Science,
Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuencho, Sakai 599-8531, Osaka, Japan. Tel.: +84 8 839 7720. E-mail address: [email protected] (Tran Thi Ngoc Lan).
Air pollution problem caused by industrial development is the reality which the modern world is facing. A huge amount of sulfur dioxide and other pollutants like nitrogen oxides, carbon monoxide, etc. are discharged into the atmosphere. Consequently, the atmospheric acidification, with high SO2 concentrations in the air and
1352-2310/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.09.074
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acid rains, becomes more and more serious, especially in developing countries (Leygraf and Graedel, 2000; Maeda et al., 2001). Damage to architectural and cultural constructions caused by acidifying atmospheric pollution imposes huge socio-economic costs (Soon-Bok et al., 1996). Therefore, considerable attention has been paid to the effect of air pollutants on architectural material, and especially on the cultural assets made of carbonate stones. Many reports concerning deterioration of carbonate stones were published (Alonso and Martinez, 2003; Boke and Lal Gauri, 2003; Graedel, 2000; Franzini, 1996; Hadler et al., 1996; Johnson et al., 1996; Katsanos et al., 1999; Komeiji, 1992; Maeda et al., 2001; Metallo et al., 1995; Tecer, 1999). Most of them were laboratory-simulated experiments and were concerned with European countries where the climate is moderate and the temperature is low. Vietnam is a tropical country. Temperature and relative humidity are rather high. The behavior of the deterioration of carbonate stones was unknown. It was found that SO2 caused deterioration (corrosion) of carbonate stones by sulfation, i.e. the reaction of SO2 with carbonate calcium and carbonate calcium was consequently changing to more soluble gypsum (CaSO4 2H2O). However, the kinetics of deterioration and sulfation of marble was not well studied. The purpose of our research is to clarify the effect of air pollutants due to dry deposition on marble and the kinetics of its deterioration. Since products of marble corrosion tends to be lost due to the washing effect of rains (Leygraf and Graedel, 2000), to investigate the sulfation process, we focused on marble that was exposed under a rain shelter. Understanding the effects of environmental factors on marble enables us to predict the material damage and to propose protective measures.
2. Experiment 2.1. Climate conditions at four exposure sites in the south of Vietnam Four exposure sites at HoChiMinh (HCMC), Bien Hoa, Vung Tau and My Tho representing urbanindustrial, urban, urban-marine and rural climate, respectively, in the south of Vietnam were chosen. HoChiMinh is the biggest industrial center in Vietnam. The other sites are located about 30 to 140 km around HoChiMinh. The climatic parameters like temperature and relative humidity were obtained from the meteorological stations where the test specimens were set up. There are only two seasons in the south of Vietnam: dry (from the end of November to the end of May) and rainy (from the end of May to the end of November) seasons. The period from January to March had no rain at all and was termed as the absolute dry period.
Table 1 Climatic conditions at the four exposure sites: HoChiMinh (HCMC), My Tho, Vung Tau and Bien Hoa Period
Jul/01–Sep/01 Oct/01–Dec/01 Jan/02–Mar/02 Apr/02–Jun/02 Jul/02–Sep/02 Jul/01–Jun/02
Temperature (1C) HCMC
My Tho
Vung Tau
Bien Hoa
28.0 27.3 27.0 29.8 28.2 28.0
27.1 26.2 25.8 28.3 27.0 26.8
27.8 26.9 26.0 28.9 27.8 27.4
27.6 26.5 26.4 28.9 27.9 27.4
Fig. 1. Three-month and 1-year (solid line) average relative humidities at the four exposure sites.
The 3-month average temperatures at the four exposure sites in the period from July 2001 to September 2002 are summarized in Table 1. In the south of Vietnam, the temperature is relatively stable all the year round and from year to year. During the period from July 2001 to September 2003, the 1-year average temperature at the four exposure sites was in the range of 26.8–28.0 1C (Table 1), while the 3-month average was in the range of 25.8–29.8 1C. Fig. 1 represents the seasonal dependence of relative humidity at the exposure sites. In principle, the 3-month average relative humidity at all exposure sites was the lowest in the absolute dry period, and highest in the period from July to September when there was a lot of rain. The lowest relative humidity of 67.3% was observed against the highest one of 84.8%. However, the 1-year average relative humidities varied a little from one site to another and were in the range of 73.8%–80.3%. Moreover, the 1-year average relative humidities at each individual site in periods from July 2001 to June 2002 and from July 2002 to June 2003 were almost the same, with the difference between them being less than 2%. In addition, at any exposure period, the
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average relative humidity was the lowest at HoChiMinh. Especially, a very low relative humidity was observed at HoChiMinh and Bien Hoa during the absolute dry period. The big difference in relative humidity depending on the season allowed elucidating the effect of relative humidity on degradation of marble based on short-term exposure. 2.2. Sampling method of SO2 A passive sampling method was used for the determination of the gaseous SO2 concentration in the air (SO2 concentration) at the four exposure sites (Tran Thi Ngoc Lan et al., 2004). A SO2 concentration was estimated from an amount of SO2 collected by the passive samplers. The passive samplers were sampled once a month from July 2001 to September 2003. 2.3. Material and exposure methods Before exposure to the atmosphere, the test specimens with a size of 20 mm 20 mm 5 mm of Italian white marble consisting of CaCO3 were washed in Millipore water, dried in a desiccator for 24 h and weighed with a precision of 0.01 mg. The duplicated test specimens were exposed to the outdoor atmospheric environments at the four exposure sites under the rain shelter with an exposure angle of 901. The exposure was carried out for each 3-month (July 2001–September 2003), 1-year (July 2001–June 2003) and 2-year (July 2001–June 2003) period. The test specimens were denoted as the 3-month, the 1-year and the 2-year specimens, respectively, to the 3-month, 1-year and 2-year exposure. To compare the effects of air pollutants on different types of marble, test specimens of Italian white marble consisting of dolomite (CaMg(CO3)2) of the same size were exposed for 3-month, 1-year and 2-year periods together with the test specimens consisting of calcite. The test specimens are designated as calcite and dolomite specimens, representing the two types of marble.
915
Normally, one immersion for 24 h is sufficient to achieve this. The washing filtrates of 20 ml were analyzed with 761 Metrohm ion chromatograph (IC) systems to measure the amounts of anions and cations deposited on the test specimens. However, the most interesting was the amount of SO2 ions on the test specimens that 4 correlated to the amount of gypsum. The amount of SO2 ions (MS—mg cm2) on the test specimens was 4 evaluated as follows: MS ¼ V C=S;
(1)
where V is the volume of washing filtrates (20 ml), C is the concentration of SO2 ions in particular washing 4 filtrates (mg ml1) and S is the surface area of the test specimens (12 cm2). 2.5. Regression analysis Regression analysis with a stepwise method was performed by using the computer software SPSS 10.0 to identify factors influencing the sulfation of marble. Regression analysis was conducted independently for the amount of SO2 4 ions on the test specimens at the four exposure sites after the 3-month, 1-year and 2-year exposure separately. The independent variables were the SO2 concentration (C SO2 ), the relative humidity (RH), and the time of exposure, temperature, and their combinations.
3. Results 3.1. SO2 concentration in the air at the exposure sites Fig. 2 shows the 3-month, 1-year (solid line) and 2year (dashed line) average SO2 concentrations at the four exposure sites. The 3-month and 1-year average SO2 concentrations increased in the order of My
2.4. Analysis of the test specimens The corrosion products formed on the test specimens were identified by X-ray diffraction method (XRD) using X-ray diffractometer (40 kV and 150 mA, Rigaku Co.) with Cu target. The scan was performed at a speed of 4.0001 min1, a scan step of 0.0201 and a scan range from 51 to 851. The Rigaku RIX 3000 fluorescence analyzer was used to determine the amount of sulfur on the test specimens (XRF method). After being analyzed by XRD and XRF methods, the test specimens were immersed and slightly shaken in 20 ml of the Millipore water for sufficient time to completely dissolve soluble corrosion products (e.g. gypsum, as described later).
Fig. 2. Average 3-month, 1-year (solid line) and 2-year (dashed line) SO2 concentrations at the four exposure sites.
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ThooVung TauoBien Hoa andoHoChiMinh with comparable ones at My Tho and Vung Tau. Nevertheless, the variation in the 3-month average SO2 concentrations at each exposure site was small in comparison with that among the sites (except between My Tho and Vung Tau). HoChiMinh was the most polluted site, since it is the biggest industrial and commercial center in Vietnam with many enterprises inside and outside the city. Bien Hoa is the urban site located about 30 km in north-east of HoChiMinh City and the atmospheric environment is affected by industrial parks outside the city. My Tho and Vung Tau have no heavy industries, hence they are clean areas. The 1year average SO2 concentrations during the period from July 2002 to June 2003 at HoChiMinh, My Tho and Vung Tau reduced slightly, but were almost the same as those of the first year of exposure. However, the 1-year average SO2 concentration at Bien Hoa during the second year of exposure increased by about 20% compared with that of the first year of exposure. The increasing SO2 concentrations at Bien Hoa was related to the fast expansion of industrial parks outside the city and moving industrial enterprises from HoChiMinh into industrial parks.
3.3. The amount of sulfate ions on the calcite specimens The washing extracts of the test specimens were 2 analyzed by IC. Na+, K+, Cl, NO ions 3 and SO4 were found on all test specimens, but not NO2 , SO2 3 and PO3 4 . However, the main anions deposited on + + marble were SO2 4 ions. Na , K , Cl and NO3 were in 2 small quantities compared with SO4 ions. Oxalate and NH+ 4 ions were also detected on many test specimens, but in negligible quantities in comparison with the other ions. This indicates that corrosion of marble was mainly caused by sulfation. The amount of SO2 ions eluted 4 from the test specimens was measured for all of the specimens and was estimated as an average value of the duplicated ones. Fig. 4 shows the amount of SO2 4 ions on the 3-month, 1-year (solid line, right axis) and 2-year (dashed line, right axis) calcite specimens at the four exposure sites. The site dependence of the amount of SO2 ions on the 3-month calcite specimens of the 4 second year of the exposure was similar to that of the first year of the exposure. The amount of SO2 ions 4 increased in the order of My ThooVung TauoBien HoaoHoChiMinh for all of the 3-month, 1-year and 2year exposures. In addition, the amount of SO2 4 ions on
3.2. Corrosion products on the test specimens Fig. 3 shows a representative result of the XRD analysis for the 3-month specimens at HoChiMinh, where the diffractions of gypsum (CaSO4 2H2O) at d ¼ 7:635; 4.287 and 2.880 A˚ were indicated by the symbols G and the other strong diffractions corresponding to those of calcite. At the four exposure sites, gypsum was found on all of the calcite and dolomite specimens. It was found that the corrosion products were only gypsum. All of the test specimens increased in weight after the exposure due to the transformation of CaCO3 into gypsum by the following reaction (Graedel, 2000): oxidation
SO2 g 3SO2 aq 3
3CaSO4 2H2 O:
SO2 4 aq 3CaSO4 wH2 O ð2Þ
Fig. 4. Amounts of SO2 ions on the 3-month, 1-year (solid 4 line) and 2-year (dashed line) calcite specimens at the four exposure sites.
Fig. 3. XRD pattern of the 3-month sheltered specimen at HoChiMinh.
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5000
XRD intensity (cps)
SO4-2(µg cm -2)
200 150 y = 3.7292x R2 = 0.9593
100 50 0 0
20
40
60
XRF Intensity (kcps)
80
917
d:7.622
4000 3000 2000
y = 69.407x R2 = 0.9115
1000 0
0
20
40
60
80
XRF Intensity (kcps)
( Fig. 5. Relationships among the amount of SO2 4 ions, intensity of the highest XRD diffraction of gypsum at d ¼ 7:622 A; and XRF intensity of sulfur on the test specimens at the four exposure sites.
the 3-month calcite specimens at individual exposure sites was the lowest in the absolute dry period lasting from January to March. An exception was the rural site of My Tho; nevertheless, the amount of SO2 4 ions on the 3-month calcite specimens at My Tho observed in the absolute dry period was almost the same as the lowest one at this site. From Figs. 2 and 4, it was found that the site dependence of the amount of SO2 4 ions on the 1-year and 2-year calcite specimens was associated with that of the 1-year and 2-year average SO2 concentration. In addition, for each identical 3-month exposure period, the variation in the amount of SO2 4 ions on the calcite specimens at the four exposure sites reflected the variations in the 3-month average SO2 concentrations. Furthermore, it was seen from Figs. 1 and 4 that at each exposure site, the change in the 3month average relative humidities corresponded to that in the amounts of SO2 ions on the 3-month calcite 4 specimens. Thus, the amount of SO2 ions on the 34 month calcite specimens was related to the 3-month average SO2 concentration, and the 3-month average relative humidity, while that on the 1-year calcite specimens seemed to relate only to the 1-year average SO2 concentration. From Fig. 4, at HoChiMinh, My Tho and Vung Tau, the amount of SO2 4 ions on the 1-year calcite specimens of the first and second year of exposure were almost the same, with the difference between them being less than 10%. However, at Bien Hoa, the amount of SO2 4 ions on the 1-year calcite specimens of the second year was about 20% higher than that of the first year of exposure. This should relate to the increase of SO2 concentration at Bien Hoa, as mentioned before. The amount of SO2 ions on the 2-year calcite 4 specimens at each exposure sites was almost twice that on the 1-year calcite ones. In addition, the amount of SO2 4 ions on the 1-year calcite specimens was comparable to the cumulative amount of SO2 4 ions on the 3month calcite specimens of the year. This indicates that the amount of SO2 4 ions deposited on calcite specimens correlated with the duration of exposure.
3.4. Relationship among the amounts of sulfate ions, XRF intensities of sulfur and XRD intensities of gypsum on the calcite specimens X-ray analysis was performed and the relationships among the amounts of SO2 4 ions, the intensities of the ( and highest XRD diffraction of gypsum at d ¼ 7:622 A the XRF intensities of sulfur on the calcite specimens are shown in Fig. 5. Linear functions fit well. Therefore, the XRF intensity was proportional to both the amount of SO2 ions and the XRD intensities of gypsum, which 4 suggests that the amount of SO2 4 ions was proportional to the XRD intensity of gypsum, that is, the amount of gypsum.
4. Discussions 4.1. Effects of environmental factors on the sulfation of marble In this section, we consider the sulfation of marble from the amount of SO2 4 ions, which was proportional to the amount of gypsum, as described in Sections 3.3 and 3.4. As pointed out in Section 3.3, the amount of SO2 ions of the calcite specimens is related to the 4 average SO2 concentration, the average relative humidity and the duration of the exposure period. From Figs. 2 and 4, for the 1-year and 2-year exposure period, the increase of the 1-year average SO2 concentrations resulted in that of the amounts of SO2 4 ions of the 1year and 2-year calcite specimens, respectively. This consideration can be applied to the 3-month exposure, from the behavior of the amounts of SO2 4 ions and the 3-month average SO2 concentrations at the four exposure sites for the same 3-month exposure period. This means that the amounts of SO2 ions were 4 proportional to the average SO2 concentrations. Moreover, it was found from the comparison of the amounts of SO2 4 ions on the 3-month calcite specimens (Fig. 4) and the 3-month average relative humidities at each
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Table 2 Amount of sulfate ions on calcite specimens exposed in absolute dry periods (January to March) and in the other periods with the comparable SO2 concentration at HoChiMinh and Bien Hoa HoChiMinh
SO2 4
2
(mg cm ) Amount of SO2 concentration (ppb) Relative humidity (%)
Bien Hoa
Jan/02–Mar/02
Jan/03–Mar/03
Other periods
Jan/02–Mar/02
Jan/03–Mar/03
Other periods
38.6 18.3 67.0
40.4 16.8 67.5
72–81 16.4–19.1 72.2–78.4
19.9 10.22 68.9
17.8 10.32 70.2
42–51 10.0–11.5 74.8–84.4
exposure site (Fig. 1), that the higher 3-month average relative humidity was correlated to the higher amounts of SO2 ions. Thus, the amount of SO2 ions was 4 4 proportional to the average relative humidity at the four exposure sites. Consequently, SO2 concentrations and relative humidity were the factors strongly affecting the corrosion of marble under the sheltered exposure conditions, although the correlation of each factor to the amounts of SO2 ions on the sheltered specimens 4 was more or less evident depending on the 3-month or 1year exposure periods. For the same exposure period (including 1-year and 3-month), the difference among average relative humidities at the four exposure sites was not large. Therefore, the correlation of the average relative humidities was not seen, although the average SO2 concentrations showed a clear correlation, as they were largely different among the four exposure sites. On the other hand, at each exposure site such as at HoChiMinh, the 3-month average SO2 concentration was rather stable, but the 3-month average relative humidity largely depended on the 3-month period. Thus, the correlation of the 3-month average relative humidity was shown more clearly. It has been pointed out above that the amounts of SO2 ions deposited on calcite 4 specimens were almost proportional to the duration of the exposure. As mentioned in Section 3.3, the amounts of SO2 4 ions deposited on calcite specimens at HoChiMinh and Bien Hoa exposed to the absolute dry periods from January to March were very low in comparison to those on the ones exposed to the other periods at these sites, although the average SO2 concentrations of the absolute dry periods were comparable with those of the other 3month periods. As shown in Fig. 1, the relative humidity at HoChiMinh and Bien Hoa during the absolute dry period was very low compared with that during the other 3-month periods at these sites and with the 3month relative humidity at the other sites in total. For comparison, the amounts of SO2 4 ions deposited on the calcite specimens exposed in absolute dry period and other periods with comparable SO2 concentrations, the relative humidity, and the SO2 concentration at HoChiMinh and Bien Hoa are given in Table 2. From Table 2, in absolute dry periods at HoChiMinh and Bien Hoa,
the relative humidity was lower than 70.2%, and the amounts of SO2 4 ions deposited on the calcite specimens was about half of those on the ones exposed in the other periods with comparable SO2 concentration. This pointed out, that at a relative humidity lower than 70.2%, sulfation occurred much more slowly than at the relative humidity above 72.2%, and there was some critical value of the relative humidity, below which the kinetics of sulfation changed to a lower rate.
4.2. Regression equations for the sulfation of calcite specimens Regression analysis was carried out as described in Section 2.4. Based on the observed results, the regression analysis was conducted separately for the amounts of SO2 4 ions deposited on the calcite specimens exposed at Bien Hoa and HoChiMinh in the absolute dry periods, and for those deposited on other calcite specimens, i.e. regression analysis was done separately for two ranges of relative humidity: (1) relative humidity above 72.2%, and (2) relative humidity below 70.2%. Regression equations obtained for the range of relative humidity above 72.2% showed that the most substantial environmental factors influencing the sulfation, i.e. corrosion of marble were C SO2 and RH. This was in agreement with the observed results. It was difficult to elucidate the effect of temperature on the sulfation of marble, since the average temperature during the exposure periods at the exposure sites was almost unchanged. Moreover, regression analysis showed that the relative humidity and SO2 concentration affected the sulfation of marble as interacting factors. We obtained the following functions for the relation between the amount of sulfate ions on calcite specimens and the environmental factors for the range of relative humidity above 72.2%: MS ¼ 0:0188 t RH C SO2 ; ð3-month exposureÞ
R2 ¼ 0:917;
n ¼ 32; ð3Þ
MS ¼ 0:0153 t RH C SO2 ; ð1-year exposureÞ
R2 ¼ 0:958;
n ¼ 8; ð4Þ
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R2 ¼ 0:946;
n ¼ 4;
120
ð5Þ
SO2 4
where MS is the amount of ions deposited on the calcite specimens (mg cm2), t is the duration of the exposure (month), C SO2 is the average SO2 concentration in the air during the exposure period (ppb), RH is the average relative humidity during the exposure period (%), and n is the number of data processed. The empirical kinetic equation of calcite sulfation derived from Eqs. (3)–(5) is as follows: MS ¼ 0:0219 t0:857 RH C SO2 ;
(6)
where the duration of the exposure is expressed in months, or MS ¼ 0:185 t0:857 RH C SO2 ;
(7)
where the duration of the exposure is expressed in years. RH and SO2 are expressed in % and ppb as in Eqs. (3)–(5). The amount of sulfate ions on unit marble surface is considered as a level of sulfation or corrosion of marble by sulfur dioxide. The fact that the time dependence of sulfation level of marble was stated in terms of time, in the power of 0.857, pointed out that the corrosion rate of marble caused by dry deposition decreases slightly with the duration of exposure. This means, that unlike some metals, the corrosion products in this case have a somewhat protective ability against further corrosion, while corrosion products of marble do not have this feature. Regression analysis was also performed for the amounts of SO2 4 ions deposited on the calcite specimens exposed for 3 months at the relative humidity below 70.2%. The following equations were obtained: MS ¼ 0:0106 t RH C SO2 ;
R2 ¼ 0:820;
n ¼ 4; (8)
MS ¼ 0:0721 t C SO2 ;
R2 ¼ 0:855;
n ¼ 4:
(9)
Regression analysis gave higher correlation coefficient if independent variable was t C SO2 instead of t RH C SO2 : It seemed that at low relative humidity, the effect of relative humidity on the sulfation of marble was minor and the amount of SO2 ions deposited on the 4 calcite specimens was correlated with SO2 concentration, but not the relative humidity. All the above obtained regression equations were in agreement with the observed data. Considering that the squares of the regression coefficient R2 were large, the proposed model for the sulfation of marble is credible. Therefore, the empirical kinetic Eqs. (6) and (7) can be applied to predict deterioration of calcite marble by dry deposition at the four exposure sites. Assuming that the amount of SO2 4 ions deposited on the marble specimens is proportional to the SO2 concentration in the air,
100 SO4-2 on dolomite (µg cm-2)
MS ¼ 0:0140 t RH C SO2 ; ð2-year exposureÞ
919
80 60 40
y = 0.6738x 2
R = 0.9357 20 0 0
30
60 90 SO4-2 on calcite (µg cm-2)
120
150
Fig. 6. Amount of SO2 ions on dolomite specimens versus 4 amount of SO2 4 ions on calcite specimens exposed under the same conditions.
marble can serve as an indicator for the pollution level at a site exposed to SO2. The sulfation rate of marble decreases remarkably at relative humidity below 70%; hence the first step to protect marble artifacts in indoor conditions from attack by sulfur dioxide is to reduce the relative humidity below the above critical value. We suggest to reduce relative humidity to lower than 65%. This has a significant social and economical meaning, especially for the preservation of cultural artifacts in museums. 4.3. Sulfation of dolomite To elucidate the effect of environmental factors on different types of marbles, 20 dolomite specimens were exposed to the environment under rain shelter together with calcite specimens at the exposure sites for 3-month, 1-year and 2-year periods. Fig. 6 shows the amount of SO2 4 ions on dolomite specimens versus the amount of SO2 4 ions on calcite specimens exposed under the same conditions. The sulfation level of dolomite was about 67% that of calcite. Thus, dolomite was less sensitive to sulfur dioxide than calcite. It was pointed out in the literature (Leygraf and Graedel, 2000) that in principle, dolomite is the most stable among the carbonate stones.
5. Conclusions (1) Deterioration of marble by dry deposition is mainly caused due to sulfation by sulfur dioxide in the air. The product of sulfation was gypsum (CaSO4 2H2O). (2) Marble consisting of dolomite is less sensitive to sulfur dioxide than marble consisting of calcite. (3) The sulfation of marble at relative humidity above 72% was related to both the SO2 concentration in
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the air and relative humidity; while at relative humidity below 70%, it was controlled by SO2 concentration in the air only. (4) We obtained the following dose-response functions for the sulfation of marble consisting of calcite at relative humidity above 72%: MS ¼ 0:0185 t0:857 RH C SO2 ; (10) where MS is the amount of SO2 4 deposited on marble (mg cm2), C SO2 is average SO2 concentration (ppb) and RH is average relative humidity (%).
Acknowledgments This research is supported by The Japan Society for Promotion of Science. We greatly appreciate the help from Osaka City Institute of Public Health and Environmental Science, Osaka Prefectural Environmental Pollution Control Center, Technology Research Institute of Osaka Prefecture, College of Engineering, Osaka Prefecture University, Vietnam National University and Meteorological Center of south Vietnam.
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