Deterloration of Marble Structures 'Ihe Role ofAcid Rain Roger J. cherrg
ArmOspheric Sciences Research Csnte~ Slale University of New York Albany, N.Y. 12222
Jih Ru Hwu Jung T. Kim show-Me1 Leu
Department of a m i s b y The Johns Hopkins Universily Banimore, Md. 21218 The city hall building in Schenectady, N.Y.,a registered historical building, was built in the 1930s of the finest Vermont marble, comparable to the Carrara marble of Italy. I t has already fallen victim to destruction by acid rain. The structure of the building has been weakened by the conversion of marble to gypsum. Even old structures that have survived several centuries have begun to deteriorate a t much more no-
which give rain an acidic character. The sulfates then convert the calcium carbonate, an insoluble component of marble, into a soluble material known as gypsum. The nitrates conv e r t t h e calcium carbonate into calciuw nitrate. To slow dam the formation of gyp sum, the destructive material must be identified and its origin determined.
mlvsis We used modern optical and classical analytical techniques to identify the destructive material. Optical microscopes
ANALYrICAL
APPROACH ticeable rates. In Yeking, China, there are marble monuments that are 500 years old (Figure 1) that contain the history of the empire. Until 40 years ago the inscriptions were legible; now they are unreadable. This clearly indicates that most of the damage to the marble structures has occurred recently. The acceleration in destruction has created an interest in discovering how the damage occnrs and generated concern about the role of acid rain in this destruction. Increased emission of sulfur dioxides and nitric oxides by the industrial and private sectors has resulted in an increase in acid rain formation and its destructive effeds on marble. These materials, which are emitted from industrial smokestacks, are converted to sulfates and nitrates, respectively, 104A
Figure China
. .
Fmty years ago me lnsalptims mlegible. as shown In me rubbing (bottom). Now. mey are unreadable (top)
with a photomicrographic system and dispersive staining capabilities were used to magnify the material several hundred times. Using this technique, we discovered that gypsum was always present in samples of deteriorated marble. For example, significant deterioration was found on the protected area of the balustrade from a 50-year-old marble building. The black crusted material on the surface consisted of massive gypsum crystals and industrial depositions. Cross sections of eroded marble, below the surface of the gypsum region, examined by a polarizing microscope showed wide, open, intergranular spaces that had been penetrated by sulfuric acid (Figure 2). This indicates that sulfates converted the marble into gypsum. Further analysis of deteriorated marble samples with ion chromatog-
ANALYTICAL CHEMISTRY, VOL. 59, NO. 2. JANUARY 15, 1987
500-year-old marble mnument in Peking,
raphy was carried out to detect the ratio of sulfates and nitrates embedded in the marble surface. Although both sulfates and nitrates exist in significant quantities in acid rain, our findings revealed that the concentration of sulfates on the marble surface was more than 20 times greater than that of nitrates. This increases the likelihood that the sulfates in acid rain are the material that destroys marble. In exploring the destructive effeds of sulfates on the marble surface, the following experiments were carried out. We placed two pieces of marble, an Italian type with a surface area of 42.1 cm2 and a US. type with a surface area of 23.2 em2, side by side in a container circulating aqueous sulfuric acid solution for 24 h. Two concentrations of sulfuric acid, 30 pM and 100 p M , were 0003-2700/87/0359-104A1$01.50/0 @ 1987 American Chemical Society
Cross section (500X) of deteriorated marble examined by a polarizing microscope revealed wide. open. intergranular spaces that had been penetrated by sulfuric acid
Flgure 2.
used. T h e marble samples were weighed before and after each trial to measure the weight loss or, in this case, the deterioration of the marble sample. The solution was circulated to mimic the effects of rain splashing on the marble surface. In addition, it was replaced by fresh solution every four hours to keep the concentration of sulfuric acid within 10% of the original concentration. The experiment with the 30 p M sulfuric acid was intended to simulate the environmental concentration of rainwater.
The experiment was repeated under the same conditions and concentrations using aqueous nitric acid solution instead of sulfuric acid. We found that the loss of marble in the 30 p M sulfuric acid experiment was almost three times as much as that lost in the nitric acid experiment. In the case of the 100 pM acid solutions, the marble loss in the sulfuric acid experiment was 13-17 times as much as that lost in the nitric acid experiment (see Table I). Using Xray dispersive techniques, we analyzed the precipitates on the marble surface
from the 100 pM sulfuric acid experiment and found them to contain sulfur and calcium. We discovered no visible precipitates, however, on the marble surface from the experiment using nitric acid regardless of the concentration applied. From the evidence collected, we concluded that the sulfates are the reactive materials in acid rain with the capability of significantly damaging marble. After reaching this conclusion, we concentrated our efforts on determining the origin of the sulfate and the pathway that it followed. It is known that industrial emission of sulfates is minimal. Most of the sulfur is expelled in the form of sulfur dioxide. It is also believed that sulfates are produced by the oxidation of sulfur dioxide. In looking for clues regarding the relationship of marble deterioration and the oxidative pathway of sulfur dioxide to sulfates, we analyzed the marble surface Figure a. Gypsum cry* tais (200X)were formed on the marble surface in a laboratoly experiment of chemical interaction of fly ash and gaseow sulfur dioxMe in a moigt environment
ANALYTICAL CHEMISTRY, VOL. 59, NO. 2. JANUARY 15. 1987
105A
Flgure 4. Characteristics of the particles revealed by scannlng electron micrographs (2000X) of fly ash sampled from 011- (left) and coal- (right)fired power plants with thelr EDXA spectra
of a deteriorating structure. Using a scanning electron microscope (SEMI and an energy-dispersive X-ray microanalyzer (EDXA), we found that fly ash emitted from industrial smokestacks was embedded in the marble along with the gypsum. This information led us to consider the possible role of fly ash in the conversion of sulfur dioxide to sulfates. Consequently we carried out the following three experiments. In the first experiment we placed a piece of marble, with fly ash on its surface, in an enclosed chamber. This chamher was filled with air containing 100 ppm gaseous sulfur dioxide. A damp sponge was also placed in the chamber to provide the moisture needed to create the acid. After 36 h, the marble was examined with the SEMEDXA system. We found that a substantial amount of gypsum had formed on the surface of the marble (Figure 3). The second experiment was carried out to determine whether the fly ash was actually needed for the formation of gypsum. This control experiment was run under the same conditions as the previous one except that no fly ash was present. After 36 b we examined the marble surface and found minute amounts of gypsum. These two experiments indicate that fly ash does play a role in the formation of gypsum. The third experiment determined whether sulfur dioxide was needed to cause marble deterioration. This control experiment was also carried out under the same conditions as the first but without thesulfur dioxide present. The analysis showed very little gypsum existing on the marble surface; the little that was present is suspected to have existed already in the fly ash. These three experiments make it evident that 106A
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both fly ash and sulfur dioxide are needed to cause considerable damage to the marble. Because we considered fly ash to be a possible catalyst for the oxidation of sulfur dioxide to sulfates, an analysis to find the components of fly ash was made. Oil and coal power plants are the largest point sources of particulate matter; samples from each of these two sources were examined using a Coates and Welter Field Emission Model 100 SEM, a PGT-1000 EDXA, and a Leitz Ortholux light microscope with photomicrographic cameras (Figure 4). We found that the particles from the coalfired power plants were usually spherical in shape, whereas the particles from the oil-fired power plants were more irregular. These particles appeared in various colors ranging from milk white, yellow, gray, orange, and brown to black. The black particles consisted of unburned hydrocarbons as the major component; the colorless particles appeared to have been burned more efficiently. Small quantities of metallic elements also were found in the particles. Special attention was given to the metallic components because their catalytic properties are well-known. Using atomic absorption spectrometry, we found that fly ash emitted from the oilfired power plants contained Mg, Ni, V, and Fe as the major elements and Pb, Cr, Ca, AI, K, Ti, and Cu as the minor. The fly ash from the coal-fired power plants contained Ca, AI, K, Ti, and Fe as the major elements and V, Mg, Ni, Mn, and Cu as the minor. Concentrations of the metals varied from one sample to another. We recognize that the metals in fly ash such as Fe, V. Cr, Mn, and Cu could possibly play a catalytic role in oxidizing sulfur dioxide into sulfates.
ANALYTICAL CHEMISTRY. VOL. 59. NO. 2. JANUARY 15. 1987
lnterpratation ol results Because metals exist in fly ash and are known to be good catalysts in their oxidized forms, we consider fly ash to have the ability to accelerate the oxidation of sulfur dioxide to sulfate. For example, vanadium pentoxide has been used to oxidize sulfur dioxide to sulfur trioxide. In addition, vanadium(V) is the most stable oxidation state of vanadium a t high temperatures. Also, the fly ash emitted from a fiery smokestack is known to contain vanadium. Therefore, the pathway for the conversion of sulfur dioxide to sulfuric acid with oxygen and water in the presence of vanadium(V) can be explained by the following mechanism:
2v5+
+ so, + 0 2 -
SO, + H,O
2v4+
--
-
+ ~,o,
2V'+
+ so,
H2S0, 2v5+
+ 02-
V"
SO,
+ H,O + '1202
H,SO,
Chromium oxide, ferric oxide, and other metal oxides are also known as catalysts in the conversion of sulfur dioxide to sulfate. In addition, Mn(I1) and Cu(1l) play a considerable role in the environmental production of sulfuric acid. The fact that fly ash contains vanadium, iron, copper, chromium, and manganese strongly supports the theory that fly ash on the surface of marble behaves as a catalyst for the oxidation of sulfur dioxide to sulfates, causing the deterioration of marble. Acknowledgment R.J.C. is grateful for a research grant from the city of Schenectady, N.Y. J.R.H. is a research fellow of the Alfred P. Sloan Foundation, 198688.
